Dedicated to the Promotion of Electric Propulsion in all types of Aeromodeling

A Multiplex Mentor Testbed for Year 2020

A Forty Size (Test Bed) Standard Electric RC Airplane

By Carl Murphy


This article begins where all the rest of the articles, tests of as specified by the manufacturer and flown only a few flights, ended.  Real world flying, where parts fail, break, wear out, fly off or don’t match the specifications.  Not shiny new hopes with failures censored out, and, what it costs, the net cost per hour of flight.  Follow along and try these easily available combinations for yourself.  And why I have been declared a bad example where ever I fly by Radio Control.

Before ever starting make sure the airframe is set up right, with components that last, then begin the tuning of the propulsion.  The setup of of the airframe follows.

By propulsion we are describing the whole package, motor, propeller carrier, propeller, motor-controller (to include any internal programming) and battery.  They have to match a specific RC airframe, location and flying style.  Ten motors were flown, seven direct drive outrunners, three geared inrunners, varying the input voltage from 3S LiPos through 5S and matching the propellers.  Several more motors, some defective, some obsolete, were rejected after bench tests.

Some combinations were later flown in more suitable airframes, see the parallel Fun Cub and the economic comparison of outrunners verses inrunners articles.

If you read a vast majority of magazines articles and their Internet equals, with the exception of competition machines, matching propulsion to the airframe is a neglected art.  You get an entirely false interpretation that the author chose, and demonstrated, the best, on the first try.  It becomes a form of censorship by omission not comparing equipment.  And the tests don’t go long enough to demonstrate stuff wearing out.  I haven’t even decided on the center of gravity or control surface throws at three flights, they are publishing everything is great at that much/little experience?  Which has led to the current acceptance that cheap equipment is just as good a real, no risking a quality/efficiency comparison.  Although many can set a motor-controller for braking the propeller to a stop when in-flight with the motor off, as for setting the motor-controller timing, it’s which-craft (zu Deutsch Wechseleri) to most.


At up to (600) watts-in, a typical (280) watts-out (on the bench measurements) decent quality outrunners in (135) to (210) grams on (2200) to (3800) mAh 3S, 4S at 2250 mAh and 5S LiPos, provided effortless power for twenty-five minutes (on 3S) to thirty-five minutes (on 4S) for this most typical, five foot wing span, medium wing loading, sport RC airplane, landing at the highest settable motor-controller low voltage cut-off.  That was flying in still air, on just the motor, near sea level.  Making use of slope lift (not usually enough to fly a glider though) or thermals sometimes extended that.

Flight times were so long, twenty to thirty minutes, that to extend the life of the batteries, most flights were stopped with about a quarter of the potential energy remaining.  A twenty minute flight out of a 4S 2250 mAh LiPo (which looks small for this size RC airplane) is enough for most RC pilots and well beyond usual.

Inexpensive outrunners at the same watts-in barely flew the plane, for about eight minutes, at a much higher net cost.

At a higher initial investment and lower overall cost, geared, competition quality inrunners (such as a NeuMotor 1107-Maxxon 4.4:1) provide the same performance at a lower energy consumption.  A net ((91)-(05))% efficiency verses a real (65)% isn’t just ((86)-(65)), it is flights a third longer at the same thrust and weight.  The reduced load on the batteries results in the batteries lasting longer which can make geared inrunners less net expensive.  The two available geared inrunners were too small for this airframe, they provided outstanding performance in a smaller and lighter Fun Cub.  It was hard to believe how much better than outrunners, in suitable airplanes, geared inrunners performed.

The hardest to define for what it costs to fly; The mean time between catastrophe crashes.  For the author, during the decade long depression and almost, but not quite, broke, that was transmitter failure at about every couple of hundred flights.  After “weeding out” house brand servos and such.  Reinforced, typical factory cast from foam, impact absorbing airframes (take five minutes to clean off the wing’s leading edge casting flash!) went fifty to a hundred and fifty flights with nominal patching, taking a couple of hours to sand, fiberglass and paint after fifteen hours of flight.  Usually the last few flights of the airframe it refitted with an inexpensive propulsion for use by a beginner pilot.

The medium wing loading Mentor was preferentially flown on breezy days, too much for a Fun Cub or Twin Star II.  The author is a former slope RC pilot who routinely flies in wind that would destroy a majority of sport RC airplanes on landing.  As a “traditional” sport airplane this Mentor was the best anybody has ever seen flown.  What it isn’t is a “floater”, much of the flight the motor has to run although with folding propellers it “coasted” about a quarter of a typical flight.  Lacking a slope within reasonable range to fly, the flight profile is heave it up there and glide down, play with and against the wind swirling up and across the hills through trees then take it way up there and glide back down for carefree flying.  The higher wing loading is an advantage in the wind, but it has it’s limits, from about five to fifteen miles an hour or up to (25) kmh.  The airframe was reinforced accordingly, landing gear was omitted, the belly reinforced with simple “scab on” fiberglass so it could be landed on farm fields around Rhein-Main (western Germany) and exclusively folding propellers.

Flying was with the ground at altitudes of about three hundred feet, with the RC airplane going another couple of hundred yards above that, where conditions allowed.  Fly at higher altitudes, add landing gear, used fixed propellers and the results change.  If you hung out with our national and international competition pilots at the Silent Electric Fliers of San Diego California (any closer to sea level would require submarines) you’d understand, they change every component, unsatisfied, until they know they have things just right.  Even at that one of our long time international pylon racing pilots, his propulsion selected for him by another pair of club members at NeuMotors-NeuTronics (the world’s best RC electric motors) was badly beaten recently.  He usually flies at sea level, the race was a mile up, that put him at the bottom of the F5D electric pylon list for that International race.  Sport pilots would benefit from some trial and error too.  There’s this relationship between motor and propeller and airframe and the air I don’t understand.  Since unlike my (140) horse power Piper I can’t adjust the propeller pitch in flight (and even less double the motor by changing voltages and propeller diameter) I try different combinations with my RC stuff until it functions best.  In case you were wondering I made it through nine semesters of university mathematics.  So if I give up; Ask the Editor, he has the Aeronautics degree.

In addition to motor tuning all my RC airplanes are “reinforced”.  Do what I do with a stock one and you are soon wondering why yours came apart.  I am considered a bad example where ever I fly, except slopes.  Having learned at the beginning of electric powered RC flight (when we were pleased if it flew at all) I know how to fly smooth, getting the most out of a battery’s charge and can put the landing right at my feet, like a hawk going after a rabbit.

Five Foot Wingspan, Looks like a Cessna 185/210

For the best of reasons, the 40 size Mentor (Multiplex) with a five foot wingspan or a 30 size Apprentice (Horizon Hobbies) with a four foot wing have been, for at least forty years, the most common size on RC airports the world over.  Fine adjustments that make the difference between bliss and frustration are easier to see and make.  They look like a “real” airplane, they handle like “real” airplanes, the size makes them easy to see in flight, the response somehow more natural, buffeting by wind and thermals more manageable.  And yet they still fit into passenger cars.  With electric power they don’t even stink of fuel lubricant rest!  Three batteries fit in a back pocket.  With a Mentor size RC airplane and transmitter you can walk for miles and fly for most of two hours!

In Rhine-Main (Germany) I mostly land on fields, in Southern California (USA) mostly Mission Bay in San Diego CA although all up and down the 15 Freeway.  I fly for a couple of hours and go home, clean, with no repairs required.  Even in still air, with the back ground noise of Rhine-Main or Mission Bay, at a hundred yards few people notice the slight prop sound.  Since I knew where the weak spots were and corrected them ahead of time there is (almost) no maintenance.

What It Does Cost

An estimate, over one hundred flights (nominal) puts the cost of this Reinforced Mentor airframe and propulsion, for an average twenty-five minute flight, at six dollars a flight.  Because the flight times of equipment as specified in this report are so much longer than ordinary, for reports starting with this one, the economics is converted to cost per hour.  At fifteen flights and again at sixty flights a couple of hours sanding, spachtle and fiberglass (from landing on water frozen into rocks and rocks hidden in mole hills in the long grass) was the only maintenance.  I failed to tighten one folding propeller combination enough so it flew off and was lost, other than an inexpensive propeller collet and using up the batteries those were the only losses and expenses.  The cost to fly a Mentor is about fifteen dollars/hour.

This Stuff Can Hurt

The propulsion combinations in this article, at about one third to one half of a horsepower, are about the power of a hand held grinder, saw or drill.  They can rip your arm open, break fingers, sever tendons and throw a propeller blade right through safety glasses.  You had best know what you are doing before you use this kind of equipment!

Within the SEFSD I’ve had three (3) first hand accounts of LiPo batteries which exploded while charging, burning down garages. The most recent from year 2017, when a club president, somebody who ordinarily hung out in his garage while batteries were charging, went in the house to get a drink.  Before he could come back the fireball was evident.  He took into account the energy potential of the many batteries in there and let it burn.  I had a 3S 2200 mAh LiPo explode next to me in my camper (I distractedly hit the charge button a couple of times) the initial ball of flame was about a yard across.  A thrill you don’t need!

A Mentor, Apprentice or Twin Star II size can be intimidating to people and horses.  Speeds and noise aren’t all that much and flown conservatively they don’t usually bother.  Although short of a direct head strike of the prop, being impact resisting foam, they probably wouldn’t hurt much, the secondary wreck as a horse rears up or a car swerves has to be taken into account.

See many archived SEFSD articles for lower powered combinations in smaller airframes.  The Mini-Mag/Fun Man or Twin Star II combinations, at about a quarter of the power of this article’s propulsion systems, could be considered ideal for beginners.

Eight Minute Flights Are for Amateurs

Just to be sure, an inexpensive combination (all too common) was tested.  Although it would have risen off ground, from landing gear, it could barely be hand launched.  That wallowing around is about what most RC pilots expect.

If you thought eight minute flights were good enough, these combinations all do a minimum of double to triple that while being hurled around.  I made no internal modifications to any of the propulsion, buy the same stuff and get the same results.

Although the pilot is part of it, the many times I’ve de-rated one of my Reinforced Fun Cubs, from (450) watts-out (135 gram Scorpion motor, 3 blade folding propeller) 4S LiPos, by just switching to 3S LiPos, to help beginners at Mission Bay by letting them fly my airplanes, confirmed that quality flies better, fine tuning is worth it.  National and international ranking competition pilots (F5B with seven horse power pylon racers and F3A precision aerobatics) with much more expensive equipment are an influence.

Most reports of comparable new ARFs put flight times in the eight to twelve minute range.  Such would be the case for the fine Horizon Hobbies Apprentice systems as delivered Ready for Flight.  Put in the E-Flite tuning motor, a better propeller, motor-controller and battery and flights go on for half an hour on up.  And not just flying easy, endless loops, touch and goes and in general the joy of a simple, stable, full house (motor, ruder, elevator and ailerons) RC airplane.  The cost per flight from the modifications went up, the cost for an hour’s flight went down.  Tap an Apprentice on it’s nose and it get’s beat up, replacement parts are easily available, for good reason.  That stabilization is the greatest for beginners, even semi-pro pilots use gyroscope assistance.

As delivered the Apprentice, even more so the latest versions with assisted stabilization, are fine fliers with the components a match for “the mean time between catastrophic crashes” of beginner pilots who may have no better reference then (often inept at using electric) fuel pilots.  That stock motor the Apprentice comes with is marginal new and quickly the bearings give out.  That square tipped (fuel) propeller which can withstand some prop strikes, is rather inefficient.  Fly the LiPos down to the Battery Eliminator Circuit’s cut off and at twenty flights they start giving out.  The simplest four channel radio it comes with should not be reused in anything flown over a hundred yards away.  You get what you pay for and what you pay for an Apprentice System is a best deal, if you are starting with nothing.

For the benefit of the SEFSD, who really, really know how to get the best out of electric powered flight.  About half of all clubs consider an electric motor just a torque replacement for those irritating, crud spewing, it takes a whole box of stuff to operate them, unreliable, hideously dragging cylinders stuck out in the wind that should be run an hour on the bench to break them in.  They just haven’t converted to optimum.  They like having to fuss with their stuff and don’t seem to mind the filth of fuel.  I’m in it to fly, preferably care free.  I was one of the first to convert to heat shrink, self sticking, plastic covering, foam airframes and then to electric.  Save the “ it isn’t a real airplane if it isn’t covered with silk and dope” for the history books.  Beat up propellers, motor-timing retarded, junk LiPos, can’t even round off the casting flash on the wing’s leading edge, are their (average) expectations.  Most of the flying clubs I guest at take pity on me when I show up with electric powered ARF airplanes, until after my first flight.

Another reference; The approximate Mentor replacement, the Multiplex Fun Cub NG (Next Generation) is half again heavier than the (under two pounds) Fun Cub it replaced.  They fixed the problems with the previous one, it costs a little more and reportedly with the standard (way wide and flat) 13X4 propeller with a (135) gram sport quality outrunner and a 3S 2600 mAh LiPo flights are in the eight minute range.  From a report out of the German magazine FMT the November year 2019 issue:  You can hover a just under three pound Fun Cub NG with that combination.  You can tow an equal weight RC glider up (250) meters ((280) yards) three times per charge with that battery.

Me, I’ll put on a folding 12X6 propeller, run it on 4S LiPos and stay up three times as long blasting around the sky like a slope soarer without a slope.  I have other airframes for hovering, there isn’t any slope to soar on far and wide.  My personal Reinforced Fun Cubs with about the same propulsion will fly for half an hour, they take me two days to get ready for a service life of fifty to a hundred and fifty flights.

To have a Fun Cub NG Radio Ready flight ready is about an hour plus getting your radio configured.  Service life to be determined, that increased reinforcement in the wing should have taken care of the “hinge” at the servos and although the wing strut is a lot of drag it adds a lot of strength (my Piper Pacer had two struts per wing) they made the landing gear functional and beefed up the fuselage to use it, that ought to make them a lot more durable.  Changing the motor of a Fun Cub NG, to anything that won’t bolt on at the rear and fit length wise, is going to require a days model building from a kit.

For “why bother” as a Scorpion or Hacker, sport quality, outrunner will bolt right in, follow along…


Eleven different propulsion systems (not including tuning the propellers and different battery voltages) were evaluated.  Three different, easily available, sport quality outrunners plus a inexpensive one were tested.  The medium size (for this airframe) outrunners would have functioned fine too, except that, due to use and sitting, the bearings were shot, performance was interpolated.  To that two competition quality geared inrunners plus a sport geared inrunner, were flown but are too small.  As testing proceeded and what was reasonable in the Mentor, or lighter Fun Cub was determined, a couple of systems were rejected without even trying them.  And, for nostalgia, some old stuff was also run up.  The one Duramax inexpensive outrunner was crummy.  There are some real p.o.deleted. available.  As in I tested some other brands that I wouldn’t give away or even use for short time between crashes beginners, that ineffective.

In the USA, for outrunners, Scorpion brand is a fine choice.  Hacker sport motors (a German company, not currently available in the USA) are either equal or a little better, they are my preference in Germany.  At a commensurately higher price Hacker will sell you geared outrunners or geared inrunners too.  Figure either brand sport outrunner, configured correctly, that the efficiency (watts-out/watts-in) at (70)% on 3S LiPos and (72)% on 4S LiPos.  Those are estimated values as I had no way to verify the output.  Neither manufacturer will give an efficiency rating at that price level.  For their better offerings (helicopter motors have to be higher quality) they venture (80)% efficiency.  At a higher price level outrunners max-out at about (85)%.

HiMax (former contract supplier to Multiplex, easily procured in Germany) isn’t quite as good, they cost less, and the bearings don’t last quite as long.  There were some problems with quality control at HiMax, that may be why Multiplex went back to motors with their own name on them, Permax.  Permax motors are a reasonable, economic, match between the expected life of the airframe and having the whole setup expire at about the same time.  The HiTek/Multiplex servos of the RR setups will generally way outlast the airframes they come in, although there were some quality control problems (some servos failed quickly) a while back.

The best inrunners are from NeuTronics out of San Diego, CA.  The efficiency of the NeuMotors inrunner was nearing the theoretical limit at (91)%.  Another geared inrunner, old stock sport quality, was at (82)%.  Both less an estimated (5)% through the transmission.


How Do You Get It To Stay Up There That Long?”

Motors constructed from cheap components, poorly assembled, which also came with already rough or miss-aligned, or quickly worn out bearings, (at least half of what is currently being flown), poor quality control (it doesn’t perform as claimed) on batteries that were low quality to begin with and motor-controllers set wrong, casting flash on the wings leading edge (takes five minutes to correct); Minus half, or more.  Three out of four RC pilots fly that way.

Ordinary “cheap looks like real” motors; Minus a third to half.  I couldn’t coax out decent performance from cheap motors, even though they didn’t look much different than real ones.

Worn down LiPo batteries from being flown at (15)C into the cut-off voltage twenty-five times, which come off the balanced battery charger at anything below (95)%, minus a third.  That creaps up on ya…

Motor-controllers with the timing set at a correct for inrunners (5) degrees lead used with outrunners which require (20) to (25) degrees lead, minus a fifth.  Add to that the correct switching frequency.

Fuel burner propellers poorly matched to electric power, minus a fifth, maybe worse than that.

Beat up propellers; Minus a fifth.  Many fuel burner pilots, when flying electric,  believe that beat up ends from the prop striking the ground and scrapes don’t matter.  Really!  And their usual combustion stuff shakes so bad they don’t believe in electric motors running smooth either…

Vibration, such as from an inexpensive propeller collet; Minus some.  Ok, just a little bit, for a flight or two, worse is that everything degrades quickly.  Many converts to electric power don’t realize that electric, correctly set up, has no felt vibration on the bench and such also cannot be heard in flight.

These things are all cheap, why bother?”

A         Don’t clean off the casting flash of the leading edge of a moulded foam RC airplane; Minus a fifth, due to poor flying caused by constantly rearing up and down.

B         Fly with the control surfaces set up only partly correct (equal throws on the ailerons) and fail to coordinate the turns, forget that properly set up that maximum power is only required for a few seconds at a time, minus a fifth.  Subtract down quickly worn out servos which didn’t position all that well to begin with and were off a whole tooth of centered, or mismatched centers…

I didn’t know those could fly that well”.

Stick an electric motor in where a fuel burner was intended, and don’t streamline that hideously high air resistance, don’t remove the no longer required extra weight of framing required to withstand the vibration of a combustion engine plus the decoration as the crud from the exhausts soaks everything; Minus a fifth.  Not quite as clear cut as it seems, many average RC pilots benefit from higher drag as it forces the motor to be run all the time providing more consistent elevator and rudder.  Combustion pilots don’t fly fuel burners “dead stick” unnecessarily (about one landing-crash per airplane) so they aren’t accustomed to the lower drag of a stopped electric motor, beginners benefit from not having to think about the power setting.

If using a decent quality outrunner, configuring it on the bench for the maximum continuous load (for these low speed, high drag airframes Hacker recommends a quarter over their continuous as set up on the bench, the virtual dyno for NeuMotors about the same) minus a fifth.  These things unload in the air and you don’t fly at full amps the whole time either.  Net efficiency is more complicated, it had to be flown to be determined, see the following…

Failing to determine your combination’s optimum, by flying a little on either side of optimum of propeller pitch and diameter, change weights and with that the power out by trying different quality and weights of batteries; Minus a tenth, often (usually) way worse than that.

Fight slope lift, avoid thermals, sling the airplane around with excessive control inputs, minus a twentieth.

The Stock, From Multiplex, Propulsion, As a Starting Reference

I was starting with an airplane for which I had no personal experience.  An first order estimate of what was needed to fly the Mentor is what Multiplex specified of a HiMax (200) gram outrunner, (37/5) mm diameter/shaft on a 3S 5000 mAh LiPo with an APC fixed 11X5.5 propeller.  Something always sets a limit, if we start with the maximum of the motor-controller at (53) amps times the ten to eleven volts under load on a 3S 5000 mAh LiPo (from back then) would deliver that sets the upper limit as (550) watts-in and about (375) watts-out.  HiMax rates that motor at (450) watts(in) maximum, that figures at (300) watts-out.  At (746) watts-in for a horse power ((752) DIN), the upper limit as specified by HiTek would be (0.6) hp in and (0.4) hp out.  That goes down to (0.3) hp in flight.

Just once, I saw a Mentor set up as with the Multiplex propulsion package fly at Mission Bay.  It was a distinctly ho-hum flight.  Follow along as a RC sport airplane and it’s propulsion are sorted out, that original combination can be great, with a modern motor-controller, modern 4S LiPo at half the capacity and a much larger diameter, higher pitch propeller.

Motor Testing

HiMax HC3522-0700  (Euro75/$90-)  (160) grams  (35/5) mm diameter (outside diameter of the motor/output shaft diameter)  (700) kV

This was the first motor I used in my Mentor.  For a majority of sport pilots, buying new, in Hacker or Scorpion quality, this weight and kV would be a best choice.

I picked up the motor new in the original box at a big meet and trade stuff around in West Germany October 2017 (Lampertheim) for a fifth of what they normally cost new, from a dealer who had a pile of them.  They were new, there was nothing (known) wrong with them.  At that price I should have gone back and bought more.  But minutes later I gave just Euro40-/$48- for a new MVVS motor that originally sold for Euro120-, and the deal included a used Aero-Naut folding propeller assembly worth Euro50- (new) while waiting for the next project with only one month of three I worked last summer paid out…

For the first flights I used the new/old 2/3 S (53) amp motor-controller which was part of the complete kit plus propulsion package that came matched to the Mentor, I just changed the battery connector.  Fitted with Aero-Naut 11X8 folding propeller on 4S 2200 mAh LiPos, this older controller with no lead setting and the brake not set it drew (26) amps on the bench at (14) volts on 4S 2200 mAh LiPos for (365) watts-in at a likely (0.65)% efficiency putting watts-out (on the bench) at around (230) watts-out.  Note that the power and weight, at least run up on the bench, go up one volt by using a 4S 3800 mAh battery.  I wanted the first flights to be a little on the light side at a slight loss of initial power and duration.

Flights one through three   Overcast and breezy down in the little valley, in the farm fields above the wind is two to three times walking speed.  Not yet cold, but we needed jackets.  This made a fine combination which blasted around the sky just like the smaller Mini-Mags, if bigger and better able to buck the wind.  A friend also flying with me has been subject to my fiddling with propulsion combinations, his Reinforced Fun Cub was flying on a favorite combination (removed from his Gemini) using a (105) gram Scorpion motor and Aero-Naut folding propeller (he also flies a Mini-Mag) he flew the Mentor too.  To our surprise we both felt “one” with the Mentor in seconds.  In still air or light wind we like the Reinforced Fun Cub or Gemini better, this was about the amount of wind where the higher wing loading Mentor takes the advantage.  We thought this combination, even with the propeller not folding (without the motor-controller set to brake the propeller windmills) was just great.  I was just playing with it as a toy, it appears a maximum climb for a count of ten takes it up around a hundred meters (the height restriction at Mission Bay in San Diego CA) to glide back down at a count of seventy.

I later tested the decade old motor-controller, it did fine with modern outrunners.  Although restricted to 2S and 3S LiPos, fine for “beginner” combinations (where we aren’t expecting much of a mean time between crashes, lower power preferred too) in a Fun Cub.  The latest Multiplex motor controllers rated as 2-4 S LiPos are much better as the available power to the servos is not dependent on the input voltage to the motor-controller.

An estimate is (230) watts-out, on the bench.  An estimate is I have no way to correlate the power out as that takes expensive measurement equipment.  It’s something the magazines started leaving out a while back.  I could measure thrust, but that would be on the bench and so not representative of what is going on in the air.  A reference is a couple of years old article about a Multiplex tuning package in a Multiplex Dog Fighter.  On the bench the 3S system overloaded everything but the seventy amp motor controller’s rating.  In flight, the motor-controller reporting back to base to be recorded and reviewed later, it was all at (80)% with much of the flights being below half.  Another reference is the virtual dyno at NeuMotors, it too compares bench power draw with in flight.  For their motors it is right on.  Watch that on the bench amps don’t burn the motor up and set things up for about a quarter over the maximum continuous power draw on the bench hits it about right.  After that fly it and work up, or down, from there.

For this Mentor in flight the first combination demonstrated to be plenty of power and duration at around fifteen to twenty minutes.  I walked up the hill to try it and I’d have flown more in the wind, but then and there, there wasn’t even a three foot grass strip between the muddy fields and the abrasive asphalt of the farm roads above our usual landing spot.

There is more to be had out of this combination with a more modern controller set on automatic and brake, cruising one third of the time motor off with the propeller folded, instead of leaving two amps on.  And maybe even more by using the higher capacity and higher weight batteries I have.  I own this motor, some interesting flights with the Fun Cub were way over-rating similar HiMax motors on 5S LiPos, which I still have…  The grass for landing the first flights was perfect, sometimes it’s a little rough.  It may well be that using twelve to fourteen inch folding propeller and loading the motor down more would be a best use of this motor in this airframe.

In the beginnings of electric powered RC flight we had to run everything on the edge of burning up to get enough power to fly.  In the here and now often for sport use carrying a slightly more powerful and slightly (as compared to overall weight) heaver motor being run at two-thirds power is better.

Wow, just three flights and already the best use of the Mentor is becoming apparent.  Fly it on breezy days where the glide speed of a Twin Star II is less than the wind speed.  In still air blast the Mentor around in medium diameter maneuvers.  However, set up right, the same size Twin Star II puts on a better show in close as in you can put on a demonstration in about the volume of a school gymnasium i.e. right in front of you and casual watchers.  Comments about the Fun Cub are that of all we fly people passing by see it as scale, as if they were in it.  Here, outside with half a mile in every direction to fly in I liked the higher power of the Mentor though as to really use a Twin Star II, with half the thrust of a Mentor at the same size and weight, you have to slowly climb out and put it in a dive, somewhere around fifteen miles an hour wind and if the battery goes dead you can’t glide back.

Flights nine through twelve

The dealer provided a printed sheet for recommended combinations for this HiMax motor.  It has a reasonable operating range of eight to thirty amps, the maximum amperage for fifteen seconds is forty amps.

If nobody has yet explained it to you, amps are the volume of electricity flowing, the voltage is the pressure.  That volume is almost all of the heat loss within the motor within it’s reasonable power range.  So, the maximum amps for 2S LiPos is only slightly higher than for 4S LiPos.  Although not that simple, the available power from 4S is more than double that of 2S.  If you aren’t familiar with your motor/propeller/battery combination; Work up to maximum power.

Try starting with a smaller propeller.  Fly it gently at first, if on landing the motor is too hot to touch, if you see the insulation of the copper wire, when new clear, turn dark, you are pushing the motor too hard.  And, as the bearings wear out the amperage draw will go up and the performance down, so that what was OK initially at near the limit, eventually burns up.   Just changing to better batteries can up the power by a fifth, measure for that too.

And duty cycle, how much of the time a piece of equipment runs verses how much it doesn’t can be right on the edge too.  I had a (28) mm diameter, inexpensive (62) gram motor in a Reinforced Fun Cub that was great fun in light breeze flying it on 4S.   I’ve flown that combination a few times a year on either still or minimal wind days for a decade.  Until an afternoon in stronger wind, when I almost burned it up, as I was running the motor for a higher percentage of flight time.  The difference was that for short to climb bursts the motor could absorb heat, cooling down during the glide portion.  Until the wind carrying the smell of burning up insulation back to me flying on a slope.   The darkened insulation wasn’t the problem, after it cooled down the magnets weren’t as magnetic.  The bearings were giving out anyway (increased power draw at reduced thrust) which contributed to burning it out.

An estimate, for the (165) gram HiMax, from the specifications sheet; 3S LiPo 14X7 at about (33) amps times (10.5) volts is (350) watts-in.  These are “economic” motors, they look nice, probably on 3S LiPos about (2/3) of the energy goes into turning the shaft and they last about as long as the airframes they were recommended for if assembled per Multiplex instructions.

It depends on your batteries under load, depending on size and quality 3S LiPos can be (11) volts or (10) volts.  That’s on a full charge after ten to fifteen seconds of full amps.  Don’t forget, that is before going through the motor-controller, where at a minimum another (0.6) Volt is lost.

Estimate, from the specifications sheet; 4S LiPo 12X7 at about (34) amps times (13.5) volts is (450) watts-in, (300) watts-out.

Again it depends on your batteries under load, 4S LiPos can be (14) volts, or (13) volts.

(450) watts-in times (0,7) efficiency is (315) watts-out or almost half a horse power.  That ought to take a Mentor up there!  Even if in flight the combination is outputting more like (215) watts-out.  Twenty minutes run up this motor on the bench, the airframe had three full flights, it was time to put some power to this Mentor!

But before that, a more modern motor-controller from Rock Amp rated at (40) amps, with the timing set on automatic and the brake activated was installed.

12X7  Aero-Naut folding blades  3S 2400 mAh LiPo  (33) amps at (10.5) volts  (225) watts-out  From experience with a geared motor this would fly the Mentor, but only just and for half the duration of 4S combinations.

12X7 Aero-Naut folding blades 4S 2250 mAh LiPo  (34) amps at (13.1) volts  (320) watts-out on the bench, estimated (225) watts-out in the air.

Not much change using the bigger capacity 4S 3600 mAh LiPo, (13.3) volts

This was too much of a good thing, the outside case heated up during testing on the bench.  From experience if it is that close to overload by running a half a minute at part load then fifteen seconds at full load, a pause and then repeating with different batteries; I don’t need to run a sport motor that close to burning up.

The problems with heat isn’t just burning the insulation on the wires, heat a magnet too much changes the crystal structure, it isn’t a magnet anymore.  The wires are inside of that outside “bell”.  By the time the heat has made it to the outside case the stuff inside is already too hot.  And overheating may damage the lubricant in the bearings too.  So; Try a less on the limit combination.

12X5  Aero-Naut folding blades

(24) amps at (14) volts  4S 2250 mAh LiPo  (336) watts-in  (235) watts-out

(23.8) amps at (14) volts  4S 3800 mAh LiPo  (330) watts-in

Not really enough.

12X6.5  Aero-Naut folding blades

3S 2400 mAh LiPo (21) amps at (11.0) volts  (230) watts-in

3S 3600 mAh LiPo (21.5) amps at (11.3) volts (240) watts-in

Interesting, sort of, but just barely staying in the air isn’t what I’m looking for.

(27) amps at (13.4) volts  4S 2250 mAh LiPo  (360) watts-in  (250) watts-out

(28) amps at (13.8) volts  4S 3800 mAh LiPo  (380) watts-in  (265) watts-out

Now that’s more like it!  The motor was just slightly warm after testing.

Did I mention I like carefree flying?  Not bragging about how cheap my stuff is or how quick out of the box it was in the air.  Part of carefree is knowing I won’t scrape up the prop or bash the foam on a medium landing.  Now it’s ok to have to “index” that the propeller is folded on the fuselage sides for landings, rather than sticking straight down, when working up a combination, but not over the long haul.  Although as first rounded off and fiber-glassed eleven inch propellers in flat pitches folded fine, at eleven inches with a seven inch pitch if they were straight down on landing they stuck out so far that damaging the propeller blades was just a question of how many landings and not many at that.

Since it was clear that twelve inch folding and probably even larger diameter propellers were going to be the long range choice, it took a couple of hours sanding off up to eight layers of (3/4) ounce per square yard fiberglass plus two component resin I’d put over the nose and put three layers of (2) ounce fiberglass back on.  I used a block of sandpaper, a belt sander would have been useful.  That took a couple of hours on a rainy afternoon.

Flights nine and ten   After Thanksgiving year 2017.  Too much wind to enjoy a Fun Cub, and too much for a Twin Star II if you went up much more than fifty yards above the valley floor, which was flooded from recent rains.  These are the conditions I wanted a Mentor for.

This combination way beats the same watts-out of the geared motors.  Flights are about half again longer and a quarter longer then with the smaller diameter propeller on this motor.  With this much, way more, power a count of ten takes it about two or three hundred yards up.  Flown first with the 4S 2250 LiPo the glide down was about one hundred thirty.  I must have caught a roll of air on the up side as the average is more like seventy.  Going into the wind it was hanging on the prop.  Flown second on the 4S 3800 LiPo the climb was flatter, the glide back down more like one hundred ten.  The longer glide is due to rising half again higher than the geared ten inch prop or the direct drive eleven inch prop.  It’s hard to tell, the wind swirls in this little valley, one side is sink, the other turbulent.

What a great sport RC airplane on a marginal day and place.  Most of this “get acquainted” flight is at one quarter to one half amps, right where the motor is most efficient, with a burst available for when wanted.  It’s not clear cut which battery is better.  Maybe the best is to be glad I had two sizes.

With this much power some other issues start showing up.  The standard motor plate with four adjusting screws doesn’t have enough down thrust.  Since the amount of down thrust required is both speed and thrust dependent, there is no perfect angle.  The only solutions are to mount a fixed motor plate or accept it and use some down elevator.  That is part of the reason all full sized airplanes have trim.  The ailerons have too much throw, time to use the computer in the transmitter to work on that.

This combination way beats the previous ones tried.  This is likely a best combination.

So, after testing the rest of the selection (three of five potential outrunners, two of three potential geared down inrunners) it was back to this HiMax HC3522-0700.  An estimate was try again at (400) watts-in with a twelve inch folding propeller.  With Aero-Naut 12X8 folding blades it drew (29) amps at (13.9) volts on a 4S 2250 LiPo.  That is (420) watts-in times (0.7) for (300) watts-out on the bench, probably (210) watts-out in flight.  That is just a little more power on the bench than last time.  And if the motor comes back down cool I’ll risk running it harder.

So far every single landing has gone perfect.  And yet, off to the side just above the belly fiberglass the foam has been compressed.  What that demonstrates is that at some recent landing it hit on a hidden rock, the fiberglass distributed the load.  Without that additional ounce of slick reinforcement I added to the belly I’d have an RC airplane with a chunk gouged out of it.  Repairs took an hour.  With balsa I’d probably have had to build another airplane…

Flights thirty through thirty-two  Just above freezing with two layers of gray clouds, no perceptible wind at ground level and not much above.  Although a goal of this RC airplane was to fly in the wind, even in these still conditions it demonstrates that bigger flies better.  I’ve seen a lot of RC airplanes fly, this is the best semi-scale sport combination in this size I have ever seen.  Although I’ve had enough thrust in half as big Mini-Mags to take them vertical (too), this bigger size is more fun and easier to fly.

There was so much power available that the launch needed to be at part amps.  The reason being that at the minimal air speed of a toss that the motor first pulls the airplane both forward and down, until the airspeed builds, at which climb sets in.  There is no perfect straight ahead down-thrust on the motor with this type of airframe where the motor is way below the center of drag.  This is the limit of what the frame rails and screws in the motor mount plate allow.  The count of ten maximum climb takes it way up there, the highest yet, the glide back down is in the one hundred range.  Verses the weakest combinations with very low pitch propellers where the maximum climb is only slightly “uphill” from level flight, with this combination you hang it on the prop, seemingly (65) degrees up from horizontal.  Acceleration is just great.  Most of the flying is at part amps as at full amps the motor is on the verge of overwhelming the propeller in level flight, part amps is just right for zooming around anyway.  That may be just as well as that puts the motor into a more efficient mode.  Flight thirty-two was with a 4S 3800 mAh LiPo., the Mentor was flown just for the fun of it, mostly motor on, no bother with climbing way up there and glide down, just let it rip.  With 3000 mAh used the flight went on for twenty minutes.  When you get to this power level time and glide start failing for comparisons, the maximum loop was about fifty meters.  It would fly that loop until the battery ran down! The fuel burners would be amazed, and if so inclined you can cut the power, let the prop fold and just drift too.

To be reminded; My stuff is “reinforced”.  The whole inside of the nose and a trough down through the middle were treated with fiberglass.  That included securing the motor mount rails to the foam airframe.  Then the whole nose is wrapped in fiberglass on down to the belly past the wing.  Put that much power to a standard Mentor and the nose rips off.  I know, I saw that happen to an Acromaster at Mission Bay.  Multiplex corrected that for the semi-replacement Fun Cub NG and probably did something similar for the Next Generation Acromaster too.  We didn’t so easily have that much power available when they laid out these airframes.

Right after all three flights everything was felt for heating, only the battery on the third flight was just slightly warm.  That drag as soon as the motor is run was still there though.

Next up was to more power to it.  Two Graupner 14X9,5 blades flashed (43) amps.  I thought about it, the combination would likely hold, but decided against it.  Part of that was as soon as this motor is run that drag is back.  And it puts that motor-controller over it’s limit too.

That would normally be the end of the 4S LiPo testing for this motor as it looks like a folding 13X6 or 13X7 (I didn’t then have them) might be the limit.  Except I have a propeller carrier not often seen, one that holds three propeller blades.

Three Aero-Naut blades (3)X11X8  (32) amps at (13.6) volts on 4S 2250 LiPo makes for (435) watts-in, (300) watts-out on the bench and probably (210) watts-out in flight.  The battery voltage fell off some at the plus ten percent over the previous combination, or there is some variation between three seemingly identical 4S batteries, the increase wasn’t as much as might at first be expected.  Something interesting, I ran it up with (3)X11X6 (that on a (135) gram Scorpion with 3S and 4S LiPos has been a great combination) but it vibrated at some rpm ranges.  Change to 11X8 propeller blades and it all smoothed out.  There’s another issue with three blades and belly landing, the “indexing”, one blade straight up puts the other two just to the sides, on landing that has to be perfect as with the smaller diameter bolt circle the blades don’t fold all that well.  Better land on long grass (or snow) is the choice.

Flights thirty-three to thirty-six   We have this awful annual event north of the Alps called “winter” and I have the flue.  Well, the Rhine glistened in the distance, the sun low in the sky at noon (twenty-one degrees above the horizon) but the wind was coming down slope over the forest.  It was a short flight, too turbulent to enjoy.  A few hours later at a different site, what a fine combination this is.  The propeller no longer makes sounds as if it is going into cavitation, in fact, it is quieter than two blades.  As for climb at a count of ten, it is climbing steeper although not quite vertical, it was hard to decide if it was flying higher.  I had the 12X8 two blade combination with me, but my fingers were too cold to change propellers.  The glide down counts varied between eighty-five and two hundred as there was some up slope wind, the surface wind across the bluff changed over a hundred degrees now and then and I may have caught a thermal, even with hail coming in.  Two 4S 2250 mAh LiPos and one 4S 3800 mAh LiPo were enough for an hour and a half in the air.

That is the upper end of watts-out testing as this combination provides more thrust to the airframe then is useful in this, at times just a walking breeze, and at times enough gusts to need better shoes on the long grass.  When the airframe starts to buffet from too much speed that’s all it can use.  Hitting full amps to avoid flying a negative ground speed (backwards relative to the ground) combined the effects of gusts and thrust and the whole airplane shook.

We call this kind of weather, mixed clouds and sunshine “Wagenerian”, from the tragic operas.  Every single landing to date has been perfect.  But it is just too much hassle to have to fly there where there is no risk to the propeller, and the horses at this location aren’t used to company.  Three blades was interesting perfecting how much power to put to the Mentor.  For carefree flying it’s time to go back to two blades.

Flights forty-eight through fifty-one   gray, a little above freezing, not as muddy as expected and I’m was feeling poorly, so, the wind was a problem for the pilot.

HiMax (165) gram (700) kV

Aero-Naut 14X6 blades

(32) amps at (14.5) volts on 4S LiPos

Maybe this combination is breaking in?  That works out to (450) watts-in, the maximum rating for this motor, (300) watts-out on the bench and an estimated (210) watts-out in flight.


Speed in level flight wasn’t all that good.  Climb was near vertical!  Surprisingly quiet.  This needs a little more pitch.  Ten seconds maximum climb made for a glide back down in the (90) range.  I was more into getting outside and flying then testing.  I ran into my flying partner on the way back.  After which I collapsed on the couch and slept early, too tired to do physical therapy in the Thermal Pool.

Flights fifty-two through fifty-four  Nominal wind at walking to double that speed.  Near freezing, loads of mud from the snow yesterday.

Graupner 14X9,5 folding blades on 4S LiPos

All four 4S batteries were already in the camper, I just swapped the blades and decided to have a go of it.  This is the greatest combination yet.  Much better speed at part amps in level flight.  Oddly, best time to climb is much flatter than any other combination.  A count of ten takes it at least two hundred meters up!  Glide back down in the (90) range.  The motor doesn’t run even warm, but the motor controller warmed up.  This might be too much at a more aggressive flights style, such as more wind, and at higher temperatures.  A suspicion is that 14X8 or 13X8 might be even better.

Flights fifty-five through fifty-eight.  Uncommonly warm temperatures for January year 2017 in Rhein-Main (think the coldest it gets in San Diego) with good visibility under the clouds   still air at ground level with a fine view of the Rhine River, although an estimated ten miles an hour at cloud height

Three counts of ten maximum climb and glide back down; (120), (135) and (140).  What a great RC airplane!

Not long afterwards I was in the flying bus and off to SoCal.  What was expected to be six months turned into almost two years.  During which I hardly flew RC.


The Mentor is, from a simulator sense, an ordinary RC sport airplane.  Pick one from the choices in your simulator and go fly it on the computer.  Get the size about the same, about the same wing loading and fly it.  If you are really into it the airfoil is a little different, undercambered from the old days when it had to carry more weight with less thrust and so a lower speed, it is more efficient and more maneuverable than a flat bottom Clark Y.  It appears that although the simulation will allow substituting electric for combustion, that the improvements of the streamlining are not taken into account, even with a fixed propeller.  Glide with the motor off and the propeller won’t be modeled from a fuel burner configuration.  In flight tests, in which the propeller was left to windmill, verses folded, the glide back down time for folded is at least half again longer.

My Mentor flew with the prop folded at least a quarter of the time.  I was having too good a time enjoying flying to quantify everything…

We used Real Flight Version (5) for a while.  That’s about ten years behind the current.  If you had the time and patience you could model the Mentor.  I suspect that with the improvements in Real Flight, now up to Version (8) (about a hundred bucks for a stripped down version with a controller, double that for the whole program) you could do a lot more.  From competition helicopter pilots they get a lot more out of computer virtual flying.  As a dynamic aerobatics, fixed wing, RC pilot; Simulations just weren’t interesting to me.

Although there are others out there, my interest at motor calculations (virtual dyno) were, for this report, restricted to the one at NeuTronics.  For their motors it was right on.  Possibly the most important was the confirmation that I had been seemingly way over powering my motors, on the bench, for a decade.  In the air the loads are reduced, the power goes back at least a quarter.  Just make sure your motor-controller can withstand the higher start up loads and don’t overdo the amps to the motor either.

I didn’t get around to trying it, Multiplex has a simpler simulator with their airplanes programmed into it.  Partly money (I was almost, but not quite, broke for a decade) and partly because only the simulations where you could virtually roam around in were interesting.  The ones where the view was from one place were quickly boring.

The Airframe’s Power Requirements Verses Duty Cycle

From the very first guess, and followed by testing, by a RC pilot who won’t let it be until knowing he has the best out of what a combination can do, the comfortable range was about (300) watts-out on the bench.  That was about the upper limit as in level flight as more than that resulted in the airframe buffeting at full amps in level flight, that is a low-medium speed sport wing!

Folding propellers from ten inch to fourteen inch were tried.  Bigger was better, optimum for the Mentor was twelve to fourteen inch.  Although larger is easily available, measurable issues such as gyroscopic start showing up, the propeller gets in the way and maybe aesthetics too.  Final testing (two years later) some ho-hum combinations first flown in the Mentor using ten to twelve inch propellers provided great performance in a lighter airplane.

2S verses 3S verses 4S verses 5S verses 6S

At this five foot size running an electric motor on 2S LiPo voltages is pointless.

At this five wing span and four pounds size running an electric motor on 3S LiPo voltages was pointless.  At most a worn down airframe with a worn down 4S combination could be reconfigured for a beginner on 3S.

Motors scaled for 3S run on 4S provided fine performance.  Even on the smallest standard size of 4S 2250 mAh performance was spirited and duration so far beyond any “normal” expectations that there wasn’t much need to go beyond that.  There’s this trade off, more voltage and the capacity of a larger, heavier battery isn’t necessarily more fun.  Just the same, I tune my RC airplane’s weight for wind with the battery weight.

5S LiPos had to await getting a better motor-controller.  My Talon 2-6 (40) amp controller was back in SoCal.  Using the NeuMotors 1105/4.4:1 on 5S LiPos in a Fun Cub was inclusive as it put out so much power already on 4S LiPos…

Although 6S LiPo would have been usable with two of the motors tested, it was not tried.

The Inexpensive, Seemingly As Good, Almost, But Not Quite, Useless

Dualsky  (165) grams  (35/5) diameters  rated as (1050) kV   $55/Euro44-

The attempts with the (165) gram Dualsky are interesting as they represent about the best that can be had out of an almost, but not quite, too cheap to be real motor.

This was the third motor tested.  I came into this lowest purchase price (for it’s size) outrunner, near new, as it would not have been satisfactory in the complete Multiplex Gemini (too heavy) it was originally installed in.  That used Gemini misadventure (I junked everything but this motor, even at that I threw the prop carrier it came with away, and I was planning on tossing this motor too) is detailed in a previous SEFSD article.  Flipping the Dualski through you hardly notice the magnets, the bearings feel OK, the insulation on the wires is clear.  This motor, still available, cost (year 2017) new $51- plus taxes and shipping.  So, somebody is buying and using them, I might as well give it a try.

The HiMax and Scorpion motors in a comparable size cost about double that, sport Hackers two and a half times as much, NeuTronics with a transmission four times as much.

As installed in my Mentor the Dualski had the benefits of a modern motor controller with the lead set on automatic, first rate batteries, a really good folding propeller that braked to fold and glide, in an RC airplane known to fly very well indeed at that weight.  Plus a pilot who knows how to fly smoothly making the most out of what he has.  This inexpensive motor got it’s best shot!

From the decision to investigate this inexpensive motor, to, it was ready for flight, was a pleasant hour puttering at the bench.  I hit the propeller selection right on the first try.  Add to that an hour’s Internet research for what it was.

3S 3600 mAh LiPo

Aero-Naut 11X4 folding propeller

(31) amps at (10.9) volts makes for (340) watts-in, I won’t guess at watts-out.  The in flight performance would indicate an efficiency of less than half the power went to turning the propeller.


The manufacturer rates it at a maximum of (240) watts-in, so, this is too much.  Except I think there must have been a mistake in the print, something more like (340) watts-in and (30) amps maximums seem more likely.  It didn’t heat up on the bench, it will “unload” in flight, that is the power draw in the air will be less then on the bench.  I own this motor, I might as well try it.  If it burns up, at least I’ll know.

At the same weight, or even less, outrunners from Hacker and HiMax are rated at (450) watts-in, Scorpion claims up to (600) watts-in.  They will withstand that for many flights.

Flights eleven and twelve  There was ice on my station wagon overnight, by the time I left the house the wind had died way off and shifted direction ninety degrees.  So, the choice of flying nine km/six miles from home with this combination proved to have been a better choice than a much further drive (and disappointment) intending to semi-slope soar with a Solius motor-sail plane.

By the standards of fifteen years ago; It flew fine.  Launching into a walking speed wind it stayed level, accelerated to decent flying speed and just went about the task of being a RC flying machine.  That’s the advantage of the flat pitch and larger diameter propeller, a lower power combination could be used, the down side was a lower top speed.  Scale, it looks like a Piper Tripacer with the fixed pitch (75) hp combination and they fly fine.  Climb was actually ok, a count of ten maximum climb took it up at least a hundred yards or comparable with the (165) gram HiMax being run at (2/3) power.  That is the AMA height restriction over surrounding terrain and enough for most people anyway.  To keep it moving in level flight and mild aerobatics required more amps then some other combinations and the aerobatics were wimpy.  After a first try at looping from level flight at full amps, from then on I dived to get enough speed.  So equipped the Mentor performed all most people would expect.  At this level of power my reinforcement of the wing makes less difference, the maneuvers aren’t as violent as what I’m prone to.

The glide down with the 3S 3600 mAh LiPo was in the fifty-five range (half of the  same weight HiMax) with the 3S 2400 LiPo in the sixty range.  It’s hard to be definitive with the wind swirling over the forest coming up the mild hill, then over farm fields and back down the other side.  With the climb out and a half circle the Mentor is up to a third of a mile away.  It will only do a vertical up loop after diving to get up some additional speed first.  Inverted flight was too much of an adventure with such a narrow speed range between flying and falling out of the sky.

Although the text description seems “OK”, the hitch is fewer climbs then the previous combinations, not even half as many.  Duration of flight is way down, just wallowing around for twelve to fifteen minutes.  Any savings on the motor are lost on using up the batteries faster, and, landings are what beats up airframes the most.

Although having to fly with more amps at nominal crusing speed in level flight can have advantages for beginner and under average pilots as that way there is some extra airflow over the tail, so the elevator and rudder stay effective.  If the combination has the power, I have the skill, experience and usually enough power to pulse the amps and get airflow over the tail for hard maneuvers at near stall speed, most RC pilots don’t.  For this combination that is useless information as it was a struggle to just stay in the air, placing the landing with the battery flown into the low cut-off voltage would for most pilots result in writing off the whole airplane.  My flying it smooth, whereas the effective combinations would keep the Mentor flying at just two or three amps, the Dualsky required more like it took five amps to stay up in level flight.

Maybe I shouldn’t be such a snob.  Much of what I fly has a way big speed range from just creeping along to “wow” climb and I stay up two to three times as long as “usual”.  But the available thrust of this combination was so low that most “on the low side of center of the bell curve” pilots would trash it due to low power.  If they got into the air at all.  I am an expert at hand launching, few pilots could get this combination to fly before it coasted into the ground.  Complete testing might have included trying a set of ten inch blades at a higher pitch for the same input.  Except that would make hand launching too much of an adventure.  At best this configuration would “rise off ground” from landing gear.

A majority of RC pilots wouldn’t realize the flight was half as long as with a more effective motor and the same battery at the same speed.  With my Mentor fitted with this inexpensive Dualski it flew about like most people’s expectations of an Apprentice with the stock motor, a beat up propeller and the batteries giving out, which is about typical after a few flights.

You can buy a new HiMax for not much more than what this Dualsky costs, or a Hacker or Scorpion for double, to fly carefree at way less cost per hour of flight.

A suspicion is that the Dualsky could handle more input power, it landed cool the two flights it was used.  But why bother to continue with such a lousy performer?  I also couldn’t be bothered try operating the Dualsky on 4S when the manufacturer doesn’t rate it for that voltage.

This inexpensive motor isn’t completely useless.  If you were setting up an RC airplane for a beginner, where we aren’t expecting wearing out to be an issue (as in crashing and destroying everything often, long before wear shows up or efficiency matters) this might make a usable first trainer combination.  With that (5) mm output shaft it can likely withstand a moderate impact and still fly.  Twenty to fifty flights in the hands of a beginner, maybe in a worn down Easy Glider, might as well use it with a dated motor-controller along with 3S 2200 mAh indifferent quality LiPos and servos left over from other projects, would be a reasonable expectation.  If flew a Multiplex Solius just fine.

I have other motors, takeouts from ARFs, which look just as good, but perform so poorly I wouldn’t even try them or give them away.  I know because I tried them in a Fun Cub.

Bigger,  Heavier, Run At Well Under Optimum, Great

I had a couple of (200) gram motors, they were given a try.  A conclusion is that if you had them, or a source at a reasonable price, that in a (1,800) gram airplane that the additional weight of (35) grams heavier motor was minimal.  It was a net thing, although (135) gram motors could also be pushed to comparable performance (and so running at relatively lower efficiency in cruise, peak power off a bit), the (165) gram size (in ordinary quality) proved the best, i.e. optimum, for this exact combination.  But the (200) gram motors were only slightly on the far side of the optimum’s bell curve.

The efficiency of a motor is usually cited when run at maximum continuous input at which, with decent cooling, it doesn’t overheat.  Follow it back to part amps and that improves a bit.  I seldom ran a Mentor at full amps for more than a count of fifteen and then only on doing a big loop and continuing on way up there.

A guess of sport quality efficiency of Hacker and Scorpion outrunners is (70)% efficiency when run on 3S LiPos, (72)% when run on 4S LiPos.  That probably flattens out on 5S and 6S, I have no references.  That no references is because starting about a decade ago a vast majority of published material left efficiency out.  Even fewer cited durability.

HiMax 3528-1000  (200) grams  (35/5) mm diameters  (1000) kV

The fourth motor tested.  This is what Multiplex specified, this is what was included with the (seven years old) propulsion package from Multiplex the Mentor came with.  The designer’s specification was to use an APC 11X5.5 fixed propeller with a 3S 5000 mAh LiPo.

As originally delivered the Mentor has a big front “chin” intake and two big rear outlets, required back then as everything heated up a lot more than modern equipment.  Ten years ago we expected to land with warm to the verge of hot batteries, the motor-controller and motor warm.  I blocked the original inlet off, around the motor plate is clearenced becoming the front air intake as with modern equipment heat isn’t the problem it once was, all I am concerned about getting rid of heat from nowadays is the motor.

Also tested with the fixed APC 11X5 were my selection of motor-controllers.

A new Rock Amp 40 amp 2-4 S with 3 amp BEC set on automatic timing and brake.

A five year old, way used, 2-6 S Turnigy with no settings activated.

A five year old, new, Multiplex 45 amp 2/3 S with no settings activated, run as it came.

3S 3600 mAh LiPo  New Rock Amp (33) amps at (10.8) volts         taken as (100)%

That works out at (356) watts-in times (0.67) as (240) watts-out.

3S 3600 mAh LiPo older Turnigy     (29) amps at (11.0) volts         down (10)%

Maybe this bargain basement cost motor-controller could be improved with the settings, however, it was among the least expensive and sold off at a discount five years ago.  There’s often a reason why something is sold off cheap, it might have had issues as manufactured.  I’ve used it plenty, it’s time to give this motor-controller a rest by letting a beginner have it.  When later use showed a big decline at the near freezing temperatures of Rhine-Main in the “dark” months of the year, it was discarded.

3S 3600 mAh LiPo older Multiplex  (32) amps at (10.8) volts        down (3)%

That was better than expected.  Time to look up what settings it might have and go ahead and use it.

I could try flying my no landing gear Mentor with a fixed propeller, but won’t.  I’m often landing in long grass with stones, I just don’t feel like breaking a fragile APC propeller.  I could run the much stronger Graupner or Aero-Naut fixed propellers, but I’m not interested, either the propeller brakes or the motor mount does.  Part of carefree flying is leaving the house without a specific destination knowing what I have can land anywhere I fly with no maintenance and no broken parts.  So, having established that the best of my (currently on hand) motor-controllers is the Rock Amp, the (200) gram new HiMax was run up with an Aero-Naut 11X5 folding propeller.   The carrier bar is (55) mm between blade hinge centers, the diameter with the blades mounted as eleven inches overall width.

With the Rock Amp motor-controller:

Aero-Naut 11X7 folding propeller  3S 3600 mAh LiPo  (27) amps at (11.3) volts  (300) watts-in  (200) watts-out  an estimated (150) watts-out in flight

After this motor has a couple of flights on it with 3S LiPos it might be worth investigating if running it at higher amps, up to (40) amps on 3S with a larger diameter propeller might be worthwhile.

4S 2250 mAh LiPo  (32) amps at (13.3) volts  (425) watts-in (300) watts-out

That is close enough to the maximum rating of (450) watts to monitor it overheating.  With the (new) bearings dragging the decision is to fly it a few times on 3S first.  There is also an additional option here.  For me, fly it at 4S and let it rip.  As a RC airplane wears down I start using it to let beginners get some experience.  It may well be that by just using it with a 3S LiPo this combination reverts the Mentor to a second trainer.

I’d have gone and flown the Mentor to get an idea of how it would have originally flown, except, the field above my apartment had frost on it and it started to rain, for a couple of days.

When this new (200) gram HiMax motor is just sitting there it turns free.  After being run up on the bench, even fifteen seconds at one quarter of maximum amps, it starts to bind, despite the outside bell not being warm at all.  That is the same as the new HiMax (165) gram motor.  It means that the bearings, or maybe their shields, are dragging.  Nothing else for it except to run a battery through at five amps and then go fly it.

Flights thirteen through sixteen   The first three flights on 3S were taken easy as it appears these motors need some break in.  I just flew it around enjoying it for what it was, this is enough power for most people.  Although the wind swirls around the ridge above and down through the forest, the overall direction was sink where I was flying, no need to do climb and glide tests under those conditions.  It flew about what expectations of 3S LiPos, that flat pitch propeller and (200) watts-out, now that I have some experience with this airframe, are, just a little better performance than previous 3S combinations provided.   Flight four was with a 4S LiPo.  Way more climb, the count of ten maximum climb was now almost hanging on the propeller, the additional height gain though not quite in proportion to the additional watts-out.  Part amps is a lot more usable, acceleration from minimum glide speed is way better.

But, it appears that on 4S this combination is close to overwhelming the propeller.  It sounds like the F5B propellers as they rev up, as though any more power and the propeller starts to just turn in it’s own turbulence not making much thrust.  This is just too much power for this amount of airframe drag, weight, propeller diameter and pitch.  Something else new, running the motor all the way up at minimal speeds during slow maneuvers (such as changing directions at the top of a hammer head) puts so much airflow over the tail that new, not dynamic, aerobatics showed up.  For the first time ever in this type of RC airplane I could have used more rudder.  Something else showed up, if you have the power you tend to use it, the 4S flight was on the short side.

With temperatures only just above freezing on landing the motor felt cold, and yet something inside it is dragging as after fifteen minutes at refrigerator temperatures it swings free again.  Although that improved over the four flights, it didn’t go away.  On returning home flipping the prop through you can again feel the magnets.  By now the grease shields of the bearings should be worn in, maybe they didn’t allow enough free play in the bearings?  Fifteen seconds isn’t time enough to heat up the aluminum case which might be expanding to preload the bearings?  I checked the brass collet on the shaft that keeps the bell from falling off, enough clearance there.  Pulling and pushing by hand with this motor at shirt sleeve temperature shows an estimated three thousands of an inch free play (just enough to see it) which seems a little on the low side.  I don’t have an explanation for why just a short run, as in fifteen seconds at under maximum rated power, would cause so much drag. Unlike the wheel bearings of a car, where the mechanic adjusts for just a little free play, sealed bearings like this don’t wear in, they wear out.

I’ve worked in factories, quality control, that is checking that things are going right, costs way more than most people realize.  In volume production it gets really tedious.  How the workers keep from going nuts making bearings beats me.  I had a tough time my six months pulling a lever making just a few parts of any one type at a time (aerospace hydraulics) although I went another five years as a technician, until solvents got to me.

Maybe the bearings of this individual motor aren’t as they need to be for this application.  HiMax (if they make anything at all) buys their bearings from a bearing manufacturer.  So too do Scorpion, Hacker, and NeuTronics all buy bearings from outside.  The difference as the price goes up isn’t just materials and expected performance, it includes checking that the product does as intended.  Maybe that nice surface finish combined with just a little tight has the bearings sticking out just enough to drag as soon as they even warm up?  But if that drag doesn’t improve I’ll set this HiMax motor aside for a beginner.

This (200) gram HiMax motor run on 4S has more power than an 11X4 can really use and going to a smaller propeller (like a 10X6) is probably a wrong direction.  The (165) gram HiMax motor was a better match on 4S.  What this airframe needs  is a twelve inch propeller with thirteen and fourteen inch diameters worth investigating.

So, back at the shop, a first attempt at a twelve inch propeller on 3S was an Aero-Naut 12X6,5, which flashed (46) amps, at which it flew off.  With 12X5 blades and the nut of the Aero-Naut collar tightened to the verge of stripping the threads it drew (36) amps at (11.1) volts for (400) watts-in and (265) watts-out (estimated).  I’d have gone for more, changing the motor controller too, except for that drag at the bearings and a general preference for working up to maximum a little at a time.  A decision was made to not remove the collet and carrier bar again.

Flights seventeen through nineteen  It was the first sunny day in a while with some wind, the day started out at freezing and warmed up a little for what may be the last day this nice for months.  Those stupid (if you come from SoCal) carols about sleigh rides in the snow, they have that around here.  I don’t know what Christmas traditions you family has, ours in SoCal includes going down to the beach to surf for a couple of hours.

Part of the flying experience is where you are flying.  They straightened the Rhine River over a century ago.  At first an improvement, they realized they over did it.  Half an hours drive away costing at a Euro for two km adding Euro20/$20- to the event (but a pleasant hour, I often ride over there on a bicycle taking the commuter train back for six bucks) is an area newly enclosed by a twenty foot high dike as an emergency retaining basin (in 1954 the Rhine, normally a quarter of a mile wide here, was a mile and a half wide) so you can see way out, that has been a fine flying site near Oppenheim.  That launching from a twenty foot high dike, with the mythical soft grass for landing, has advantages for testing.

I wasn’t expecting much better from this combination, instead it turned out to be just the right direction.  Not effortless power, it takes a couple of seconds after pushing the amps stick up to get to full speed, this is enough.  This is sport scale flying, the wing, intended for upright flight, is asymmetrical, so you have to fly the airplane like a scale General Aviation airplane, paying attention to the direction of lift.  You don’t have instant power, it can’t go vertical up or hover on the motor.  However, unlike the pure acrobatic RC airplanes with their neutral stability, you can back off and let the Mentor fly itself too.  As in as you count to nine at maximum climb, push the elevator forward and let it begin the glide at ten.  I don’t know any pilots of full RC aerobatic ships who fly at half a mile out and half that far up, which is just fine with a Mentor.  That way out there, knowing the RC airplane is stable enough, lets you enjoy contemplating the scenery.  The big jets out of FfM (ten miles away) and the gliders out of Oppenheim (two miles away) are prohibited from flying here.  There are big egrets around, pure white, one of which I confused with a man carrying glider for a few seconds.  Climb at a count of ten took the Mentor way up there, a hundred and a half yards, probably higher as my visual estimates are likely off that high up.  The glide back down counts varied between sixty-five and one hundred and thirty (the wind is doing both vertical and horizontal boils coming in and down to the Rhine River here) with an estimate of ninety-five average.  For the first time the 3S 4500 and 3S 2600 LiPos had better climb/glide ratios than the 3S 2400.  There was buffeting from ground level all the way on up, the Mentor just flew right through it.

I’ve flow whatever I had here.  Today was too turbulent to enjoy a Fun Cub or Gemini.  There were no thermals, due to turbulence from trees the dike can’t be slope soared, so no need for a Solius, even though it could be landed in the calm at the backside of the base of the dike.  It was too bouncy to really enjoy a Fun Man or Easy Star II (both would require more than the stock motor) or Zaggi.  The air was calmer up high, too high to enjoy a smaller airplane.  A Twin Star II, even with upgraded motor(s), would only be just able to keep up with the wind while you had power on, nothing more, although I could have put on a show with it flying just above the dike.  Even though the grass was fine to land on, I’ve landed here on snow, remelted snow tramped down and turned to ice rocks, this Mentor is (slightly) modified to take that.

These flights were the here and now I wanted the Mentor for.

It would take a stop watch and repeating a pattern back to back swapping combinations (easy enough to do taking about five minutes in the field) between this (200) gram motor on 3S and the (165) gram motor on 4S to decide if this is the best combination yet as it was that good.  And a careful look at costs to decide if this (200) gram 3S combination or the first (160) gram 4S combination was the best value.  It might come down to a personal preference, style or what you had already.

Flights 59 and 60   Mensfelderkopf Wind coming up the wooded slope

On the bench with 14X6 Aero-Naut blades it drew (50) amps.

Up it went and up it went well.  Great performance although less diameter and more pitch are in order.

However, that binding on warming up is still there.  This motor is for sale, an Andrew Jackson, contact the Editor, who gets ten percent.

HiMax HC4220-0510  Outrunner   (510) kV   (207) grams   (42/5) diameter/output shaft   Euro66/$80-  recommended 5S and 6S LiPo   This was the eighth motor tested.  If you stay with outrunners; Get a Hacker or Scorpion like it.  This was the most fun combination.


This motor is still available new for Euro66/$80-.  It was the stock combination for the Multiplex Tucan, rated as (680) watts(-in).  Multiplex recommends running it on 5S LiPos with a 15X8 propeller at (37) amps or 6S LiPos with a 13X8 at (35) amps.  Another vendor recommends 4S LiPos with a 14X7 propeller.


Out buying new folding propeller blades, in person, in a store (!), there aren’t many left, that one went to by appointment by the Corvid-19), there it was on the shelf.  Out of box slightly used, the magnets are better them modern stuff, no felt drag at the bearings turning it through by hand, for Euro20/$24-.  You can feel the magnets of this one when you flip it through.


At which, to use this motor, I start running into having to purchase too much new stuff, or so I thought.  My currently available motor controllers are a Talon 2-6 LiPo at (25) amps and a Rock Amp 2-4 LiPo at (40) amps.  I had neither usable 5S nor 6S batteries.


Run it as well as it can on 4S LiPos at not more then (45) bench amps was the decision.  Since that is what I settled on for the (165) gram and (200) gram HiMax motors at about the same on the bench (630) watts-in, a reasonable comparison.


Set up with 13X8 Aero-Naut folding propeller blades, on the bench it pulled (27) amps at (14.8) volts.  That is (400) watts-in, (280) watts-out (estimated) for (200) watts-out in flight.  This motor can easily put out way more.  Less that strange drag as soon as the motor is run up.  From previous testing that is all this Mentor can use.


Something quickly apparent, as soon as run up on the bench, afterwards the motor drags.  An estimated three times as much resistance to turning over as either of the other two HiMax motors of about the same weight.  I was wondering if it was worth the risk of using it.


I’ve been using HiMax motors for a decade.  They were part of the packages Multiplex specified for their airframes.  I found them to be a very reasonable initial price compromise between efficiency and durability about matched to the expectations of how long an airframe would last.  Half of them were acquired after the airframe they were in was wrecked.  Two of three HiMax motors in this test I bought new, right about the time Multiplex decided to go back to their own “house brand” of Permax, all three tested have something wrong in that as soon as warmed up there is something dragging.  This was a hobby adventure, not representative of what might be, valuable for bringing up things which don’t always function right.  That showed up at an expensive, encapulsated motor too.  The three motors were used anyway and provided spirited performance.  That was a goal; At what price level do things work well enough?


It isn’t just our little hobby bearings, a friend who keeps Ferrari motor cars going reports the latest available wheel bearings, in SoCal where you can’t go near as fast as the cars are capable of, are now lasting just a thousand miles.  Try explaining that to the customer of a two hundred and fifty thousand sports dollar car.

Flights 60 and 61  North Overlook  Last of January year 2017   First and last not warm, but not cold and sunny day of the year so far.


The wind had picked up too much for a Fun Cub, although here is normally too narrow for the much heavier and faster Mentor, to get anywhere else would have taken too long.  Wow, yep, the prop drags, otherwise it doesn’t heat up.  It’s hard to set to words, what a great compromise of level flight and steep climb performance.  I put on a show.  What effortless power, what dynamics in flight.


Got to take a look at the aileron servo settings as after having the twin antenna long range receiver in the Fun Cub things were off, it took some fiddling to fly with the last of daylight.


Flights 62 through 65  Valley under the Village   early December 2019  The plane had sat for two years.  Hooked up to a 4S 2200 mAh LiPo it ran up fine.


Something odd about the configuration of the ailerons, way left trim evened them out.  I’m going to have to go back through the instructions and figure that out.  For right here right now, from experience with similar airplanes a Mentor can be landed with the elevator plus just one of two ailerons or just the rudder.  Fact is throws on the ailerons looked just about the same, better trust I had it right.  It was too breezy for a Fun Cub and marginal for a Twin Star II, these are the conditions I wanted this airframe for.  I flew just for the fun of it.  Four batteries made for an hour and a half of flight.  Great flights, the motor still drags.


That drag buged me, there is a retaining ring (to keep the bell with the shaft from falling out) which although the jam screw would not back out, a light rap with a hammer created about (1) mm of clearance, that will not be the source of binding again.


If what you want is carefree flying, and, insist on using outrunners, this was the final and best choice.  My Mentor had a hundred flights on it, it was for sale.  As has happened repeatedly, part of the trade included several cases of local white deleted, no tax stamps on the bottles.



A result of being out of practice, a friend stalled a Radio Ready Fun Ray (semi-hotliner from Multiplex) on the first flight.  The motor was run up on the bench with the Reinforced Mentor.  Something not previously commented on is that the distance between the hinges of folding propellers changes their performance.  The Fun Ray propeller carrier is way wider then what I had been using.

(37) amps  (xx) Volts  3S 3200 mAh LiPo  (400) watts-in on the bench.  So much for 3S systems being obsolete, that moved the Mentor along just fine.  Something odd, again the bearing dragged as soon as the motor was warm.


Scorpion (135) gram motor (35/5) diameters   (1050) kV and (730) kV  about $100-

These could have been the seventh and eight motors tested.  If you wanted a recommendation for the Mentor sized RC sport airplane, of what you could easily source in SoCal, this motor on 3S and 4S would be a fine choice.  I was going to try three different Scorpion motors of the same size, I have them with three different kVs, on 3S, 4S and 5S in the Mentor.  Or so I first thought, as it turns out only the one with the lowest kV had potential in the Mentor.

Except that it is doing just fine in a Multiplex Solius turning a 12X6 Multiplex folding propeller blades on 3S 2400 mAh LiPo at thirty amps at eleven volts.  The experience from other combinations was; No need to try it in the Mentor where it would have provided decent performance.  Going to 4S wouldn’t likely have brought it about even with the (165) and (200) gram HiMax(es) for flying performance, at a shorter duration per flight as it is being flown at a relatively lower efficiency (both the motor on it’s efficiency curve and the less effective ten inch propeller diameter) the one ounce weight savings much less important in the Mentor, so why bother was my choice.  I had one other motor, a veteran of many, many flights in Reinforced Fun Cubs, which having sat for most of three years, just didn’t turn right.  What happened is that the daily change of temperature drew moisture in, the front bearing had rusted.

All my HiMax motors at the (135) gram weight the bearings had either been worn out or impact damaged.  Twice failed radio equipment and the hard stall of a Blizzard, most reports with that discontinued airframe put the average time between catastrophic failure at five flights, mine made it ten.  After testing there was just no need for this report to extend it out to motors needlessly lighter and less efficient when weight wasn’t much of a factor.  Scorpion and Hacker motors were enough more effective to cease with HiMax motors.

My choice was rather then push my (135) gram Scorpion outrunners hard (Scorpion rates it as up to (600) watts-in, a third more than HiMax or Hacker at the same size) to just leave well enough alone.  Although my sample rate is too low to be valid, and extends years back into the past, all three manufacturers, HiMax, Scorpion and sport Hacker you may expect that at around fifty to seventy flights, used at the limit of what the manufacturers recommend, that the bearings start giving out.  Scorpion and Hacker sell, at a reasonable price, replacement bearings.

We also flew (135) gram motors in the Fun Cub and Gemini, where they were excessively heavy, excessively powerful and great fun.  See my parallel report on optimum Fun Cub propulsion.

Just the same, this Scorpion motor has made a hundred flights in a Reinforced Fun Cub, half of them with (450) watts-in on 4S with 3X11X6 Aero-Naut folding blades (for short bursts, as in vertical up) already.  But in that airframe full power for more than a few seconds was unusable, in a Mentor with the higher duty cycle lower efficiency and wear would become a factor.

A Try At, Now Exotic, Geared Down Inrunners

I had three geared inlinner motors in inventory.

There are two current electric motor options.  The vast majority of what is currently available for use for RC flight is called an outrunner.  As it is the reverse of a vast majority of other electric motors, with the outside spinning the output shaft instead of the inside.  They took over RC flight a decade and a half ago.  In general outrunners have higher torque at a lower rpm making them a better match to the propeller requirements.  As in attach the propeller directly to the shaft of an outrunner on direct drive.  They are currently almost, but not quite, all you see in use.

The original type of electric motor, the inside turns the output shaft.  However at the rpm range useful for RC flight they need to turn too fast for the propellers.  An engineering trade off, spin an in-runner motor at way too high rpm for the propeller, so the motor is really effective using the energy input (electrical watts-in) for shaft mechanical (watts-out) output.  Mechanical watts-out, which is always less than the electrical watts-in, ranges from not even two thirds typical of cheap, indifferent outrunners on up to over ninety percent for well matched, top quality inrunners.  But getting that maximum efficiency, by using an almost as expensive as the motor (and it consumes some power) transmission, to match it to the propeller size the airframe needs.  Note that even top quality stuff that the efficiency falls off if you don’t match input voltage (which is quality of the battery and motor-controller depended too) and propeller.

See NeuTronics, racing grade Hacker and Plettenburg inliners, which can be run at over (94)% efficiency.  Configure them “wrong” and that quickly drops into the seventies percent efficiency.

Sport Hacker and Scorpion outrunners are in the (70)% efficiency range (when perfectly matched), and below that for not getting things at optimum.  Their helicopter grade motors, better quality (and more expensive) than the general purpose direct drive outrunners, are in the (80)% efficiency range and also available (at a reasonable price) for use in airplanes with either a Maxxon (4.4:1) or Reisenauer (6.7:1) transmissions.

Brace yourself, the efficient, effective combinations cost more money then you paid for the Mentor airframe.  But, are you in this to fly, or talk?  A surprise was that the geared inrunners, when the lower use-up of the batteries and that they hardly wear out was taken into account, they are the lowest cost per hour of flight.

I was never able to get a clear decision on how much power current RC typical transmissions consume, it has to be some.  If world champion pilots and manufacturers can only comment, “it can’t be much, they don’t heat up” (that is a first person quote from a motor designer himself) it may be down in the five percent level.  You want an as narrow as possible fuselage for speed (F5B and F5D) or all out thermals (F5J) its clear cut, geared down in-runners are the choice.  I note that none of the virtual dynos I’ve used reduced the net output when adding a transmission and yet there has to be some loss.

From the F3A competition aerobatics pilots (similar propulsion requirements to a Mentor although half again to twice as big) it isn’t as clear cut which option, in-runner or out-runner, has the net advantage.  Both outrunner and geared inline motors are in common usage with more of the top pilots using outrunners.  Consensus is that when EVERYTHING is maxed out quality (no cheap stuff at their level) that geared inline has an advantage over outrunner of five percent less weight at the same watts-in for the five percent more power output (watts-out) at a higher purchase price.  Five percent is too small to be definitive as the transient loads (you can position the amps stick in-between up and down) may give the outrunners in the constantly varying load of aerobatics an advantage.  And maybe a slightly aerodynamically blunt nose required for an out-runner, plus their own flywheel effect, is an advantage too.

Even NeuTronics gave in, they now make outrunners in the same great quality as their in-runners always were.  A decade ago they marketed large diameter in-runners which were more effective, but the customers didn’t buy them…  Some manufacturers (among others, Hacker) even make and sell geared outrunners!  F3A pilots use motor controllers costing as much as my whole Mentor airframe kit, they have to be programmed through a computer…

I ran the two different geared inrunners in a Fun Cub repeatedly alternating between them.  That airframe you notice the higher weight of the “ordinary” (still way better magnets then usual) inrunner verses the competition motor.  And if you have been flying the NeuTronics when switching to the Permax you really notice that the duration is off a bit.  To anybody else the performance is outstanding.

NeuMotors 1105/3000 kV (that winding no longer available) with a Maxxon 4.4:1 transmission   (135) grams   200/350 watts(-in) held to not more than (100) C constant and burst rating, comparable in year 2019 about $230- and 200/400

I bought this maxed out quality motor (from the Editor) just before the Second Great Depression started.  It was used for some flights in Easy Star (I)s and some more in a Fun Jet (I) where it provided outstanding performance.  If you haven’t used this level of equipment it comes as a surprise how smooth they run, how long they last and how much better quality flies!  Over a decade later, some cash and some fresh lobster traded hands, NeuTronics got some fine dining and the motor a Maxxon 4.4:1 transmission.  Current production has a slightly different kV and the motors are rated at 200/400 constant/burst.  According to the manufacturer nobody has ever reported wearing one out.

1105-4.4:1 NeuMotor-Maxxon  Aero-Naut 12X6.5 folding propeller (350) watts-in on (14.8) volts 4S 2250 LiPo  From the virtual dyno that is operating this motor at (91)% efficiency!  I took (5)% as losses through the transmission.    That calculated out as (200) watts-out in flight at about (15) amps, at maximum, with the motor turning (37,000) rpm.

This little motor was flown, with my “pulsed” flying style, knowingly after a burst letting it cool, in just above freezing air it was enough.  What could happen in a more average usage would being run at too many amps too much of the time.  Calculations of running it on 5S LiPos would require a smaller diameter propeller (less effective) at only a slight increase in motor efficiency, which would represent a net decline in overall efficiency.  Conclusion, get a bigger (1107 series) motor at 300/600 watts-in.  Using the virtual dyno for selection to be a future article.

Used in a Fun Cub (a little smaller and quite a bit lighter) the performance of the 1105-4.4:1 combination was outstanding!  I’ve been flying a Fun Cub for a decade and this not only surprised me, it shocked every other RC pilot who witnessed it.  As in an equally experienced RC pilot, who had just optioned that for just fun flying that fuel was better, after seeing what a professional airliner pilot did with it (he had never flown a RC Fun Cub before) changed his expectations.  See the Fun Cub outrunner verses inrunner article (following this one) for more details.

For the Mentor it figured in that although less efficient, the available outrunners could turn a (13) or (14) inch propeller which had a better net couple for the airframe.  And that running (135) gram direct drive outrunners on 4S was pushing them harder then optimum.  That placed both sides of optimum.

Next up, I’m going to fly the stuffing out of a Radio Ready Fun Cub NG (€290/$310, add receiver, battery and go fly) and putting that NeuMotor 1105 with the Maxxon 4.4:1 transmission in a Fun Cub NG kit (€130/$145-).  It will cost me more for new parts assembling a Fun Cub NG from new parts than buying the Radio Ready one.  Except that the building the Fun Cub NG and Easy Glider 4 are manufactured in burned down in early November 2019…

Multiplex 480/4D with Maxxon 4.4:1 transmission  (170) grams including the transmission  (32/4.0) mm diameters  rated at (340) watts(in) on 3S LiPos 

This was the second motor tried.  It sat behind the counter in Orange County for a couple of decades, I bought it, with a motor-controller, new in box, for $60-, during the Second Great Depression.  If you can find a source for them separate, those transmissions cost, year 2019, at least seventy-five bucks.  Removing the transmission though requires destroying the pressed together motor, even pulling the output shaft gear requires a specific puller.  As old grease dries out and turns abrasive, it was replaced with high rpm stuff (not generally available).

Without a transmission about all this motor is good for is a pylon racer as it turns a 5X5 prop on 3S.  4S was uncommon, back then (twenty years ago) for fixed wing RC flight, they didn’t rate this motor at 4S.  What you don’t see from a picture is a good, solid case with good bearings and good magnets.  Although they had smaller motors, this one is built to last, at slightly heavy and more expensive, with better than most efficiency.  Past experience is that motors in this size can run 4S just fine, if not as efficient as motors designed for the higher voltage.  Quality control was fine, those motors are well built.  The cost of rare earth materials has gone up, so they reduced the amount in most motors currently available, this old Multiplex has better magnets then common today and so, even with the losses through the transmission, it might have better net efficiency.

It’s first used was in a Reinforced Fun Cub, operated as specified by Multiplex i.e. limited to 3S LiPos (this motor was current when NiMh batteries about equal to 2S LiPos were the standard) at not more than (340) watts-in on the bench, it provided ho-hum performance when I first used it back in year 2014.  Actually it was pretty good, by most people’s expectations, but compared to a (135) gram Scorpion outrunner on 4S, it paled.

Year 2017 the original specifications were easily found on the Internet.  Rpm allowable was up to (50,000) rpm.  I tried it on both 3S and 4S in the Mentor, just not enough the conclusion.

Watts-in on the bench was the same, or higher (final was (340) watts-in on the bench) as the lighter NeuMotors, and yet it didn’t fly the plane as well.  Since the (25) grams weight difference is near trivial, what wasn’t?  The efficiency of the NeuMotor was being operated at a calculated (91)% efficiency, the Permax at more like (82)%.  Both less a guessed at (5)% loss through the transmission.  For the final tests, me now “one” with the Mentor, the 1105/4.4:1 NeuMotor was swinging a two blade set of Aero-Naut 12X6.5 blades, the Permax 480-4D/4.4:1 Aero-Naut 12X6 blades, both on the same 4S 2250 mAh LiPos.  There was some fine tuning of swapping blades and adjusting the width of the blade carrier bar, none of which would have changed the conclusion.  The NeuMotor was a size too small, although it provided performance which astounded most people, the old Permax was ok, but a size too small too.  Both had to be run way too high up on their duty cycle for most pilots.

First run up on the bench with Graupner 10X6 folding propellers it drew (15) amps at (15) volts on 4S 2200 mAh LiPo, except that the vibration was excessive.  There isn’t anything to do about balancing folding propeller blades, except to find ones that don’t shake, if you know your carrier and mounting system is good.  In Graupners defense those blades weren’t intended for that much power or rpm, they have been doing fine at lower rpm.  I’ve had dozens of Graupner propellers over the years, that was the only time balance was an issue.  That is mentioned as almost nothing in print mentions things that didn’t work right.  I had a whole pile of propeller blades and loads of experience tuning with them.  Somebody with just one set might have drawn a very wrong conclusion about folding propellers.

From then Aero-Naut folding propeller blades (same carrier assembly), everything ran smooth.

11X4  4S LiPo (17) amps at (15.5) volts  (265) watts-in  not enough

11X4  5S LiPo (18) amps at (18.1) volts  (324) watts-in (260) watts-out  But that pitch is too flat.

11X7  4S  flashed (31) amps  too much

11X6  4S  flashed (27) amps  marginal on the edge of too much


10X6  4S  (20) amps at (15.3) volts  (310) watts-in  (250) watts-out

10X6  5S  (24) amps at (18) volts     (432) watts-in  (350) watts-out  evidence on the bench of overheating.


The old Permax 480/4D was interesting, but after flying two configurations and five flights it was set aside.  Even after trying propeller combinations on 3S, 4S and 5S LiPos on the bench the conclusion is that a direct drive outrunner in the same weight beats it.  A lot of which is that the much more efficient couple of the twelve to fourteen inch propellers much better matched the Mentor’s airframe.  I later bought one of the last Fun Cub kits and specifically configured it for this Permax with transmission motor.  That (4)mm output shaft is more durable than a (3)mm one, when a beginner hits the ground with it, but more than anything else the customer (a professional airline pilot, so close to two thousand flights a day S. Fraport often encountered on the west side of Rhein-Main) liked the feel and sound, there was something about the whine of the transmission that reminded him of the hundred thousand pound thrust turbines he flew and that much power using the flaps and landing gear was entertaining.

Something to watch for, as the any electric motor heats up and cools down, any tiny unevenness in plywood (even laminated with fiberglass and sanded smooth) and plastics get flattened, the screws loosen up.  Check the screws holding it all together after a first run and often for the first flights.  Often after a first flight the screws take a quarter of a turn to re-tighten.  Don’t just tighten, loosen first, then re-torque.  If you need help, some of us have the required “feel”, some of us don’t!

What Does It Cost To Add a Transmission?

In case you were wondering, it wasn’t until I started using transmissions on the brushed can motors and larger diameter folding propellers of my first few years with electric propulsion RC flight (mid 1990s) were satisfactory.  We found the last brushed combination I used (traded with the motor-sail plane to a friend) a year after it went missing having hung the whole time in a tree.  Years later I refurbished that RC car motor, sixteen turn double wound with an inexpensive three to one transmission and 11X8 folding propeller, twenty amps on eight NiMah cells.  Even with a modern propeller and LiPo batteries it is lame compared with even an inexpensive outrunner of today.  I have a couple of top quality RC brushed flight motors with the transmissions of back then, sitting new in boxes, I didn’t even try them.

If there is a current source of Maxxon transmissions, just the transmission, I haven’t found it.  If you are buying a NeuMotor plus a transmission and you have to ask; Pay NeuMotors mount the transmission.

In Germany, the Reisenauer Company will fit a 6.7:1 ratio transmission to most anything.  That costs, if you have them do the fitting, about €105/$111-, for the smallest size.  They will also sell you the parts.  Although at first that seems reasonable, an exact fit, which varies between motors, requires specific adapter plates.  And you need the skill to press on a little tiny gear at just the right distance.  Hacker offers various combinations of their motors already fitted with transmissions at such an attractive price that, unless you just to have a motor you already own fitted, it isn’t economical to buy the transmission and add it on yourself.

I had four, two pairs, of 20 series (nominal just over an inch diameter) Hacker motors, new in box (two sport quality, two helicopter rated) for which I paid $27- each, on what should have been $75- motors.  Wound too high for direct drive, with the availability of factory mounted ones with transmissions they aren’t yet worth it to me to retro-fit them with a transmission.  A clue from a magazine report was that for a burst to climb use in a F5J thermal ship that those little motors could be run at up to (38) amps on 3S LiPos with sixteen inch folding propellers.

MVVS “encapsulated” outrunner  (177) grams  (40/5) mm (1200) kV (claimed)  (180) grams   €120-

This was intended to be the third motor tried.  MVVS only sells really decent, if not competition, stuff, they may have had a quality control failure with the batch that included this one.  I paid €40/$48 for that motor, new, but not in the original box, plus an Aero-Naut (5) mm folding propeller collet assembly and two new Aero-Naut blades at a meet and trade stuff around swap meet.  That carrier and blades were worth what I paid, even without the motor.  A flip through the magnets’ cannot be felt indicating a recent production.

A neat idea, enclose an outrunner in a case, use a fan for cooling, with the wires coming out the back.  That way neatens things up, the advantage in a crowded nose, or a owner who doesn’t want to fiddle with stuff dangling around getting chewed up, that can be worth the additional weight and expense.  New these cost half again more than “bare” common sport motors, it appears the quality was commensurate.  This appears to be an approximately (135) gram motor (if it were bare) in a nice case.

But it turned up too high, the kV may be higher than marked as it doesn’t turn as large a propeller as the description it came with.  From the specifications reasonable was, on 3S LiPos, 11X7 to12X6 propellers.  The specifications are that a reasonable operating range is twenty-five amps to thirty-seven amps with the maximum as forty-five amps for fifteen seconds.

Since I had it already as part of the from Multiplex as part of the Mentor propulsion package, a fixed APC propeller was tried;

3S LiPo 3600 mAh  APC fixed 11X5.5 propeller  (39) amps at (10.6) volts   almost too much

That was an insufficient quality propeller mounting system.  It repeatedly flew off when tightened reasonably, when tightened enough to hold on the threads stripped.  A relic of long ago, it would have been sufficient for the original power it was shipped with as later determined by testing that combination.  My most recent from Multiplex, in a Solius produced in year 2017, functioned just fine at this power level and no doubt cost a lot less then the Aero-Naut.

Aero-Naut folding propellers;

3S LiPo 3600 mAh  12X6.5 Aero-Naut folding  “flashed” to (46) amps at; I didn’t check the voltage, it was too many amps.

3S LiPo 3600 mAh  10X8  Aero-Naut folding  “flashed” (45) amps at; I didn’t check the voltage, it was too many amps.

What “flashed” in this case means is that something, this time the (40) amp rated motor-controller, maxed out first, for no longer then me to realize it was too much.  Power went on exponentially, just the last few up clicks on the amp stick of the transmitter did this combination draw real power.

3S LiPo 3600 mAh  11X4 Aero-Naut folding  (30) amps at (10.9) volts  (325) watts-in (210) watts-out   This motor as wound is for use on 2S LiPos.  Although it could have been tried with an 11X6 propeller; I didn’t bother with a test flight.  After later realizing what a (210) gram, direct drive, outrunner can do (even being operated at way under it’s maximum) with no need for the fan nor clearance, this motor was put up for trade.

In all likelihood something was off in manufacturing, the kV is higher than intended.  The most likely mistake was made in reusing the setting of the machine for the number of wire turns per pole from a previous run, resulting in not enough turns for this one.  I have no 2S LiPos for this size, at which it could turn a decent diameter propeller and just barely fly a Mentor, so I won’t get any.  Later testing demonstrated that although (135) gram, direct drive, decent quality outrunners, run on 4S LiPos can function great, this MVVS encapsulated motor is not useful for this Mentor.  The economics are you would be better off buying an off the shelf combination, unless you had to have the clearance, rather than having a transmission added.

Who would enjoy this motor plus transmission combination is the gentleman flying next to me above the Rhine River the day before, experience equal to mine, just as demanding from his equipment, if a lot less experienced at electric power.  For him this “encapsulated” MVVS motor, and the maximum sized battery that will fit, would correct requiring tail weight in his iron on covering over balsa over foam core motor thermal ship, an ARF.  A sort of medium liner with clearance in the nose tight.  That encapsulation isn’t all that much additional weight, nor is the drag from the fan to insure cooling.  This might be fine for a short nose scale airplane, such as the WWI Triplane.

The moral of the story;  Get paid for working (finally) and quit buying seemingly ok stuff at swap meets.

Airframe Costs  

Something I find unreasonable in other test reports, omitting what it cost to fit an airframe out making it a complete RC flying assembly (easily researched) and how long they last.

Note:  Costs for airframe stuff are what I paid when I paid it, / for what they would have cost new at that time or the €uro to USA dollar conversion.  This project took place just as the use of the Internet killed off most walk in hobby stores.  The Euro/dollar ratio(s), the rate goes up and down.  Costs for propulsion stuff are listed separately, adjusted (when still available) for year 2017 prices to make comparisons valid.

Euro230/$270-X2   February 2017         delivered by the post office, new in box   complete with four servos (but not the required extensions) and propulsion but no glue(s), receiver, transmitter, flight batteries(s) nor battery charger.  With what I already had, including three 4S 2200 mAh LiPos, a modern Castle Creations (35) amp 2-6S LiPo motor controller, radio and charger.  I had most of an entire RC airplane system!  X2 because that’s not even half the original cost.  I’d have bought new, but this airframe was discontinued about the middle of the second great depression.

Euro0/25-     two extensions for the aileron’s servos and two for the rudder and elevator

Euro40/$48-/X  Multiplex Zackie CA glue, Hobie Poxy two component resin, fiberglass in (3/4) and (2) ounce per square yard thicknesses, a dozen single use paint brushes and sanding paper, and two cans of spray paint. /X because it didn’t all get used up.

Euro18/$22-  One new HiTek HS-82MG servo for the elevator.  The metal gear doesn’t change either the speed or torque of the servo, it improves the resistance to being broken in transport or worn out in use.  I just didn’t see any need to use either of the Euro40/$48-, great big and heavy, metal geared servos originally part of the Multiplex package for the rudder and elevator.  They were later bartered away.  At a hundred flights it seem to be working about as well as new.  An estimate puts changing the elevator servo out as soon, when the little variable resistor wears out and shorts out the whole airplane gets trashed.  That happened on a Fun Cub, the third airframe that servo was installed in, at six hundred flights.  The loads of a Fun Cub are considerably less than a Mentor.

Euro0/15/$18-        One used HiTek HS-81MG servo for the rudder.  It worked fine on the bench, once installed in the airframe this one constantly growled as the electronics couldn’t quite complete the hunt for center.  I was about cut it out and replace it, in the air it calms down and performs fine.

What was an unexpected irritation was first one, then both of the wing servos, new HS-81s.  I cycle any new servos a hundred times on the bench.  They were then glued in and as run up on the bench with the aileron attached, so far so good.  Then a fiberglass “patch” was put over it to make up for the missing foam and hold it in place plus a Depron filler (to build up the wing underside to flat) and another layer of fiberglass.  They chatter trying to find center when the battery is first connected, wiggle the control stick quiets them down.  I hear well, gliding the Mentor overhead I don’t hear either servo in flight.  Evidently the slight buffeting in flight rocks the servo just enough that the electronics can establish zero and hold it.


The predecessor to the Mentor was the Magistrate.  It was at the forefront of Almost Ready to Fly (ARF) which actually functioned better then what most us could built ourselves.  The Magistrate was a little bigger than the Mentor which replaced it, approximately 50 size verses 40 size.  For the benefit of the SEFSD (all we were ever allowed at Mission Bay, San Diego CA, USA  since starting there the mid 1990s was electric power) those are in cubic inches displacement for reciprocating fuel burning engines.  Realistically it took about six to eight hours the first time to assemble a Magistrate, or Mentor, plus the usual fiddling with the radio.  A fuel burner’s engine needs an hour of run time to wear in, which few fuel pilots can be bothered with.  A basic four channel flew a Magistrate, or Mentor fine.

The Magistrate was laid out for a geared down “can” brushed motor and D size Nickel-Cadmium batteries, or, a combustion motor and fuel tank, it was supplied with parts for both.  Flight times on electric power back then were in the four to eight minute range (we cheered back then if a new electric powered airplane flew at all) it was climb and maneuver as about all the propulsion could do was take off and land.  The fuel pilots remained smug.  You can’t give that kind of propulsion away today.

The brushed “can” motors needed about a battery flight to set the brushes, after thirty to fifty flights the can motor was worn out.  It took a couple of flights for the Nickel based batteries to get going too.  Set up correctly, modern outrunners there is almost no maintenance (the bearings wear out though) nor is there any break in for the motor or LiPo battery.  The fuel burners need at least an hour of running at no load to wear in the microscopic unevenness inside them.  There is a slightest wear in of brushless electric motors, as the metal plates shielding the bearings grease clearance themselves.  I play with my electric motors for a few minutes on the bench before putting them to the test.  Modern LiPo batteries require no break in of any kind.

To build up a comparable fuel burner from sticks and iron on covering is more like forty hours, it takes way more than that your first time.  Even though you can now buy ARFs, built up for you the old way, you still have to repair them the old way.  As in if not after every flying session then every third, hours of repairs required.  Multiplex molded foam was and is impact absorbing Elapor (a modified and improved Expanded Poly Propoleyene) with reinforcement.  Set up right foam requires almost no maintenance.  Scrapes almost don’t happen, hitting stuff that would wreck a balsa and spruce covered with shrink-wrap is bounced off or a dent.

Today, you can make fools out of combustion pilots (most, but not quite, all of them) with electric power, using affordable, easily sourced equipment.  Most of them as if the fuel pilots tune their equipment right they come up about even.  Although at this power level (half a horse power out) electric power is not silent, only the most carefully muffled fuel burners are comparable at not irritating.

My Mentor can shame a majority of fuel pilots with similar RC airplanes and most other electrics too.  Depending on combination it can climb straight up, although half that angle of climb is the most pleasant combination, even electric carrying too much power has down sides.  As it ages it will be used as a secondary trainer (the foam is good for about four years, flown or not it slowly goes brittle and in use the wing foam flexes out) by just putting in a lower voltage battery to cut back the thrills.  I don’t have to carry a whole box of stuff, stink up my car and garage with fuel and oil to get it going or clean off the excess fuel lubricant after flying.

For reference my Twin Star II(s) use two (40) to (70) gram motors, ordinary stuff (affordable, fifty to eighty dollars each) from Hacker and Scorpion (some are inrunners, a modified mount for outrunners) with a combined (200) to (280) watts-in for (130) to (160) watts-out.  Flights with a 3S LiPo, sizes from 2200 mAh through 5,800 mAh run twenty minutes to over an hour.  And not just balancing it around flights, heave it around in a ten meter horizontal circle, vertical up loops from level flight at twenty meters and careen around the sky like a “hit and hope” cue ball.

Let us be aircraft designers.

Any airframe will have its own net requirements for propulsion.  Full size airplanes the design begins with general requirements then starts backwards to that from the available engines and propellers.  Even then as the customers could afford it they varied the combinations.  I’ve flow full sized Piper Pacers at (125) horse power fixed pitch propellers on up through our (140) variable pitch propeller and (160) with no need for variable pitch.  The original post WWII combination was (75) hp with a fixed propeller, they did all right too although higher altitude on hot days takeoffs with a full load were too much of an adventure.  All configurations had advantages and disadvantages.  If you could afford it though the (160) combination was most people’s favorite.

I was over at Mainz-Finthen recently, a formerly military airfield and once a center in Rhine-Main for owner modified and built general aviation.  Do it yourself is fading out in Germany too.  If there was any part of that Carbon Cub that matched my six decades older Pacer, or the fifties era Cub of a friend, I didn’t see it.  It has a modern, current production motor (no magnetos) and two blade carbon fiber propeller that probably works better, weights half what my Hartzel did and costs less.  Even “homebuilt” now look like they came out of factories.  We can extend that out to the progression of the Pico through the current Fun Cub NG, which looks a lot like a Carbon Cub.

Although heavier and more expensive, for ground clearance in SoCal on my Reinforced Fun Cub, where I use landing gear, I often fly with three blade folding propellers.  Until I tried an Apprentice with it’s forward “tricycle” nose wheel, since all my RC airplanes were either belly or tail dragers, I made all my landings “dead stick” i.e. motor off.  Motor off landings, as are crusing around with the motor off, are routine for electric power. Except that with the most efficient for use with the Fun Cub two blade propellers are hanging down for the landing, jamming the tips into the ground.  Three blades made for carefree flying.

Landings with the engine stopped are often ruin for the fuel-burners.  The airflow over that upright cylinder is bad enough, the open section they most commonly use for mounting is the highest possible aerodynamic drag, without that prop pulling them the glide angle of most combustion RC airplanes is terrible and without the engine’s additional airflow over the tail the handling is unexpectedly sluggish.  Engine powered RC pilots do not willingly practice power off landings as a majority of the time they result in destruction of the entire airplane.  Even with power on prop strikes on landing often break the propeller.  That’s why many of their propellers are delivered with (inefficient) blunt ends.

If you are flying a stock Mentor off a decent runway, on the stock 3S LiPo,  at or not much above sea level with not too much wind, then the supplied as part of the balanced package from Multiplex APC fixed 11X5.5 is about right, for a first or second trainer and pleasant sport flying.  Many RC pilots are old enough that just enjoying steering their model around in the air is all their reflexes can enjoy anyway and it is important to them that what they are steering around looks like one they could sit in.  What they chose for the propeller is acceleration from the ground at the expense of a little lower top speed.  That relatively flat, fixed propeller also functions as a brake in downhill sections, an advantage at keeping the speed down when used as a trainer.  And LiPo batteries back then weren’t as good.

If I were flying a stock Mentor with the recommended 3S mAh LiPo I’d try varying the pitch and diameter of the propeller anyway, same for the size and weight of the battery.  In the here and now I don’t see much reason to fly an airframe this size on 3S voltage when 4S and 5S offer better efficiency.  Although the airframe might benefit from a larger diameter propeller, too much more would result in propeller strikes on the ground, that eleven inch fixed propeller is about the maximum with landing gear.  Smaller diameter propellers have a less efficient couple between thrust speed and the Mentor airframe speed, which may negate any gain in higher voltage efficiency.  And I prefer a “burst” climb to glide, verses cruise around on the motor, not the usual  “power” flight profile at all.

By omitting the landing gear my airframe has less drag, so a little more pitch to the propeller is often better.  As I’d otherwise build two Mentors, just a little different from each other (I’d like one with landing gear too) this is likely the only Mentor airframe I’ll ever get.  The choice was I have a dozen fine, if slightly rough, grass fields to land on five miles away without being a member of a club in Rhine-Main.  I can neither afford the further drive to a club runway (an additional $10/Euro8- a session plus an extra forty-five minutes driving) nor to be a member.  Carefree to me means affordable, net affordable, RC flying.

Able to afford more than me, a friend had to try bigger is better, after hundreds of flights in the Fun Cub sized aerobatics airplanes we moved from the comfortable field with people going by, to across the public road although still on an asphalt, if narrow, farm access road.  That cut back some of the fun of it, just the half a mile of the other site was separation from road noise and we enjoyed the other people passing by at the other site too.  A conclusion was that the bigger airframe, the five foot wingspan of the sport aerobatic airplane, was twice as big as his usual size, required landing gear and a runway, bigger then what we had available in Rhine-Main farm fields we are surrounded with.  What would have been great fun, except he didn’t enjoy the longer drive (five minutes verses twenty minutes, times two for each way) or the company of club pilots, at the nearest RC airport, was too big for where he and I flew right near where he lives.  After just a couple of flights transmitter failure, decreasing range that sneaks up on ya, wrecked the airplane.  He now uses a modern Spektrum transmitter, the one with two antenna integrated into the case.  About the four foot wing span works best for us.

Were we at Mission Bay in San Diego CA it would have worked out different, five foot wingspans fit right in.  For my SoCal off field flying the whole airplane must be able to withstand rough field “bush” landings, sport aerobatic RC airplanes can’t be used.  A Reinforced Mentor or an Apprentice, with bigger wheels, would be about the upper limit.

Regarding propellers, although fragile, APCs fixed propellers don’t cost much and are among the most effective at converting watts-out from the motor into thrust.  And they are most of what you can get in SoCal.  In Rhine-Main I prefer the more durable, more expensive and slightly less efficient fixed propellers from Aero-Naut and Graupner.  Don’t bother me with Master-Airscrew propellers, although if we meet at the airfield and that is what you have (after sneering at you) I’ll help you select the best one for your stuff.  I’d even likely loan you a better propeller to demonstrate the difference.

My “Reinforced” Mentor is going to be belly landed on grass, before the first flight the nose will have been modified (rounded and fiberglass reinforced) to accept folding propellers.  Easily available in Germany, folding propellers from Aero-Naut and Graupner are difficult to source in the USA.  In San Diego CA buy them at Discount Hobbies.  Mine are sport airplanes, the carbon fiber racing ones as seen on F5B racing airplanes, sourced from NeuTronics, are a tad expensive for my purposes, and usually too large diameter.  F5B propellers are also thin, great for their minimum drag airframes, they are too narrow (and too high pitch) for this application.   That box of “tuning” stuff I carry on flights between “The World” aka SoCal and Rhine-Main includes a couple of hundred dollars worth of folding propeller stuff.

My flight profile is not typical, I throw my airplanes around more than most, I often fly at part amps and I glide them more than most.  In the rolling hills of west Rhine-Main in between lengthy periods of still air we often have wind, if nowhere near enough with a decent updraft to slope soar on, so my powered flight is in a sense a replacement for the slope soaring I would prefer.  Thermals; Yawn, although I use them if I find them.  Way up there, suddenly (did the radio go nuts(?)) tumbling, with the light wing loading Fun Cub, as it is flung about on the edges of a thermal, are just fun with the more heavier loaded Mentor sized airplanes.  Climb hard, glide during maneuvers, cruise around, usually my motors are “proped” for maximum output (with the motor-controller correctly dimensioned to that) but run at the more efficient partial amps.  That left stick has positions between all up and all down, ya know.  Electric motors efficiency peaks at about a quarter of its maximum output, which partially explains why my airplanes stay up so much longer than yours, as I often fly at part amps…

The recently available propellers for drones, although like slo-fly propellers they seem to fall in the suitable range for a Mentor, are optimized for hovering.  In downhill sections of even a simple loop a Mini-Mag will exceed such propellers speed range so much they go into flutter.  Even on the bench some of them can be driven at higher rpm then they are intended for, they vibrate at the full power required for a Mentor.   Don’t use slo-fly or drone propellers in a Mentor, or an Apprentice.

The Fun Man/Mini-Mag  Same Thing at Half the Size

Not long after the Magistrate came out Multiplex sold a half the wingspan size, seemingly the same airplane, called the Mini-Mag.  I bought the fifth one Sureflite sold in San Diego back two decades ago, it lasted over four hundred flights through half a dozen propulsion combinations.  I assembled a dozen for others.  Originally they flew OK with the then typical Speed 400 can motor and a 5X5 propeller on seven NiMh cells.  That was about one hundred watts-in, and fifty watts-out. You read that right, half the power disappeared as waste heat.  That was after the first flight when the brushes were worn from flat to round and before the whole thing wore out.  The available power was so minimal that back then I recommended leaving out the rudder servo to save weight, something no longer necessary.

It was replaced by the nearly identical Fun Man with a decent brushless outrunner system.

My favorite combination was an affordable brand Roxxy (62) gram outrunner on 4S 1200 mAh LiPo with a 10X5 folding propeller.  That launches a Fun Man straight up or it can float/cruise around for three quarters of an hour.  A decade ago at a “Foam Waffel” meet the Multiplex sponsored pilots came over to inquire “what do you have in that thing?”. That combination works in a twice as big Fun Cub, in still air, to hunt thermals, too.

The Fun Man is the Mini-Mag renamed for use with modern brushless motors, controllers with the lead correct for outrunners and LiPo batteries.  They are great little airplanes, but bigger flies better.

Motor Controllers and Efficiency

There have been some big improvements since I started RC flight with batteries powering the propeller back in 1995.  My first couple of electric powered RC airplanes had a servo activated mechanical on-off switch.  Since the switch was not dimensioned for the required amps they quickly burned out.  And by that, where we’d now use two (16) gram servos for the rudder and elevator of a six foot wingspan motor-glider, back then we used “full size” (45) gram servos plus an additional “full size” servo for on and off.  Well, that’s what we had, the fuel burner pilots with the endless vibration and the excess lubricant sprayed out the engine’s exhaust soaking the wood didn’t need the strength, they still need the durability.  Small servos were rare and expensive two decades ago.  Those mechanical on-off switch(s) the first kits came with were way too small for the twenty amp load, so they burned up.  My first purchase from the, now founder of NeuMotors, was an electronic on/off switch.  It went a thousand flights before I gave it to another pilot who left it hanging in a tree for a year with the motor dangling from it.

With Direct Current (DC) brushed motors we had to mechanically select a compromise fixed lead.  You could see it, electricity “leaking” between the brushes and the commutator.  In the air was different than on the bench, although adjusting for the lowest level of arcing was usually the best choice.  Oh the hassle of wearing the brushes (delivered square, to fit the round rotating assembly) in and out and having to true up the comutator.  We knew from testing that “can” motors had an efficiency of (50)%, car motors were a little better.  About 1995 the first of the brushless RC airplane motors became available.  At that time competition quality brushed motors (inrunners) were in the (75)% range.

It’s complicated (on the same level as Energy equals Mass times the Speed of Light Squared) the difference between the sine waves fed into brushless (alternating current, pulsed DC actually) of modern motors, and the constantly varying impedance of the rotating assembly, which requires that the electrical force drawing the magnets be constantly (thousands of times a second) varied.  There’s some interplay (feedback) between the motor-controller and the motor best described by the technical term expletive deleted magic too.  I let the electronics experts do their part, most of the time for our Rhine-Main play the automatic setting of the latest motor-controller’s functions best.  It’s comparable to the fiddling between throttle, fuel mixture and propeller pitch, while watching the exhaust gas temperature, of my Piper, except at thousands of times a second.

When the first of brushless motors came out in the mid 1990s half the cost was the sensors and the complex electronics to use them.  Then came sensor-less motor controllers, but until about year 2010 most came with the (5) degrees lead for inline electric motors, although the vast majority of outrunner motors require more like (20) to (25) degrees of lead.  It’s natural to not know it, what they fused, hidden away, into the modern chips is way more complicated then the individual parts we could still see of twenty years ago.  Modern controllers from Multiplex, Scorpion and Hacker and even cheap motor-controllers now have adjustments for timing suitable for outrunners with the choice automatic being the usual best.  The Talons from Castle Creations have no owner settable timing lead adjustments.  That automatic timing did just fine running all the outrunners and both inrunners.  I had a settable motor-controller, it had to be reset for either type and didn’t function as well in the air with the lead requirement constantly changing.

The racing motor-controllers, such as supplied by NeuTronics, Hacker and others, have better, and more expensive, transistors (the semi-conductor part that varies the amps at the final stage) then the sport ones, and some other refinements too.  If you are racing that percentage is important.  They are also smaller and lighter, although that few grams is hardly any advantage for a Mentor.  From trying cheap verses “real” sport level Hacker motor-controllers in a Twin Star II the difference between motor-controllers didn’t show up, at least not in a way I could co-relate with perceived performance flying a Twin Star II, until getting rid of the cheap motors and using twice as expensive (sixty to eighty bucks, each) Hacker motors.  For outrunners in the fifty gram/two ounce size(s) suitable for Twin Star II stock airframes:  Flights went on for twenty minutes (and were a real show for onlookers) with twenty dollar motors (bearing problems at twenty flights) they went twenty-five minutes with decent forty dollar motors (bearing problems at fifty flights) they went half an hour to an hour with sport Scorpions (bearing problems at seventy flights).

How long do you fly between hitting the ground and trashing your RC airplane?  If you are using inexpensive stuff set up half-wrong and fly for eight minutes a flight does durability matter?  Radio problems cost me my fifth Twin Star II, with Scorpion motors and Castle Creations motor-controllers (oh wow did it fly well though) on flight fifty-four.  I had expected a couple of hundred flights before selling it to a beginner as a trainer.  I trashed two brand new motors (unknown brand, bought from Hobby People as they quit the business) their first flight, on flight thirty of my sixth Twin Star II (see the archived article in the SEFSD Newsletter) again transmitter failure.  Both times the power of the transmitter slowly gave out, until at “reasonable” ranges contact was lost.

The current production house brand Multiplex Permax motors at Twin Star II sizes are less expensive than Hackers (you could hear bearings) they don’t perform as well or last as long and they don’t cost as much.  Permax motors are just about matched to how long a standard airframe holds up, their bearings start giving out at around fifty flights.

Another advantage to modern-controllers (the last four years or so at the affordable level) the Battery Eliminator Circuit has been improved, most are no longer input voltage derived.  If your motor-controller is marked 2/3 S it puts out less power to the servos on 3S then when on 2S.  Starting with motor-controllers marked 2-4 S what that means is go ahead and run the higher input voltage(s) without the reduction in power to the servos.  So, my 2-6S Castle Creations motor controller in (25) amps will do just fine with the servos of my Mentor running my motors on Four and Five cell LiPos as the available power to the servos is now a constant (3) amps at (4.8) volts regardless of the input voltage to the motor-controller.  I flew a slower Twin Star II on a servo and radio supply of (1.5) amps at (4.8) volts for long enough to trust it.  So, modern controllers starting at (3) amps at (4.8) volts for the radio and servos should do fine for the faster Mentor.

If I get paid for last summer I’m going to build the next RC airplane with new stuff including servos that run on (7) volts, not the current (4.8) volts.

There is a minimum loss of (0.6) volt across any silicone semiconductor (the output stage of the motor-controller) regardless of input voltage, which is part of why higher voltages have, in general, higher efficiency.  That (0.6) volt loss on a (7.4) volt 2S LiPo is twice as much loss across the motor-controller verses the same (0.6) volt loss on a (14.8) volt 4S LiPo.  That is at maximum current, the transistors at full through, the situation at part amps is even worse.  The bottom line is to run as much input voltage as the rest of the combination will stand.  The engineering is that sometimes, 3S and (10.5) volts under load with a bigger propeller is more effective than 4S and (13.0) volts swinging a smaller propeller.

For the helicopter pilots reading this; Try not to laugh.  The electric powered RC helicopters are using twelve to fourteen LiPo cells, that’s (36) to (42) volts under load.  At the other end, the indoor stuff is using (1) S and (2) S LiPos.

To convert watts-in, which can easily be measured, to an estimate of watts-out I use a conversion of (0.70) for sport outrunners on 3S LiPos, (0.72) for sport outrunners on 4S LiPos.  That isn’t just a guess, from having used virtual dynameters (computer calculations) you seldom get an ideal match, the limits are in the (0.8) range these motors.  Often, or actually almost always, we run our little RC motors at less than peak efficiency for the motor to get a better match of the propeller to airframe.  My combinations may be off from yours as I often fly with no landing gear and use a burst to get to altitude and cruise down motor off, propeller folded.  And I’m flying either at sea level or not much above it.

To repeat; If the kV of the motor will allow it, and the internal construction can use the higher magnetic density, and the resulting propeller diameter is suitably matched the airframe, higher voltages are almost always more efficient.  That’s why the Heli-fraction starts at 6S LiPos.

Multiplex, back when the Mentor was new, laid out their stuff for the then common 2S Lithium Polymer (about like six or seven Nickel Hydride cells) or 3S LiPos (about like ten NiMh cells).   Keep in mind that ten cells (in Nickel-Cadimum) was the realm of advanced pilots when the Magistrait came out, most of us started with six cell car packs.  The HiMax motors can use more voltage then 3S, if the resulting propeller matches the airframe.  A further advantage of the better efficiency is the reduction in battery size and weight.  But you have to manage the whole system, just plugging in a higher voltage battery can be burned up parts.


Although I mention, for historic reference, older Nickel type batteries the current choice for RC flight of the last decade for batteries is Lithium-Polymer.  Further, the ratio of battery capacity to how much it can output has gone way up.  From a ratio of three too one when they first became available to more like seventy-five to one.  I started using LiPos when they claimed a twenty to one ratio and had an actual limit of twelve to one.  They held up about forty flights back then at ten to one and flown all the way out.  Even modern inexpensive LiPos run well under their “C” rating may not go twenty flights at full output if operated all the way down to their minimum voltage i.e. fully discharged.

To get enough amps back when there was a too low limit per cell we used to have to run LiPos in parallel, that’s over.  And with less internal resistance more of the batteries’ power goes into moving the airplane, rather than heating itself up.

Aveox “Cuisinart”  1406/2Y SE inrunner with an Astro-Flite 2.67:1 transmission   (3000) kV  (35/5) diameters   (240) grams with transmission

This was the seventh motor tested.

This “Cuisinart” (so named as production of more than the food processor company bought) was sold new, by the Editor, to a friend of mine, back about 1998 as one of the first brushless motors.  Back then we often “borrowed” from the electric RC auto stuff, most of us used batteries intended for the wheeled stuff on either six NiCad cells (no load (7.4)) volts or seven cells, therefore the 2S LiPo operation expectation.  Although the wires going inside the case have since been cut off, the hall effect sensors are still in the case, which has no cooling air circulation provisions.  All Aveox stuff was well made.  I had a smaller one in an Easy Star (I) which, when last seen (from an camera taped on to a Twin Star II with brushed motors) was hung up in a tree about sixty feet above the ground up.  The search plane crashed on the following flight (to narrow up the search area) I never again located that airplane.

A decade and a half ago, Aveox, one of the first manufacturers to make available brushless motors to hobiests like us, decided to abandon the hobby market.  No doubt a good decision as the cheap stuff soon after displaced their always top quality.  NeuTronics can still service Aveox motors, although more out of nostalgia than profit.  As in twenty years ago when a Japanese RC magazine published an article of the Best of the USA (although none of us can read it) our one time F5B champion (back when he worked for a company and not himself as NeuMotors) was featured with having (wow, that many) three of their motors, and a milling machine!  The Japanese fly F5B and F5D too.

Most of what use this individual motor has had was in a RC car (on six, seven and ten NiCad cells) where it provided, by the expectations of back then (it is brushless, at the forefront of the transition away from brushed motors) astounding performance.  As in sponsored RC car drivers couldn’t believe it, for the first time with electric they could blow the fuel burners out of the way.

Eighteen years ago I had “Dr. Jet” fit it with an Astro-Flight 2.67:1 transmission (paid for by; How many lobster? It depends on how many he catches.) and time it (you had to rotate the case relative to the sensors, the transmission reversed the direction of rotation) with the then hall sensor motor-controller.  It took a couple of tries to get a (5) mm output gear on the shaft right, the Editor helped too.  Dr. Jet’s recommendation was (40) amps with the expectation I would run it on the then common combination of seven to ten NiCad cells.

Since then I used it for just one flight, eight years ago, the hall sensors cut off on a modern motor-controller, mounted in the nose of a Twin Star II as a single motor conversion on 4S.  Unfortunately, that’s when the decreasing range of the transmitter resulted in that Twin Star II’s flight (454) being it’s last.  Weight wasn’t a problem, since the one Aveox (reshaped the nose) weighed only a little more than the two Speed 400 brushed motors otherwise (then) in a Twin Star II’s wings and it ran on the same battery.

Since then it sat, too heavy for anything I flew.  I whittled down an old, large bolt circle Multiplex motor mount, one made from white plastic, first sanded flat and built back up with wood (to build in some down thrust) and fiberglass.

Aveox “Cuisinart” with 2.3:1 Astro-Flite transmission  (35/5) 3000 kV  (240) grams

Aero-Naut 9X5 folding propeller blades on a narrow carrier

3S LiPo  (31) amps at (10.2) volts

4S LiPo near or over forty amps, make a try with 3S first.

Flights forty-two and forty-three

The day before Christmas I was feeling better and there was surprisingly little traffic on the road, great visibility under the thin cloud cover, only a trace of wind and warm enough that the snow was all gone, if muddy.  Pushing the Mentor into the air (fine view of the Rhine River from there) if flew just about as expectations were, ho hum, by today’s standards.  It did do very controllable loops and I enjoyed the couple of flights in somewhat turbulent air coming across and through forest and trees.  A Fun Cub would have been too light, even with gyroscope stabilization, a Twin Star II brushless ARF about even with it.

The count of ten maximum climb and glide ration was in the fifty-five range.  Just  what was to be expected with that propeller, watts-out and the additional weight.  However, the combined flying time of the two 3S LiPos (3800 mAh and 2400 mAh) only added up to just over half an hour.  All of the modern, good quality combinations will fly for twenty minutes to half an hour (or more) at higher performance on the same batteries.  For the first time with the Mentor I ran a battery into the BEC low voltage limiter.  That wasn’t enough to make it back over the hedge to the side I was standing on although the landing went prefect, again.  The motor was slightly warm.  A short flight and a warm motor, just no reason to mount more propeller even though it could take it without a bigger battery too.

Almost two decades later, having it fit with a transmission way back when and two hours of fiddling on the bench (including shaving down a metric allen wrench to fit the USA inch fraction plus having to rework a mounting plate, then another half an hour to build the plate back up for modern motors) I finally got half an hours use out of this combination!  That is enough nostalgia for a while.  The Aveox motor and transmission were placed in retirement.  Maybe give it to NeuMotors for parts?

Jeti Phasor 15-3   (37/5) diameters (140) grams

This could have been the eighth motor tested.

Another two decades old relic of the era of brushless replacements for the Speed (500/600) series “can” brushed motors being run on six or seven NiCad cells.  This motor has sat, never used.  More modern than the Aveox (still no cooling holes) it has no internal position sensors.  It was originally purchased by a friend, two of them, for use in a model B-25.  The other one the bearings rusted shut.

First run up with a folding 9X6 propeller (the smallest reasonable match to the Mentor) on a 2S LiPo it drew (47) amps on a 2S 2200 mAh LiPo.  With no cooling holes too much.  Next up with an 8X4 was (31) amps times (6.5) volts times (0.65) efficiency puts the watts-out in the (130) range, not enough to fly the Mentor.  With that small a diameter of a propeller it won’t fly a Mentor and isn’t even reasonably in a Fun Cub.

Although I ran the calculations on a virtual-dyno twelve years ago as the motors were still available new, thrust doubles with a transmission and larger diameter propeller, there just isn’t any reason to spend a hundred bucks gearing this one down as my other stuff on 3S and 4S just buries it performance wise.  To my knowledge no modern transmission will (easily) fit this case, with no cooling holes I doubt it could be run as hard as modern motors.

Brushed Motors

Although I have two new, twenty year old, never used, Astro-Flite brushed motor (top stuff before brushless) with a new 2.67:1 transmission sitting there (same transmission as the Aveox) there appears to be no reason to ever make use of those motors, or transmission.