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

Propeller and Motor-Controller Selection, Even An Unknown and Inexpensive Motor Benefits From Tuning

By Carl Murphy


This is why my RC airplane has seemingly double the power and duration of yours.  Even this unknown, inexpensive, motor turned in enjoyable performance.  Getting there required using and evaluating including a motor that burned up, a pending equipment failure motor-controller, a cheap propeller carrier collet that caused vibration, a new propeller carrier collet that didn’t just bolt together, overheating a battery and it required propeller tuning and duty cycle determination to get the best out of it.

All I knew about it was from a tag on the bell, brand Finwing, intended for use on 2S and 3S LiPo (batteries) with a kV from the data sheet on the bell and that it was about the right size to fit the RC airplane.  Is it good for anything?  As was determined, resoundingly, yes.  Most people would be delighted with it.  About the power of a sport (20) sized fuel burner on (10)% nitro-methane with a decent muffler for double the duration.  That from having watched a similar sized fuel burner at the field right against the USA/Mexico boarder.  Sometimes, this time, inexpensive and not all that efficient is still fun.  On modern high discharge 3S 3500 mAh LiPos with an APC 9X6 (fixed, not folding) propeller it flew a Reinforced Fun Cub well.  Duration was about two thirds of a preferred, lighter, more efficient motor with a folding propeller.  Or half the duration at half the net power of a more expensive motor on 4S LiPos.  Even more if glided, slope soared or thermaled, something the fuel burners can’t do.  Part of that it the better streamlining possible with electric power and an RC pilot who flies coordinated.

But it took a while, twenty flights, to get it sorted out. That is what this article is about.  I do not believe reports based on a single flying session where everything is perfect on the first try.  

The Test RC Airplane is (40) Size, (20) Weight

The test bed RC airplane is a Reinforced Multiplex RC Fun Cub with additional reinforcement to correct original weak stuff and make a bush plane out of it.  It has a four foot wing span and weighs about two pounds fitted out for flight.  With a motor plate between the motor and air-frame changing the power system starts with just twenty minutes removing two screws and an electrical connection.  I have fitted a dozen different E-power combinations to various ones of my own and friends.  Ours, (the new Multiplex RC current production Fun Cub NG replacement is a better) are outstanding performers.  Even with reinforcement (we fly the stuffing out of ours) the basic Elapor foam (EPP similar) gives out in about a hundred flights at the power levels and duration of this article and if that doesn’t after five years the plastic goes hard, brittle and warps.  Until then air-frame maintenance is only required if you hit something, used cheap servos that wore out, the gears broke or etc., the bearings of the motor either started as junk or wore out and exhaustion of the LiPo batteries.  That’s fifty hours or so of carefree RC flying enjoyment at about six bucks a flight.  Figure the original plus a replacement set of motor bearings and four or so LiPo batteries as consumed for typical qualities.  If if you bought decent servos they are good for another similar RC airplane.  See previous archived SEFSD articles for more details.

This is an original year 2021 article for the Silent Electric Fliers of San Diego (SEFSD). 

Flight 70  Santa Ana, CA (USA) Overlook March year 2021  At sundown I put on a show for a young family of three.  This is a great place to fly and impressive even to casual viewers RC airplane.  Even more so for being able to land it on this rock strewn parking lot.  This was the first flight with a new 3S (70) C 3500 mAh LiPo.  

Injury Warning

The propellers used in this test at these power levels can damage an arm, wrist or fingers resulting in blood splattered all over and injuries that never completely heal.  If you don’t already know what you are doing please get help from somebody who does.  The APC propellers of this report can do more damage to flesh and tendons then the blunter, less efficient propellers that come with the most common trainers, to that the higher power of these combinations.   A universal quote from emergency rooms; “It happened faster then I could think”.  

Can’t Get Aero-Naut Replacements On this Continent

I returned from Rhine-Main (western Germany) with the “light medium” optimum wide and shallow brand Hacker (sport) brushless outrunner at (80) grams using an Aero-Naut folding propeller system (wide bar, 10X6 folding blades) which performed very well indeed on 3S LiPos.  As in flights from half an hour to forty-five minutes made it clear that finding the right combination had its benefits even for this “sport” combination.  

Until, a couple of flights later, for the first time in my decades (thousands of flights) of using folding propellers, a screw holding one (of two) folding blades came off in flight.  Although I had spare propeller blades and in fact found the one that came off in flight, the (4) mm conus was damaged on removal.  An Internet search had the results that individual blades were available by Internet order, but not the carrier assembly.  I was going to have to make use of what I could get here.  The search for use of alternate motors limited to what I had on hand, with a budget of just twenty-five dollars, began.

Motors On Hand

On hand as motors, with (5) mm output shafts and large diameter bolt circle to fit the available motor mount plate, were three electric motors:

HH Standard, it looks like about (120) grams.  Although not a (5) mm output shaft, it came with an ARF motor-sail plane, matched folding propeller, and an E-Flite (30) amp 2S-4S  Pro motor-controller.  It bolted up to the motor mount plate.  At the (few remaining) hobby shops this motor sells for $45- plus taxes, that makes for $(48.65).  The used motor burned up seconds into the first flight even although it drew only (18) amps at (11.7) volts on the bench.  Well, I was given it and I have seen dozens and dozens of these motors provide fine for their purpose performance in Almost Ready to Flys.  A disappointment was being unable to remove the adapter for the folding propeller, it had rusted in place.

HiMax HC3516-1130 kV (135) gram with worn out bearings.  For a decade this was the standard Multiplex RC Fun Cub motor.  Decent efficiency when new, maybe as much as (65)% when configured exactly at optimum on 3S LiPo voltages while the in-flight loads and battery voltage were exactly matched.  Something often forgotten, that peak efficiency only occurs at just the peak of the curves.  In flight loads and falling battery voltage reduce that.  The best outrunner motors of this size hit no more than (75)% electrical watts-in to mechanical watts-out on 4S LiPo voltages.  

While the bearings turn free this moderately priced HiMax brushless out-runner motor can take the Fun Cub straight up a thousand feet/three hundred meters at a count of ten when run on a 5S LiPo at (25) bench amps with a suitable propeller and motor-controller followed by a three minute glide back down.  Run at (30) bench amps on 3S LiPos in the 2200 mAh size after around seventy flights the bearings are giving out.  I already knew the bearings were used up, as soon as run up this motor bound.  It flew the plane well but that turning stiff bothered me.  These  $75- motors are not quite as good as Scorpion and E-Flite in the hundred USA/ninety Euro range at current prices.  The most cost effective way to get new bearings for this motor is to buy a new Scorpion motor and order new bearings for it at the same time.

Finwing (135) gram motor acquired at a SEFSD swap meet in December year 2016 for ten bucks.  From flipping it through it is a low end (but not cheap) brushless outrunner in the (35) mm diameter class that would bolt right in.  This was a near new “take out” when the original owner of an ARF decided that “quality flies better”.  From the information on the case it looked like a likely choice.

In case you were wondering, in Rhine-Main (western Germany) I when I need an affordable outrunner I use brand Hacker (sport) of brand Scorpion.  In Southern California (USA) I use either Scorpion or E-Flite.

The available motor-controllers were:

E-Flite (production date unknown, probably seven years old) which came matched to a HH standard motor and a folding propeller with a (30) amp Pro 3S-4S.  It may be software settable, no attempt was made.  Twice the physical size of the Castle Creations Talon (and still not very big) it ran warm even when run at just (15) amps which touched off this report.  Used as supplied it is no doubt at optimum for the sport RC airplanes that come with it.  But it ran too warm for comfort, even at amps well within the sticker rating.  Please take note that a single sampling of anything like this is not representative.  Even more so for one that sat three summers in (120) F. heat and was manufactured likely four years before that.  It was likely operated with a motor wound defectively with the result that the original owner had not had a successful flight with it as part of an ARF.

Castle Creations Talon (25) amp 2S-6S (current production) with no settable timing, they are on automatic lead and the BEC can spike up to eight amps.  These have performed wonderfully.  They are so good that the manufacturer of the worlds best racing RC electric motors (NeuMotors) recommend them for anything less than all out competition.  

Castle Creations Phoenix (35) amp 3S with BEC or 4S without BEC  This series is out of production by about ten years.  No attempt was made to set anything.  It was thrown in on a deal years ago which included some cash and fresh lobster in exchange for some new stuff and some old stuff that wasn’t selling or laying around.

3S LiPos

Two brands of 3S LiPo batteries were available.

New production NewEnergy 3S 3500 mAh LiPos.  With the notice these are racing capable batteries with very high discharge rates.  That comes with the consequence that the battery gives very little notice (almost none at this discharge rate) that it about out of charge.  That is why as the end of the charge is approaching I occasionally give full amps.  If the acceleration has fallen off it is time to land.  

To that an easily available brand Venom 3S 3200 mAh LiPo which provided plenty of satisfaction and a plenty high C rating.  It actually put out slightly more power, but, it ran hot at medium discharge rates of twenty-five amps at (40)% duty cycle.

LiPo batteries have some extra power for the first fifteen seconds or so then start a long fall off on power.  For motor testing you want the motor-controller to be “dimensioned” to be about equal with the maximum the combination can draw, that being with a peaked LiPo battery on the bench.  For motor-control testing the initial “burst” was used up before taking measurements.  After that for the short runs (ten seconds per combination) the available battery power may be taken as constant for three ten second runs.  

For the motors used there was no other reasonable voltage choice except 3S or about (11.7) volts before the (0.7) volt loss (minimum, usually more) as measured before the motor-controller.  I seldom bother with 2S LiPos (for this size E-power) and none of these motors will hold up at 4S.  From long experience, dozens of motors, if the manufacturer rates it for operation on 2S and 3S LiPos about all that running them on 4S does is wear them out twice as fast.  Need more power by using the better efficiency of 4S then get a motor rated for that.  

Net efficiency favors better quality after the MTBCI (Mean Time Between Catastrophic Impacts) reaches the fifty flights range.  

Although the battery capacity was selected for use in a “long range” maxed out Twin Star II, performance at these power levels for the more typical 3S 2200 mAh LiPos would be only a little less power at two thirds the duration.  That noted these were new, racing quality LiPos.  

One of the first flights, operated at (33) bench amps (the HiMax motor) and an estimated duty cycle of (35)% on landing not only was the E-Flite motor-controller too warm for comfort, the 3S 3200 mAh LiPo battery brand Venom was too warm too.  Switching to the competition batteries, even although they deliver slightly less power, took care of the battery heating problem.  

What to do if what you can get is brand Venom LiPo batteries (which I recommend) or any other brand that overheats?  Back off the duty cycle a little or use a little less propeller.  There are more than two positions to the amps stick ya know!

Used up LiPos need not be used for performance testing (or much else) and that includes most common brands after twenty-five flights run down into the motor-controllers low voltage limiter.  

Propeller Selection Was Limited

The available propellers were fixed APC in a limited selection.  I started with APC 9X7.5 E and APC 8X4 E.  For just sixteen dollars that was expanded to APC 9X6 E, APC 9X4 E, APC 8X8 E. and APC 8X6 E, what I could buy over the counter in Corona, CA.  Test flights were made, and enjoyed, with all five propellers.  Near, or at, sea level in wind from none to fifteen miles an hour the best were the APC 9X6 E and APC 8X6 E.

All four propellers used in this article, once the system was configured to use them, provided results that even “fussy” me enjoyed.  Smaller diameter and lower pitch did not necessarily provide longer flight times, nor higher pitch higher level flight speeds.  The angle of climb was all about the same (not vertical but plenty) although the rate of climb went up with the watts.  That APC  9X7.5 E and APC 8X8 E made for flights straight out of the “simulator” reality in the “real” world.  So much power that speed at low altitude flinging it around was fun knowing that full amps would put it into near straight up in just a few feet forwards.  What happened with that combination was a best match between motor, battery and air-frame zooming around at speed in level flight.

APCs propellers are now standard on both American and German shelves.  Here in SoCal (Southern California) that is about the entire selection.  Back in Rhine-Main I prefer the better durability, at higher initial cost, of Aero-Naut.  Please take note that just because two different propellers are marked the same diameter and pitch does not mean they have the same current draw or performance.  APC has a great selection of diameters and pitches.  APCs have excellent efficiency at converting mechanical shaft watts-out into thrust and cost little.  They are however fragile.  Better carry an extra.  

Wood propellers have varying material density, we used to balance them.  At this eight and nine inch sized nobody I know of balances APC propellers, they don’t seem to need it.  What little vibration the final combination had was at just off no amps during acceleration of the propeller (first the propellers rotational inertia has to be overcome, then thrust accelerates the whole airplane) at almost all usable thrust it ran smooth.  Its just that most of my stuff there is zero harmonic vibration.   

This Was Enough

All five propellers tested with this Big Foot motor gave time to the maximum height at Mission Bay in seconds you can count on your fingers, or half that.  All four propellers tested with the Big Foot can climb at a rate that will surprise a majority of sport RC pilots.  But, don’t forget, temperatures were LiPo correct (not the winter cold of Rhine-Main) and this was at or only just above sea level.  Constant wind can change the required combination too.

The Multiplex RC Fun Cub Will Not Computer Model

There are now two “Worlds”.  The physical one we live in and the one in the computer.  Even pilots older than me confuse them as about everybody either plays video games or used computer simulations as part of learning to fly RC.  

As noted in previous articles with its notably low wing loading and under-cambered wing profile unlike any other “model” RC Piper Cub (for the Multiplex Fun Cub and NG Fun Cub the tail surfaces way enlarged compared to real scale) the Multiplex RC Fun Cub will not model on computer simulations.  Kill the thrust and this is a “floater”, momentum is lacking, maneuverability way better then comparable, particularly at low speeds.  Although the more modern, more expensive, RC computer simulations will allow changing E-power, this RC airplane will not model on them.  A further restriction is that the RC simulations do a poor job of modeling the E-motor performance, the center of gravity balance is always perfect, the battery is always at peak voltage, they do not simulate flying with ruined propellers (a favorite of fuel burner RC pilots when they switch to electric) or LiPo batteries that are exhausted (another common problem, it peaked, it must be OK?) etc.

That is specifically mentioned as if a “standard” Piper Cub with a Clark Y flat bottom airfoil (often observed with the casting flash of the wings leading edge still making stability problems) and half again heavier is fitted with seemingly the same power, using a junk battery and an inefficient or damaged propeller, the resulting wallow around the sky for eight minutes would render my accounts of fifteen to twenty minutes of blast it around like it was on nitro-methane performance seem like fiction.  

Something always sets a limit in the physical world.  In this case I was not comfortable with that basic E-Flite “economy” motor-controller, an old one subject to unknown already, no matter what amps.  After that the LiPo battery and duty cycle set the limit.  Standards, the motor, motor-controller and battery may be warm to the touch after the flight, but not hot.

Reusing a HH Motor-Glider Combination Burned Up   

Three years before I was given a simple, “full house” motor sail plane with an HH basic motor, the HH basic motor-controller and a house brand folding propeller which were six or seven years old.  On examination the EPP airframe wasn’t all that good new and time had rendered it worse, it had time curing bent to uselessness.  The servos were too cheap to want to reuse.  There was almost, but not quite, no evidence it had ever been flown. 

The HH propulsion was transferred to the Reinforced Fun Cub.  With the matched proprietary folding propeller on 3S 3200 mAh LiPo it drew (18) amps at (11.7) volts on the bench.  It barely flew the plane with an altitude gain of thirty feet from a hand launch, then the motor quit.  Running it up on the ground produced white smoke.  The motor had burned up.  There was no easy way to remove the adapter from the (4) mm output shaft to work with the folding propeller assembly.  expletive deleted  That left the motor-controller still possibly usable.

There must have been some reason why an otherwise average RC pilot gave up on his nice looking motor-glider.  From what I knew of his skills if a package didn’t perform correct out of the box he and his friends didn’t have a clue what to do.  It was for people like him I wrote this up.  

When it was over every bit of that ARF went in the trash.  This one used motor burning up is not representative of the basic quality of this series of motors.  That motors do burn up is instructive.

The HiMax HC35160-1100 kV (135) Grams With Worn Out Bearings, Won’t Due

The HiMax with know to be worn out bearings was installed drawing (30) amps at (11.7) volts on the bench with the (used) E-Flite Pro 2S-4S (30) amp motor-controller and a 9X6 E APC propeller.  The amp draw goes down at least a quarter in the air.  I flew it in a steady fifteen mile and hour/twenty-three km/h wind for three flights.  Although it flew the plane well, as soon as the motor warmed up the motor bound.  I want the energy from the battery to go into moving the airplane, not heating up the bearings.  Further, use of these bearings are impending doom as some flight the motor binds so much that available thrust to the air-frame falls off as the amp draw increases until something burns up and quits in the air.  It creeps up on ya.  Performance falling off is compensated by running the motor more of the time (higher duty cycle) as the amperage draw increases.  The increasing bearing drag becoming the same load as a larger propeller, until something gives out.

That worn out HiMax was too much risk, it was time to try motor number three.  The decision was to try the unknown Finwing motor, at first with a smaller propeller and see how the motor took it. 

There were two more problems.  The motor-controller was landed all three times with it too warm for comfort.  

And that cheap propeller carrier collet.  It had vibrated with every motor it was used on.  With the HiMax there were harmonics at about the one third of full amps in the air which were with pilot use avoidable.  That is from an RC pilot who understands that such vibration should not be there at all.  Vibration not only wastes energy, it ruins equipment.  Such as the little tiny wires breaking, one by one, until the whole airplane goes out of control and slams into the ground.

Of Cheap and Mediocre Propeller Carrier Collets

I started with an on hand (5) mm propeller carrier of unknown origins.  See my archived report about a used MP Gemini that was too far gone it came with.  No mater what motor or propeller it was used with there were always phases of vibration harmonics.  That mentioned as converted over RC pilots seem to expect things to shake apart.  They even fly with damaged propellers!

For $(8) plus California taxes I bought an E-Flite (5) mm propeller carrier collet assembly.  It flew with an APC 9X6 E just fine.  But, when I tried to tighten with an APC 8X4 E when it bound there was a little gap between the clamping nut and the propeller hub.  With the centering bushing it would not tighten enough, the propeller was loose.  The difference is that the 9X6 E had a thicker hub,  enough that it tightened, the thinner 8X4 E when the nut bound there was an air gap with the 8X4 E.  As it turned out the collet diameter was the same as the forward hole, leaving the spacer out and it worked out fine.  That noted as not everything works right on the first try!  It was my first time ever having to leave out the centering bushing having used fifty different sizes of APC fixed propellers and a dozens of propeller carrier collets.

Although there was still some vibration at low power, it was one twentieth of the previous.

Scorpion and Hacker (sport) outrunner motors come with a group of additional parts including their own propeller carrier collet, for good reason.  They want their quality products to have a chance to perform as advertised.  Something even the best motor can’t do with a cheap propeller carrier collet that is so poorly made the whole assembly shakes.

Finwing M2825 kV 1200 7.4V to 11.1V  approximately (135) grams 

APC 8X4 E (fixed) cheap propeller carrier collet

E-Flite 2S-4S Pro (30) amp motor-controller

amps (19) at (11.7) Volts

A couple of test flights were made at which it was determined that although the motor ran cool, the motor-controller ran just as warm at (19) bench amps/(14) amps in the air at as when being run at (30) amps bench/(22) amps in the air on a (45)% duty cycle.  

The vibration was lower intensity then with the HiMax running at higher power, but in the same RPM range.  That propeller carrier collet had to be replaced.

I enjoyed this combination as it accelerated the Reinforced Fun Cub and flew it well although the shallow pitch limited the speed.  This would be a fine combination for a beginner once the problems were fixed.    

If It Operates Hot Change It Before It Fails

What worried me too is that despite the reduction of amps to about half what the E-Flite motor-controller was rated for that the motor-controller still ran hot.  It was possible to establish that duty cycle was not the problem.  That left the Battery Eliminator Circuit as the problem. 

Finwing M2815 kV 1200  7.4V to 11.1V  Motor-Controller Tests

APC 8X4 E fixed propeller with a new E-Flite (5) mm collet

3S 3500 mAh LiPo from NeuEnergy

Values as taken on the bench, with a watt-meter, just like any hobbyist could.  Air temperature about (80) F/(27) C.  Altitude about (500) feet/(135) meters.

E-Flite Pro 2S-4S (30) amps, timing unknown (19) amps at (11.7) volts

Castle Creations Talon 2S-6S (25) amps, timing automatic (19) amps at (11.7) volts

Castle Creations Phoenix (35) amps (approximately ten years old as usage on 4S requires the red lead be disconnected and an additional battery to power the radio be used) timing unknown Long since obsolete. (18.7) amps at (11.7) volts

The battery was a current production 3S 5300 mAh LiPo, new, from NeuEnergy.  The battery was first run at part amps until the initial fifteen seconds or so of higher power was used up.  After that for the few seconds of each test run the power available from the battery would be constant.  This is what an hobbyist does without a constant voltage power supply.  If you successively test combinations without first running out that “initial burst” which is about a tenth more power, you get messed up results.

Even worse is trying to test with a shot battery.

It would appear that there might be a slight disadvantage to the old Phoenix motor-controller.  Since of the currently available propellers the best compromise is the APC 9X6 E fixed rather than risk the modern Castle Creations Talon motor controller on too many amps (even although power draw goes down in the air) the CC Phoenix was the motor-controller of choice.

There are two components that can be responsible for excess heat in a modern brushless motor-controller otherwise being operated per the manufacturers recommendations and sufficient airflow for cooling: Overloading or defective power transistors, or, the Battery Eliminator Circuit is overloaded or defective.  All electronic devices include power storage devices called “capacitors” some of which with time and heat degrade.  A best guess is that this individual E-Flite motor-controller that the BEC was giving out.  It was retired before it failed in the air.  As it still put in half a dozen decent flights with other motors it evidently was not the cause of the demise of the HH basic motor.  Since unknown motors bearings felt OK and it was operated well within the reasonable for that motor with a from the factory propeller something most have previously gone wrong.  See my series of archived reports where I reconditioned a used RC airplane, there were always lots of hidden, not right stuff.

At this low of an amps (twenty amps on the bench, probably fifteen in flight on 3S) the motor-controller should run cool, the inexpensive E-Flite didn’t.  I wasn’t comfortable with the duty cycle of how warm the E-Flite motor-controller was on landing.  The felt temperature was the same at (19) bench amps as at (33) bench amps and in any case seemingly too warm.  The Castle Creations Talon series have almost no subjective warmth over air temperature when operated per the manufacturer’s instructions.  

Watts-in Electrical Never Equals Watts-out Mechanical

Finwing motor with APC 8X4 E propeller

Amps (19) times Volts (11.7) makes for (220) Electrical Watts-in.

Electrical watts-in times (2/3) efficiency at converting is (150) mechanical watts-out estimated.  

Less a quarter as the power draw goes down in flight makes for (110) mechanical watts-out to the propeller in flight.

That flies a heavier than the ARF, Reinforced Fun Cub just fine, thank you.  Subjectively this combination limited the level speed flight with fine climb.  A ten second burst from minimum gliding speed took it up to at least (350) feet level starting at (500) foot above sea level.

The down side compared to the half as heavy, wide and shallow Hacker (sport) out-runner turning an Aero-Naut 10X6 folding propeller;  Two-thirds the duration at the same subjective performance.  

Were I to transfer this to a beginner this would be the combination. That flat pitch of the prop keeps forward speed down (the last couple of amp clicks don’t get hardly any increase in forward speed in level flight) with way enough climb to get back up, if the RC pilot knows how.  And cheap enough if he/she doesn’t.  At about two pounds of weight that (5) mm output shaft can withstand some impacts, the APC propeller none at all.

Although the power draw (amps included) drops off by at least a quarter in the air compared to on the bench and I have the skill to launch at part amps, long life out of my (25) amp Castle Creations Talon made me decide to not risk it with an initial amps load of (33).  A consideration was that that the lowest losses to heat of a motor-controller (that (0.7) volt across the transistors) are at full amps and this combination is mostly used at part amps.  But, I glide my stuff more than most.

Although there is a slight loss with the old Phoenix as compared to the more modern motor-controllers.  Since cash out of pocket was limited, it does fine. 

Now That’s More Like It!

Finwing 2820-1200 kV (135) gram motor, operation on  2S and 3S (LiPo) operation recommended.  No recommendation known on how many amps or power.  At (33) amps (on the bench) on 3S the motor ran cool, the LiPo battery became the limiting factor.  The CC Phoenix was chosen because preferably the motor-controller should be sized to match the bench power draw as that is the current on launch.  Yes, the CC Talon (25) motor-controller would likely have done fine.

Finwing M2815  1200 kV 

APC 9X6 E fixed   

Amps (33) at (11.1) volts makes for (360) electrical watts in

Times (2/3) for average efficiency of an “inexpensive” brushless outrunner motor makes (240) mechanical watts-out on the bench, estimated.

Times (¾) for the reduction in the air verses on the bench (where the propeller is turning in air coming in with flight speed) makes (180) watts-flight, estimated.

The APC 9X6 E fixed draws half again more then the “soft” APC 8X4 E combination.  The performance is convincing.  At this power and pitch the air-frame becomes the limiting factor as does this proportionally large (average would be 3S 2200 mAh LiPo) battery if the duty cycle goes too high.  Better watch the battery temperature if flying in a twenty mile an hour/thirty-two km/h wind as the Author does all by himself.  Nobody else will take to the air under gusts to half again higher…  

Most people would be delighted with either choice of propeller even although the first owner rejected this motor and truth is even I am undecided which prop I liked best.  If you didn’t know either propeller can exceed the level flight speed of a Reinforced Multiplex Fun Cub, so, the motor-controller is operating at it’s worst efficiency of half amps much of level flight, the motor’s efficiency though goes up at part amps more than the losses through the controller.

For good reason both Scorpion and Hacker (sport) motors come with decent propeller carriers (and some other small parts such as a motor mount and screws) as part of their package.  If you wanted similar results get yourself a Scorpion (35) mm diameter motor in the (135) gram weight with a kV of about (1,200).  The SEFSD Editor is the best source.  If you can land what you have with a bigger diameter propeller (“stock” for the Fun Cub was an APC in 13X4 E) I’d get a numerically lower kV (about a thousand) and use a bigger diameter propeller.  

A comparable motor from E-Flite, at comparable cost, is also a fine choice.  That one I have (back in Rhine-Main) does just fine.

At which, you get to repeat the above, for your combination.    

Assessment and Conclusion

There is no need for this much thrust in this air-frame with the top half of the battery charge using the Finwing 1200kV motor on a 3S LiPo with an APC 9X7.5 E or APC 8X8 E.  The best compromise for me was the APC 9X6 E.  All but a few seconds were at either part amps (half was plenty) or coasting with the motor off.  The flights went on for a good long while, they were satisfying.  Duty cycle estimated at (30)% in a twice walking speed breese comparable to afternoons at San Diego’s Mission Bay.  That is mostly coasting or at part amps.  

To check the system, one flight, just before landing five big loops (estimated diameter fifty yards) were flown.  Knowing that these high discharge batteries can abruptly give out, the newest landing profile is to come in high, use the maximum flaps as brakes to come down steep, then cut the flaps back to (45) degrees or zero to get enough forward air speed for good control.  The Finwing motor, the Castle Creations Phoenix motor-controller and NewEnergy batteries all landed cool.  The trace of remaining vibration can be lived with.  I had my combination.  Three different motors, two different motor-controllers, two different batteries and five different propellers over twenty flights.

On recharging the LiPo though it was discharged (3100) mAh or down to (10)%, too much for long life of the battery.  This modern high discharge LiPo still had great power clear to the end of the flight.  But even a just a little longer, had I come in without the flaps and needed some power to get back to the landing parking lot, could have resulted in a total loss in the street!  With its taped leading edges and slow speed one flight landed in a tree with no damage to the RC airplane.

How, in words to compare the different propellers.  The identical combination with the APC 8X4 E at full amps flies from maximum level flight speed loops about thirty yards in diameter.  Full amps level flight speed you hear the propeller rpm higher than the speed which is just fine for this air-frame.  The same combination with an APC 9X6 E the loops can to to sixty yards.  Level flight speed at full amps is excessive.  Duration was not measured, subjectively was just less with the smaller, flatter combination.  

That happens with the ones you can sit in too.  Net fuel consumption of a Piper Pacer flown at maximum take off weight (full fuel for a long flight, two passengers and some luggage) using a fixed pitch propeller with (125) horse power is higher than either a variable pitch propeller with (140) h.p.  Or a fixed pitch with (160) h.p.  Although part of that is the (160) hp system uses a more modern, flex pitch, propeller and the reduction in weight and complexity makes the (160) hp the best performer.  About like substituting a (fragile) APC for that durable (epoxy stirrer) most trainers come with. 

Flights (50) to (52) September 01, 2018 (Sa.)  Samoa Airfield near Eureka California.  This is still summer and yet with the gray and overcast, no wind at ground level, I had to put on long pants!

At sixty years old I am the youngest of the four of us, the most senior of we threat to society is twenty years older.  It was cute, they checked me out while I took a good look at them.  Who’d have guessed that two of us flow our own small airplanes (a discussion of local general aviation airports ensued) and a third was a mechanic on military jets during the war in Korea.  A vision of my own future, these gray, once capable men are now in decline with little to show what they once were.  It was cute, one of them wanted to help with my airplane.  True to the stereotype, RC pilots are notoriously well behaved and polite, but for good reason, want to establish that a newcomer has the right stuff.  They reported (another) competition level (so he said) pilot who only flew with them once.  When his combustion engine quit he crashed seconds into his only flight there. 

And they should check me, as I did them.  They have a below average grip on electric power, whereas I am just behind current.  As for flying skills, time is catching up on us, we flew accordingly, just glad to still be here.  They are so used to being so few that having two airplanes in the air at the same time required agreement.

Later I showed one of them that one of his batteries was defective.  It checked as charged, the no load voltage was only a little off, but hit the amps and it fell off so bad the motor quit.  That tipped the nose over damaging the propeller, twice.  Typical of fuel pilots, whose stuff has so much surplus power they usually fly with damaged propellers, he didn’t realize how much in performance that costs and that it is dangerous.  I helped them set up a gyroscopic stabilization.  I have a power meter, I showed the once jet fighter mechanic how to measure the performance of his batteries under load, the results a surprise to the three of them.  With nobody else to compare to they thought they were doing all right.  As indeed they were, it’s me who flies next to national and international champion pilots who have rated me a bad example.  

As we picked up a piece that fell off of his airplane on landing, bought built (all of us once built our own RC airplanes, one of them has one in a museum in Alameda) I reminded the other pilot to throw out his bad batteries, replace his chipped on landing propeller, spit out the gum and to start sharing himself with more members of the opposite sex.  We all laughed.  What ever we once were, right here right now we were what we are.  Four old men doing what we started as boys.