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

A Multiplex Mentor as Motor Testbed Not Just a 40 Size Electric RC Trainer/Sport Airplane for the Masses

This is an original year 2014 article

for the Silent Electric Fliers of San Diego. It includes a lot of things missing from the magazine like articles, such as what things really cost for both initial investment and as cost per flight, how well things actually hold up, and, perhaps the most important, the adventure of getting a Mentor to really fly well.


I’d seen Mentors flown back at Mission Bay and wasn’t impressed. But then a standard Mini Mag/Fun Man isn’t all that interesting either, what our slightly modified ones with modern propulsion can do is a shock. As in a Mini Mag/Fun Man with slightly better propulsion can take off almost straight up, fly at just above walking speed, do most dynamic aerobatics and stay up for twenty to forty five minutes on a 4S 1100 mAh LiPo with just a medium priced electric motor and a folding propeller. It was time for something which could buck some wind, able to make use of my accumulation of medium sized electric motors and go for at least a couple of hundred flights.


Bigger Flies Better and is Dangerous

Take note: The use of any RC airplane the size and power of the Mentor is DANGEROUS.


Screw up with a similar in appearance one pound Mini Mag, or two pound Twin Star II, both with typically one tenth of a horse-power motors, and in all likelihood all you get is a hurt finger. Even the two or three times as powerful setups in a two pound Gemini or Fun Cub, with their larger propellers, don’t in practice do all that much damage. At that light weight even a direct hit on a pedestrian, horse or car mostly just startles them, the foam often keeps injury and damage minimal.


The available power in a Mentor can destroy a finger, sever tendons, rip open blood vessels or fling the broken off half of a propeller between your ribs or through safety glasses into an eye. Hit something with a Mentor’s four or five pounds and you may cause injuries and damage all out of proportion to the increase in purchase cost over a one or two pound flyer. Ignite a Lithium Polymer battery in the Mentor size and anything near it may go up in flames as initial the initial fireball is bigger and the duration of it burning longer. You really must either have a full grip on using everything connected with things this size yourself, or get some competent help. So, think it over well before you buy and use anything in this five pound size and half a horse-power scale; Can I safely use it? Even experienced pilots should have another set of eyes check it over for first flights and every now and then.


Just the same, for the best of reasons, this was once the most common size on a RC flying field. The combustion powered ones were at least as dangerous. The worst issues with the combustion RC airplanes were that the engines required extensive full throttle adjustments every flight, done with your fingers and arm right next to the propeller. During the initial setup it was essential to hold the whole RC airplane, with the motor running all out, at all possible angles to verify fuel draw, with a substantial potential for injury. At the end of a flight all that concentrated metal of the engine seemed to do more damage when it hit something. I often wondered if the fuel wasn’t bad for us as in automobile dragster people using similar fuels wore gas masks. At least if you screw up and start an electric powered RC airplane with the battery connected it takes a little bit for an electric motor to come to full torque, combustion propellers were usually at full rpm when they hit something. When the engine of a combustion RC airplane stops running in the air it’s reason for a lot of excitement as a ruined airplane is the most common result. Fuel burner pilots almost never practice engine off landings, so they have no clue how to glide them in. With electric it’s routine to stop and start the motor in the air, modern (this is 2014) electric power is more reliable in the hands of ordinary pilots than combustion engines.


Among other improvements modern digital transmission GHz radios made interference from other radios almost an issue of the past.


A Mentor from Multiplex is,

 in modern, able to withstand unbelievable impacts and be easily repaired, expanded foam with reinforcement using electric propulsion a nearly five foot wingspan version of the 40 size high wing trainer/sport type popular for so many years of RC flight. Bigger flies better, costs more and is more dangerous. For down at San Diego’s Mission Bay a Mentor may be a below average choice as the easy landing and minimal wind tolerance requirements make other better performing, if more fragile, airframes, and smaller, easier to transport (and be blown around by the wind), less expensive combinations more desirable.


Mentors assembled as specified by Multiplex are reliable, if docile, performers good for about a hundred flights before the bearings in the motor wear out and the foam past the wing spar fatigues creating a hinge. Power off the glide speed is at the low end of medium, top speed is only somewhat faster. The power off glide is about average, which makes landings less of an adventure. A tendency to flare up as the speed increases tends to protect average pilots as it sends the airplane away from impending doom instead of accelerating into it. Although depending on available power (easily changed) they can be flung around, Mentors aren’t actually acrobatic, at least not in the sense of the many fine foam ARF F3A airplanes available for the same investment. Although mine is visually identical to standard except for omitting the landing gear to belly it in on grass farm fields, there were a whole series of simple modifications to improve durability, and, it was fitted with a wide range of propulsion. Either put enough power to a Mentor, or use more efficient lighter propulsion to get the weight down, and it changes character to more interesting.


That not fiddling with things and being too easily satisfied that the first combination works at all is a major difference between my articles and nearly anything else except reports from competition pilots. Most published tests they didn’t even vary the propeller! There is more to be had out of a Mentor then what the You Tube videos of stock ones on their maiden flights demonstrate.


Who Needs a Mentor?


During my 2014 look for work trip on a Saturday morning in March (perfect flying conditions in San Diego, snow still on the ground in Germanys Wiesbaden) I helped three pilots gain some experience by letting them fly my Fun Cub down at Mission Bay. The air was so still it was hard to decide if it was wind aloft or thermals bouncing the airplanes around just a little.


Two slightly experienced middle aged pilots enjoyed the experience, I even let them land. They commented it seemed to get bounced around by the wind (at ground level only just above walking speed) and was slow to respond to control inputs. As noted in a previous article of mine, with its light wing loading the Multiplex version is very different then all the other Cubs out there.   I took the controls for a little to show them, use full throw on the sticks mine can chase it’s on tail, it’s plenty agile at the reasonable slow speeds. As it can flat turn around in six times it’s own wingspan, do an up loop up the diameter of four times the wing span and a down loop in six. A Fun Cub isn’t really a model of a Piper Cub, it just looks like one. Given time and experience they’d likely become “one” with the Fun Cub and automatically dampen out the gentle rocking, with the Mentor that wouldn’t even come up. Whereas a Fun Cub has to be coaxed onto the runway, ready to make big stick movements to cancel out it’s floating onto the ground, with a Mentor you cut the amps and steer it onto the runway flaring just like the big ones you sit in do.


My personal Fun Cub was (then) fitted with a Scorpion (135) gram outrunner and folding prop. Even on 3S, at only (450) watts-in of a rated up to (600) watts-in of that motor there was enough thrust to take off straight up, the “guest” pilots had the self restraint to use not full amps. But the Fun Cub, even that relatively heavy one, floats on landings and gets blown around by every little puff of wind. They would have been better served with a Mentor at about a hundred bucks more investment with its steeper, more stable glide and directer responses to control inputs.


The third pilot was a retirement age gentleman who I later found out had just wrecked five airplanes in quick succession, including one just before I arrived at the field. In his case he needed some help and maybe inexpensive equipment. I put the Fun Cub up about a hundred feet, set the power for a slight climb and let him have a go at it. Although from ten paces out he seemed to be doing fine, for him it was such a struggle that after twenty minutes his concentration gave out, I landed it. I may be a (flying) fool, but at least I’m a helpful one. That twenty minutes got him some confidence back. That combination can hang out up there being flown that way for about twice that long. He needed the slight lag of a response to control inputs of the lighter wing loading of the Fun Cub verses a Mentor.


From Multiplex USA the Fun Cub wing loading is (10) ounces per square foot, the Mentor comes in at (15) ounces per square foot. Most people fly a Fun Cub in burst and fly mode, in mine the motor runs less than a fifth of the flight as they glide well. With a Mentor the motor, for most pilots, runs at least half of the time. Most of our under forty years of age pilots have significant computer game experience. Even the retired set enjoys the simulators. Bottom line, if you have been playing computer games when you move the stick you expect a response, a Mentor fits in closely to that conditioning. Not needed at Mission Bay, a Mentor can buck wind better than anything smaller. And, important to many pilots, a Mentor is a model. Almost, but not quite like a Cessna 180 to include similar ground handling. I don’t personally taxi my Fun Cubs, a Mentor though tracks just fine on the ground.


We could put together a stock Fun Cub with an (80) gram motor on 3S for about two thirds the cost of a Mentor with its (200) gram motor and half again bigger battery. Not only would the lower wing loading make it slower to match lagging reflexes, they don’t splatter as much on impacts as the higher wing loading and just generally bigger Mentor. Bigger flies better, but bigger, and higher wing loading, crashes harder too. Durability hadn’t yet had a chance to be an issue for the retired pilot. The over four foot wingspan Fun Cub is medium sized, slow and easy handling. Even my choice of garish colors were picked to make it easy to orient way out there. But flying a Fun Cub in any wind much over twice walking speed is out.


Way out there is relative. As we get older far isn’t as many distance units as it once was. That may be extended out to how quickly and directly a radio controlled flying object responds to control inputs. Some lag, a curse to people with their original, even if average reflexes, is a benefit to the longest lived set. What that means is that although a Mini Mag, with its two foot wingspan half that of a Mentor, and a quarter of the weight, has to be flown so close to the ground to be seen by a beginner pilot that the interval between making a decision and correcting for it puts the Mini Mag into the ground, you still have enough time with negotiate with a five foot wingspan Mentor.


The semi-beginners didn’t needed that hundred bucks worth of motor, forty bucks worth of automatically set variable timing, forty bucks for the folding propeller assembly and servos with metal or Karbonite gears of my personal Fun Cub. Which, on 4S or 5S would power a Mentor just fine too. Think back to the Editor’s “Big Red” or a Senior Telemaster for what a modern Mentor is. For pilots with reflexes and eyesight in the ordinary range, and by that I mean the vast majority from just above inept on up to, like me, were never going to be in the running for a champsionship, a Mentor is just fine. You won’t get a chance to need it down at Mission Bay, but out where there is constant wind is where a Mentor’s size and wing loading shine.


Although seldom represented, I would expect that the RPV (remotely piloted vehicle) gang might take use of Mentors where the resistance to wind is an advantage and there is a fair amount of room in there. However, most of them go to flying wings with no landing gear as they are easier to land.


Almost All Other Reports Are Censored        


My modified foam airplanes fly better and last longer than a vast majority of anybody else’s of the same kind. The improvements are easy. As for why I put this down in print; It just isn’t that much more effort to extend my record keeping out to articles like this one.


When you read other reports keep censorship by omission in mind. You shouldn’t accept a test of five flights (not even one with twenty flights), all with the same propulsion and RC components, as being representative of the whole reality, because it isn’t. In that short of an interval durability, as in stuff that tears and wears quickly out, isn’t a factor, efficiency is seldom discussed. I can’t even get the balance and control throws sorted out in just five flights! Even though just a few flights is the de facto (zu Deutsch amerikanische Rechtssprache für in der Tat, auch in Druck als Umgangssprache häufig zu treffen) standard in print. Then to a single report based on few flights often neglects significant differences in the quality of servos and other components.


We had a pair of Multiplex Gemini’s. Although they can be flown with “house brand” servos they are then imprecise, often frustrating, fliers. Go to at least the recommended servos (approximately the HiTec HS-55 at eight grams) and not only are the servos a lot more reliable and consistent, they don’t break near as often. Go to the stronger gears (Karbonite, the additional weight at twelve grams is trivial) and you get a more precise gear train and positioning, the whole airplanes then does what the pilot instructed. Not the hunt for centers (mismatched between servos) and the aileron and rudder geometry doesn’t match of the cheap stuff.


Most RC authors, a vast majority, just don’t do a representative analysis of how to get things to really work well. That is so consistent it becomes a form of censorship by admission.


Down at Mission Bay you have the benefit of associating with a whole group of competitive pilots and a couple of manufacturers. They spend many flying sessions refining just one part of their airplanes and often try different components and changing the programming of their transmitter in their goal of perfection. Take notice of the occasional article from competing pilots where they go over details left out of everything else. Such as the amount of energy at the output shaft is never as much as what went into the motor-controller! The SEFSD actually has people with dynamometers who can quantify the otherwise censored out watts-in to watts-out efficiency. Although most of us don’t need that last couple of percent of the competition pilots, some basic quality improvements and fine tuning to get twenty percent more can make a big difference to even simple airplanes. Our competitive pilots and I can fly rings around the people satisfied with their first try and stay up twice as long by using better quality equipment. Well, maybe really knowing how to fly helps too.


A while back flying a really beat up looking Multiplex Mini Mag with (300) flights on it down at Mission Bay I listened in on a discussion about my flying between a new member, himself well experienced, and two former club presidents. That plane looks like shaving cream, but it flies really well and seems to stay up there a long time. The club presidents didn’t even look up. It must belong to our fool.


The Mentor as a Time Capsule


When the Mentor, originally as the Magistrate, came out a vast majority of the marked for RC flight trainers were “40” sized with two stroke combustion propulsion which looked about like a Cessna hi wing man carrying airplane. Foam was distained, the joys of Elapor and EPP weren’t as generally known. Worse, electric propulsion was just getting going. The original Magistrate was laid out for a geared down brushed motor and nickel based batteries. Not even half the thrust at twice as much weight for about three minutes. A decade later you can’t give that kind of electric propulsion stuff away as the new stuff performs so much better. Multiplex kept most of the Magistrate when they did the conversion to the brushless power Mentor including a dedicated motor mount consistent throughout the medium sized line of airframes to come, but kept most of the rest of it.


Slight Modifications and Slight Crashes are Easy


Censored out of a vast majority of articles in print, is that Multiplex system of mounting the motor behind the propeller, instead of hanging the whole (moment arm) assembly off of the back, often saves both the motor and cowl in a light crash. Smack an ARF, lighter and spiffier, into the ground the whole motor/collet/propeller and lightweight cowl gets pried out/smashed. There really isn’t much to be done to improve that. Beef up that box most ARFs use to mount the motor to and when the nose hits the ground you just trash the motor worse. You can try adding material to the cowl, it still breaks and being secured to the fuselage aft of the motor, can’t absorb an impact the way foam can. Even a stock Mentor can just shrug off impacts that splatter everything from the wing forward of a built up ARF.


Break the whole nose off of a Mentor, and often it goes back to together with just glue, in minutes, right at the flying field. Scrape the wing tip of a built up from sticks covered with film RC airplane, all it takes is a puff of air at the airplane out there where you can’t feel it transmitter in hand, and you need to make repairs. Drag a Mentor wing tip, even stock although mine get hardened, and you probably don’t need to do anything. Mine get layers of fiberglass knowing that the front end is going to get bashed and wing tips dragged. That bit with layers is important, don’t want an abrupt change in rigidity ya know.


For my purposes, which are mostly because I land on farm fields (I often glide and sometimes slope soar or thermal) my foam airplanes all wear folding propellers. Although doing a precision re-shape on an inherently flexible and compressible material like Elapor is difficult, a slightly rougher reshape to clearance allowing a folding propeller to fold flat to the underside of the cowl only takes a couple of hours. You can’t do that with the thin injected cowl of an ARF.


For better aerobatics, and for beginners on the landing, sometimes the drag from the disk of a fixed propeller windmilling is an advantage. For those not familiar with electric flight, with the amp stick all the way back, that’s no amps to an electric motor, unless the motor-controller is set to hold the shaft still, in an electric RC airplane the propeller continues to turn creating substantially more drag than a propeller held fixed. Unlike fuel burners, electric motors require no idle running mode. It takes about five minutes to change propellers and reconfigure the motor-controller between braked and free turning.


My Whys for a Mentor


It was time for me to try something new able to use motors in the (80) to (200) grams size with diameters from (28)mm to (40)mm diameter on 3S, 4S and 5S LiPos. I had no suitable airframe in inventory, the motors, batteries and suitable motor controllers were sitting there waiting. Every airframe I had was not only worn out, they were too small. A Gemini flies best with motors in the eighty to hundred grams size. Fun Cubs fly best with equal or even less thrust and weight. Although I’ve flown Fun Cubs at six hundred watts-in those soft wings won’t sustain even half that. I am going for from two hundred up to a thousand watts-in combinations in my Mentor. For the high power setups the nose must be reinforced. I once watched the nose of a Multiplex Acromaster fly off independently at twice the rated watts-in.


The many fine factory built framed up airframes (ARF almost ready to fly) would quickly be scrap under the intended landing conditions, and motor changes to nearly all ARFs are a expletative deleted. That expletative deleted of changing motors often and easily rules almost everything else from a factory out. Even a stock Mentor won’t last, at least not the way I’m going to use it. See the following build up followed by experience beefing this one up.


A friend presented me a 3S 4500 mAh LiPo (twice our “standard size” of 2200 mAh), intended for an uncompleted thermal motor sailplane, which was about half again too big and too heavy for a Twin Star II. Our brushless Twin Star IIs fly fine on 3S 2200 mAh LiPos, a long thin 3200 mAh would fit well, but not that big thing. Any airframe performs best in a specific weigh range. Although that 3S 4500 weighs less than the eight NiMh cells I used to use, is no bigger (won’t fit where a brushless version requires for balance though) and contains far more energy, it puts the weight up and we have taken a like to a mix of blast around and glide that modern light weight components allow. I already had four 4S LiPos in two sizes and a pair of 5S LiPos, all useless for a Twin Star II, Gemini or Fun Cub.


Later on that nice late October 2014 day I stepped down wrong, completely unexpected, the internal parts of a knee broken in an on the job injury 1996, went under load at a wrong angle, it hurt. I was going to have to take it easy for a while, quite a while. The combination of wanting to use bigger motors I already have in a bigger RC airplane, having accumulated the desired batteries and impending hurt knee inactivity put things over the edge from barely affordable under my meager finances, into reasonable.


Unlike back at Mission Bay (San Diego California) where I’d be enjoying the Mentor’s landing gear on sandpaper like, flat packed sand, here in Western Germany it’s going to be belly it in on farm fields. Although I would have liked a Pilatus Porter in the same size, from prior experience with Multiplex and other foam airplanes, improved with suitable fiberglass reinforcement and folding props, even at this, for us here, twice as large as up to now size, a Mentor looked to fill the bill. Yes, Multiplex offers a nice scale Pilatus Porter, but too small for my current purposes. Those cool ARFs in the five foot wingspan size would be a pile of parts in about one to five landings. My Depron one was unsuitable.


In addition to both motors and batteries being too heavy and too hard to fit, that thousand watts-in would rip the wings off of a Solius motor sailplane. A Fun Cub wings won’t take those kinds of loads, the wings would fold back, nor does it tolerate that kind of weight. Blizzards only need about three hundred watts-out (among other things the wings bend right where the spar ends, since they don’t last long most people don’t comment on that) and it’s an expletative deleted fitting it with different motors,   nor would the batteries fit and Blizzards are a expletative deleted to land. Not only are the Blizzard landings ruinous, erosion of the wings leading edge if they aren’t taped is why don’t last long.

All my foam airplanes get taped leading edges and select additional reinforcement. Ever read any other reports which include that virtually every Twin Star II ever assembled that the elevator horn tears out at around fifty flights if not reinforced, or that running both of two motor-controllers BEC in parallel is a no no? If you knew that why not improve it on assembly.


My shortest lived Multiplex airplane is the Fun Cub(s) as the wings give out at around sixty flights even when flown at reasonable weights and thrusts. If you read my older reports I cite about eighty. The difference is that I flew my Fun Cubs with motors down as low as (60) grams on 4S 1500 mAh LiPo batteries. Everybody else I’ve seen them flying with combinations weighing three times as much with twice as much thrust.   The expectations of the wings were adjusted down, but then the flights go on for longer too. Geminis, suitably reinforced, the wings out past the struts start giving out at hundred hundred flights (although our pair have respectively a hundred and fifty and two hundred and fifty flights on them) and that’s from in flight loads. I averaged three hundred and fifty flights from my Easy Stars I and II: Mini Mags and Twin Star IIs come in between the Gemini and Easy Star.


And then there are places, such as Jamul out beyond San Diego, where the fun of flying justifies more risk (rocks hidden in the long grass), a shorter airframe life is expected. Wind would do the same thing.


Why to Buy a Foam Kit


Back when we had no other choice I was a proficient builder. I investigated fitting a self made fuselage assembly with a bought as spare parts Mentor wing, but they are neither reasonably available as replacements nor does the economics work out. I could have assembled a self built wing, but I’m not into reinventing the wheel, nor can I reasonably assemble a modern wing profile. Flat bottom Clark Ys don’t do it for me anymore. There are services which offer foam cores for wings to sheet yourself, or will cut EPP for you, but what a hassle. If a Multiplex wing is the right size and type might as well use it and might as well buy the whole airframe while I’m at it. I’m just not into designing wings, or airplanes. Not when such good stuff is available at such reasonable prices and durability.


Do the arithmetic of building a similar airplane up from sticks and iron on covering, that’s a lot of hours (around forty to sixty for a Mentor sized airplane) not only for the initial assembly, the cost of constantly making repairs quickly gets tiring. It took a decade or two for the ARF importers to get it together. They can now factory build a more then decent airplane and ship it half way around the world for not much more then what the just the materials cost here. That’s why few traditional wood based kits are offered any more.


I bought a unfinished framed up “standard” intended for a combustion engine a while back in about the .20 size and completed it with additional fiberglass. I even fitting the Clark Y wing profile with ailerons. Using mediocre electric power although it flew rather well, to me, it started getting dull on the first flight. I sold it after five flights to somebody who was delighted with it as a third trainer. Size and appearance wise it was about half way between a Mentor and Mini Mag, but that flat bottom balsa, spruce and heat shrunk plastic wing doesn’t perform as well as the molded in foam ones. Just the same the extra reinforcement made for a much more durable airplane. My creations in Depron went no better, durability where I fly was unacceptable without covering them with fiberglass.


Although I note that those flat plate indoor fliers are a blast to fly, even outdoors down at Mission Bay.


Even if a used Mentor came up on the Internet they are, because of the size, problematic and so expensive relative to their value to ship. The big twice a year RC swap meet in Lampertheim Germany, so often a start for my projects, had come and gone for another five months. Going and getting a used RC airplane the drive usually is so expensive it negates any savings, even if one came up for sale at a reasonable price. And there is often hidden damage and poor initial assembly to a used RC airplane, see my Blizzard report for a specific example. Then too I like to reinforce the “trough” down the inside of the fuselage of Multiplex airframes. That added crash resisting strength at a nominal weight and expense along with a desired to make more room in there are more difficult on an airframe already assembled. I have taken to reinforcing the wings at the “hinge” of the abrupt change in rigidity where the spar ends, less satisfactory if the foam is already fatigued. Most of the few Mentors I sourced on the Internet were so complete, often with stuff I don’t want, that for me there would be no net savings buying them used. I really don’t need a ho hum (200) gram HiMax motor or a cheap outrunner from Brand X.


Here and now for me the best purchase was a new in box kit with on hand new servos. I am a master scrounger, if there was to get a new airframe by spending less I would have already done it that way.


Begin Initial and Cumulative Cost


Keep in mind following this that I fly the s