I emailed you a while back about trying the 3-cell 700 mah battery with the twin motor systems and the 5" props on the P-38.

The plane flies wonderfully with good power, but as it turns out I am apparently burning out a motor or two after a couple of flights

From : Don Stackhouse

That's definitely abnormal. I have quite a bit of time on the Ryan PT-16 with three 700 mah cells feeding a single MPS-2A with no problems, although the motors can get quite hot if I overuse full throttle during a flight. I also have a lot of flight time on the Triplane with a 250 mah 3-cell pack and both the MPS-2A (6" prop) and the -2B (5" prop). Two or three flights per motor failure is definitely abnormal.

Your problems with the pinion gear suggest a binding problem somewhere in the gearbox. A worn pinion can definitely increase the load on the motor, but the pinion shouldn't be wearing in the first place. Make sure you have enough end play in the shaft screw so that the propshaft spins freely.

Make sure there isn't an alignment problem with the frames. The propshaft screw should be perpendicular to the frames and the long sides of the ply plate.

Another issue that can sometimes cause binding on tractor installations in particular is the edge of the hole in the aft frame. The hole has a slight taper to it, and the base of the head of the propshaft screw has a small radius in the corner. If you assemble the frames such that the small end of the hole in the aft frame is against the head of the screw, and if it happens to be a frame where the hole is a little tighter fit than normal (an awful lot of little factors obviously have to all add up the wrong way here!), when the prop starts making thrust and pulls the shaft forward, the edge of the hole rubs against the radius in the base of the screw head, causing a big increase in friction. It's also possible (though very rare) for the head of the screw to have a burr in the radius from when they formed the screw head during manufacture.

Remove the prop, and slip the motors aft in their frames so that the pinion gears disengage form the prop drive gear entirely. Spin the shaft with your fingers. It should spin freely. Now, pull forward on the shaft as you spin it with your fingers. Do you feel a big increase in the friction? If so, there are several ways to fix it.

First, remove the screw and make sure the underside of the screw head doesn't have any burrs or rough spots.

It's possible to flip the aft frame around so that the taper in the hole faces the other way, but if you glued the frames in like it says in the instructions, it may be difficult to get the aft frame out without damaging the ply plate. Instead, try inserting the screw from the other direction. The frame assembly on the -1A and the twin-motor MPS systems is symmetrical, so turning it around and using the other frame for the aft frame may run more freely.

If that doesn't work, you may be able to CAREFULLY shave the edge of the offending hole with a knife. The idea is to bevel just the edge of the hole that's rubbing against the corner of the radius under the screw head, but not to enlarge the hole in the frame.

The other thing that may help, particularly if there are any burrs or out-of-roundness in the frame holes, is to line-ream the two holes together with a new, SHARP #44 (.086" diameter) drill bit.

However, I hesitate to recommend these last two suggestions because it's easy to open up the hole too much, causing a sloppy fit, with a corresponding increase in gearbox noise and possibly an increase in gear wear rate. Proceed with caution.

Worn gears can definitely increase the friction in the system, resulting in more current draw and shortened motor life. Although we have not generally seen problems with pinion gear wear that weren't actually the result of some other problem, once the gear starts to wear noticeably it can deteriorate much more rapidly from that point on.

I also began having the same problem with the B-17 with the 3-cell 1200 mah battery. Whatever the initial cause is (too much wattage?), I do notice on inspection of the problem motors that the small spindle gear is usually marred enough to make the prop bind. Is it possible the added rpm's are too much for that plastic gear and causes the prop to bind which in turn would probably cause the motor to overheat? Also, I am wondering if you have had any feedback on this kind of problem using the 2-cell batteries. I would sure like to find the right configuration to keep these planes flying reliably - they sure look and sound great in the air !!

Approximately one minute of continuous full power on an MPS-1A with a 3-cell pack will melt down the motor. We have not seen any significant gear damage, just catastrophic internal failures in the motor, but it's reasonable that given time it could. The prop and the battery pretty much determine how much power is generated, and whatever motor(s) are between the prop and the battery have to make that power. If there's only one motor, then all that power has to come from that one motor, and all of that power has to go through a single pinion. Other than the losses due to lower motor efficiency, a single-motor system on three cells makes nearly as much power as a twin-motor system on the same setup. Obviously, if you ask a single motor and pinion to do the job of two, they're likely to get a little "hot under the collar" about it!

The first time we melted down a -1A (single motor) on a 3-cell pack in the Triplane, I'd already accumulated a great deal of flying time with that setup, with no problems. However, I've developed the habit with electrics (and, BTW, a very good habit to have!) of never using more power than I really needed to get the job done. On the 3-cell Triplane I almost NEVER needed more than about 3/4 throttle at the most, even for "yo-yo" maneuvers. I'd used full throttle occasionally, but never for more than 5 -10 second bursts at the most, and with enough time between bursts for the motor to cool down again. Used in that manner it held up quite well.

Even full-scale F-15's only use afterburner when they really need it, and then only for the minimum time they really need it. Any more than that gets hard on the engines and even harder on the fuel bill. Doesn't do their range much good, either.

So, we know that about 60 continuous seconds of a single motor running at full throttle from a 3-cell 250 mah pack will cause a motor melt-down. What about the Boeing, with four single motor systems dividing up the power? Well, if you're using a 3-cell 1200 mah pack, that's 300 mah per motor, more than I had in the Triplane. The pack is capable of putting out enough current to feed all four of those motors as much as they can handle and then some. Just based on the ratios, that suggests we could expect meltdown on that system in about 50 seconds of continuous full power. At first glance it seemed to many (including me, I admit) that four single motors should be able to properly load a 3-cell pack, but in the case of the 1200 mah pack, obviously not! A 3-cell 700 mah pack is only 175 mah per motor, which should work better. The setup we've been using most of the time in the Boeing is a 2-cell 1200 pack, and it's very comfortable with that. The total energy content of a 3-cell 700 mah pack is almost 90% of that in a 2-cell 1200 pack, so flight times should be about the same (it might actually be better, since the extra voltage of the 3-cell 700 mah pack would allow you to use more of the total energy in the pack before the battery voltage dropped too low to sustain flight), but you would still see a benefit in the available power at full throttle, assuming you really need that much power in the Boeing in the first place.

The other option would be to put in four twin-motor MPS-2Bs. I would not recommend the -2A, because the current draw with the larger prop could total about 8-9 amps at full throttle, more than the Pixie 7-P ESC is rated for. That also equates to about 150 watts per pound, which is an absolutely insane amount of power for a park flyer, especially a scale WW II bomber. Planes of that category were not known for their hovering ability! If it's top speed you need, then a much lighter, cheaper and more effective solution is to cover the underside of the wing with something like Litespan or Monokote to make the airfoil flat-bottomed instead of undercambered. This will reduce drag at high speed, which is far more effective for that than adding more power.

Getting back to the motor life problem in your P-38 and other twin-motor planes, the lessons from above with the single-motor power units still apply, especially if/when one of the two motors starts to go bad. I found this out on the Triplane recently. At first it seemed to be acting sluggish, then it fried a motor. I replaced that motor, but the replacement fried only a few flights after that. At that point I started investigating in more detail. I discovered that the OTHER motor had been gradually going bad (apparently brush wear problems), and was not doing its fair share of the job. Instead of two motors sharing the job equally, one was doing all the work and the other was pretty much along for the ride. The result was something that acted like an MPS-1A, with about the same total current draw, but less power at full throttle actually delivered to the prop (because the motor's electrical efficiency is worse at the higher per-motor current), and with excessive motor heating in general and melt-down at sustained high throttle settings. It all made perfect sense! The thing that had set up the problem was that the only partial failure of the first motor made it look like it was still performing when I tested them. When I changed the motor on the other side, the new motor melted down very quickly from overwork, just like its predecessor. The moral to the story is that the defective motor in the system is not necessarily the one that burned out.

Since then I've gotten in the habit of checking the temperature of both motors on the twin-motor unit immediately after landing. They should both be equal. If one of them is either unusually warm OR unusually cool, that's a warning sign. Also, when I replace motors in a twin-motor system, I've gotten in the habit of replacing both, not just one. It's a little like battery packs, and the importance of having cells in them that are reasonably closely matched. Unequal load sharing ANYWHERE in the system, be it cells, motors or whatever, can obviously cause problems.

The same advice theoretically should apply to multi-engine models such as the P-38 or the Boeing. Although it can be easier to spot individual motor problems if you really pay attention to the symptoms, if you're seeing some odd, unexplainable behavior or unusually short motor lives, it's probably a good idea to change ALL the motors simultaneously. I also like to do a full throttle static run occasionally and check the prop RPM on each motor. They should all be reasonably equal. Any significant discrepancies indicate a probable motor problem. Even an unusually high RPM on one motor may indicate a problem with it or with one of the others. If you don't have a tachometer, get one. It's as essential a tool for diagnosing problems and health of your plane's power system as a voltmeter or ammeter.

So, let's summarize:

Use only as much power as you need, develop flying habits that emphasize finesse instead of brute force.

You can use single motor systems with 3-cell batteries, but you need to keep the use of high throttle settings to a minimum.

On a twin-motor system, if you have to change one of the motors, change both.

I hope this helps. Good luck, and please keep in touch!

Don Stackhouse
DJ Aerotech