Two-motor, one shaft experiment


New member
I wanted to get some use out of some old cheapo 1000kv 2212 motors and 30A ESC's I had lying around. I liked the idea of seeing how much performance could be squeezed out of these traditionally slow-fly entry-level motors and ESC's on 3s voltage. Since I've already done a couple of traditional twin motor planes, I wasn't looking to go that route.

Anyway, I don't know if I saw it somewhere online or what, but the idea of using two motors to drive one shaft (thus being able to swing a 10x8 prop to give me some speed) came up.

So I got some 3.17mm shaft and configured the motors as you see in the photo, with one "outboard" (in front of the firewall) and the other inboard. Thinking that the inboard motor would be in danger of overheating without the propwash/airflow a motor normally gets, I made an internal heatsink out of some scrap aluminium and put a chopped-off prop on the back end of the shaft to draw some air over the motor internally.

To complicate things, though, I also got it into my head that I'd like to build something reminiscent of the U2 spyplane (but with a prop up front rather than a pusher prop or EDF).

I like working with duct-tape-covered foamboard, which is great for durability but does get fairly heavy. And with two 50g motors, 25g heatsink, and two ESC's, it came in fairly heavy for its high-apect ratio 44-inch wingspan - about 890g with a 1300mAh battery. Wing loading worked out to about 19oz/sq ft.

Admittedly the 19oz/sq ft wing loading is at the higher end of what I've flown, but I think the high taper of the U2-esque wingshape made this thing more tip stally than I was prepared for (apparently the real U2 was similarly scary to fly).

How it flew on the first try:

Though I had built in some right thrust angle on general principles, the torque roll tendency on this baby was much more of a bugger than I had anticipated. 10-inch prop, the mass of two motors spinning, and then with the narrow U2 wing tips not providing as much anti-torque-roll as I was used to, meant that she pulled majorly to the left the whole flight. She absolutely just laughed at me when I tried to bank right.

(Question to you experienced guys: would the fact that the nose on this U2 shape is so much longer than the average single-prop plane serve to make small changes to the motor's thrust angle have a greater effect than usual - located that far forward does give it more moment arm with respect to the cg, right?)

I got her back down on the ground - barely - and remounted the motor to give her more right thrust, which did help on the next flight.

I thought I'd "take it easy" speedwise on these first flights, so flew it on a 10x5 prop instead of the 10x8 - big mistake. After a minute or two of tipstall/catch/recover events, physics and gravity eventually won and she spun to the ground. Crunch.

After a rebuild/repair, the second try went like this: on the 10x8 prop she flew fast and true until excessive vibration (from a wonky prop adapter) caused the motor mount nuts to unscrew themselves, and/or the prop starting slipping on the shaft, and possibly the rear motor overheated (version one didn't have the internal cooling fan) and seized. Right roll over (result of the left torque tendency suddenly disappearing and me not realising what was happening and reacting in time?) Crash.

Anyway, I'm rubuilding with a shorter nose and have grafted some extensions onto the wings to increase area and reduce the taper. Not as sexy as the U2 shape, but hopefully one that will fly a bit less scarily.
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Well-known member
This is a really interesting project! It looks great for what is presumably an early version of a new design. I'm not experienced with the relationship between thrust angle and nose length, or thrust angle in general aside from trying to eliminate it on my original builds so far, because they're both twins.


Legendary member
2 motors to one axle/prop been done...specially before with brushed motors, in plane/boats eg. A couple of gear wheels should fix that.


Techno Nut
Have you tried running those on the bench like that? You would probably get better efficiency out of two separate motors with two props than trying to run them inline like that. The timing and phasing of brushless motors can cause some problems with them tied directly together like that. Here's a video of a slightly different scenario where two motors were run off a single ESC.



New member
I guess two separate 1000kv 2212's could probably spin 8"-pitch props if I stepped the diameters down a few inches, and that may well be a more efficient setup than my little rig. But as I say, I've done twin motors, and they were fun, but I just wanted to try out this idea this time. And just to clarify, I am running two ESC's, not trying to run both motors off of a single one.

Haven't done any proper bench testing, but I have recorded/compared the sound of each motor (your basic 2212 14-pole 1000kv 50g cheapo outrunners) spinning the 10x8 prop individually - versus the sound when both motors are driving it. The two motors running together seem to spin that prop about 20% faster than either motor alone, which is consistent with what eCalc predicts a single 100g motor like a 3536 would manage.
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New member
Not sure, but my best guess is that:

you can hear something going wrong in the drive train about 20 secs before the crash (the prop started slipping, or the rear engine fried or motor mounts loosened or something). I thought I could gently bring her down, but then I think it suddenly lost all thrust, and with it the left-roll torque (that I would have been countering with right aileron throughout the flight to keep the wings level).

With the ailerons set for right roll but with suddenly no left torque to counter, she rolled hard right and had dug a hole in the grass before I could say "what the f...?"
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