Motor Throttle Behaving Strangely

Hello all!

I just finished building a quad for the first time (started on a Hubsan X4 to learn to fly). It’s on a 450 frame, using SunnySky 2212 motors and the MultiRotorMania 20A 2-4S Atmel ESCs.

I’m attempting to use a very old TX/RX pair to avoid buying a new one (it was my dads, maybe upwards of 20 years old). The TX is an Airtronics VG6DR and the RX is an Airtronics 92765. When I did the first time setup for my APM, I noticed that the throttle bar was moving backwards (down on the TX showed a full green bar, and up on the TX emptied the green bar). All the other controls appeared normal.

Because of this, I switched the CH3 reverse parameter. The copter would actually arm at this point; before I couldn’t get it to arm at all.

The weird thing, however, is that the throttle behaved very strangely. 0% on the throttle would idle the motors (as expected), but then even 1% on the TX throttle gave 100% on the motors. As the TX throttle increased, the motors decreased, so max throttle on the TX was probably 1% on the motors.

This is insanely confusing to me because it’s almost backwards, but not completely. That’s what I don’t understand.

Later, I looked at the actual reading the APM was receiving from the RX in Mission Planner, and it appears that that TX was setup such that full throttle was low values and off was high values, but all the other channels were standard (I think it was just one of the standards at that time?).

I’d switch it on the TX but it doesn’t have that option. I’d be willing to open up the TX and swap the wires on the potentiometer, if necessary, but this seems like something that can be fixed within the software. Fixing it in just software worries me, however, if it happens to revert to manual mode and my throttle switches to backwards.

Any thoughts? This is super non-intuitive to me and I can’t figure out what might be causing this. I don’t think it’s the ESCs because from what I understand they were calibrated at MRM.


You always calibrate the ESCs for your transmitter. A factory calibrate does nothing to help the ESC understand your transmitter. I’m not saying this will resolve your issue, but it is necessary.

I calibrated them several times and it didn’t fix it. I ended up opening the TX and swapping wires on the throttle channel, and all is good with that now.

However, now I have a new problem. I’m going to preface this with I already checked the propeller types and motor spin directions several times; I went through the unstable takeoff checklist on the APM documentation.

Whenever I try to take off, it wants to roll really hard right. Is is plausible that the APM being slightly off level when calibrated could cause this? I’m not sure how aggressively stabilize mode will try to correct.

My APM is mounted on a vibration dampener, and where it is on the frame, the USB cable can’t fit unless the APM is nudged a little. This is my only theory. Any other ideas?

While I’ve not calibrated the APM with it being anything but level, being a tiny bit off…I don’t think it would respond as badly as you indicate. It sure sounds like a motor is spinning in the wrong direction or the motor order is incorrect. I know you said you checked it.

They’re all spinning the correct direction, and you can’t see in the image, but they’re the right type of propellers, I promise.

My theory at the moment: the integral term of the PID controller basically maxes out pretty fast, but the adjustment the flight controller wants to make is huge, even though the amount off from level isn’t very big.

Any other ideas?

Your motor and props seem to rotate the correct way. Any chance you might have messed up the esc connection order to the APM? That could explain this.