Mini APM 3.5 Tricopter no tail servo movement



Anybody with experience in APM Tri copters? I’m sure this has been asked before, but I really need to get this thing off the ground.

I’m set to go except one thing. My Tri Copter has no yaw servo movement. The signal seems to have no pass through. The motors spin up with the throttle, and adjust in different attitudes, but no tail servo. I have no where to begin, and need suggestions.

It has power supplied from a separate step-down converter to the tail servo, and retracts. I am running an FRSKY module in a football 9C with a FRSKY D8R-XP in CPPM mode.

The radio calibrates, and all the green bars move through Ch 8, here are no warnings at all.

Is there a way to check the firmware while connected to make sure it is tri copter firmware?
It seems that when you select the frame type in MP, the tri copter frame is mixed with a whole bunch of others kind of generally…

I am suspecting that the tri copter firmware is not going into my board, which might explain no tail servo movement. (maybe)

Any suggestions?




The first thing you should check is the actual servo, can you plug it directly into a PWM receiver or a servo tester to check its working properly.

You mentioned that your servo is powered via a step down voltage regulator, why is it not connected directly to your APM as that is how most people connect their tail servo directly to the APM.


Hello, Thank you for responding.

I did not connect it to the board because I heard of brown out issues, and board voltage warnings, on previous versions of the APM.
I just figured that why not just avoid the possibilities of this, as I had an untapped power source in the Lipo balance plug anyway.

Here are some additional notes, observance and progress I have made, since my last post.

My tail servo plugged into ch7 on receiver, and the servo is powered by and external step-down converter, plugged into the balance plug of a 14V lipo.

The motors arm, spin, and change RPM when the frame is moved in a different orientation. I have no warnings. The servo has proper voltage.

I found this info below on the APM Tricopter wiki, but it seems it is a bit vague.
I believe this page is the issue.

I wish there were clear, concise directions to solve this problem on the wiki. I see people with this problem all over the forums I frequent, but no solutions. I am going to take it upon myself, once I kill this problem, to post on all of the forums I frequent. The solution for others.
It seems that this problem has been over looked, or most likely people like Quads, and other frame types better. No biggie, I like Tricopters for their maneuverability.

Wiki Link to info below…

I suspect these settings are in the advanced parameters, correct? The APM Wiki doesn’t mention where to look.

Can anybody make sense of them? If so, can you explain what I need to change?


1 Copter 3.3 (and higher)

2 Copter 3.2.1 (and earlier)

Copter 3.3 (and higher)

MOT_YAW_SV_MIN: yaw servo’s lowest PWM value before binding occurs.
MOT_YAW_SV_MAX: yaw servo’s highest PWM value before binding occurs.
MOT_YAW_SV_TRIM: yaw servo’s PWM value close to what is required to keep the tail from spinning.
MOT_YAW_SV_REV: yaw servo’s reverse setting. +1 = servo moves in default direction, -1 to reverse direction of movement.

This is the firmware I am using

Copter 3.2.1 (and earlier)

RC7_MIN: yaw servo’s lowest PWM value before binding occurs.
RC7_MAX: yaw servo’s highest PWM value before binding occurs.
RC7_TRIM: yaw servo’s PWM value close to what is required to keep the tail from spinning.
RC7_REV: yaw servo’s reverse setting. +1 = servo moves in default direction, -1 to reverse direction of movement.


Sorry, I had a typo in the above… The servo IS NOT plugged into the receiver, it’s signal wire is coming off of the APM.

I am running FRSKY CPPM


In terms of brownout, this will be very unlikely if you are just using 1 servo connected to your APM/Pixhawk

So your main question is where to change those settings?

If so you find them on hte full parameter list as shown below, and my suggestion is to use the search feature to find MOT_ values.

Also have you managed to confirm your servo is working as it should when connected directly to your receiver or via a servo tester?


Hello, I found two bad connections in the circuit for the servo, I now have it working. I am running the servo off of the BEC for the tail servo. The board is powered by an APM power module. I finally have this thing working, and now can move foward with tying the rest of the loose ends to get it in the air.
Thank you for responding, your answers are always helpful an informative, and I always use the drone trest forum when I have questions.
Thank you again


Hi Ted

Thanks for letting me know what the problem is and happy to hear its all working. All the best with the rest of your tricopter build :smile: