Quadcopter Coupled Pitch and Roll

I have been testing a new quad build for a couple weeks now and I can’t seem to get rid of this issue. The problem is that when I make a pitch input, I get a pitch output coupled with a roll. Making a roll input produces the same effect (roll output with coupled pitch), but it is not as drastic as on the pitch axis. I have tried different props, swapping motors around, and checking that my center of gravity is coincident with my center of thrust.

The quad is a custom built FPV frame similar to the QAV400, but slightly larger. It is running a Pixhawk on 3.1.2 with 30A Afro ESCs, T-Motor MT-2216 900kV motors, and APC props.

Can anyone provide some insight on what might be wrong?

For a QAV style frame, should the frame type be set to X or H? I am currently running X.


Couple things off the top of my head:

-Flight controller not square with the frame.

-CG: how exactly are you adjusting it? Does this frame have a trapezoid, square or rectangular motor layout? There is a some erroneous information about CG location given by some of the multirotor manufacturers. Particularly those with trapezoidal frames. They say the CG should be on an intersection of the lines drawn cross-wise between the motors, but this is false.

-Thruster imbalance: Motors, props, esc’s, something wrong with one of them. You said you’ve been looking for this already.

-Thruster misalignment. If for example, one of the motors is misaligned, when you pitch, it could create a yaw torque, which needs to be counteracted, and then results in a roll.


The flight controller is square to the frame but set backwards (Yaw 180 set for AHRS Orientation). Accelerometers calibrated after AHRS orientation set. When doing the accelerometer calibration with a nonstandard AHRS orientation, should the quad be positioned on its front, left, right, etc. or should the Pixhawk be positioned on its front, left, right, etc. relative to its “forward” arrow?

Frame has a square motor layout, but there is definitely more inertia on the pitch axis than the roll axis. I am setting the CG at the center of the motor layout, which is also where the Pixhawk is positioned. Where should the CG be?


False alarm.

Turns out the frame type somehow got set to + instead of X. I know I set X initially :blush:
Thanks for your help Rob.