I am flying a tandem helicopter, everything was fine at first. but after several flight tests, the two swash plate tilts inversely on the ground after armed and waiting for run-up. looking through the log, i find an unexpected yaw target value.
Then find the code in mode_stabilize_heli.cpp, reset_yaw_target_and_rate(false), motors->using_leaky_integrator(), if someone could tell me is it result in the unexpected target_yaw value?
@hututu_duan i put this in the traditional helicopter section of ardupilot rather than copter 4.2. It will be seen more by those that fly dual heli’s.
@picoflug told me about the same issue that he had for his tandem heli’s.
Yes what you found in the code is what contributes to the target moving while on the ground after run up is initiated. This caused by how the integrator is handled. Most traditional heli’s don’t see this because the heli may move on the skids or they just don’t see enough integrator build up to notice. It is more evident on tandem heli’s because the pilot can see the rotor heads tilting in opposite directions.
If you have arducopter version 4.1 or later then you can use a different method of integrator handling that was introduced. You can read about it here. The important point about using the new integrator method (H_OPTIONS=0) is that you want to ensure it is detecting landing properly. With copter version 4.2, this should be better as the land colllective min parameter is being used and should make land detection more reliable. It also doesn’t use the leaky integrator and the ILMI parameter. So the integrator is only limited by the IMAX parameter. I would recommend that you reduce your IMAX parameters to less than 0.2 initially while you get to understand how your aircraft behaves with this new integrator scheme. Depending on your I gain, the integrator can build pretty quickly and potentially cause high pitch and roll moments on the ground as it takes around 1 to 1.5 seconds to detect landing and reset the integrator.
@picoflug did say that the new integrator scheme helped eliminate the behavior you are seeing. Let me know if you have any questions.