I am working on tailsitter and I have written a 6dof fdm simulation code that connects to the ardupilot through json interface. The problem is when all moments and forces are set to be zero in fdm except the x-force which is upward in tailsitter, in takeoff the plane starts the motors and an acceleration in x-direction is passed to the autopilot. But it interprets the vertical acceleration as lateral one and thus the dcm and ahrs estimate lateral acceleration and change in pitch angel which is not true! I tried to find where is the problem in the code but I have not found it yet.