I changed from an X525 frame to an own-design H-frame. Same motors, props, ESC's & APM2. I've used the X525 aluminium arms but with a ply-box for the center fuse portion of the H. Same positioning of the motors except the center section of the H is about 30mm longer than the 2 arms.
The quad flies fine with roll and pitch normal but it yaws slowly left and right rudder doesn't stop it nor does it cause any right yaw at all. Left rudder doesn't increase the slow left yaw speed. In fact either rudder direction just makes the whole quad get a bit unstable and out of shape.
Very strange, any ideas?