I have just built and flown a lightweight delta/flying wing hybrid. It has no up or down thrust, nor any left or right thrust, and even more surprising no reflex. It flies straight and true with no trim needed a type of result that RMH was talking about. The first flight had an issue with torque pulling the plane left, but launching it with a catapult eliminated that problem. Once in flight the spiral slipstream or P-Factor does not seem to be an issue at all. I think that any real problem is caused by the torque of the motor, not the spiral slipstream.