Hi, I wonder how the motor installation height affects to flying wing stability in reality? When the motor is above the wing chord line it might give nose downward pushing moment and below the chord line it is the opposite way. Many flying wing concepts has motor installed to same level with the wing chord line. Has anyone got experience of installing motor above or below chord line in a flying wing?
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Hi Veikko,
I haven't installed motors on my flying wings, but used to do a lot of slope soaring with un-powered wings and have designed and built fixed wing powered and un-powered models. The principals here aren't any different from any other powered fix-wing craft; there are two factors to consider (actually many, but two relevant to your query). One is the the pitching moment induced by not having the thrust vector pointing along the line between the thrust application point and centre of mass of the aircraft (which induces a moment about the CoG) and the other is the pitching moment induced by the lateral difference between the centre of mass and the centre of pressure (the point at which lift effectively acts).
Typically you want stable straight and level flight (so all natural moments cancel) at a given flight speed and typically a slight nose-up pitching moment as speed increases (so that when you're rocketing toward the ground taking your fingers off the stick induces the aircraft to naturally pull up out of the dive). Since lift is proprotional to (the square of) airspeed, having the CoP slightly forward of the CoG in gliders is good. In gliders we obviously cannot adjust the thrust vector to balance against the pitching moment induced by CoG-CoP differences, so we use added weight to shift the CoG to an appropriate spot. You *could* do this balancing by shifting the thrust vector up or down, although it would be better to simply change the angle of thrust by making a motor mount that was not perpendicular to the longitudinal axis. Note that this is how motor induced torque roll is countered in many aircraft, by having the thrust vector angled slighting in the horizontal plane to induce a balancing yaw. Ultimately it would require flight testing, which can get a bit hairy when you get the balance wrong!