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On Fri, 25 Jan 2008 19:33:58 -0800 (PST), Tina
wrote: There's a physics issue here called 'frame of reference.' Think about an external stationary (with respect to the air mass) observer, and the airplane is made to fly a loop. The center of rotation to that obsever is the center of the loop. Or, if the observer tracks the airplane, or even its CG (assume it's marked on the airplane) he'll see it move laterally at airspeed, then at the start of the climb will probably see it dip a little bit, then start assending. I'm having a hard time imagining a frame of reference where the airplane would appear to rotate about its CG, where I take 'rotate' to mean a point about which the tail end goes down and the other end goes up, because the CG itself will be moving, first down a little (I think) then up. Down first because the elevator is adusted so it loses upward lift of increases downward thrust, effectively making the airplane heavier. As the aoa increases the wings (making a huge number of assumptions (assume a spherical cow?) increase lift. Note also that the 'center of lift' of a wing may change with aoa, so even that model -- all effective lift concentrated at a fixed point -- may fail. A more minor point, (but why not pick nits?) is that it's unlikely the CG, center of lift of the wing, and center of lift of the elevator are all in a straight line. In a high winged airplane the center of lift, about a third of the way back from the front of the wing, and probably pretty close to the wing's underside skin, is well above the CG. That vertical displacement will not affect computing moments for horizontal flight, but will as directions of flight different from horizontal take place. Think for a moment or two about a helocopter in horizontal flight transitioning to a nose up attitude. When I've seen that, it appears the center of rotation is well above the hellcopter. I know, there's nothing like adding some mud to the water. no mud at all. consider another situation that may provide some insight. when an aircraft is falling in a spin what does it spin about? |
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