powderburner
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If you guys are going to deeply into a technical subject and want to start splitting hairs, then we need to start with correct terminology. We seem to have several technical concepts confused. Let's go through a short list:
AoA is angle of attack of the wing mean chord line relative to the oncoming air. Note that the wing may be "pointed" in a slightly different direction than the rest of the aircraft. For most airfoil sections the wing MUST be at a positive AoA in order to create lift, regardless of the apparent orientation or motion of the rest of the aircraft. The glider may appear to be "pointed" downward but its MOTION (flight path angle) may be at an actual angle that is even steeper; this can leave the wing at a positive AoA even while the total aircraft is descending.
Angle of Incidence describes the angle of the offset between the wing mean chord line and a longitudinal reference line passing through the rest of the aircraft. AoI is often positive at the wing exposed root and smaller and smaller toward the wingtip (sometimes even to a negative AoI); this wing twist is called "washout" and is an aerodynamic design feature to improve spanwise lift distribution and aircraft stall behavior.
Flight path angle is the difference between the path of the vehicle and the "world" coordinate reference system in which it is operating. This is the visible angle that most of us see and think of. It is not the same thing as AoA or AoI. Obviously a glider must descend at a negative flight path angle in order to use the force of gravity to accelerate the aircraft, but at the same time the wing must be at a positive AoA to generate lift.
The only aircraft that comes to mind that flies (and climbs!) while appearing to be at a negative AoA is the B-52 at takeoff. This is because of that monster-thick wing, the huge trailing edge flaps, and the very large TEF deflection. This creates a mean chord line (from airfoil leading edge point to airfoil trailing edge point, which is deflected to a very low position with flaps in takeoff setting) with a relatively severe positive angle. Because the airfoil generates too much lift when the B-52 is oriented horizontally (as when it has just lifted off of the landing gear) the aircraft is pitched nose-down to a more usable AoA as it accelerates after takeoff. This has the startling visual effect of causing the B-52, flaps down, to move to a significant nose-down-tail-up orientation while climbing off of the runway. And yes, it does look weird.
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