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Lift and Angle of Attack
"Dave Daniels" wrote in message
... [...] Firstly, can the lift acting on an aircraft go negative? Yes, though a non-symmetrical wing (the most common type) will not have an exactly reversed lift-to-angle-of-attack profile for negative angles of attack. That is, negative five degrees of AOA won't produce the same amount of lift downward as positive five degrees of AOA produces upward. Most wings are designed to optimize lift in the upward direction and so will get more lift for the same angle of attack in the positive direction, compared to the negative direction. [...] Secondly, does the angle at which an aircraft is banked affect the angle of attack? The only thing that affects angle of attack is the airplane's attitude relative to its path of flight. If I were writing a simulator, I would just use the velocity vector and the attitude (bank, roll, and pitch angle) to determine angle of attack. Whether roll angle affects the angle of attack depends on your definition of "affects" and how exactly you're calculating angle of attack. But generally speaking, if the flight path is not exactly aligned with the longitudinal axis of the airplane, the roll angle does affect the angle of attack, albeit in a small way. Aerodynamically there's another thing to consider, which is that the force called lift is directed perpendicular to relative wind. In anything other than straight and level flight, this means lift is not exactly opposite gravity. In order to calculate vertical acceleration, the simulator code will have to break the lift vector into multiple components, one of which will be the vertical component acting against gravity. Again, roll angle (among other things) needs to be accounted for to figure out the vertical component of lift. [...] Thirdly, induced drag: I have read that this is inversely proportional to the square of the aircraft's velocity, but I do not see how this follows from the equations I have seen. It appears to me that the greater the velocity, the greater the induced drag. I must be missing something fundamental here. Can anyone explain it? For a constant angle of attack, increased airspeed means increased induced drag. However, it also means increased lift. Induced drag is a by-product of lift. In normal cruise flight, induced drag actually goes *down* as airspeed increases, because lift remains constant. The only way to keep lift constant is to reduce angle of attack as the airspeed increases, and reducing the angle of attack reduces drag faster than increasing airspeed increases it. Pete |
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