PRINCIPLES OF FLIGHT
LIFT REQUIRES POWER In classic two-dimensional aerodynamics, one learns that a wing requires no work to produce lift and that there is no net downwash behind the wing. This is illustrated through theory and calculations that are done with two-dimensional airfoils. These two-dimensional airfoils are in fact infinite wings. This is so because a two-dimensional, or infinite, wing is much easier to calculate than one of finite span. The efficiency of a wing increases with the span of the wing because the amount of air diverted increases with area. Thus an infinite wing diverts an infinite amount of air at near-zero velocity to produce lift and thus is infinitely efficient. The net vertical velocity of the downwash is essentially zero. Therefore, the infinite wing requires no power to produce lift. Of course, this is not the situation for a real threedimensional wing. Some people argue that there is no work done in producing lift. Work is force times distance. The argument goes that since the wing in straight-and-level flight does not change altitude, no work is done against the downward force of gravity. However, if you push a heavy box along a flat floor, you must do work against friction. Induced drag is the horizontal force produced as a consequence of lift, which, like friction in the box example, is opposite the direction of motion. So the work done in producing lift is the induced drag times the distance the wing travels.
duce enough thrust to lift the craft vertically off the ground before they are rotated forward for horizontal flight.
Wing Vortices The downwash behind a wing is sometimes called the downwash sheet. This downwash sheet has a curl in it, producing the wing vortex. Near the tip of the wing, the wing vortex curls very tightly, creating the wingtip vortex. The wingtip vortex itself contains only a small portion of the total energy of the wing vortex. Because of the tight curl, it behaves like a small tornado with very low pressure in its core. Low pressure is tied to lower temperature, and so, under the right conditions, moisture condenses, making the wing tip vortex visible.
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