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Unformatted text preview: Chapter 15 Flow Over Bodies: Drag and Lift Lift 15-71C The contribution of viscous effects to lift is usually negligible for airfoils since the wall shear is parallel to the surfaces of such devices and thus nearly normal to the direction of lift. 15-72C When air flows past a symmetrical airfoil at zero angle of attack, ( a ) the lift will be zero, but ( b ) the drag acting on the airfoil will be nonzero. 15-73C When air flows past a nonsymmetrical airfoil at zero angle of attack, both the ( a ) lift and ( b ) drag acting on the airfoil will be nonzero. 15-74C When air flows past a symmetrical airfoil at an angle of attack of 5 , both the ( a ) lift and ( b ) drag acting on the airfoil will be nonzero. 15-75C The decrease of lift with an increase in the angle of attack is called stall . When the flow separates over nearly the entire upper half of the airfoil, the lift is reduced dramatically (the separation point is near the leading edge). Stall is caused by flow separation and the formation of a wide wake region over the top surface of the airfoil. The commercial aircraft are not allowed to fly at velocities near the stall velocity for safety reasons. Airfoils stall at high angles of attack (flow cannot negotiate the curve around the leading edge). If a plane stalls, it loses mush of its lift, and it can crash. 15-76C Both the lift and the drag of an airfoil increase with an increase in the angle of attack, but in general lift increases at a much higher rate than does the drag. 15-77C Flaps are used at the leading and trailing edges of the wings of large aircraft during takeoff and landing to alter the shape of the wings to maximize lift and to enable the aircraft to land or takeoff at low speeds. An aircraft can takeoff or land without flaps, but it can do so at very high velocities, which is undesirable during takeoff and landing. 15-78C Flaps increase both the lift and the drag of the wings. But the increase in drag during takeoff and landing is not much of a concern because of the relatively short time periods involved. This is the penalty we pay willingly to takeoff and land at safe speeds. 15-79C The effect of wing tip vortices is to increase drag (induced drag) and to decrease lift. This effect is also due to the downwash, which causes an effectively smaller angle of attack. 15-80C Induced drag is the additional drag caused by the tip vortices. The tip vortices have a lot of kinetic energy, all of which is wasted and is ultimately dissipated as heat in the air downstream. The induced drag can be reduced by using long and narrow wings. 15-81C When air is flowing past a spherical ball, the lift exerted on the ball will be zero if the ball is not spinning, and it will be nonzero if the ball is spinning about an axis normal to the free stream velocity (no lift will be generated if the ball is spinning about an axis parallel to the free stream velocity)....
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- Spring '08