On modern aircraft, a large number of aerodynamic devices, such as slats, slots, flaps, spoilers, and dive brakes, affixed to a simple wing serve to increase or decrease lift and drag. With all these devices hanging on a wing, the unsuspecting air traveler might well think that the wing is a piece of modern art. The sound of flaps and slats opening as one approaches for a landing combined with a visual inspection of the wing "coming apart at the seams" may unnerve the unknowledgeable. But a purpose exists for all of these devices, and the safety and economy of air travel depends on them. It is in the interest of safety to perform takeoff and landing at as low a speed as possible. But also, one does not want the normal flying characteristics to be affected. Consider a near-level flight condition in which the airplane weight is equal to the lift (L = W). For minimum flying speed (takeoff or landing), the wing would be operating at maximum lift or CL,max. From the equation for total lift on a wing, L = 1/2 p¥V2S CL
where 1/2 p¥V2 = the dynamic pressure, S is the surface area of the wing, and CL = the coefficient of lift, after some manipulation, it is possible to calculate the minimum flight velocity needed for takeoff or landing Vmin,
The density r ¥ is considered to be constant and if the weight (W) is considered a fixed characteristic of the airplane, then the only way to reduce the minimum velocity Vmin is to increase CL,max and/or the wing area S. Slots and flaps are used for this purpose. The maximum coefficient of lift CL,max may be increased through the use of a slot formed by a leading-edge auxiliary airfoil called a slat. When the slot is open, the air flows through the slot and over the airfoil. The slot is a boundary-layer control device and the air thus channeled energizes the boundary layer about the wing and retards the separation. The airfoil can then be flown at a higher angle of attack before stall occurs and thus get a higher CL,max value....
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