Page 191 - Understanding Flight
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CH07_Anderson  7/25/01  9:00 AM  Page 178




                 178  CHAPTER SEVEN



                                       Takeoff Performance


                                       The takeoff of an airplane is a fairly simple thing. The airplane accel-
                                       erates down the runway until it has reached a speed comfortably
                                       above the stall speed. The pilot then pulls back on the controls, or
                                       rotates, and off the airplane goes. The takeoff speed is typically about
                                       20 percent above the stall speed, but it can be as little as 5 percent for
                                       some military aircraft. If you were an airplane designer, what other
                                       things would you consider in takeoff performance?
                                         The most obvious figure of merit for takeoff performance is takeoff
                                       distance. If you want to design an airplane that can take off from a
                                       short dirt field you will have to include certain features. If you have
                                       unlimited runway, you might design a different airplane. As a general
                                       rule, airplanes that have short takeoff distances will fly at lower cruise
                                       speeds. Faster airplanes usually need longer runways. Let us examine
                                       why this is so.
                                         Since we are concerned with takeoff distance, it is obvious that one
                                       can shorten this distance by increasing the engine power or by reducing
                                       the takeoff speed. The problem with increasing the size of the engine is
                                       that it adds weight and cost. Also, since the parasitic power goes as the
                                       speed of the airplane cubed, the increased power will do little for the
                                       cruise performance of the airplane. The takeoff speed is decreased by
                                       reducing the stall speed, by reducing the wing loading, and by adding
                                       high-lift devices such as slats, slots, and larger flaps. These add cost and
                                       weight to the wing and can degrade the cruise performance.



                                         YEAR    NY–LONDON TICKET  COST OF A BOEING 747
                                         1970        $316               $20M
                                         1980        $595               $57M
                                         1990        $462              $123M
                                         2000        $436              $180M



                                         The biggest factor in takeoff performance is the weight of the airplane.
                                       The takeoff speed is proportional to the square root of the airplane’s
                                       weight. A 20 percent increase in weight will cause approximately a
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