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