Page 65 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 52

52 CHAPTER TWO

Fig. 2.23. Armament goes on the bottom of the wing of an F-16. (Photo courtesy
of the U.S.Air Force.)

comes to within about a wing’s length of the ground. The effect
increases with the reduction in the distance to the ground. A low-wing
airplane will experience a reduction in the induced drag of as much as
50 percent just before touchdown. This reduction in drag just above a
surface is used by large birds, which can often be seen flying just
above the surface of the water. Pilots taking off from deep-grass or soft
runways also use ground effect. The pilot is able to lift the airplane off
the soft surface at a speed too slow to maintain flight out of ground
effect. This reduces the resistance on the wheels and allows the air-
plane to accelerate to a higher speed before climbing out of ground
effect.
What is the cause of this reduction in drag? Many have the
misconception that it is air piling up between the wing and the
ground, and that the airplane is flying something like a hovercraft. To
understand ground effect we first examine the air flowing over a wing
in greater detail. Notice in Figure 2.24 that the air bends up from its
horizontal flow to form the upwash. Newton’s first law says that there
must be a force acting on the air to bend it. Since the air is bent up,
the force must be up as shown by the arrow. Newton’s third law says
that there is an equal and opposite force on the wing which is down.
The result is that the upwash increases the load on the wing. In
straight-and-level flight the bending air over the top of the wing must
now lift the weight of the airplane plus the additional load caused by