The concept of ground effect
is well known to pilots. This effect is the increase in efficiency of a
wing as it 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 asurface 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 airplane to accelerate to a higher speed before climbing out
of ground effect.
What is the cause of this reduction in drag? There are two contributions that can be credited with the reduction in drag. The ground influences the flow field around the wing which, for a given angle of attack, increases the lift. But, at the same time, there is a reduction in downwash. It can be surmised that this additional lift must come from an increase in pressure between the wing and the ground. In addition, since lift is increased for a given angle of attack, the angle of attack can be reduced for the same lift, resulting in less downwash and less induced drag.
Ground effect introduces a fundamental change from the discussion of flight at altitude. When no ground is present, the relationship between lift, drag and downwash is straight forward. But, near the ground, there is an action-reaction between the wing, the air and the ground. At altitude the ground is so distant that this effect does not exist. Near the ground this interaction helps produce lift and reduce downwash due to an increase in pressure below the wing. The details of ground effect are extremely complex. Most aerospace texts devote a paragraph or two and don’t attempt to describe it in depth. The truth is that so much is changing in ground effect that it is difficult to describe by pointing to a single change in the air flow or a term in an equation. There is no simple way to describe how the airflow adjusts to satisfy the change in conditions.