### Re: how does gound effect flight work

Date: Thu May 18 12:26:03 2000
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Physics
ID: 958612404.Ph
Message:
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Greetings:

The following text in QUOTES is taken from Section 8, of Level 3, Flight
Performance, on the NASA ALLSTAR (Aeronautics Learning Laboratory for
Science, Technology and Research) web site. This web site has a great deal
of educational material for all levels of education.

http://
www.allstar.fiu.edu/aero/AirPerf08.htm

For helicopter aerodynamics in ground effect, the U.S. Army Field Manual 1-
51 presents well illustrated information about ground effect for rotary wing
pilots.
http://www.copters.com/
helo_aero.html

QUOTE:

GROUND EFFECT

The total drag of an airplane is divided into two components, parasite drag
and induced drag. Induced drag is the result of the wing's work in
sustaining the airplane. The wing lifts the airplane simply by accelerating
a mass of air downward. It is perfectly true that reduced pressure on top of
an airfoil is essential to lift, but still that is but one of the things
that contribute to the overall effect of rushing an air mass downward. The
amount of downwash is directly related to the work of the wing in pushing
the mass of air down and therefore to the amount of induced drag produced.
At high  angles of attack, induced drag is high. As this corresponds to
lower  airspeeds in actual flight, it can be said that induced drag
predominates at low speed.

When a wing is flown very near the ground, there is a substantial reduction
in the induced drag. Downwash is significantly reduced; the air flowing from
the trailing edge of the wing is forced to parallel the ground. The wing tip
vortices that also contribute to Induced drag are substantially reduced; the
ground interferes with the formation of a large vortex.

Many pilots think that ground effect is caused by air being compressed
between the wing and the ground. This is not so. Ground effect is caused by
the reduction of induced drag when an airplane is flown at slow speed very
near the surface.

Ground effect exerts an influence only when the airplane is flown at an
altitude no greater than its wing span, which for most light airplanes is
fairly low. A typical light airplane has a wing span of perhaps 35 feet and
will experience the effect  of ground effect only when it is flown at or
below 35 feet above the surface (ground or water).

A low wing airplane is generally more affected by ground effect than a high
wing airplane because the wing is closer to the ground. High wing airplanes
are, however, also influenced by this phenomenon.

Pilots get into trouble because of ground effect when they precipitate take-
off before the airplane has reached flying speed. Take the scenario of a
pilot trying a take-off from a poor field. He uses full power and holds the
airplane in a nose high position. Ground effect reduces induced drag and the
airplane is able to reach a speed where it can stagger off. As altitude is
gained, induced drag increases as the effect of the ground effect
diminishes. Twenty or thirty feet up, ground effect vanishes, the wing
encounters the full effect of induced drag and the struggling airplane which
got off the ground on the ragged edge of a stall becomes fully stalled and
drops to earth.

Ground effect is also influential in landing. As the airplane flies down
from free air into ground effect, the reduction of induced drag as it nears
the runway comes into, effect to make the airplane float past the point of
intended touchdown. In the common case of an airplane coming in with
excessive speed, the usable portion of the runway may slip
by with the airplane refusing to settle down to land. A go around will
probably be necessary. On short fields, approach as slowly as is consistent
with safety.

An airplane also tends to, be more longitudinally stable in ground effect.
It is slightly nose heavy. The downwash from the wing normally passes over
the tail at an angle that produces a download on the tail. Ground effect
deflects the path of the downwash and causes it to pass over the tailplane
at a decreased angle. The tailplane produces more lift than usual and the
nose of the airplane tends to drop. To counteract this tendency, more up
elevator is required near the ground. During take-off as the airplane climbs
out of ground effect, the download on the tailplane increases and the nose
tends to pitch up.
END QUOTE

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