MadSci Network: Physics

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


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. 


For helicopter aerodynamics in ground effect, the U.S. Army Field Manual 1-
51 presents well illustrated information about ground effect for rotary wing 



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.

Best regards, Your Mad Scientist
Adrian Popa

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