|MadSci Network: Physics|
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 Best regards, Your Mad Scientist Adrian Popa
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