Ground effect has a significant effect on how your airplane takes off and lands, but depending on the airplane you might not ever notice. Aircraft with wide wingspans are obviously impacted. Aircraft engine idle characteristics can mask landing ground effect impacts. The key is you have to think about it and deal with the idiosyncracies of the airplane you are flying.
Most of us identify ground effect as causing any tendency to float during a landing. The solution, of course, is to fly the airplane onto the runway or go around. But a more critical problem happens during takeoff. Many modern aircraft are powerful enough to muscle their way through the increased induced drag that occurs when leaving ground effect. But at heavy weights on a hot day with an engine failed, that may not be so easy. After an engine failure during takeoff achieving V2 takeoff safety speed becomes critical. Flying slower can be fatal. More about this: Case Study: Gulfstream G650 N652GD.
Everything here is from the references shown below, with a few comments in an alternate color.
Photo: Albatross landing, from Duncan (Creative Commons).
[Hurt, pg. 379] When an airplane in flight nears the ground (or water) surface, a change occurs in the three dimensional flow pattern because the local airflow cannot have a vertical component at the ground plane. Thus the ground plane will furnish a restriction to the flow and alter the wing up wash, down wash, and tip vortices. These general effects due to the presence of the ground plane are referred to as "ground effect."
Figure: Ground Effect, from [Hurt, pg. 380], figure 6.9.
[Hurt, pg. 379]
It's no wonder ground effect is so misunderstood and some believe all this talk about down wash is hogwash. We can do better. . .
Figures: Wing tip vortices (top two) and airflow about an infinite wing (bottom two), from Dole, figures 2.37 through 2.39.
[Dole, pg. 50]
You have more pressure under the wing than over it, so the pressure below wants to wrap around to the top. Of course the wing is moving forward so this movement from bottom to top tends to trail aft.
Figures: Aerodynamic force, from Eddie's notes.
[Dole, pg. 131]
Since less of the aerodynamic force is pointed aft, you have less induced drag and more lift. To the pilot, the wing appears to have become more efficient because, really, it has!
[Hurt, pg. 379]
We, as pilots, are often taught that ground effect starts at one-half the wing span of the aircraft. This is not really true, it actually starts at just over one wing span but the effect is negligible. At one-half the wing span it reduces induced drag by about 9 percent and the effect builds as you get closer to the ground.
[Hurt, pg. 381]
This "floating" sensation really depends on the airplane and its sensitivity to being precisely on target threshold speeds. Aircraft with very long wing spans behave differently and engine idle characteristics also impact this sensation. A B-747, for example, will float the distance of the runway without a loss of airspeed at some weights. This is a combination of the long wing span and high flight idle of the engines. A GV, on the other hand, has very long wings but lower relative idle engine power. If a GV crosses the threshold on speed and the throttles are allowed to "auto retard" starting at 50 feet, there should be no float at all.
[Hurt, pg. 382]
This is what happened to Gulfstream G650 N652GD. The test crew was attempting to achieve unreasonably low V2 speeds and when the aircraft left ground effect, it stalled asymmetrically.
More about this: Gulfstream G650 N652GD.
Figure: Airplane lift versus angle of attack in and out of ground effect, from NTSB AAR 12/02, figure 5.
[NTSB AAR-12/03, ¶2.3] As stated in section 1.1, ground effect refers to changes in the airflow over the airplane resulting from the proximity of the airplane to the ground. Ground effect results in increased lift and reduced drag at a given AOA as well as a reduction in the stall AOA; thus, the stall AOA is lower for airplanes in ground effect compared with the stall AOA for airplanes in free air (out of ground effect). Ground effect decreases as the distance from the ground increases and is generally negligible above a height equivalent to the wing span of the airplane (which is about 100 feet for the G650). [The figure] depicts the changes in the airplane‟s lift and stall AOA due to ground effect.
Dole, Charles E., Flight Theory and Aerodynamics, 1981, John Wiley & Sons, Inc, New York, NY, 1981.
Hurt, H. H., Jr., Aerodynamics for Naval Aviators, Skyhorse Publishing, Inc., New York NY, 2012.
NTSB Aircraft Accident Report, AAR-12/03, Crash During Experimental Test Flight, Gulfstream Aerospace Corporation GVI (G650), N652GD, Roswell, New Mexico, April 2, 2011
Copyright 2019. Code 7700 LLC. All Rights Reserved.