Photo: Albatross landing, from Duncan (Creative Commons).

Eddie Sez:

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: Mishaps / Gulfstream G650 N652GD.

Most of this comes from the references listed at the bottom of the page. My comments are shown in blue.


Ground Effect

[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."


Aerodynamic Influence of Ground Effect

The classic explanation

Figure: Ground Effect, from [Hurt, pg. 380], figure 6.9.

[Hurt, pg. 379]

  • While the aerodynamic characteristics of the tail and fuselage are altered by ground effects, the principal effects due to proximity of the ground plane are the changes in the aerodynamic characteristics of the wing. As the wing encounters ground effect and is maintained at a constant lift coefficient, there is a reduction in the up wash, down wash, and the tip vortices. These effects are illustrated by the sketches of [the figure]. As a result of the reduced tip vortices, the wing in the presence of ground effect will behave as if it were of a greater aspect ratio. In other words, the induced velocities due to the tip (or trailing) vortices will be reduced and the wing will incur smaller values of induced drag coefficient, CDi, and induced angle of attack, αi, for any specific lift coefficient, CL.

It's no wonder ground effect is so misunderstood and some believe all this talk about down wash is hogwash. We can do better. . .

Another explanation

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]

  • Wing tip vortices are formed when higher pressure air below a wing flows around the wing tips into the lower pressure region on top of a wing that is developing lift. The vortices are strongest at the tips and become weaker progressing toward the centerline of the aircraft, as shown in [the figure].

  • Consider an aircraft with an infinitely long wing. This (infinite) wing has no wing tips and, consequently, no wing tip vortices. The absence of wing tip vortices on the infinite wing means that the up wash, in front of the wing, and the down wash, behind the wing, cancel each other out, and there is no net down wash behind the wing.

  • The relative wind (RW) ahead of this wing is horizontal, and the RW behind the wing is also horizontal. The RW at the aerodynamic center (AC) is the average of these two and must be horizontal. Life is 90° to the RW, so it acts vertically.

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]

  • The most noticeable effect is caused by a reduction of the up wash ahead of the wing and the reduction of the down wash behind the wing. as we learned in basic aerodynamics, the down wash is caused mainly by the wing tip vortices. These, as well as the down wash, are greatly reduced by the proximity of the ground.

  • Without so much down wash, the local relative wind at the aerodynamic center is more nearly horizontal. The lift vector is more nearly vertical, and the induced drag is greatly reduced.

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!


Proximity to the Ground

[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.


Landing Influences

[Hurt, pg. 381]


Takeoff Influences

[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: Mishaps / 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.


References

Creative Commons

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