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."
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. . .
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]
- In order for ground effect to be of a significant magnitude, the wing must be quite close to the ground plane.
- When the wing is at a height equal to the span (h/b = 1.0), the reduction in induced drag is only 1.4 percent. However, when the drag is at a height equal to one-fourth the span (h/b = 0.25), the reduction in induced drag is 23.5 percent and, when the wing is at a height equal to one-tenth the span (h/b = 0.1), the reduction in induced drag is 47.6 percent. Thus, a large reduction in induced drag will take place only when the wing is very close to the ground.
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]
- The overall influence of ground effect is best realized by assuming that the airplane descends into ground effect while maintaining a constant lift coefficient and, thus, a constant dynamics= pressure and equivalent airspeed. As the airplane descends into ground effect, the following effects will take place:
- Because of the reduced induced angle of attack and change in lift distribution, a smaller wing angle of attack will be required to produce the same lift coefficient. If a constant pitch attitude is maintained as ground effect is encountered, an increase in lift coefficient will be incurred.
- The reduction in induced flow due to ground effect causes a significant reduction in induced drag but causes no direct effect on parasite drag. As a results of the reduction in induced drag, the thrust required at low speeds will be reduced.
- The reduction in down wash due to ground effect will produce a change in longitudinal stability and trim. Generally, the reduction in down wash at the horizontal tail increases the contribution to static longitudinal stability. In addition, the reduction of down wash at the tail usually requires a greater up elevator to trim the airplane at a specific lift coefficient.
- Due to the change in up wash, down wash, and tip vortices, there will be a change in position error of the airspeed system. In the majority of cases, ground effect will cause an increase in the local pressure at the static source and produce a lower indication of airspeed and altitude.
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]
- During the takeoff phase of flight ground effect produces some important relationships. Of course, the airplane leaving ground effect encounters just the reverse of the airplane entering ground effect, i.e., the airplane leaving ground effect will (1) require and increase in angle of attack to maintain the same lift coefficient, (2) experience an increase in induced drag and thrust required, (3) experience a decrease in stability and a nose-up change in moment, and (4) usually a reduction in static source pressure and increase in indicated airspeed.
- These general effects should point out the possible danger in attempting takeoff prior to achieving the recommended takeoff speed. Due to the reduced drag in ground effect the airplane may seem capable of takeoff below the recommended speed. However, as the airplane rises out of ground effect with a deficiency of speed, the greater induced drag may produce marginal initial climb performance.
- In extreme conditions such as high gross weight, high density altitude, and high temperature, a deficiency of airspeed at takeoff may permit the airplane to become airborne but be incapable of flying out of ground effect.
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