How high does the ground effect last?

Question

Up to which height is the ground effect still significant enough to be taken into account? Would this height be sufficient to actually fly above the ground, overcoming at least trivial obstacles like trees?

The early transatlantic flights were done with heavy loaded planes that had difficulties taking off (only at the end of the runway, and only just). Could it be that these planes were flying within ground effect for rather extended distances, before picking enough speed to complete the take off?

Answer 1

There is no height at which ground effect is switched off. It just becomes less and less effective. There are two factors which determine how strong ground effect is.:

1. Height relative to wingspan: The downwash cannot move into the ground so the induced flow field around the wing is distorted. In a first order approximation, the wing affects mostly the air which flows through a stream tube with the diameter of the wing span. For an explanation see this answer. Once the height is above one semispan, the distortion of the flowfield comes to an end.
2. Trailing edge height relative to wing chord. For very small heights, the gap between trailing edge and ground becomes small enough to more or less block the proper exit of the flow below the wing. The result is an increase in ram pressure on the lower side and further distortion of the flow field such that the stagnation point moves down and the suction peak of the nose flow becomes stronger, adding nose thrust and reducing drag.

When moving up from zero height, ground effect diminishes rapidly because the gap between trailing edge and ground opens up and the ram effect vanishes. Moving up further will also weaken the first effect until at a height of one semispan the flow will be very similar to that of the airplane out of ground effect.

To illustrate how much happens on the last centimeters (or feet), look at the two pictures which I took from this YouTube video:

In the top picture the wing is clearly closer to the ground than its semispan, but the vortices from the flap edge trail straight behind the plane. Now move forward to the lower picture when the trailing edge is maybe half a chord above the ground. Now the trailing vortex is swept to the side because the flow, which is compressed below the wing, expands when it exits the gap between wing and ground. Additionally, the downwash becomes a sidewash, because it has no space below the wing left in which to drop.

This should demonstrate that most of the ground effect happens when the ram pressure below the wing builds up. Once the trailing edge is less than half the wing chord from the ground, this part of ground effect kicks in, and the aircraft sees a significant reduction in drag.

Answer 2

I remember studying that ground effect happens within one wingspan. Although it reduces when you gain altitude.

As mentioned in FAA’s Pilot’s Handbook of Aeronautical Knowledge:

When the wing is at a height equal to its span, the reduction in induced drag is only 1.4 percent. However, when the wing is at a height equal to one-fourth its span, the reduction in induced drag is 23.5 percent and, when the wing is at a height equal to one-tenth its span, the reduction in induced drag is 47.6 percent.

It is also referenced here:

Ground effect only begins to show up when you're within one wingspan of the ground. But, it really reduces your drag when you're within 20% of your wingspan to the ground. At that height, your wing only generates 60% of its normal induced drag.

Answer 3

Ground effect height is primarily a function of the airfoil, and the "ground" surface. Smooth desert lakebeds have "better" ground effect than forest tree tops. Calm water is better than open water for smoothness.

As I recall, there have been several times where planes have operated in ground effect for substantial distances, such as military aircraft loosing engines at sea.

So to the OP questions:

1. The general rule is that 1/2 the wingspan is the practical limit of ground effect. My spin is that the surface of the ground, and the airfoil, vortex and downwash factor into that distance.
2. Obstacles such as trees are not practically overcome by ground effect.
3. Numerous documented instances have historically occurred where aircraft have extended operations over large bodies of water with engines out, or lower power settings, using ground effect. As fuel is burned off, sometimes the ceiling of the aircraft will increase notably.

Answer 4

Although it depends of the characteristics of the wings (shape, wingspan,...), as you said it can be useful to gain some lift. I would say from 5 to 10 meters (for a medium sized plane), the effect loses a lot of its efficiency. Unfortunately, I have no mathematical proof to back this up.

I don't know if you ever heard about them, but ekranoplans use almost exclusively the ground effect to "fly", however, they can't gain altitude like an actual plane would do. As @mongo stated, smooth surfaces are much better to take advantage of this effect.

More examples of Ground effect vehicles here