3305: Ground Effect

Ground Effect

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Title text: Runners looking for aerodynamic advantage typically wear sneakers because some fancy dress shoes can create wingtip vortices.

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Explanation

This is one of 51 incomplete explanations: This page was created by an AIRBORNE RUNNER. Don't remove this notice too soon. You can help by editing the explanation!

Ground effect is an aerodynamic phenomenon, which reduces lift-induced drag when an airfoil is close enough to the ground (usually within 1 wingspan) that circulation is significantly affected. Some vehicles make use of this effect to increase their efficiency of operation.

This comic uses a different interpretation of the term 'ground effect'. Runners depend on their feet making contact with the ground in order to push against it and generate forward motion. In order to do this, all of them must be within one (of their own) body length of the ground. If they were not, their feet would not touch the ground and they would not go anywhere. This could be considered to be reducing their 'efficiency' to zero. This process has little to do with aerodynamic effects.

The title text claims that runners avoid wearing fancy dress shoes because some dress shoes can create wingtip vortices. Wingtip vortices are longitudinal vortices (horizontal tornadoes parallel to airplane fuselage) that a wing creates near the tip (where lift ends) when they generate lift. These create unhelpful drag (without adding lift), so modern airplanes generally have winglets at the end of the wings to make these vortices smaller. There is a style of dress shoe called wingtips with small holes in the upper leather in a pattern that somewhat resembles a bird's wings. In reality, runners would not wear dress shoes because they would be uncomfortable and restrictive to run in, and typically have soles that would not provide sufficient grip on the running surface.

Detailed discussion of ground effect

An airfoil can be modeled as a wedge with flat bottom, vertical leading edge, and sloping top. Undisturbed air has molecules (mostly nitrogen) moving in all directions, some as fast as the speed of sound. (Air molecule motion conveys sound and sets its speed.) A hand placed in a stream creates a depression in the water behind the hand, as the flowing water takes time to go under the hand. Likewise the leading edge pushes forward air molecules moving forward slower than the wing. The air molecules left to hit the sloping top are those moving down and faster forward than the wing. The flat bottom pushes down the half of the air molecules moving up. Every action has an equal and opposite reaction. Pushing air forward, drags the airfoil backward, Pushing air down below (more than above), lifts the airfoil. Circulating the air forward and down, rotates the leading edge of the airfoil upward. (An airplane's empennage or tail counters the airplane wing's tendency to nose up.)

An anemometer (airspeed meter) sees slower airspeed above the wing and faster airspeed below the wing. The airstream above the wing takes more time to reach the trailing edge than the airstream below. Aviation engineers call this circulation. Assuming the air pressure is only changed about 1%, this means for an airplane moving left to right over the ground, the wing encounters a right to left airflow, and adds a clockwise vortex (circular wind) to the otherwise undisturbed right to left airflow. In the wake of the airplane, downwash (i.e. air thrown downwards) compresses air below before bouncing back up. Trailing geese and airplanes in close formation take advantage of this downwash bounce back to fly with less effort or fuel. If the relative humidity is high enough, the compression will condense water vapor into a contrail. ("When contrails trace across the sky, expect that rain might be nigh.")

When the airfoil is close to the ground, this circulation is reduced by friction with and interruption by the ground. The circulation partially becomes downwash from the leading edge, and bounces back like a cushion to counter the loss of form-factor lift. This is why air hockey pucks slide almost without friction, ground effect vehicles slide across rivers and beaches, and light aircraft land gently.

Ground effect is very relevant for aircraft: excess airspeed when landing makes the aircraft float along the runway, potentially causing a late and dangerous touchdown. When taking off, the ground effect reduces form factor lift. Without enough airspeed to create non-ground effect lift, the airplane will stall as soon as it climbs out of the ground effect layer, and loses the downwash cushion. When taking off from short or soft (grass) runways, airplanes will use ground effect to float like a ground effect vehicle and accelerate more quickly to flying speed before initiating climb.

Incidentally, a reverse effect is known in ships. When a ship accelerates in shallow water, it is sucked towards the floor, and its draft increases.

Transcript

[Five Cueball-like people are shown in the panel. At the left, two of them are floating in the air, flailing their arms and legs as they float at least 12 feet off the ground. At the right, three others are running to the right in a normal way. A dotted line is shown above the tops of these three runners' heads, and a regular line below them indicates the ground. An arrow points down toward the dotted line and another arrow points up toward the regular line behind the runners.]

[Caption below the panel:]

The "ground effect" explains why sprinters can run most efficiently if they stay within roughly one body length of the surface.

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Discussion

And as the drag is insufficient for the runners depicted above the ground to actually stay above, they would ultimately fall. 2001:4C4E:1C0F:1B00:3056:6C17:EB81:88B4 11:33, 1 October 2026 (UTC)

Ground effect in the context of racing is essentially an issue for cars. While it can be explained in terms of planes, that's a bit of a weird thing to do. F1 cars etc. exploit ground effect to increase cornering grip, which seems like a far more relevant reference point for "fast moving racing on the ground that's being described in terms of ground effect" than concentrating on something that flies through the air. Yorkshire Pudding (talk) 13:17, 1 October 2026 (UTC)

Except that the comic appears to be suggesting that less 'efficient' runners do routinely fly through the air. And the 'within one body length' bit does seem to be a direct corollary to within one wingspan. Not to say that it's not worth adding something about that as a genuine case where true ground effect is important for ground-based activities. 86.22.132.121 13:45, 1 October 2026 (UTC)

I'd been floating in the air during my marathons till I saw this comic, very good strategy.--Death cab for me (talk) 16:01, 1 October 2026 (UTC)

In the second sentence of the second paragraph, I doubt that 'friction of the legs...on the ground' is a normal part of running. Friction of the feet (or shoes) is essential. At the end of the third sentence, I wonder about 'generate momentum'. 'Generate forward movement' seems more accurate to me. Nekoninda (talk) 16:19, 1 October 2026 (UTC)

The section 'Detailed discussion of ground effect' contradicts a few of the standard explanations of airfoil lift (explanations that may be wrong). For example, 'An anemometer (airspeed meter) sees slower airspeed above the wing' contradicts the Bernoulli Effect, which is normally given as a part of how an airfoil generates lift. In the previous paragraph, I don't understand the assertion that 'The air molecules left to hit the sloping top are those moving down and faster forward than the wing.' Classical explanations say that the wing is moving faster than most of the air molecules above and below it. Nekoninda (talk) 16:34, 1 October 2026 (UTC)
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