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

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Ground effect is an aerodynamic phenomenon, which reduces induced drag when an airfoil is close enough to the ground (usually within 1 wingspan) that circulation is significantly affected.

An airfoil modeled as a wedge with flat bottom, vertical leading edge, and sloping top, pushes air forward and down. Every action has an equal and opposite reaction. Pushing air forward, drags the airfoil backward, Pushing air down, lifts the airfoil. Circulating the air forward and down, rotates the leading edge of the airfoil upward.

In flight, the air molecules are compressed in front of the wing, leaving fewer and slower air molecules to impinge on the upper surface of the airfoil. The airstream above the wing takes more time to reach the trailing edge than the airstream below. Assuming the air pressure is only reduced about 1%, this means for a wing moving left to right, a clockwise vortex (circular wind) is added to steady right to left airflow.

When the airfoil is close to the ground, this circulation is reduced by friction and interruption with the ground. The interrupted circulation means both drag and form factor lift are reduced. This is why air hockey pucks slide almost without friction, hovercraft (ground effect machines) slide across rivers and beaches,r and light aircraft landing with excess speed or at high density altitude need longer runways.

This comic reverses the aerodynamic feature called ground effect. Runners depend on friction of their legs and feet on the ground to propel themselves forward. Rather than the ground interfering with their operation, they must be in contact with the ground (within their length) to use the ground.

The title text claims that runners avoid wearing dress shoes because some dress shoes can create wingtip vortices. Wingtip vortices are vortices that a wing creates near the tip when they generate lift. These unfortunately create drag so modern airlines 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 letter in the pattern of a birds wing.

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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