What's the simplest explanation of ground effect which is still accurate?

Steve V.
  • What's the simplest explanation of ground effect which is still accurate? Steve V.

    We've all heard the "acts like a cushion of air" explanation tossed casually around by CFIs.

    There's also plenty of books and reference materials that give a detailed, accurate, and complete explanation of ground effect, and which are incomprehensible to most student pilots.

    What's the simplest way to accurately explain ground effect?

  • Simple explanation:

    When out of ground effect, air on the bottom of the wing has plenty of room to move around. When in ground effect, the air on the bottom of the wing can't move around nearly as well and therefore pushes the plane upward to try and make more room.

  • The ground acts as an aerodynamic mirror. When the wing approaches the surface it is as if an inverted wing comes from below. Their high pressure areas amplify each other, increasing the efficiency and thereby reducing the drag.

  • It's impossible to give a correct AND simple explanation I think. I would explain it like this if I was asked:

    The downwash from the wings and the downward component of the wingtip vortices create a higher than normal pressure area below the wings because the air hits the ground and can't "escape", increasing lift.

    Induced drag is reduced because wingtip vortices (causing drag) can't fully develop close to the ground.

  • Near the ground, the ground can push up on the airplane, giving it more lift.

  • Here's my simplification, it's not the full story, but it covers the essentials.

    Lift: When you are in free space, the high pressure below your wing dissipates into the surrounding air. When you're in ground effect, the high pressure below the wing encounters an incompressible solid, and therefore cannot dissipate as quick, causing higher pressure below the wing, and therefore more lift.

    Drag: When you're in free space, the wake (turbulent, low pressure) behind your aircraft dissipates unimpeded until it becomes uniform with the surrounding atmosphere. When you're in ground effect, the air near the ground absorbs this dissipation quicker because it has to shear against the ground (more friction that air in free space). Additionally, wing-tip vortices encounter the ground and dissipate much quicker than in free space.

    I suspect the reason GE is tied to wing length is because the longer your wing is, the stronger your wing tip vortices, and the further air has to travel laterally to exit the high pressure zone under the wing.

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