In ground effect, up to what altitude is it typically observed?

In ground effect, up to what altitude is it typically observed?

• A. One rotor disc diameter above the surface
• B. One rotor disc diameter measured from rotor disk to surface
• C. Half a rotor diameter
• D. Two rotor diameters

Answer: (B)

Ground effect happens when the rotor’s downward flow is disturbed by the nearby surface, creating a cushion that makes the rotor more efficient by reducing the induced velocity and the power required for a given thrust. This interference is strongest while the rotor is within about one rotor diameter of the ground; above that height, the downwash expands into free space and the effect fades away. So the altitude where ground effect is typically observed is a distance from the rotor disk to the surface equal to one rotor diameter. Within this zone, you’ll notice the performance benefit; beyond it, the effect diminishes and performance returns toward free-air behavior.

Ground effect happens when the rotor’s downward flow is disturbed by the nearby surface, creating a cushion that makes the rotor more efficient by reducing the induced velocity and the power required for a given thrust. This interference is strongest while the rotor is within about one rotor diameter of the ground; above that height, the downwash expands into free space and the effect fades away.

So the altitude where ground effect is typically observed is a distance from the rotor disk to the surface equal to one rotor diameter. Within this zone, you’ll notice the performance benefit; beyond it, the effect diminishes and performance returns toward free-air behavior.