ground effect

i. The change in the aerodynamic reaction on an aircraft caused by the proximity of the ground. An aircraft, helicopter, or any other flying machine flying very near to the ground experiences an increase in lift. This additional lift or cushioning is caused by an effective increase in the angle of attack by the deflection of the downwash and without an increment in the induced drag. The ground effect diminishes rapidly when the aircraft is higher than about half a wingspan or three-quarters of a diameter of rotor disc above the ground. Other factors that influence the ground effect are the nature of the ground, its slope, and the prevalent wind. In the case of rotorcraft operating near the ground, induced drag is reduced, and the lift vector becomes more vertical as a result of the reduced inflow velocity. The vortex ring becomes smaller, and hovering can be sustained with less power.

In-ground effect.

In-ground effect hover.

ii. All unwanted effects caused by ground interference on radars, radio NAVAIDS (navigational aids), and other electromagnetic systems. See ground clutter.

iii. The modification of the directional pattern of an antenna system, especially at the very low, low, medium, and high frequencies, by the presence of the surface of the earth underneath the antenna. The effect is more pronounced in the vertical, or elevation, plane than in the horizontal plane. Ground effects have a large influence on the optimum distance at which the communication is realized.

The ground effect on a half-wave, horizontal dipole antenna, viewed from the axis of the wire. At A, the radiation pattern in the absence of ground; at B the radiation pattern for an antenna height of 1/2 wavelength over perfectly conducting ground; at C, a typical radiation pattern for antenna height of 1/2 wavelength over normal ground.