Definition

The United Kingdom's National Physical Laboratory defines G-force as a term to convey values of force and acceleration. The values are expressed in terms of the earth's gravitational force. For example, if a pilot experiences three Gs while executing a steep turn, that is three times the force of the earth's gravity.

Description

G-forces are experienced by the human body as perceived weight. During the rapid acceleration of a spacecraft during launch, for example, astronauts feel themselves pushed down in their seats as if suddenly they weigh more than they actually do. G-forces come into play for other manoeuvres, as well. If a pilot banks into a steep turn, G-forces increase, and the amount of G-force varies with speed and bank angle. Pilots in high-performance military jets can experience forces as high as nine Gs.

A brochure by the U.S. Federal Aviation Administration's Aerospace Medical Education Division identifies three types of acceleration:

Linear Acceleration reflects a change of speed in a straight line. This type of acceleration occurs during takeoff, landing, or in level flight when a throttle setting is changed (increased).

Radial Acceleration is the result of a change in direction such as when a pilot performs a sharp turn, pushes over into a dive, or pulls out of a dive.

Angular Acceleration results from a simultaneous change in both speed and direction, which happens in spins and climbing turns.

G-forces can also be experienced in deceleration. The U.K.'s National Physical Laboratory says on July 13, 1977, British race car driver David Purley survived a deceleration from 173 kilometers per hour to zero in a distance of approximately 0.66 meters, enduring 180 Gs.