National Geographic : 1955 Aug
243 Kodachromes by National Geographic Photographers Robert F. Sisson and Donald McBain Skin and Muscles Sag Under Centrifugal Force, Twisting and Aging the Face Flight surgeons refer to centrifugal pressure as the G force: one G equals the pull of gravity, or a pilot's weight. Sharp turns multiply that value. This engineer, circling at 36 miles per hour, endures four G's (below). Now he "weighs" four times his normal 220 pounds. His face is distorted compared with its natural appearance (above). Downward force, draining blood from the head, will black out his vision in about 10 seconds. Upward force would pool blood in his skull and cause temporary visual "red out," a more serious problem.