A g-suit is worn by aviators and astronauts who are subject to high levels of acceleration ('g'). It is designed to prevent a black-out and g-LOC (g-induced Loss Of Consciousness), due to the blood pooling in the lower part of the body when under g, thus depriving the brain of blood.
A g-suit does not so much increase the g-threshold, but makes it possible to sustain high g longer without excessive physical fatigue. The resting g-tolerance of a typical person is anywhere from 3-5 g's depending on the person. A g-suit will typically add 1 g of tolerance to that limit. Pilots still need to practice the 'g-straining maneuver' that consists of tensing the abdominal muscles in order to tighten blood vessels so as to reduce blood pooling in the lower body. High g is not comfortable, even with a g-suit. In older fighter aircraft, 6 g was considered high, but with modern fighters 9 or even 10 g can be sustained aerodynamically making the pilot the critical factor in maintaining high maneuverability in close combat.
A 'g Suit' is a special garment and generally takes the form of tightly-fitting trousers, which fit either under or over (depending on the design) the flying suit worn by the aviator or astronaut. The trousers are fitted with inflatable bladders which, when pressurized through a g-sensitive valve in the aircraft or spacecraft, press firmly on the abdomen and legs, thus restricting the draining of blood away from the brain during periods of high acceleration. In addition, in some modern very high-g aircraft, the Anti-g suit effect is augmented by a small amount of pressure applied to the lungs (partial pressure breathing), which also enhances resistance to high g. The effects of Anti-g suits and partial pressure breathing are straightforward to replicate in a simulator, although the continuous g forces themselves can only be produced artificially in devices such as centrifuges.
Various designs of g-suit have been developed. They first used water-filled bladders around the lower body and legs. Later designs used air under pressure to inflate the bladders. These g-suits were lighter than the fluid-filled versions and are still in extensive use. However, the Swiss company Life Support Systems AG and the German Autoflug GmbH collaborated to design the new Libelle suit for use with the Eurofighter Typhoon aircraft, which reverts to liquid as the medium and improves on performance. The Libelle suit is under consideration for adoption by the United States Air Force.
If blood is allowed to pool in the lower areas of the body, the brain will be deprived of blood leading to temporary hypoxia. Hypoxia causes first a brownout (a dimming of the vision), also called grey-out, followed by tunnel-vision and ultimately a blackout (unconsciousness), that is g-induced Loss of Consciousness or 'g-LOC'. The danger of g-LOC to aircraft pilots is magnified because on relaxation of g there is a period of disorientation before full sensation is re-gained.
G-force induced hypoxia has resulted in a number of fatalities in which the aircraft and crew are lost.
As early as 1917, there were documented cases of loss of consciousness due to g-forces in pilots.
In 1931 a Professor of Physiology, Frank Cotton, from the University of Sydney described a new way of determining the center of gravity of the human body. This made it possible to describe the displacement of mass within the body under acceleration.
With the development of high-speed monoplane fighters in the late 1930s, g-effects in combat became more critical. In the Battle of Britain in 1940, some German aircraft had foot-rests above the rudder pedals so that the pilot's feet and legs could be raised during combat, in which large use of the rudder was often not necessary but turning inside the opponent, was.
Air-based g-suits were very common in NATO aircraft of all nations from the 1950s onwards and are still in common use today.
Later jets such as the BAe Hawk, F-16 Falcon, F-18 Hornet, Eurofighter Typhoon and the Dassault Rafale can sustain high-g for longer periods, and are therefore more physically demanding. However, by using a modern g-suit a pilot can now be expected to sustain flight forces of up to 9 g without blacking out.
Astronauts wear similar g-suits to aviators but face different challenges due to the effects of microgravity. Aviator g-suits apply uniform pressure to the lower legs to minimize the effects of high acceleration but research from the Canadian Space Agency implies there might be a benefit in having a suit for astronauts that uses a "milking action" to increase blood flow to the upper body.