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Understanding G Force

Understanding G Force

When you fly a fighter jet, you are subject to G force. G force can be positive and negative, which implies that the weight of your body will be submitted to variations of weight depending on acceleration. Here, we explain in a bit more detail how G force works and its effect on the human body.

Check the effect of G Force in flight:

The expression, “g-force,” suggests that forces are involved whenever masses accelerate. Einstein, in his Theory of General Relativity, showed that gravity is actually a warpage of spacetime whereby even light—a massless beam of energy—follows a curved path when traveling past a massive body. Whereas Einstein’s theory of gravitation is the most accurate one known, it is far from intuitive. Isaac Newton had a different view that, for most practical purposes, is an excellent scientific description of gravity and the laws of motion. Newton gave the world an equation that, when reduced to its simplest form, is written F = ma. The formula means that a force F acting on a body is equal to the mass m of the body times its acceleration a.
In an airplane, the pilot’s seat can be thought of as a hand holding a rock; the pilot as the rock. When flying straight and level at 1 g, he is being acted upon by the force of gravity. His weight (a downward force) is 725 newtons (163 lbf).

Simultaneously, according to Newton’s third law, the law of reciprocal actions, the plane and the seat underneath the pilot provides an equal and opposite force acting against the force of gravity; the seat is pushing upwards with 725 N (163 lbf) of force. If the pilot were to suddenly pull back on the stick and make his plane accelerate upwards with respect to the earth at 9.8 m/s2, the total g force on his body is 2 g. His body is now generating a force of 1,450 N (330 lbf) downwards into his seat. According to Newton’s third law, this inertial acceleration is only possible because the seat is simultaneously pushing upwards with an equal force. Moreover, the relationship of acceleration, force, and mass is always in accordance with the teachings of his second law of motion: F = ma. The term “g force” reflects the fact that accelerating bodies entails forces.

Human tolerances depend on the magnitude of the g-force, the length of time it is applied, the direction it acts, the location of application, and the posture of the body. The human body is flexible and deformable, particularly the softer tissues. A hard slap on the face may briefly impose hundreds of g locally but not produce any real damage; a constant 16 g for a minute, however, may be deadly. When vibration is experienced, relatively low peak g levels can be severely damaging if they are at the resonance frequency of organs and connective tissues. To some degree, g-tolerance can be trainable, and there is also considerable variation in innate ability between individuals. In addition, some illnesses, particularly cardiovascular problems, reduce g-tolerance.
Vertical axis g-force: Aircraft, in particular, exert g-force along the axis aligned with the spine. This causes significant variation in blood pressure along the length of the subject’s body, which limits the maximum g-forces that can be tolerated. In aircraft, g-forces are often towards the feet, which forces blood away from the head; this causes problems with the eyes and brain in particular. As g-forces increase a Brownout can occur, where the vision loses hue. If g-force is increased further tunnel vision will appear, and then at still higher g, loss of vision, while consciousness is maintained. This is termed “blacking out”. Beyond this point loss of consciousness will occur, sometimes known as “G-LOC” (”loc” stands for “loss of consciousness”). Beyond G-LOC, if g-forces are not quickly reduced, death can occur.

While tolerance varies, with g-forces towards the feet, a typical person can handle about 5 g (49m/s²) before g-loc, but through the combination of special g-suits and efforts to strain muscles—both of which act to force blood back into the brain—modern pilots can typically handle 9 g (88 m/s²) sustained (for a period of time) or more (see High-G training). Resistance to “negative” or upward g’s, which drive blood to the head, is much lower. This limit is typically in the −2 to −3 g (−20 m/s² to −30 m/s²) range. The subject’s vision turns red, referred to as a red out. This is probably because capillaries in the eyes swell or burst under the increased blood pressure.

Horizontal axis g-force: John Stapp was subjected to 15 g for 0.6 second and a peak of 22 g during a 19 March 1954 rocket sled test. The human body is better at surviving g-forces that are perpendicular to the spine. In general when the acceleration is forwards, so that the g-force pushes the body backwards (colloquially known as “eyeballs in”) a much higher tolerance is shown than when the acceleration is backwards, and the g-force is pushing the body forwards (”eyeballs out”) since blood vessels in the retina appear more sensitive in the latter direction. Early experiments showed that untrained humans were able to tolerate 17 g eyeballs-in (compared to 12 g eyeballs-out) for several minutes without loss of consciousness or apparent long-term harm. The record for peak experimental horizontal g-force tolerance is held by acceleration pioneer John Stapp, in a series of rocket sled deceleration experiments in which he survived forces up to 46.2 times the force of gravity for less than a second. Stapp suffered life-long damage to his vision from this test.

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