Raising the feet, a technique used in the Battle of Britain, will raise tolerance by about 0.5 G. Several procedures are employed by fighter pilots to maintain the blood pressure at head level, and hence consciousness and vision, on exposure to high +G z. At +5–6 G z the blood flow to the brain of a seated, relaxed individual ceases, and unconsciousness supervenes in about 5 sec. At about +4.5 G z the pressure in the arteries supplying the retina of the eye falls below the pressure within the eyeball (20 mm Hg), blood flow to the retina ceases, and loss of vision, termed blackout, follows in about 5 sec. The pressure of blood in the vessels above the level of the heart is decreased whilst the pressure below the heart is increased, and the blood moves from the upper to the lower parts of the body. Of even greater significance on exposure to +G z is the increase in the weight of the blood. Accurate movements of the fingers can be performed, however, at +9 G z, provided that the hand and forearm are well supported. The increase of the weight of the tissues and organs of the body produces sagging of the soft tissues of the face at +2 G z makes it impossible to stand up from the seated position at +3 G z makes upward movement of the upper limbs very difficult at +5 G, and impossible above +7 to 8 G z and makes it very difficult to raise the head at +5 G z, once the neck has flexed, and impossible at +6 to 8 G z. The direction in which an accelerative force acts is described by the use of a three axis co-ordinate system (X, Y, and Z) in which the vertical axis (Z) is parallel to the long axis of the body (see figure).Ĭommon aircraft manoeuvres such as co-ordinated turns or pulling out of a dive apply an accelerative force parallel to the long axis of the seated pilot which tends to displace tissues towards the feet (+G z). It is useful to classify accelerations into short duration (where the force acts for less than 1.0 sec), when the main determinant of the effect is the structural strength of the body and long duration, where the force acts for several seconds or longer and the effects are due to the sustained distortion of tissues and organs, and alterations in the distribution of blood within the body. The effect of an accelerative force upon the body depends upon the magnitude of the force, its duration of action, and the direction in which it is applied. Finally, aircraft and spacecraft may crash on landing, exposing the occupants to accelerative forces of the order of 50 G or greater (see crash impact.) The pilots of modern combat aircraft are exposed to accelerative forces up to 9–10 G for many seconds, whilst the crew of a space vehicle which is in orbit around the earth or which has attained escape velocity will be exposed to microgravity - 1 × 10 -4 to 1 × 10 -5 G. Thus the passengers of a wide-bodied jet aircraft will be exposed to accelerations of 1.2 to 1.3 G sustained for several seconds during take off, landing, or a tight turn. The ranges of accelerative forces to which the occupants of aircraft and space vehicles may be exposed in flight and the durations for which these forces may operate are extremely large. Thus a body which has a weight of 1 kg at 1 G will weigh 5 kg when exposed to an accelerative force of 5 G. The ratio of the acceleration of a body to the gravitational constant is the ‘G’. It is convenient to state the latter in multiples of the acceleration due to gravity, the gravitational constant (9.81 m/sec 2). For a given mass the force generated is directly proportional to the acceleration of the mass (Newton's Second Law of Motion). Changes in the speed or direction of travel of a vehicle such as a car or an aircraft generate accelerative forces which may be many times the accelerative force of gravity. G and G-suit Life on earth has evolved in the accelerative force of gravity, which attracts all material towards the centre of the earth and gives a mass of material the characteristic which we term weight.
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