G and G-suit

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. 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. For a given mass the force generated is directly proportional to the acceleration of the mass (Newton's Second Law of Motion). It is convenient to state the latter in multiples of the acceleration due to gravity, the gravitational constant (9.81 m/sec²). The ratio of the acceleration of a body to the gravitational constant is the ‘G’. 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 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 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 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. (See space travel). 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 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. 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 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).

Common 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). 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. Accurate movements of the fingers can be performed, however, at +9 G_z, provided that the hand and forearm are well supported.

Of even greater significance on exposure to +G_z is the increase in the weight of the blood. 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. 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. At +5–6 G_z the blood flow to the brain of a seated, relaxed individual ceases, and unconsciousness supervenes in about 5 sec.

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. Raising the feet, a technique used in the Battle of Britain, will raise tolerance by about 0.5 G. Reducing the amount of blood which pools in the lower limbs and abdomen by applying counter-pressure to these regions by a G-suit will increase tolerance by 1–3 G, depending upon the area of the lower body covered by the G-suit. Raising the pressure in the chest, either actively by performing a hard expiratory effort against a closed glottis, or passively by means of positive pressure breathing, can greatly increase tolerance of +G_z by raising the arterial blood pressure. When active expiratory effort is employed it must be interrupted by taking a rapid breath once every 3–4 seconds, to allow blood to flow back into the chest to the heart and to maintain respiratory gas exchange. The pilot also tenses the muscles of the trunk and limbs while performing this manoeuvre, which is termed the Anti-G Straining Manoeuvre (AGSM). The AGSM together with a G-suit will increase the tolerance to 8–9 G. It is, however, very fatiguing. Positive pressure breathing combined with a G-suit which fully covers the lower limbs and abdomen will maintain performance at 9 G for many seconds.

Much less common in flight is for a pilot to be exposed to –G_z which forces the blood towards the head. –G_z is produced by simple inverted flight and outside loops and spins. Tolerance of –G_z is much lower than tolerance for +G_z acceleration. Thus exposure to –2 G_z causes severe discomfort in the head, and is followed usually by a severe headache which persists for several hours. Exposure to –2.5 G for only a few seconds causes rupture of blood vessels in the skin of the head and neck and on the surface of the eye. It frequently causes bleeding from the nose. The increased pressure in the arteries of the neck acting through the carotid sinus baroreceptors causes very marked slowing of the heart and often produces loss of consciousness.

John Ernsting

Bibliography

Ernsting, J.,, Nicolson, A. N., and and Rainford, D. J. (1999). Aviation medicine, (3rd edn Butterworth–Heinemann, Oxford.

See also crash impact; flying; space travel.