TP 13312

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Acceleration

• G Axes
• Physiological Effects
• Figure 9
• Positive Gz
• Negative Gz and Jolt
• Traverse and Lateral G

Doctors often feel that an understanding of acceleration (G) and the effects of gravity (g) are only of importance to aerobatic or high performance aircraft pilots. This is a mistake. Because we are normally terrestrial creatures, bonding to the earth has taught us that gravity exerts a downward pull. In an aircraft however, G-forces are often upward or outward and as they are associated with changes in both acceleration and direction, what is experienced is a resultant force. It is these forces and their effects on the vestibular organs which give rise to our recognition of position in space. In the review of orientation the importance of this will be explained.

G Axes

Speed is the rate of movement of a body while velocity is a vectorial quantity made up of both speed and direction. Acceleration (G) is a change in velocity either in direction or in magnitude. It is described in three axes in relation to the body, x, y and z. Each axis is described as positive (+) or negative (–) according to an international convention.
Considerable confusion can arise if a clear distinction is not made between the applied acceleration and the resultant inertial force as these, by definition, always act in diametrically opposite directions. Thus a headward acceleration tends to displace tissues such as viscera and the eyes, footward and the resultant force is termed positive G, +Gz. (See Fig. 9).

Physiological Effects

The physiological effects of G vary with its magnitude, duration and axis of application and are modified by the area over which it is applied and the site. Tolerance to acceleration varies from day to day and is modified by body build, muscular tone and experience. It is decreased by poor health or conditioning, fatigue, hypoxia and alcohol. It can be increased by continued exposure and education. Pilots exposed to heavy G loads soon learn to use a modified Valsalva manoeuvre with controlled breathing and muscle contraction to increase their tolerance (the M1 manoeuvre) . G-suits mechanically
increase resistance to positive Gz by exerting pressure on the lower limbs and the abdomen to prevent pooling of blood. Unfortunately there is no mechanical device to counteract negative Gz.

Figure 9

Positive Gz

Positive Gz forces the pilot into the seat, draining the blood towards the lower part of the body. A 150 lb. pilot exposed to +4G has a weight equivalent to 600 lbs. This interferes with muscular movement, aircraft control and the ability to change position or to escape in an emergency. As G comes on and blood is drained from the head, the first symptom is visual. The normal intra-optic arterial pressure is 20/25 mm. of Hg. and under loads as low as 2-3G peripheral vision is decreased due to retinal anemia. This leads to “grey-out”, a condition in which peripheral vision is progressively lost and central vision begins to lose its acuity. As the G load increases the retinal arterial flow is further reduced until “black-out” occurs. At this point, although vision is absent, the cerebral blood flow is often maintained and the pilot may remain conscious. At 5-6G however most pilots become unconscious unless they are protected. This is referred to as G-LOC. (G-Loss of consciousness). When the G load is reduced, consciousness will be regained although there is often a brief period of confusion before full awareness is reached. If the G load is high and the onset is of short duration, G-LOC can occur without warning. This has been determined as the cause of several accidents in high performance aircraft.

Negative Gz and Jolt

Negative Gz, acting from the foot to the head, is poorly tolerated by the body and in most cases the threshold is below –5 Gz. As might be expected the visual symptom is “red-out” as blood is forced towards the head and into the retinal arterioles. Excessive –Gz leads to hemorrhages into the conjunctiva and ultimately into the brain.

A special form of G is known as “jolt”. Jolt is the rate of change of acceleration. It is descriptively used in relation to short, sharp accelerations. This type of shock can give rise to serious spinal injuries and must be minimized in the design of ejection seats. 

Brief alternating positive and negative Gz forces are experienced in turbulence and may be a serious problem when flying light aircraft in hot weather or flying high speed aircraft at low levels. G-forces not only interfere with precise flying but are also a potent source of fatigue. 

Transverse and Lateral G

Tolerance to transverse G (Gx) is much higher. It is for this reason that the astronauts in the early vehicles were placed in a recumbent position during lift-off. Forces as high as +60 Gx have been experienced over short intervals without injury. However, G interferes with both lung inflation and respiratory movements and forces greater than +20 Gx quickly lead to breathing difficulties. –Gx is less well tolerated. Gy is not of great enough amplitude to cause problems in consciousness and is not a problem with modern day aircraft. It does come into account however in VTOL aircraft such as the Harrier which is able to “VIFF” (Vector in forward flight) sideways to evade attack. At present, head restraint is the only problem experienced with Gy.