Motion in flight presents two main problems. First we can develop both positive and negative forces on the body in excess of what we normally encounter in moving on the earth. Second, motion not only produces these forces but also directs them in ways that we don’t expect, which can be disorienting. Our balancing mechanisms receive many confusing stimuli that we must rationalize. Our primary mechanism for this is our eyes and learned intellectual processes.
G is the symbol for these forces. Although commonly associated with the force of gravity we have many ways to generate this force artificially. The force of gravity on earth is generally described as 1 g and operates directly towards the centre of the earth. The artificial G forces may act in three planes relative to the body. The transverse (Gx, Front-To-Back), lateral (Gy, Side-To-Side), and longitudinal (Gz, Head-To-Toe) axes. By convention forces acting from head to foot are considered positive, as are accelerations from front to back, and right to left. Conversely forces acting from foot to head, back to front, and left to right, are considered negative.
Since the body tissues and fluids are not rigid and possess certain inertia or resistance to movement, the principle effect of g forces is the displacement of these fluids and tissues. Light aircraft are usually built to take sustained loads in the range of +6g to -4 g which represent the extremes that the normal pilot can tolerate without special equipment. Generally the body is quite tolerant of transverse loads in terms of resisting damage. The main problem in this axis is disorientation due to accelerations of the fluid in the vestibular apparatus of the inner ear used for balance. These effects are usually short lived as the brain compensates when the accelerations are continuous. The same effects are felt in the lateral axis, however since this is not generally associated with vision they are less pronounced. In both the lateral and transverse axis the pooling of fluids such as the blood does not result in marked impairment until well outside normal loads. Loads developed in the longitudinal axis are an entirely different matter, as humans have to move blood up to the brain, and retrieve it from the lower body and limbs. For the most part our circulatory system is better adapted to pumping blood up as we spend more of our time on our feet rather than on our hands. However we are not well adapted to forces greater than +4 gs in the longitudinal axis and even less so to negative gs in this axis.
The most serious effect of positive g is to drain the blood away from the head and towards the lower body causing hypoxia in the brain due to loss of blood flow. This is called stagnant hypoxia and usually causes deterioration in vision marked first by loss of colour perception (grey-out) followed by total loss (blackout). With unprotected, untrained pilots, under sustained loads grey-out starts around +2g and blackout around +4g. Usually consciousness is lost when the loads approach +6g. Further, even when conditions return to normal, the pilot can expect to have diminished intellectual capacities for periods up to tens of seconds. Of course there is a time factor involved. We can tolerate a 2g manoeuvre for a longer time than a 5 g one.
Negative longitudinal loads are tolerated even less. At about -2g the symptoms of pooling blood in the head may be seen with reddening of vision known as red-out. If -5g is attained small blood vessels may begin to rupture in the eyes. One only has to attend an aerobatics meet to see the ghoulish effect of this in the competitors. The lack of white in their eyes is telling of the stresses they have been enduring. Higher negative loads can cause brain damage. However even relatively small negative loads cause extreme discomfort in many humans. Some may be sensitive to the point of panic. This effect will be further discussed in flight training for those sensitive to these “sub-gravity sensations”. Aside from the physical effects, this distaste for negative accelerations may be related to basic survival mechanisms that dictate that you must not fall headfirst. The “natural” reaction is panic and avoidance.
With some training, we can protect ourselves in the normal load ranges. The most common way to cope with increased positive g is to bear down against a closed glottis, the flap between the throat and chest. This is known as the Valsalva manoeuvre (the same manoeuvre used in clearing the Eustachian tubes when your ears are blocked) and has the effect of elevating blood pressure to counteract pooling away from the head. This manoeuvre offers temporary relief, but tolerance to increased g also benefits by good physical condition and diet. Diet tends to affect blood volume and our nutrient levels, while conditioning improves the condition of our blood return and pumping system (heart). In the higher g ranges it is necessary to use G-suits and further continual training to operate effectively.