NOTur species is acclimatized to a world under the yoke of constant gravity – in this case, an omnipresent accelerating force born of the earth’s attraction (the earth’s unit of gravity, denoted g, equal to 9.81 m /s2). There are however circumstances where our body is subjected to stronger than the traditional terrestrial gravity… It is there still a matter of acceleration.
In aeronautics or in the automobile, specialists refer to the G (for Gravitational), or load factor, as a unit of acceleration. And its effects can be dreadful. As children learning to walk, we very quickly discover that one misstep will eventually result in a painful impact with the ground due, precisely, to gravity. When we get on a plane, this time short of crashing, everything we’ve learned about gravity and what we’re used to suddenly changes. Just look at Pete “Maverick” Mitchell’s latest aerial convolutions in the latest Top Gun to be convinced.
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Flight is indeed about overcoming gravity to rise through the air, and speed is essential. Any aeronautical maneuver can therefore expose our body to significant accelerations, with significant repercussions on the cardiovascular, cerebral and joint levels. Some aircraft are thus capable of reaching 12G, with acceleration climb speeds greater than 15 G/s!
How many Gs do we experience on a daily basis?
Such figures are of course extremes. While remaining motionless on the ground, the acceleration felt is 1G. Everything is fine. At 2G, for example when taking a 60 degree inclined turn, we already have a feeling of moderate compression on our seat, difficulty in moving. A person weighing 80 kg, or a weight of 784.8 N on Earth (considering it to be a situation equal to 1G), will feel like they weigh 1,569.6 N if they experience 2G (the kg being the unit of mass, the Newton that of weight). From 8-9G it is impossible to mobilize its limbs, except for the extremities.
In fact, there are three main types of G present in three axes of space. We can experience lateral Gs (Gy) when turning as a result of centrifugal acceleration pushing us outward. For a horizontal acceleration or deceleration, we speak of Gx. Finally, Gz occurs during a descent of the aircraft or following a sudden climb. We are more particularly sensitive to these accelerations undergone in the vertical axis (Gz), that is to say from head to toe, since it is there that we feel the force of the earth’s gravity necessary to maintain its balance.
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To further complicate the situation, for the three axes, positive but also negative Gs are possible… the actual weight due to gravity to give the “apparent” weight of the aircraft in flight. When the apparent weight in motion is greater than the actual weight, the load factor is greater than +1G. On the other hand, if the plane flies inverted, for example, the load factor is expressed in negative, – G.
To calculate the G to which they are subjected, the pilots of plane, particularly exposed, are equipped with accelerometer three axes: they can thus know in real time what they undergo.
How our bodies normally deal with gravity
The airplane pilot is indeed subjected in flight to a wide variety of physiological effects due to the combinations of acceleration and gravity. They are inherent in the forces of inertia generated by accelerations and apply to all the organs of the body, and in particular to the cardiovascular system: the heart (the pump), the vessels (the circuit), the blood (the fluid).
However, blood circulation ensures the transport of oxygen, which is essential for the proper functioning of the organs. The brain is particularly demanding in this area, both in terms of consumption (it is greedy) and the regularity of its supply. He doesn’t like jolts, surpluses or shortages!
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On Earth, there is a complex mechanism for controlling and adapting all the machinery that ensures regular and well-oxygenated blood circulation at a constant flow to the brain, whether at rest or in full effort: this is the cerebral self-regulation. Any variation in blood pressure is thus without consequence. But this beautiful balance has its limits… Acceleration in turns, braking or a fortiori the practice of aerobatics will disturb it greatly. The ability to maintain a cerebral blood supply, resilient in the face of repeated exposures to increased load factors, is therefore a critical issue for pilots who come out of normal everyday conditions.
When our physiological adaptations are no longer enough
The risks were identified, though poorly explained, more than a century ago. In 1918, the first disturbance induced by acceleration was thus felt during the air race of the Schneider Cup where a sharp turn had to be taken. First described as “unwellness in the air”, it is now known as “G-induced loss of consciousness”, or G-LOC, and results in confusion and impaired judgment at the following a temporary abolition of cerebral circulation. A state that occurs from +4.5-6G in the trained pilot.
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As the heart is in the thorax, in a vertical position (standing or sitting), the vascularization of the brain, positioned above it, forces the blood flow to fight against its own weight (hydrostatic pressure) to rise from one to the other. In the presence of + Gz, the force of inertia oriented on the head-feet axis will be added to the hydrostatic force and aggravate the situation by opposing the movement of blood from the heart to the head.
Beyond + 3Gz maintained for more than ten seconds, our self-regulation mechanisms are overwhelmed with the immediate consequence of a drop in vision and mental performance. This can result in visual disturbances such as the “grey veil” (from 3-4.5G, due to the decrease in blood circulation in the retina and peripheral vision) and the “black veil” (from 4.5 -6G, with cessation of blood flow). Negative accelerations (-Gz) cause opposite adaptation mechanisms to those of + Gz, accompanied by a more unpleasant feeling and greater perceived fatigue.
But the main problem lies in the rapid succession of – G and + G at high values (“push-pull” or pitch-up effect), as in aerobatics, which is particularly poorly tolerated. This stems from the disturbance of our adaptation mechanisms and our greater sensitivity to the phenomena of veiling and/or loss of consciousness which can occur from + 2Gz.
Identify the limits…
If the response of the cardiovascular system does not keep pace with the appearance of the Gs, the pilot’s performance will be degraded to the point of causing loss of consciousness. To avoid this dangerous extremity, studies have helped to better identify the limits of our adaptive capacities and to develop techniques to overcome them. The establishment of tolerance + Gz-time curves made it possible to compare asymptomatic and symptomatic individuals. The upper limit of these curves, marked by the loss of consciousness (LOC-G), is an essential factor in our physiological response to accelerations.
It appeared that if the increase in acceleration is gradual, the visual symptoms precede the cerebral symptoms. However, for accelerations greater than +7Gz reached quickly, loss of consciousness is not preceded by warning signs. Indeed, if the rate of increase in acceleration is sufficiently low, the cardiovascular reflexes can, at least partially, compensate for the modifications of the circulation. The tolerance threshold is thus increased. In general, it has also been found that everyone’s sensitivity to these effects is variable and can be modified with practice. Several factors can affect tolerance to accelerations.
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If the heat is not too much, a well-rested, hydrated and physically fit rider will be able to tolerate +5Gz. This is explained by the fact that the volume of blood circulating in the body is greater and available: it is then easier for the cardiovascular system to keep the brain perfused with oxygenated blood.
… to overcome them: the training of expert pilots
Expert pilots use in addition to musculo-respiratory movements: head tucked into the shoulders leaning forward to reduce the height of the hydrostatic column, contraction of the abdominal muscles and lower limbs to slow the flow of blood, intrathoracic overpressure by expelling the air or closed glottis with very contracted diaphragm and neck muscles.
A regular physical training program that includes a mix of endurance and strength exercises also increases the pilot’s tolerance to G effects. Important factors to consider are core strength and aerobic capacity. Any aerobic endurance activity (even snorkeling or at altitude) is good for the cardiovascular system.
Core-strengthening exercises (cladding, push-ups, pull-ups, sit-ups) and, above all, those that strengthen the neck muscles are a must: high Gs make the head weigh more than normal, and with a helmet, that makes a lot of weight to bear. The pilots of the fastest and most agile aircraft must constantly monitor their outward bearings and change their head position during their maneuvers.
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Aerobatics is responsible for the onset and/or aggravation of spinal pain. Muscular reinforcement to cope with repeated strong accelerations is essential for these pilots, considered to be high-level athletes who, moreover, evolve in extreme environments. Several tools can also improve individual tolerance to accelerations. Developed very early during the world wars, the anti-G pants, by applying counter-pressure on the lower part of the body in response to accelerations, ensure sufficient venous return. However, these devices only process + Gz and are unsuitable in aerobatic aircraft because of their weight.
Other innovative devices are being developed in research centers and companies in the sector. This is the case of the work carried out by EuroMov Digital-Health in Motion and the Semaxone company, which are developing algorithms and sensors to measure cerebral oxygenation directly to anticipate changes in pilots’ tolerance to acceleration.
* Stephane Perrey is PR, Director of the EuroMov Digital Health in Motion Research Unit, at the University of Montpellier.
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