The challenges that aerobatics pose to pilots' bodies
Our species is acclimated to a world under the yoke of constant gravity—in this case, an omnipresent force of acceleration born of Earth's gravitational pull (the unit of Earth's gravity, denoted by g, is 9.81m/s2). However, there are circumstances in which our bodies are subjected to forces greater than normal Earth gravity... Once again, it's a matter of acceleration.
Stéphane Perrey, University of Montpellier
In aeronautics and automotive engineering, specialists refer to G (for Gravitational), or load factor, as a unit of acceleration. And its effects can be formidable.
As children learning to walk, we quickly discover that a misstep will eventually lead to a painful impact with the ground due, precisely, to gravity. When we board an airplane, without going so far as to crash this time, everything we have learned about gravity and what we are used to changes abruptly. You only have to watch Pete "Maverick" Mitchell's latest aerial maneuvers in the latest Top Gun movie to be convinced.
Flying involves overcoming gravity to rise into the air, and speed is essential. Any aeronautical maneuver can therefore expose our bodies to significant acceleration, with notable repercussions on the cardiovascular system, the brain, and even the joints. Some aircraft are capable of reaching 12G, with acceleration rates exceeding 15 G/s!
How much G-force do we experience on a daily basis?
Such figures are, of course, extremes. When standing still on the ground, the acceleration felt is 1G. Everything is fine. At 2G, for example when taking a 60-degree banked turn, we already feel moderate compression on our seat and find it difficult to move. A person weighing 80 kg on Earth (assuming a situation equivalent to 1G) will feel as if they weigh 160 kg when subjected to 2G. At 8-9 G, it becomes impossible to move any limbs except the extremities.
In fact, there are three main types of G forces present in three axes of space. We can experience lateral G forces (Gy) during a turn, resulting from centrifugal acceleration that pushes us outward. For horizontal acceleration or deceleration, we refer to Gx. Finally, Gz occurs during a descent or following a sudden climb. We are particularly sensitive to these accelerations experienced in the vertical axis (Gz), i.e., from head to toe, since this is where we feel the force of Earth's gravity necessary to maintain balance.
To complicate matters further, for all three axes, both positive and negative G-forces are possible... Whether turning in a car or flying vertically in an airplane, resistance to movement, known as inertial force, is added to 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 aircraft is flying upside down, for example, the load factor is expressed as negative, -G.
To calculate the G-forces they are subjected to, airplane pilots, who are particularly exposed, are equipped with three-axis accelerometers: this allows them to know in real time what they are experiencing.
How our bodies handle gravity under normal circumstances
During flight, pilots are subjected to a wide variety of physiological effects caused by a combination of acceleration and gravity. These effects are inherent to the inertial forces generated by acceleration and affect all organs of the body, particularly the cardiovascular system: the heart (the pump), the blood vessels (the circuit), and the blood (the fluid).
Blood circulation ensures the transport of oxygen, which is essential for the proper functioning of organs. The brain is particularly demanding in this area, both in terms of consumption (it is greedy) and the regularity of its supply. It does not like sudden changes, surpluses, or shortages!
On Earth, there is a complex mechanism that controls and adapts all the machinery that ensures regular, well-oxygenated blood flow at a constant rate to the brain, whether at rest or during exertion: this is cerebral autoregulation. Any variation in blood pressure is therefore inconsequential. However, this delicate balance has its limits... Accelerating when turning, braking, or, even more so, performing aerobatics will greatly disrupt it.
The ability to maintain cerebral blood flow, resilient to repeated exposure to increased stress factors, is therefore a critical issue for pilots who find themselves in conditions that deviate from normal everyday situations.
When our physiological adaptations are no longer sufficient
The risks were identified, albeit poorly explained, over a century ago. In 1918, the first acceleration-induced disorder was experienced during the Schneider Cup air race, where a sharp turn had to be taken. Initially described as "air sickness," it is now known as "G-induced loss of consciousness, " or G-LOC, and results in confusion and impaired judgment following a temporary cessation of cerebral circulation. This condition occurs at +4.5-6G in trained pilots.
Since the heart is located in the thorax, in an upright position (standing or sitting), the blood supply to the brain, which is positioned above it, requires the blood flow to fight against its own weight (hydrostatic pressure) to rise from one to the other. In the presence of +Gz, the inertial force oriented along the head-to-foot axis will add to the hydrostatic force and aggravate the situation by opposing the movement of blood from the heart to the head.
Beyond +3Gz sustained for more than ten seconds, our self-regulating mechanisms are overwhelmed, resulting in an immediate decline in vision and mental performance. This can result in visual disturbances such as "gray veil" (from 3-4.5G, due to decreased blood flow in the retina and peripheral vision) and "black veil" (from 4.5-6G, with cessation of blood flow).
Negative accelerations (-Gz) cause adaptation mechanisms that are the opposite of those caused by +Gz, accompanied by a more unpleasant sensation and greater perceived fatigue.
But the main problem lies in the rapid succession of -G and +G at high values (the "push-pull" effect), as in aerobatics, which is particularly difficult to tolerate. This results from the disruption of our adaptation mechanisms and our greater sensitivity to blackouts and/or loss of consciousness, which can occur at +2Gz.
Identify the limits...
If the cardiovascular system's response does not keep pace with the onset of G-forces, the pilot's performance will deteriorate to the point of causing loss of consciousness. To avoid this dangerous extreme, studies have helped to better understand the limits of our adaptive capabilities and develop techniques to overcome them.
The establishment of +Gz-time tolerance curves enabled us to compare asymptomatic and symptomatic individuals. The upper limit of these curves, marked by loss of consciousness (LOC-G), is a key factor in our physiological response to acceleration.
It has been found that if the increase in acceleration is gradual, visual symptoms precede cerebral symptoms. However, for accelerations greater than +7Gz achieved rapidly, loss of consciousness is not preceded by warning signs. This is because if the rate of acceleration is low enough, cardiovascular reflexes can, at least partially, compensate for changes in circulation. The tolerance threshold is thus increased.
In general, it has also been found that individual sensitivity to these effects varies and can be modified with practice. Several factors can influence tolerance to acceleration.
If the heat is not too intense, a well-rested, hydrated, and physically fit pilot will be able to tolerate +5Gz. This is because the volume of blood circulating in the body is greater and more readily available, making it easier for the cardiovascular system to keep the brain supplied with oxygenated blood.
… To overtake them: training expert pilots
Expert pilots also use muscle-breathing movements: pulling their head back into their shoulders and leaning forward to reduce the height of the hydrostatic column, contracting their abdominal muscles and lower limbs to slow down blood flow, and creating intrathoracic pressure by expelling air or closing the glottis with the diaphragm and neck muscles tightly contracted.
Stephane Perrey, Author provided
A regular physical training program that includes a mix of endurance and strength exercises also increases the pilot's tolerance to G-forces. Important factors to consider are core strength and aerobic capacity. Any aerobic endurance activity (even while holding your breath or at altitude) is good for the cardiovascular system.
Core strengthening exercises (planks, push-ups, pull-ups, sit-ups) and, above all, those that strengthen the neck muscles are essential: high G-forces make the head weigh more than normal, and with a helmet, that's a lot of weight to bear. Pilots of the fastest and most agile aircraft must constantly monitor their external reference points and adjust their head position during maneuvers.
Aerobatics are responsible for the onset and/or aggravation of spinal pain. Muscle strengthening to cope with repeated high accelerations is essential for these pilots, who are considered to be high-level athletes operating in extreme environments.
Several tools can also improve individual tolerance to acceleration. Developed early on during the world wars, anti-G pants apply counter-pressure to the lower body in response to acceleration, ensuring sufficient venous return. However, these devices only treat +Gz and are unsuitable for aerobatic aircraft due to their weight.
Other innovative devices are being developed in research centers and companies in the sector. This is the case with the work carried out by EuroMov Digital-Health in Motion and Semaxone, which are developing algorithms and sensors to measure brain oxygenation in real time in order to anticipate changes in drivers' tolerance to acceleration.
The author would like to thank Mr. Jacky Montmain, University Professor (IMT Mines Alès, EuroMov Digital Health in Motion), for proofreading; Mr. Gérard Dray, University Professor (IMT Mines Alès, EuroMov Digital Health in Motion) for his review; and Mr. Guilhem Belda, Engineer (CEO Semaxone), for the data provided and his review.
Stéphane Perrey, PR, Director of the Research Digital Health in Motion Research Unit, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.