The science of G-force
After having been hit in the face with G-force for Lab Rats,...
Watching films like Top Gun it's easy to see the appeal of being a fighter pilot, but would Tom Cruise have made a real fighter pilot, let alone Zeron and me? The cockpit of a fighter plane is a very hostile environment.
Because of the altitude at which they may fly there is little naturally available oxygen and the outside temperature is very low. The movement of the plane can result in motion sickness, and the forces exerted on the body by one of the most advanced aerobatic aircraft in the world is literally crushing.
Motion sickness and the vestibular system
During our competition for a flight in one of the United States Air Force F-16s, Zeron and I had our first experience of flying in an aerobatic aircraft with a pair of Extra 300 stunt planes. We spent under half an hour reeling around the sky in a simulated dog-fight with each other.
My pilot was Andy Cubin, an ex-Red Arrow pilot and he really showed me what a modern piston engine stunt plane can do. He tried vertical rolls, break turns, barrel rolls and loops.
Concentrating on reading a line from a piece of paper while doing a wicked vertical roll nearly did the trick for me, because my eyes focused on what looked like a stationary object, but my ears told me that I was being moved about quite vigorously.
When we landed we felt suitably sick, as many people would, but why? Why is the modern aircraft pilot told to believe their instruments and not their senses?
The vestibular system is situated in the inner ear (see Figure 1 below) and together with our eyes, and various other parts of our body (for example, our toes) it helps us to balance. 143811
A cross section of the human ear
As our eyes read the visual signals around us, channels of fluid within our inner ear called the semi-circular canals sense movements through small hairs, called cilia, which are suspended within the fluid.
If you get hold of a bucket of water and spin it, much of the water it contains will remain still because of inertia [i.e. because it has mass and wants to remain at rest unless affected by a force]. Much the same happens within our ears. As we spin our heads the semi-circular canals, and the cilia they contain, move with our heads, whereas the fluid contained within the canals tends to stay still.
This causes the cilia to move through the fluid, and they sway in a similar way to reeds in a fast moving stream. This movement is picked up by sensors at the bottom of the cilia which pass messages to the brain. Thus the semicircular canals are able to detect rotation of the head (angular velocity).
In addition to the semi-circular canals, we have devices called otoliths within our inner ear. These look like golf balls on tees and they enable us to detect the static position of the head and changes of speed and motion in a straight line (linear acceleration) by bending slightly.
This tells the brain the position of our head relative to gravity. Just about all of these systems can be fooled when flying a plane, especially if it is changing direction rapidly, or accelerating.
Should the messages from the eyes not match the messages from the vestibular system the brain becomes confused. It has been suggested that the sensation is very similar to that which would be experienced had we ingested a neurotoxin (a toxin which attacks the nervous system), a real danger in prehistoric times.
For example, if you ate the wrong type of berry, the body would defend itself by vomiting. If we are to make fighter pilots this is one of the first things that we will have to overcome.
There are two methods. The first would be to use drugs, but that would be cheating. The second would be to keep going up in planes until our bodies, in particular our senses, got used to it. Hence, we were spun around in gyroscopes for the best part of an afternoon.
G-force is a killer. In fact, the effects of g-force was, as long ago as the Second World War, causing the death of pilots who either lost consciousness or were unable to bale out of their planes.
The 'g' refers to 'gravity' and while the force has little to do with gravity it provides an easy to understand measurement of what g-force is - essentially acceleration. Most people think of acceleration as an increase in speed.
This is how the word is generally used when thinking about cars and motorbikes, but in purely scientific terms acceleration is a change in velocity (change in speed and/or direction). It's weird to think that a car that is braking or turning a corner is actually accelerating if the word was used in its scientific sense.
We measure the force we feel as we accelerate in multiples of gravity - gs. The force you feel under the influence of gravity is 1g. Put simply, if you were to weigh 80kg (like me) then at 1g you will still weigh 80kg. In most people's day-to-day life they may feel a little ' g ' force when accelerating hard, cornering or braking in a car.
This would probably never exceed 1g or so, although forces as high as 12g can result from car crashes. At a fairground you might experience a couple of g and if you were straightening up at the bottom of the big descent in 'The Big One' roller coaster at Blackpool Pleasure Beach you would experience about 3g. At that moment I would weigh in at 240kg (that's about forty stone).
In the Extra 300 stunt planes in the UK, Zeron and I experienced 6.5g, with quite a fast onset, but only for a few seconds. If we were to go out and play with the big boys in their F-16s we would have to endure over 9g for as long as ten or twenty seconds.
Before doing that safely we would have to prove that we were man enough, and the only safe way to do so was in a controlled environment - a horrible giant centrifuge in the USA.
The centrifuge works much like a spin dryer, but instead of squeezing water out of clothes against the side of the drum, as this centrifuge spins faster and faster we will be pushed harder and harder into our seat. Under these conditions anything that can move will move, including the blood in your body. In an F-16 fighter jet pulling an aggressive break turn it is possible to experience 9g. That means that in a tight turn I would weigh 720kg (nearly three quarters of a tonne).
Such forces are bound to mess up the body. The first effect that is noticed by the pilot is that it is difficult to breathe. This is because the g-force is pulling the ribs down, which empties the air from the lungs. This isn't the most dangerous effect, but it does wear you out. The most dangerous effect is that blood is pulled away from the brain and pools in the legs and feet.
This is exacerbated because the internal organs tend to be pulled down through the body, meaning that blood has to be forced further to get to the brain. After a short time experiencing 'high-g' turns, the eyes lose peripheral vision - giving tunnel vision and you may only be able to see in black and white (greying out). If the turn continues all vision is lost.
This is called a blackout. Should the turn keep going the pilot would risk losing consciousness. It is called a g-LOC (g-induced Loss Of Consciousness).
A healthy person would expect to start suffering from a loss of vision and other g induced problems at 5 or 6g. After that they usually need help.
The first line of defence, if technological aids aren't available, and certainly the most important aspect of resisting the negative effects of 'g' force is the 'Strain'. It's an exhausting exercise which involves contracting as many muscles as possible in your feet, calves, upper leg, stomach muscles and butt cheeks while allowing your upper body to remain relaxed so that breathing is relatively easy.
It sounds easy enough, but when each of us was brought out to the front of the 'class' at the USAF base to practise it, we found it was far from straightforward.
First, my feet had to be positioned in a pigeon toed way. Then I had to curl up my toes and pull my feet back using my calf muscles. My stomach and upper leg muscles then had to be contracted as if I were expecting a blow to the torso. All this while relaxing my shoulders and letting them drop. After four or five attempts I managed to synchronise all these movements quickly enough, so that if I knew a hard turn was coming during flight I could 'put on my strain' almost instantly.
Other, less critical effects of high-g turns include many burst blood vessels, leading to a rash known as the 'geezles', piles and bruised butt cheeks.
Altitude, pressure and oxygen availability
At altitude, atmospheric pressure is greatly reduced. Put simply this is because there are more molecules of any gas per unit volume of air at sea level than high in the atmosphere because they are being 'squashed' down by the column of air above them.
This means that it becomes difficult to breathe because one of the molecules we need is oxygen, and at altitude there is far less oxygen available. In addition to oxygen deprivation, the lack of pressure itself causes problems.
It's worth noting that should an aircraft suddenly de-pressurise at an altitude of fifty thousand feet some gasses that had been absorbed by the blood would instantly return to their gaseous state, causing the blood to behave almost as if it were boiling. Hopefully we won't ever have to deal with that problem, but should we be at altitude and suffer some sort of technical problem it is worth knowing about.
G-suits. Before we tried the centrifuge we still needed to learn about the technological aids we may be able to use when we go up in a fighter jet. The first was the g-suit. These look like a pair of cowboy's chaps - trousers with nothing around the butt, a bit like one of the Village People used to wear.
The only difference is that it has a tube, like a flaccid and convoluted member hanging from one leg. Apart from the tube the g-suit was pretty unobtrusive at first, but in the cockpit of a plane it becomes 'alive'. The tube is plugged into a supply of compressed air, and when the pilot experiences in excess of 5g or so the legs of the suit inflate. This presses down on the blood vessels in the legs, preventing blood pooling, and helping to avoid loss of blood from the head. It is estimated that you can increase your g-tolerance by 1.5g when using these.
Combat Edge. The second piece of equipment was the 'Combat Edge' suit. This consists of two parts. Firstly, there is an inflatable singlet, with a bladder at the back of the neck, and rather than merely inflate under the influence of g-force it makes sure that the blood stays in the right place with a more sophisticated inflation strategy.
This is supplemented by a 'Combat Edge' mask, which forces oxygen into a pilot's mouth at forty pounds per square inch, should the aircraft exceed a certain g-force. I tried the mask at sea level under neutral-g conditions (i.e. standing on the ground).
Without my chest being subject to 6 or 7g it was difficult to breathe out, which is the opposite to what would happen under g-force, and as soon as I relaxed oxygen forced its way back into my lungs. Under high-gs this should help equal things up a bit and help the pilot to breathe easily. After doing the breathing exercise I could see how the 'Combat Edge' equipment would be a definite help.
Oxygen supplies. To overcome the lack of oxygen at altitude, oxygen is fed through masks from oxygen cylinders to compensate. In addition to supplying enough oxygen to breathe easily, extra oxygen helps to reduce some of the effects of g-force. When less blood is getting to the brain due to the effects of g-force, extra oxygen can be supplied to help make sure that what blood gets there is well oxygenated, even when breathing is difficult.
Would either Zeron or I have made good fighter pilots?
Although I got to go up in the F-16 I could never have been a fighter pilot because I suffer from asthma and am slightly colour blind (in as much as I know that blood is red and grass is green, but I can't pass any of the tests). Zeron might have made the grade. The g-loc events he suffered may have been 'one-offs' due to him choking on a glass of water, but I think that the real thing to remember is that it's definitely not as easy as Tom Cruise makes it look in Top Gun and although I didn't suffer from g-loc I didn't have to operate a combat fighter at the same time with all its complex instruments and weapons systems.
About the F16:
About gravity and the body:
Copyright & revisions
Originally published: Thursday, 25th August 2005
Last updated on: Saturday, 1st July 2006
- Body text - Copyrighted: The Open University
- Image '' - Copyrighted: Used with permissions
- Image '' - Copyrighted: Henry Brett | flickr
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