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HIGH-TECH ANTI-G-SUITS

Dr. A. P. Thurston was aware only of the take off and landing of his "test flight". When the scientist tried out a tethered aircraft in 1903, which had been built as a fairground attraction, he lost consciousness when his body was exposed to 6.78 times the acceleration of gravity. According to a NASA study, this event is the first historically documented case of unconsciousness, which has been induced by centrifugal force. The technical term for it is G-LOC, i.e. gravity-induced loss of consciousness.

From its very beginnings aviation had to tackle the problem of the effect acceleration has on the pilot during flight manoeuvres. It has become common practise to express centrifugal forces in a multiple of gravitational acceleration, the so-called g forces. One g corresponds with the normal gravitational acceleration at sea level.

Accelerations along the body's axis are especially damaging. Fighter pilots are primarily affected by g forces, which are directed towards their feet, which might occur during interception or in tight turns. Blood, a liquid organ, cannot offer any resistance to centrifugal forces and "sinks" accordingly. Upper body parts suffer most. The heart must work extra hard in order to supply the head/brain region, i.e. the body's control centre, with blood.

Without any added measures the blood pressure within the head decreases with increasing g forces. The body starts to fail. The "system" that becomes impaired first is the pilot's vision. He experiences first of all grey-out, i.e. loss of colour vision, then tunnel vision and finally black out. Depending on the individual's "g-tolerance" the oxygen supply to the head stops completely at 5 to 6 g resulting in unconsciousness, G-LOC.

The G-LOCK effect has been known to military aviation since World War I. It was then known as "blackout in the air". Only during World War II, doctors with a special interest in aviation began to study the effects of g forces on the human body using a device known as a centrifuge. These tests helped to develop personal and technical safeguards against centrifugal forces. "anti-g-trousers" and special breathing techniques were developed.

The pneumatic "anti-g-trousers" consist of five interconnected air chambers, which cover the lower abdomen, thighs and lower leg. The trousers are connected via a hose and an anti-g-valve to the pressure system of the aircraft, (air pumped away from the engine). If the aircraft accelerates from between 1.5 to 2 g the trousers are automatically inflated, depending on the strain the centrifugal force has on the body.

The increased outer pressure decreases the "sinking" of the blood into the lower body parts. A human's natural g-tolerance is increased by about 1.5 g by wearing well fitting "anti-g-trousers". A further improvement of tolerance of one g can be achieved with special breathing techniques, the so-called "anti-g straining manoeuvre". After quickly breathing in and out the pilot has to press his breath for several seconds against the closed vocal cords. Simultaneously he has to tighten his leg and stomach muscles as much as he can. This technique is extremely tiring during long exposure to g forces, and in addition to this, communication is made difficult.

The "anti-g-trousers" and breathing techniques are today - more then half a century after they had been invented - still the most important precautions, to save pilots from dangers posed by centrifugal forces. It is a fact that aircraft have been improved steadily and the fourth generation of fighters is about to be introduced with the Eurofighter. However, "up to now nothing has been done to improve the pilot's position", says Dr. Kurt Hauser, Head of the FIA - Flight Medicine Institute - Flight Medicine Section of the Swiss Air Force.

The "g-onset", which is the rate of increase in acceleration, plays a central role for the pilot's tolerance towards the effect of g forces. The more agile the plane and the quicker it can generate g forces, the shorter is the time delay between the occurrence of impaired vision caused by centrifugal forces and the onset of blackout.

If g onset ist more than 5 g per second, as they were experienced during operational flights for the first time at the end of the 70s in fighters like the F-16, blackouts can happen instantaneously and without warning, i.e. initial symptoms. This can occur, if flight manoeuvres are very abrupt, because the warning function of the eye will not come into action.

Even after acceleration is complete, it will take about 30sec for a pilot to be able to act and regain his orientation - a truly life threatening state, especially if the aircraft is moving close to the ground. Between 1982 and 1997 the US Air Force lost twelve F-16 through G-LOC. As a result nine pilots lost their lives.

From the beginning of the 90s the US Armed Forces have deployed a better safety system. The so-called "Combat Edge" has a special oxygen mask, which supplies the pilot with excess pressure respiration. It has a similar effect as the above mentioned pressure breathing and takes away part of the necessary effort the pilot had to use before. In order not to sustain lung damage the crew have to wear a pressure jacket, which in principle works like the "anti-g-trousers". It exerts counter-pressure on the chest.

Since last year the Canadian Air Force has been using its own improved system for their CF-18 Hornet pilots. STING (Sustained Tolerance and INcreased G) consists of "anti-g-trousers". 90 per cent of the legs are covered, as compared with only 40 per cent previously. "This measure alone, doubles the effect of the suit", explains Dr. Fred Buick of the Military and Civilian Medical Institute of the Canadian Defence Ministry.

STING also incorporates positive pressure breathing, which suits the requirements of F-18 pilots, who have to work at 5 to 7 g. There is an additional pressure vest, to which an integrated cooling vest will be added in the near future.

Thermal protection is seen as extremely necessary, since firstly most of the body is covered with materials, which do not let through any air and secondly the pilot is generally under a lot of stress. The cooling vest requires a second connection to the onboard systems as well as pressure air system.

Eurofighter pilots will have a similar system at their disposal, i.e. "anti-g-trousers", which cover the legs completely, positive pressure breathing and a cooling vest. Test pilots are already using the new trousers, and the pressure vest is still being tested. EF2000 pilots will also be wearing inflatable socks in their boots. These will be connected to the "g-trousers". At high g exposures of up to nine times the acceleration of gravity, haematomas in the feet are experience d first. Feet are exposed to the highest blood pressure during positive g force.

Doctors interested in aviation are currently questioning whether a safety suit relying on the classical pneumatic pressure principle will suffice for pilots of the latest fighter generation. Once the Eurofighter starts series production, it is said to reach up to 15 g. The aircraft can increase its maximum turn acceleration in less than one second.

However, the pneumatic pressure system of the "anti-g-suit" will still work mechanically and there is therefore a delay, before it starts functioning. In older aircraft like the F-4 Phantom this delay lasts about 1.5 sec. In extreme cases the system will only react when the aircraft is accelerating in a curve at full throttle and darts sideways. Experience shows that many pilots have severe pains in their arms at 7 g - arms are not protected at all. This could mean that crews will not make use of the full potential of their fighters whilst wearing "anti-g-suits.

A novel suit system made by the Zurich Firm Life Support System (LSS) might solve this problem. The Physicist and LSS Boss Andreas Reinhard has been working on his version of the "anti-g-suit", the "Libelle", which has a liquid filling. In principle the body of a Pilot wearing "Libelle" is surrounded by liquid of the same density, which compensates any difference in pressure caused by centrifugal forces almost entirely and without delay. The "Libelle Suit" makes sure that organs do not become displaced, as is the case with the classical pneumatic suits when the pilot is exposed to g forces.

In 1988 the first prototype of the suit still needed 28 litres of water. Reinhard has been able to reduce the amount of the needed liquid to 1,1 litres, which are distributed to less than 10 per cent of the surface area. New fabrics for the suit are being tried as well.

The system has been tried in over 200 flights with the Pilatus turboprop trainer, a Learjet and fighters belonging to the Swiss Air Force. The "Libelle Project" has also been supported by the Flight Medical Institute of the German Air Force.

On the centrifuge at the Institute for Flight Physiology in Königsbrück near Dresden the suit has been evaluated by test pilots and fighter pilots of the German and Swiss Air Forces. The new suit's performance was being compared with conventional pneumatic suits. There were also tests on the centrifuge without "anti-g-suit".

With acceleration rates of up to 5 g/sec the centrifuge in Königsbrück is one of the most powerful in the world. One pilot wearing the "Libelle Suit" managed to achieve the top result of 12 g, the centrifuge's maximum output. Assistant Medical Director Dr. Heiko Welsch, Head of the Institute for Flight Physiology, claimed that "the test person's actions were not impaired in the least". Dr. Welsch's colleague, Dr. Jost Sutter, Head of the FIA, points out the aims intended for the "Libelle": "We are not looking for record results but are attempting to maintain optimal performance over longer periods of time for the pilot when he is exposed to medium g forces".

According to Andreas Reinhard the lower strain on the pilot has a positive effect on articulation, which is impaired when the pilot is exposed to high centrifugal forces. The pilot can concentrate again on his main task, i.e. flying. Pilatus test pilot Andreas Ramseier commented, "You are fully alert and not distracted by pain". The arms do not hurt because they are incorporated in the suit. "During a mission, this fact can have a bearing on success or failure", said Reinhard about the effectiveness of the self-regulating liquid suit.

Pilots who had a chance to test "Libelle" welcome the suit with open arms. Andy Ramseier, who has tested most suits on the centrifuge, claims that "you feel like an unborn in its mother's womb".

In the meantime the development of the suit has gone a step further. It is at the last stage before it goes into series production. As well as the liquid system pneumatics guarantee an optimum fit. "Libelle" is fully operational if it fits like a glove. The pneumatic system, which is fed by a semi-compressible seat cushion - an integral part of the suit - is also connected to the inflatable inner lining of the boots. This guarantees that the suit is self-sufficient and does not need to be connected to any board systems.

This is an important marketing element for the LSS Chairman. Reinhard is convinced that "Libelle" will be of use not only in modern high performance fighters, "even older fighters become twice as attractive to the purchaser if the pilot is fitted with this suit as standard equipment".

From page 84 of FLUG REVUE 8/99