Past Talks : April 2024
“Protecting Fighter Pilots from High ‘g’, by Sue Adcock (FAST Trustee – Farnborough Air Sciences Trust)
Since a very early age, our speaker, Sue Adcock, was fascinated with flying and military aircraft and she was determined to work in the industry. Unsurprisingly, Sue had an interest in science at school and she went on to gain a mathematics degree at Oxford University, which enabled her, in 1979, to become a research scientist at the RAE (Royal Aircraft Establishment) – a most unusual achievement for a woman at that time.
Sue became involved in several aircraft system trials, which of course involved flying in a wide variety of aircraft including the DH Devon, Varsity and numerous fast jets such as the Hunter, Jaguar, Lightning and Hawk – enviable and exciting stuff. Not only that, but Sue also gained her PPL (Private Pilot’s Licence) enabling her to undertake her own aerobatics.
Meanwhile, in 1989, Sue’s husband Terry, an RAF test pilot, was posted to the IAM (Institute of Aviation Medicine) at Farnborough, where he was soon acting as a subject in various IAM trials, including the centrifuge. Being a gentleman, he volunteered Sue to participate in a series of centrifuge tests and that is how it all began for Sue, who went on to accumulate around 600 runs and an incredible depth of knowledge and experience on the effects of gravity on the human body. Interestingly, Sue asked our audience if anyone had been tested in the centrifuge and the few that responded said it was not a pleasure trip.
The human body is designed to inhabit an environment based on 1g (gravity). Any changes in speed or direction create forces that act on the body. Awareness of this first became known in WW1, when fighter pilots were known to faint in the air while dogfighting. Little research was undertaken in Britain until mid 1939, just as war with Germany was about to break out. Trials in service aircraft were difficult to measure, so it was decided that more needed to be done to understand the problems of high ‘g’ and to develop protection, equipment and training within a safe environment, under medical supervision.
The resulting centrifuge was all-British, designed by The Air Ministry with its main features finalised in 1947. Construction took place at Farnborough between 1951 and 1955 at a total cost of £350,000 (£6½m in 2024). It continued in operation until 2019 when it was replaced by its new successor (from Austria) at RAF Cranwell. The Farnborough centrifuge completed 122,133 runs throughout its life, with very few changes made to its systems.
Its construction is centred around a circular base of reinforced concrete to a depth of 27ft (8.2m), The centrifuge itself comprises two arms (totalling 62½ft long) with a gondola at each end, which is very unusual as centrifuges go. The test subject sits in a simulated cockpit within a gondola, while the medical officer sits at the centre of the rotating arm to monitor the human subjects throughout.
In all, the rotating weight is 40 tons and can generate up to 30g, at 54rpm. In reality the system is limited to around 9g, at 30rpm and a speed of around 63mph. The acceleration can be terrific (if desired) and the gondolas pivot outwards at speed. Nearby is a Plant Room drawing 6,600volts from its sub-station; the DC motor drive of 1350hp is directly beneath the centrifuge driving a 12 ton flywheel with a 12ft diameter vertical spindle. Six control cabinets are a piece of history in themselves, with good old-fashioned wires and valves proliferating – right up to the centrifuge shutting down in 2019.
The Control Room has remained original, classic 1950s throughout its life, using analogue systems, etc.. When constructed, existing proven technology was used, including for example that used in commercial passenger lifts. Little has been changed, because it worked and did the job.
The control of the centrifuge to operate between say, 1.2g up to 10g, was via a joystick operating a simple cam, shaped rather like a comma. Rotating the control cam moved an arm enabling the motor speed to be varied. Slowing the centrifuge arm was not by a brake, but by slowing the electric motor, much as modern electric trains and EV cars do today.
‘g’ effects on the human body
The human body is created to operate at 1g. If it is subjected to 9g that is the equivalent of weighing 9 times more, which affects mobility, risks soft tissue injury, particularly putting neck muscles at great risk. By way of example, the distortion it imposes on a human subject also makes you look 25 years older, as Sue bravely demonstrated via a film of her when under test.
Without any protection, blood pools in the legs and feet and the blood pressure to the brain drops, initially causing reduced vision (grey/brown out), followed by loss of vision (blackout), followed yet again by unconsciousness (G-LOC, ‘g’ induced Loss of Consciousness), which has been the cause of death for many pilots who do not ‘come-round’ soon enough.
‘g’ Protection and Countermeasures
In the early days, a suit containing water was found to be effective, but the downside was its weight and general clumsiness. Anti-g inflatable trousers were introduced therefore, which contained bladders that inflated automatically to apply pressure to the limbs, to reduce blood flow from the brain during high acceleration.
A typical, fit human can tolerate between 3½ to 5g and a g-suit will add 1g for example, making 6g, which is acceptable for the Jaguar, Lightning, etc.. Aircraft today require a tolerance closer to 9g. However, the pressure suit itself is not a complete solution and greater effectiveness is achieved by the pilot adopting AGSM (Anti-g Straining Manoeuvre), which requires tensing the abdominal muscles and holding them tense to raise blood pressure. It is an immensely tiring exercise to undertake though and it is also a distraction when the pilot is already under combat pressure.
The next step was to introduce PBG (Pressure Breathing for g-Protection) which supplied the breathing gas under pressure to the pilot, which in turn increased the pilot’s blood pressure and what a difference that made. Throughout the talk Sue showed examples of the topic under discussion on film. In this instance we watched a person undergoing a test and first of all black out, then fainting. The next sequence showed a subject wearing a full pressure suit, undertaking AGSM and PBG, accelerating to 9g and showing no effects at all. Very convincing.
One interesting aside related by Sue, was a single occasion when the centrifuge started to gain speed without any control input. The big red emergency button was immediately and decisively pushed. The result – nothing, the speed continued to build up, to the great discomfort of the human subject and to the alarm of the observers. Fortunately, an in-built motor cut-off took effect when the centrifuge reached 10g and order was quickly restored. In the way that these things happen, a failure in the circuit of one of the control cabinets affected both the speed control and emergency systems.
The centrifuge plays an essential role in the tests and development of techniques and equipment, all within a safe environment, but flying itself is not a safe environment. Everything has to be tested in the air also, under conditions similar to those that would be experienced by pilot’s on operations. To this end Farnborough operated its own two Hawk aircraft and Sue and her husband, Squadron Leader Terry Adcock, worked as a team to undertake the carefully planned schedule of tests up to 9½g. During this time Sue accumulated around 150 Hawk flying hours – so easy to say, but all under extreme conditions that would make most of us queasy to just watch on film. Most impressive.
Much of their later work was to test g-suits for Typhoon pilots, but such was their experience that they also tested the US Navy and US Airforce ‘Combat Edge’ suits in an appropriately modified Hawk aircraft, prior to the suits entering service in US aircraft.
Likewise, Sue and husband Terry undertook trials in the Hawk with the US CBRN system (Chemical, Biological, Radiological, Nuclear) with pressure breathing within a ‘goldfish bowl’ over your head.
The new centrifuge based in RAF Cranwell simulates pressures from 10g to 15g and the UK is much involved with testing the pilot suits and equipment for the F35 aircraft.
In terms of testing, the physical difference between men and women is negligible, although woman as a rule are shorter than men, meaning that the space between the brain and the heart is shorter, consequently the effects can be less, but otherwise no different.
Enabling pilot’s to withstand high G forces has to be tempered with the structural strength of the aircraft itself. In the past this was a concern that pressurised pilot’s could inadvertently cause over stress of an airframe, but design and materials have come a long way since then. Red Bull aerobatic aircraft are unpressurised, so the pilots all wear appropriate g-suits, once again filled with liquid bladders.
High ‘g’ is a technical subject, but Sue put it across with simple and very interesting, gripping, ease. Thank you Sue Adcock and to the backroom SOFFAAM team.