# G-Suits



## MIflyer (Jan 29, 2018)

During my first assignment in the USAF I had an interesting encounter with G-Suit valves..

For those who do not know, G Suits, actually formally called "Anti-G suits" consist of a rubber "balloons" wrapped around a pilot's lower legs by means of a kind of "chaps." They fill with air when activated and force the blood that pools in the lower legs back up to where it can keep the brain running. At least that is the theory; some who have worn them during centrifuge testing (ever see The Right Stuff?) think that actually its causes you so much pain that you can't pass out.

The G-Suit balloons are filled with air by means of a valve that senses when G-loads are imposed and inflates the balloons. There was a specification, developed during WWII, for how much air pressure was imposed for a given G-load, with the max being 8G's. From what I recall the max pressure was around 12 PSI.

We had a problem overhauling the G-Suit valve used on the F-111. Most aircraft used a standard G-suit valve that was about the size and weight of a brick, hogged out of a solid aluminum casting. It was one tough piece of equipment; in the event of a crash the pilot probably could remove the valve and use it as a weapon. You could kill someone with it.

But the F-111 used a special valve, a much smaller and lighter one that could easily fit inside a closed fist. . A golf ball probably is a better lethal weapon. The F-111 had some serious weight problems in the USN version so I guess that explains the special valve.

Someone apparently decided the field was throwing too many of the valves away and decided that the depot should overhaul them. And there was a problem getting them through overhaul. The specification on all the valves built since WWII said that when operating at full output pressure the valves could not leak more than 600CC's per minute of air outside the valve. Now, obviously it is very important that the valve not leak into the suit, activating it when there was no high G load. But this was a requirement that the valve not leak air outside of the valve body, as into the cockpit. And 600CC/min ain't much, less than three 8 ounce cups; in a jet fighter you are pumping hundreds of pounds of air into the cockpit to cool and pressurize it, so that aspect clearly is not a problem.

Not only that, the manual on the valve specified a Rube Goldberg procedure for testing the external leakage, equivalent to having to pat your stomach while rubbing your head and driving a car. Even a helicopter pilot probably could not do that. Even a drunk helicopter pilot probably could not do that.

While working on the F-111 valve problem I was contacted by a company hired by the USAF to study G-suits. The idea was that the WWII spec might not be adequate for the F-15 and F-16. They found that the 600CC/Min leakage requirement seemed to come from the use of G-suit valves in aircraft like the P-51, where, lacking a jet engine to provide tons of air, they used a rubber bag filled with air with the airplane's battery sitting atop it. My own research showed that on the P-82 they used the exhaust from the vacuum pump to not only operate the G-suits but to pressurize the drop tanks as well; the manual warned that when the drop tanks were in use the G-suits only worked at half capacity.

The original specification was based on G-suits being used in piston powered aircraft. They did not want the valve leaking overboard because they had so little air.

I opened up the F-111 valve leakage to 1 liter/min; if it leaked more than that something inside probably was broken. I also found that the Rube Goldberg test procedure was not required and specified a much simpler method that did the job just as well.

I also looked at the old files and found that the Air Force and manufacturer engineers had been struggling to make the 600CC/Min requirement for decades. Nobody ever thought to ask, "Whathell is this stupid requirement for, anyway?"

By the way, the tests they were doing in the centrifuge showed that the F-111 valve was a real POS. They offered to let me ride the centrifuge and try it out but higher priorities intervened. An F-105 crashed and we had to limit the fleet to 30 min flight times; this made deployments to Europe rather challenging.

I sure hope they revised that stupid G-suit valve specification!

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## XBe02Drvr (Jan 29, 2018)

MIflyer said:


> Anti-G suits" consist of a rubber "balloons" wrapped around a pilot's lower legs by means of a kind of "chaps." They fill with air when activated and force the blood that pools in the lower legs back up to where it can keep the brain running. At least that is the theory


If the suit is properly fitted, that is. If you're an occaisonal back seat rider, not regular aircrew, and don't have your own custom fitted G-suit, you're at the mercy of whatever is on the loaner rack when it's your turn to go flying. If you're a 6'5" beanpole and the average fighter jock is a stocky 5'8"-5'10", you're likely going flying in an ill-fitting suit. The belly bladder is stretched across your pelvis and squeezing your own bladder, the thigh bladders are working on your crotch, and the calf bladders are pressurizing your knees. Does wonders for your G tolerance!
Cheers,
Wes


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## XBe02Drvr (Jan 29, 2018)

MIflyer said:


> The F-111 had some serious weight problems in the USN version


The F-111 had some serious weight problems, PERIOD! The supposed "do everything machine", "MacNamara's Flying Switchblade", that could land slow, fly fast, beat any fighter in the world in a dogfight, and deliver more ordnance than a B-17 more accurately than an Intruder, turned out to be an overweight maintenance hog that was adept at disappearing without a trace on low level night attack missions. Only SAC choosing it as their "baby bomber" saved its bacon.
Cheers,
Wes


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## DarrenW (Apr 5, 2018)

A rather in-depth history of the pressure suit (530 pages worth):

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120014266.pdf

Great topic by the way...


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