Forum: F-16 Design & Construction

Why is 9g usually the design aim for modern fighter aircraft? Is it simply because the F-16 set this high level, so anything less is seen as a bit of a step backwards and that perhaps a 10g airframe would have performance shortfalls e.g weight?

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This number has nothing to do with the aircraft whatsoever. Off course, the more G's you pull on an airframe over its lifetime, the shorter that lifetime will become if nothing is undertaken to strenghthen the airframe at a certain point.

The limiting factor is the human sitting inside the airframe. An F-16 for example is limited to 9G just to protect the pilot. The airframe itself can sustain up to 13 of 14G, but the pilot simply can't. This is the only reason why aircraft are build on a 9G standard and not higher.

Greets,

Bjorn wrote:

This number has nothing to do with the aircraft whatsoever. Off course, the more G's you pull on an airframe over its lifetime, the shorter that lifetime will become if nothing is undertaken to strenghthen the airframe at a certain point.

The limiting factor is the human sitting inside the airframe. An F-16 for example is limited to 9G just to protect the pilot. The airframe itself can sustain up to 13 of 14G, but the pilot simply can't. This is the only reason why aircraft are build on a 9G standard and not higher.

Greets,

Hi Bjorn

My understanding is that modern g-suits and accessories such as the Combat Edge vest allow pilots to handle greater loads?

True … Combat Edge is an improvement, but basic Human Life Sciences is still limited to 9G to be safe.

Example: Indy Car drivers can hit the wall at 200 mph and survive thru modern technology … but he just can’t do it all day or for the duration of a sortie.

G-loc is still taking lives!

Actually, 9G is only a USAF requirement. The Navy only has a 7G requirement. And agreed, 9G is a human threshold requirement, not an airframe requirement.

slaute

sorry about laptop with fubar keyboard, but here goes.

john-boy has more to say here as he was a structural engineer in the early days.

my thots are that it was a structural limitation more than anything. at the time, the eagle and hornet and whatever had about a seven point something limit. that didn't keep the pilot from pulling ten gees unless the flcs got in the way.

some of us in older planes pulled ten gees if we snatched the stick. maintenance was pissed, as we had to do all kindsa inspections or even replace rippled skin on the wings.

so i stick with the airframe maintainability and lifetime position as far as the gee limit.

the pilots can easily stay awake at ten or more gees if they don't 'snatch' the stick and are 'prepared'. but they can't hold that gee for long - maybe ten or so seconds if onset rate was high, longer if a slow pull.

see threads about gee-loc. it bit a blue angel in the butt a few years ago.

gums sends ...

9G isn't necessarily the human threshold - modern aerobatic aircraft routinely go to 10-12G in many maneuvers without a g suit, although they don't sustain that kind of G load for more than a few moments. 9 may simply be what 'human factors' engineers decided the average 'conditioned' person could sustain through use of a g suit and proper technique.

I was thinking more along the lines of a best determined average for G tolerance. And yes, I hated doing over-G inspections!! And I also agree that mantainability and lifetime costs must be factored in.

Cheers...

I'd read that the Luftwaffe Eurofighter pilots will use the Libelle G-suit that uses fluid-filled chambers to cushion G effects independently of any interface to the aircraft. There used to be a video showing a pilot in a Libelle having a relatively normal conversation when in a centrifuge at 9Gs. It seemed to work pretty well.

Back in the days when Dozer was posting, he said there were mixed reactions re the new fluid-filled suit vs what US pilots were using. IIRC he said that pilot used to the older G-suit would have a difficult time transitioning to a LIBELLE but a novice pilot should manage.

It would seem to me that if it's a liquid, it's much less compressible than air, so it might be more painful to the pilot... Really don't know, though.

Last edited by FDiron on May 31, 2009 - 07:43 AM; edited 1 time in total

Humans can actually survive unharmed up to 40g's. 200g's is survivable, but with many injuries. The big factor is time spent at the high g. The test pilot who tested the rocket sled at 200g's (for a few milliseconds) had his retinas temporarily detach.

http://www.scientificamerican.com/artic ... 9EC588EEDF

Actually, a liquid cannot be compressed, only a gas can.

FDiron wrote:

The test pilot would tested the rocket sled at 200g's (for a few milliseconds) had his retinas temporarily detach.

Yup ... can't allow that to happen during a dog fight! JHMCs ain't that good!

cywolf32 wrote:

http://www.scientificamerican.com/article.cfm?id=0009B40B-48D2-1CC6-B4A8809EC588EEDF

Actually, a liquid cannot be compressed, only a gas can.

Can't open the link for some reason, but anyways, I heard in a discovery channel program that seawater @ many hundreds of feet below the surface is denser (because of compression) than the surface water. If I'm not mistaken a liquid CAN be compressed but not anywhere near the level you see with air. You won't have any appreciable degree of compression in the amount of water you'd use for the bladders in a G-suit. The G-Suit (let alone the pilot wearing it) wouldn't be able to withstand that kind of pressure anyways, I'm talking about many atmospheres worth, the kind you find in the deepest ocean trenches. The program was about deep sea fish I think. Anyways my point was that the air pressure would be felt differently than liquid pressure, because liquids cannot be compressed much (very very little) compared to air. This might be more painful and very hard for a pilot to adapt to.

Actually, if you read the post added, the pliot involved loved the suit to the point that he wanted to buy one on the spot. The liquid involed in the G-suit has the same viscosity as blood, since the inventor was thinking of a child in womb and a dragonfly when coming up with the concept. A liquid would react much more quickly and effectively than air since there is no waiting for the G-suit to fill up with air to counteract the forces involved and that the air itself would compress under G rather than translate the force directly to the fluid involved. Simple and very effective. Think about car brakes being pneumatic rather than hydraulic for a simple analogy.

Forum: F-16 Design & Construction

Why does 9g seem to be the design aim?