Why does 9g seem to be the design aim?

Always wondered why the F-16 has a tailhook, or how big a bigmouth F-16's mouth really is ? Find it out here !
  • Author
  • Message
Offline

Obi_Offiah

Active Member

Active Member

  • Posts: 241
  • Joined: 28 Mar 2004, 00:09

Unread post30 May 2009, 18:54

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?
Offline
User avatar

Bjorn

F-16.net Webmaster

F-16.net Webmaster

  • Posts: 1879
  • Joined: 27 May 2003, 18:56

Unread post30 May 2009, 20:30

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 Claes
F-16.net Editor
Photo Library Admin
Aircraft Database Admin
Offline

Obi_Offiah

Active Member

Active Member

  • Posts: 241
  • Joined: 28 Mar 2004, 00:09

Unread post30 May 2009, 22:02

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?
Offline

VarkVet

Elite 1K

Elite 1K

  • Posts: 1451
  • Joined: 30 Oct 2006, 04:31

Unread post30 May 2009, 22:16

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!
My eyes have seen the glory of the Lord and the esthetics of the Flightline
Offline

cywolf32

Forum Veteran

Forum Veteran

  • Posts: 623
  • Joined: 21 Nov 2005, 12:04
  • Location: USA

Unread post30 May 2009, 22:41

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.
Offline
User avatar

Gums

Elite 2K

Elite 2K

  • Posts: 2512
  • Joined: 16 Dec 2003, 17:26

Unread post30 May 2009, 23:21

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 ...
Gums
Viper pilot '79
"God in your guts, good men at your back, wings that stay on - and Tally Ho!"
Offline

VPRGUY

Forum Veteran

Forum Veteran

  • Posts: 851
  • Joined: 24 Apr 2005, 18:03

Unread post30 May 2009, 23:21

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.
Remember that a lone amateur built the Ark. A large group of professionals built the Titanic.
Offline

cywolf32

Forum Veteran

Forum Veteran

  • Posts: 623
  • Joined: 21 Nov 2005, 12:04
  • Location: USA

Unread post30 May 2009, 23:35

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...
Offline
User avatar

popcorn

Elite 5K

Elite 5K

  • Posts: 7724
  • Joined: 24 Sep 2008, 08:55

Unread post31 May 2009, 03:45

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.
Offline

Tinito_16

Forum Veteran

Forum Veteran

  • Posts: 764
  • Joined: 31 May 2007, 21:46

Unread post31 May 2009, 06:09

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.
"Like the coldest winter chill, heaven beside you...hell within" Alice In Chains
Offline

FDiron

Active Member

Active Member

  • Posts: 147
  • Joined: 28 Apr 2005, 13:20

Unread post31 May 2009, 06:25

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.
Last edited by FDiron on 31 May 2009, 06:43, edited 1 time in total.
Offline

cywolf32

Forum Veteran

Forum Veteran

  • Posts: 623
  • Joined: 21 Nov 2005, 12:04
  • Location: USA

Unread post31 May 2009, 06:33

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

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

VarkVet

Elite 1K

Elite 1K

  • Posts: 1451
  • Joined: 30 Oct 2006, 04:31

Unread post31 May 2009, 06:35

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!
My eyes have seen the glory of the Lord and the esthetics of the Flightline
Offline

Tinito_16

Forum Veteran

Forum Veteran

  • Posts: 764
  • Joined: 31 May 2007, 21:46

Unread post31 May 2009, 08:12

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.
"Like the coldest winter chill, heaven beside you...hell within" Alice In Chains
Offline

cywolf32

Forum Veteran

Forum Veteran

  • Posts: 623
  • Joined: 21 Nov 2005, 12:04
  • Location: USA

Unread post31 May 2009, 08:25

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.
Next

Return to F-16 Design & Construction

Who is online

Users browsing this forum: No registered users and 3 guests