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Limit/ultimate Load In Fighter Aircrafts

Mon Jan 16, 2012 10:07 pm

After having done some reading, I got a bit confused with the way some fighters are designed in terms of their structural strength.

Rafale, Typhoon and many other fighers are generally rated 9G/-3G.

I assume this refers to their limit load therefore their should be sized against 1.5 * 9G = 13.5G.

In civil aviation limit load is supposed to be the very maximum load that the aircraft is gonna see in its whole life. The ultimate factor then adds some extra fat in the structure to make sure the design covers for any scatter in the assumptions used in the stress analysis or funny things like build stress, manufacturing defect, ect...
The structure has to be designed so that no detrimental damage occurs until limit loads. Passed limit loads, permanent deformations, local cracking, delamination (composite) can occur as long as they don't compromise the safety of the flight until ultimate load. Clearly in principle you dont want to exceed limit load, and usually the FCS would prevent you from doing so.

In this video of Cedric Ruet prior to his Rafale display at Le Bourget 2009 (in french sorry)
the pilot does not seem to bothered when he says he pulls manoeuvres "up to 10, 10.5, even 11G sometimes" which if we believe the generic 9G/-3G above would be exceeding Limit loads....

I also came accross the presentation below which states (last page) that the Rafale static test specimen broke at 1.85 LL, which suggest the airframe has some healthy margins in static (one could provokatively say it is well overweight....)
What does this mean? Has the actual Limit Load is not 9G anymore but something around 11G?

Even more suprising, I have been told (I cannot find any source to verify this) that the Eurofighter major static test specimen unfortunately broke at 1.4LL.
The link below explains how clever Eurofighter eventually negociated to get the usual ultimate factor of 1.5 relaxed down to 1.4 not to impact structural weight:
"The ability of carefree handling to control g limits precisely has allowed designers to reduce the ultimate load factor to 1.4, from the normal 1.5"
But still I am pretty sure I read somewhere about pilots pulling 10G on Typhoon....

Can anybody familiar with military plane design offer me some clarifications?
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RE: Limit/ultimate Load In Fighter Aircrafts

Mon Jan 16, 2012 11:27 pm

Quoting flagon (Thread starter):
In civil aviation limit load is supposed to be the very maximum load that the aircraft is gonna see in its whole life.

I'm an engineer but not an aircraft designer but you've more or less answered your question right there. The ultimate limit state for civil will be a safety factor on the max design load. A fighter is expected to be able to pull max G every single flight so thats effectively its Servicability limit rather than its Ultimate limit. I'd expect that Ultimate limit strength will be measured rather designed since it will be a function of the fatigue loading of the Servicability limit rather than a design load case. The pre-FBW fighters didn't have a 'soft' limit, the max G being effectively what the pilot could pull. I remember reading tales of F-14's pulling 10+ but the pilot declaring whatever was the max allowable at the time (6.5/7.5), theres a particularly good anecdote in 'Roger Ball' that more or less describes the situation perectly. But FBW will be limited by whatever the computer says no matter how hard the pilot pulls on the stick, if a Rafale or Typhoon has exceeded that then it was via an over-ride. I think Mig-29's (and maybe F-18's) had something like that built in to allow a pilot bend the plane if it meant survival, I'm not sure but I didn't think the Typhoon had.

Re the actual measured values, it wouldn't be surprising to see the Rafale having a higher ultimate strength than the Typhoon, carrier fighters by their nature need to be stronger than land based with a carry through structure for Catobar operations but they pay a significant weight penalty for that extra strength. And again a relaxation of the ultimate strength requirement wouldn't be so serious in a FBW aircraft since its a set limit that the pilot can't get past for normal operations once the fatigue requirements have been satisfied. On a pre-FBW it would have been a much bigger problem.
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RE: Limit/ultimate Load In Fighter Aircrafts

Tue Jan 17, 2012 3:25 am

Don't forget that the high G manuvers are done clean or minimal payload. The big stress is low/moderate G when loaded with tons of bombs and missiles and full fuel. Different loadings on the frame, but if your worried about whats going to crack parts, load up the plane heavy and then pull out of a divebombing run. Thats quite harsh on an airframe.

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