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DocLightning
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Is A LAF Useful On Cfrp Aircraft?

Fri May 04, 2012 10:53 pm

So many aircraft have a load-alleviation function built into their flight controls to improve the fatigue life of the wing.

Question is: does the 787/A350 need this? Is there any benefit at all when there is a fatigue "floor" on the material?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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tdscanuck
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RE: Is A LAF Useful On Cfrp Aircraft?

Fri May 04, 2012 11:49 pm

Quoting DocLightning (Thread starter):
Question is: does the 787/A350 need this? Is there any benefit at all when there is a fatigue "floor" on the material?

Even if you don't have a fatigue problem, load-alleviation allows a lighter wing by reducing maximum loading.
Tom.
 
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DocLightning
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RE: Is A LAF Useful On Cfrp Aircraft?

Fri May 04, 2012 11:56 pm

Quoting tdscanuck (Reply 1):
Even if you don't have a fatigue problem, load-alleviation allows a lighter wing by reducing maximum loading.
Tom.

Is that allowed even thought it's an electronic system?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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tdscanuck
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RE: Is A LAF Useful On Cfrp Aircraft?

Sat May 05, 2012 1:18 am

Quoting DocLightning (Reply 2):
Is that allowed even thought it's an electronic system?

Yep. You certify for control system failures and ultimate load separately; there's no requirement to withstand ultimate load while you've got control system failures.

Tom.
 
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DocLightning
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RE: Is A LAF Useful On Cfrp Aircraft?

Sat May 05, 2012 1:34 am

Quoting tdscanuck (Reply 3):
Yep. You certify for control system failures and ultimate load separately; there's no requirement to withstand ultimate load while you've got control system failures.

That seems kinda odd. Wouldn't you expect that a control system failure might lead to ultimate load?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
tdscanuck
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RE: Is A LAF Useful On Cfrp Aircraft?

Sat May 05, 2012 1:08 pm

Quoting DocLightning (Reply 4):
Wouldn't you expect that a control system failure might lead to ultimate load?

Not really. One of the first instructions to pilots with a control system failure is *don't* make large or rapid control surface inputs because you may not have full authority around at least one axis anymore.

The control surface failure may, itself, cause a big load (a rudder hardover, for example) but that's taken into the design so single hardovers aren't supposed to get you to limit load, let alone ultimate.

Ultimate load is almost always in pitch (that's why you see the wings bend up so far in ultimate load tests), which means it's a pitch issue. I can't think of any real-world pitch control failure that would get you to ultimate load. Total flap hardovers are essentially unheard of because they're so heavily monitored for skew and differential motion (partial flap hardovers are tested and accounted for in the loading). Total soiler hardovers reduce, not increase, lift. Total elevator hardovers are nearly impossible because you've got dual or triplex control back there. If you were at MTOW and high speed and you had all elevators go full nose-up you might go over ultimate load but that would require multiple independant failures.

Tom.
 
CaptainKramer
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RE: Is A LAF Useful On Cfrp Aircraft?

Thu May 10, 2012 7:52 pm

I imagine the down side to load alleviation is that the hydraulic actuators that dappen out the rough ride in turbulance would see a shorter service life, grant you it would be the cheaper option over the airframes life?
 
tdscanuck
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RE: Is A LAF Useful On Cfrp Aircraft?

Thu May 10, 2012 8:40 pm

Quoting CaptainKramer (Reply 6):
I imagine the down side to load alleviation is that the hydraulic actuators that dappen out the rough ride in turbulance would see a shorter service life

The more likely path is to beef up the actuator to keep the actuator life the same. Actuators are usually on-condition maintenance (they don't have a proscribed life, you just replace them when the fail).

Quoting CaptainKramer (Reply 6):
grant you it would be the cheaper option over the airframes life?

If the trade study is done properly that should be the result. However, it would be a really complicated trade so no guarantee that the result is correct.

Tom.

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