Dufo
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Certification Testing Extremes

Tue Sep 11, 2012 12:50 pm

During flight testing for certification (and also later), aircraft are pushed beyond normal limits. For example DC8 going intentionally supersonic, on my current type Saab 340 I found that it has been test flown up to 313 knots, even though it's limited to 250 etc.

What other extremes have been achieved? Altitudes, speeds, operating from unusually short runways?
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tdscanuck
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RE: Certification Testing Extremes

Tue Sep 11, 2012 4:37 pm

Quoting Dufo (Thread starter):
For example DC8 going intentionally supersonic

That wasn't actually on purpose (Md for the DC8 is below Mach 1).

Quoting Dufo (Thread starter):
What other extremes have been achieved? Altitudes, speeds, operating from unusually short runways?

Yes. In order to show that you're good to the certification limit, you actually have to go slightly past it. So all aircraft have gone higher, faster, heavier, lighter, more fwd CG, more aft CG, colder, and hotter, than they're certified for.

Tom.
 
Dufo
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RE: Certification Testing Extremes

Tue Sep 11, 2012 5:42 pm

I thought that DC8 mach1+ was intentional - this article claims so: http://www.airspacemag.com/history-o...here-Boeing-Will-Never-Try-It.html
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tdscanuck
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RE: Certification Testing Extremes

Tue Sep 11, 2012 6:08 pm

Quoting Dufo (Reply 2):

I thought that DC8 mach1+ was intentional - this article claims so: http://www.airspacemag.com/history-o....html

Fascinating article, I hadn't seen that before. I agree, it claims it was intentional. I'm not sure what to think about that; there are several thing in that article that hint at absolutely atrocious test conduct practices at Douglas at the time (intentionally taking off flaps-up, intentionally blanketing the elevator, doing enough engineering to get speed tables but not enough to realize you'd lock up the stabilizer, etc.). The event, as they described it in the article, is a monumentally stupid testament to bad test design and execution.

Tom.
 
citationjet
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RE: Certification Testing Extremes

Tue Sep 11, 2012 9:40 pm

Part 25 requires a demonstrated airspeed margin of 0.07 Mach above Mmo per 14 CFR 25.335(b)(2). Mmo is the maximum operating Mach. Mmo + 0.07 = Md.
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KELPkid
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RE: Certification Testing Extremes

Tue Sep 11, 2012 11:04 pm

Does Vmca have to be demonstrated to the point where the aircraft starts to roll over? If so, that's probably where the tail chute that many prototypes have comes in handy (or at least it gives the flight crew assurance that it's there if needed    ).
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tdscanuck
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RE: Certification Testing Extremes

Wed Sep 12, 2012 1:20 am

Quoting KELPkid (Reply 5):
Does Vmca have to be demonstrated to the point where the aircraft starts to roll over?

Not to roll over, you just need to get to the point that the pilots can't arrest the yaw. Since you still have full aileron authority you generally shouldn't have too much roll (C_N_beta) if you're just above Vmca but you won't be able to control the heading. This is a tricky test because the regulator really doesn't care if you certify a Vmca above the true aerodynamic Vmca but, for performance reasons, the OEM wants Vmca to be as low as possible.

Quoting KELPkid (Reply 5):
If so, that's probably where the tail chute that many prototypes have comes in handy (or at least it gives the flight crew assurance that it's there if needed    ).

That's a spin chute; it's needed if you lose directional control for any reason. That could be Vmca tests, flutter, or any of a wide variety of lateral/directional stability tests (steady heading sideslips, rudder hardovers, stalls, etc.).

Tom.
 
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HAWK21M
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RE: Certification Testing Extremes

Wed Sep 12, 2012 12:03 pm

By Testing something beyond its limit & then restricting it to some percentage below that limit,ensures a buffer that things can work within certain ranges & a bit beyond.
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