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0.98 Mach 747-8

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0.98 Mach 747-8

Old 23rd Feb 2011, 03:50
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0.98 Mach 747-8

Seem to remember reading somewhere that during recent testing in a dive the 747-8 was flown to 0.98 mach in a dive.
What would have happened if they had crossed the sound barrier?
Mach tuck, flutter - any damage?
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Old 23rd Feb 2011, 03:58
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I don't know the eact design details of the aircraft, but a 727 [supposedly] went supersonic in an uncontrolled dive and received a bit of damage, a DC-8 was flown past Mach 1 in a dive and came out ok, so based on past performance of similar (though smaller) aircraft, I would imagine nothing too dramatic would have happened if they had planned for it (and if they had I'm sure they would have done the math beforehand).
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Old 23rd Feb 2011, 04:35
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I suspect very little damage would be done, maybe the engines flame-out that's all.
Boeing has tested the original prototype up to M 0.991 anyway.
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Old 23rd Feb 2011, 05:16
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Both the DC8 and the DC9 have been taken supersonic(intentionally) during testing. Both fantastic planes.

A 747 operated by some chinese airline (not red china) had a huge upset and went supersonic, or parts of the air frame went supersonic/transsonic and some pieces of the horizontal fin broke off...plane landed safely in San Francisco.
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Old 23rd Feb 2011, 05:53
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Certain locations will begin to have localized supersonic flow when you get around Mach 0.8-0.9, depending on the aerodynamics of the airplane in question. This is the transonic region. Though if you get past design Mach limit, all promises from the manufacturer go out the window.
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Old 23rd Feb 2011, 08:07
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Certain locations will begin to have localized supersonic flow when you get around Mach 0.8-0.9,
In cruise for the 747-400 these "certain locations" include 60% of the wing upper surface. Under certain lighting conditions you can see the shadow of the shockwave.

Qantas QF6 FRA-SIN 747-400
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Old 23rd Feb 2011, 08:45
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Picture's a little large but your correct that most airliners fly around everyday with supersonic airflow over large parts of their wings and fuselage. Dive tests are designed to establish a safe margin by identifying when the mach effects become a little too interesting.

Notice the second shadow and the area where the two overlap. They relate to the crank in the trailing edge. These shadows were often very prominent on the l1011 in the cruise at .83
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Old 23rd Feb 2011, 09:07
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The photo is lovely, please leave it.

I remember my 727 Captain father telling me that sometimes when they were cruising fast the FE would go down the back and occasionally walk back into the cockpit and casually say, "the wing's on fire."
He was, of course, referring to the shock lines that you can see in the photo above.
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Old 23rd Feb 2011, 09:41
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"Mach tuck" is old hat for old airplanes. Certification requirements assure that there are no adverse handling issues at high Mach numbers.

Certification also requires that, from M_mmo, the airplane is put into a 7° nose-down dive for 20 seconds. It is not a routine manoeuvre. (K9 is thereby mistaken that "all promises from the manufacturer go out the window" above M_mmo. But more than 20 seconds at more than 7° nose-down is something that will likely be looked at in the tunnel but not in flight test.)

So-called "supercritical" wings are designed such that at operating speeds some of the airflow over the wing is supersonic in some regions. To put it in symbols, M_crit < M_mmo. Many wings desiged nowadays are supercritical, including all Airbuses.

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Old 23rd Feb 2011, 19:41
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How the heck do you do quotes in this forum? Clicking the reply button on an individual post doesn't do it.

Anyway, all certification involves demonstrating that the airplane can safely go beyond the limits imposed on the operators, so I suppose in the strictest sense what I said could be seen as incorrect. However, there is going to be some limit beyond which the aircraft will either have structural damage or loss of control, otherwise why would we have any limits at all?
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Old 23rd Feb 2011, 22:00
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Cool

Originally Posted by K_9 View Post
How the heck do you do quotes in this forum? Clicking the reply button on an individual post doesn't do it.
delete the 1 at the end of the url so it says noquote=
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Old 24th Feb 2011, 02:22
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Vd/Md are the maximum demonstrated limits which form the hard upper limit for aircraft performance before you become a test pilot. You should not use these numbers in normal flight, because they were obtained and measured using instrumentation specially calibrated for accuracy. Your Vmo /Mmo numbers give you a small safety margin to help keep you out of dragon territory, whether it be 20 seconds of 7° nose down beyond Mmo, or a few extra knots from an off calibration airspeed.
The 747 going into San Francisco mentioned earlier this thread suffered tattered tail feathers from flutter and significant other damage. It pulled significant excess G on recovery. The FDR did not return much good data due to interference.
Those of you flying subsonic aircraft, need to believe in those limit speeds you have been provided. Swept wing aircraft really do pitch down when transiting M 1 accelerating and pitch up when transiting while decelerating. If your flight control system isn't intended to control this transition, then things may end poorly.
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Old 24th Feb 2011, 05:30
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Tks guys - v interesting - and love the photo.
I realise now I think I may have seen something similar while SLF on 747s before - the line appears to dance and ripple at times?
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Old 24th Feb 2011, 06:31
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Originally Posted by tartare View Post
Tks guys - v interesting - and love the photo.
I realise now I think I may have seen something similar while SLF on 747s before - the line appears to dance and ripple at times?
Yep.

YouTube - Shockwave over 747 wing
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