redcordes
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Aerodynamic "Coffin Corner"

Fri Mar 17, 2006 8:16 am

Can someone explain the "coffin corner" effect and give examples of the width of the safe operating speed between Vc, Vmo and Vs ? Also, please explain the affect of increasing altitude on this phenomenon. Thanks.
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Starlionblue
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RE: Aerodynamic "Coffin Corner"

Fri Mar 17, 2006 10:59 am

"Coffin corner" refers to when an altitude limit for safe operation.
- As altitude increases, air density, and thus lift, decreases. It follows that stall speed increases. This defines the lower speed limit at which an aircraft can operate.
- As altitude increases, mach speed decreases. The speed at which mach buffet sets on defines the upper speed limit at which a subsonic aircraft can operate.

So at a certain altitude stall speed and mach buffet speed meet. The area just under that altitude is coffin corner, in which the operating margin is very small. The altitude in question varies depending on the aircraft and atmospheric conditions.

The U-2/TR-1 routinely operates in coffin corner, with mach buffet and stall speed only 10 knots apart.
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jspitfire
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RE: Aerodynamic "Coffin Corner"

Fri Mar 17, 2006 12:47 pm

I think Starlionblue covered it pretty good. Basically, as altitude increases, stall speed increases, and Vmo decreases, until they meet.

In smaller, slower aircraft, the aircraft's ceiling is determined by it's engine performance. On large aircraft, the ceiling is where Vmo and Vs meet. When you are in coffin corner, it can be very difficult to tell whether the aircraft is stalling or experiencing mach buffet, which could make life very interesting very fast!

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mikkel777
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RE: Aerodynamic "Coffin Corner"

Fri Mar 17, 2006 3:15 pm

Might want to clarify a couple of things here.

-Indicated stallspeed remains the same, while true stall speed increases when you go higher.
-Vmo stays the same, in respect to % of speed of sound, but as Mach is dependent on (only) temperature, going higher will give Vmo as a lower TAS (the tropospause will of course influence on the TAS, but aside with that now). Going higher will always give lower indicated airspeed, due to less dense air.

(The following numbers are just examples, they are not meant to be accurate, not assuming ISA)
Say you are cruising at 34 000 ft, at Mach 0.8, and indicated airspeed 260kts. You will for example have another Mach 0.02 up to the mach-buffet, and 40kts down to the stall speed buffet (220), i.e a wide margin.
Say you go up to 43 000 ft, increase the speed to Mach 0.815. Since the air is a lot less dense, your indicated airspeed will be a lot lower, maybe 225kts. That means you have 5kts down to stallspeed and Mach 0.05 to machbuffet. You are more or less placed in a corner, where a small change in airspeed will make you buffet.
 
aeroweanie
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RE: Aerodynamic "Coffin Corner"

Fri Mar 17, 2006 3:17 pm

This illustrates the "coffin corner" of the Lockheed U-2C:

 
mikkel777
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RE: Aerodynamic "Coffin Corner"

Fri Mar 17, 2006 3:40 pm

I see that the stall IAS actullaly increases a little bit with altitude. Because of less thrust at higher altitude, higher stallspeed, I suppose.
Power off stall-speed (IAS) should be the same, but of course, coffin corner relates to power on.
 
viv
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RE: Aerodynamic "Coffin Corner"

Fri Mar 17, 2006 10:54 pm

Quoting Starlionblue (Reply 1):
So at a certain altitude stall speed and mach buffet speed meet

Correct. This is why operating the Lockheed U2 at operational altitude calls for VERY accurate flying.
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rsbj
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RE: Aerodynamic "Coffin Corner"

Fri Mar 17, 2006 11:38 pm

I believe stall speed increases with altitude because air has momentum. To stall at 170KIAS at sea level means the air flow detaches at 170KTAS, but 170KIAS is about 315KTAS or so at FL400 and the air's momentum won't let it "make the turn" so to speak.
Another theory of mine says Mmo decreases above a certain altitude because the wing has to deflect less air through an increase in angle of attack, further accelerating air on top of the wing.
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viv
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RE: Aerodynamic "Coffin Corner"

Fri Mar 17, 2006 11:44 pm

Quoting Rsbj (Reply 7):
I believe stall speed increases with altitude because air has momentum

Nope. At high altitude, air is thinner and so provides less lift. So the wing stalls at a higher speed.
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wingscrubber
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RE: Aerodynamic "Coffin Corner"

Sat Mar 18, 2006 12:50 am

Rsbj, I understand your idea about air having momentum and causing the airflow to detach earlier, but remember its the aircraft which has the momentum, not the air  Smile
The air being thinner, or rather the lower ambient pressure would be the main factor though...
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Starlionblue
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RE: Aerodynamic "Coffin Corner"

Sat Mar 18, 2006 12:57 am

Quoting Wingscrubber (Reply 9):
Rsbj, I understand your idea about air having momentum and causing the airflow to detach earlier, but remember its the aircraft which has the momentum, not the air Smile
The air being thinner, or rather the lower ambient pressure would be the main factor though...

Indeed. To understand how thinner air provides less lift at the same speed, one can lok at a denser medium: water. Hydrofoil wings are tiny in comparison to aircraft wings, and operating speeds much lower, and yet lift is achieved. This is due to the much greater density of the medium.

How many human powered aircraft have you heard of (well, there is one, the Gossamer Albatross)?


Or Wind powered:


Here are some more conventional hydrofoil:
http://www.navsource.org/archives/12/12160201.jpg

[Edited 2006-03-17 17:00:58]

[Edited 2006-03-17 17:04:09]
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Grbld
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RE: Aerodynamic "Coffin Corner"

Sat Mar 18, 2006 6:32 pm

Quoting Jspitfire (Reply 2):
In smaller, slower aircraft, the aircraft's ceiling is determined by it's engine performance. On large aircraft, the ceiling is where Vmo and Vs meet.

Not necessarily true. This only happens when you're semi-heavy and you go up into the higher regions (FL380+, ballpark figure). If you're heavy and your ceiling is FL330, you'll probably not even find the coffin corner there. The ceiling is just based on your residual climb rate (how fast you can still climb with full climb thrust). And on the other side, you can be light and go up to your max certified level (FL410 or whatever) and not see the coffin corner either, because you're limited by max cabin altitude and pressure differential.

I just posted a pic with my new wide-angle lens on Flickr: http://www.flickr.com/photos/17634053@N00/112957256/
(click the all-sizes button to get a better view)

On digital airspeed indicators you get an excellent view of when you're in the coffin corner because you see the low-speed and high-speed amber bands creeping up. In the situation in this picture at FL360, you see the low-speed amber band WAY down below our current airspeed and the high-speed amber band is not even in sight.


Grbld

[Edited 2006-03-18 10:33:27]
 
broke
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RE: Aerodynamic "Coffin Corner"

Wed Mar 22, 2006 1:25 am

Correct if I'm wrong, but I think the upper limit of "coffin corner" is Mmo not Vmo. The buffet is caused when you near the critical Mach Number for the particular airframe.
On the U-2, I've talked to a couple of individuals who have flown the airplane and they told me that they usually only have a 5 knot margin between stall speed and Mach Buffet, when flying at very high altitudes.
In those conditions, it is possible in a turn to stall the inboard wing and reach Mach Buffet on the outboard wing. Definitely takes some super flying skills to avoid those conditions.

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