AirSpare
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Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 2:59 am

I should know the answer to this question, but I don't. Nor when I was working a flightline where the people that would know the answer, could I find a person that could or would answer.

The question is regarding the U-2/TR-1.

The higher the altitude, the lower super sonic airspeed is. So, the higher you fly, the faster you must go to gain/maintain altitude.

So, the question is, a U-2 flying at the very edge of it's maximum altitude is balancing between a wing stall due to generating the maximum lift it can due to thin air and airspeed, and a engine compressor stall due to supersonic air entering it's inlets?

So I U-2 pilot, would have just a couple of knots between stalling the wings or the compressor?

Sorry in advance if this is a dumb question, I worked avionics as a tech, I'm not an aerodynamicist. Just pondering time "back in the day".

cheers~AirSpare
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Starlionblue
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 3:54 am

Quoting AirSpare (Thread starter):
So I U-2 pilot, would have just a couple of knots between stalling the wings or the compressor?

Actually the limits for the U-2 are stall and mach buffet. U-2s at cruise sometimes operate with only a 10 knot margin between these. You can imagine how much fun that is for the pilots since IIRC the U-2 has no autothrottle.

Don't know about compressor stall on the U-2.
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futurecaptain
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 4:02 am

Quoting AirSpare (Thread starter):
The higher the altitude, the lower super sonic airspeed is

Speed of sound is depends on air temp, not altitude.

Quoting AirSpare (Thread starter):
So I U-2 pilot, would have just a couple of knots between stalling the wings or the compressor?

You are correct. I'm not completely sure but the way I hear it is the U-2 had a few knots between a stall and overspeed. I havn't heard of compressor stalls on the U-2, but I would think it's a legitimate concern.
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futurecaptain
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 4:04 am

Oops, Starlionblue beat me to the post button. Cost of slow internet here.
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Starlionblue
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 4:09 am

Quoting Futurecaptain (Reply 3):
Oops, Starlionblue beat me to the post button. Cost of slow internet here.

I'm just a bit bored at work today so I hit refresh every 12 seconds.  Wink
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tlfd29
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 12:58 pm

It's called coffin corner. If you were to look at a graph of altitude vs. stall speed and Vne, the point where the two lines meets represents this "corner". Lots of jets out there have a very small margin once they get to that point.
 
FredT
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 4:22 pm

Quoting Tlfd29 (Reply 5):
If you were to look at a graph of altitude vs. stall speed and Vne, the point where the two lines meets represents this "corner".

To be picky it is Vmo and not Vne.  Wink
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rwessel
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 4:37 pm

Quoting FredT (Reply 6):
To be picky it is Vmo and not Vne.

To be really picky, it's usually Mmo.  Wink
 
FredT
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 4:49 pm

Quoting Rwessel (Reply 7):
To be really picky, it's usually Mmo.

Mmo and Vmo is the same speed, just expressed in two different ways. The speed limit on the roads stays the same, whether you choose to express it in km/h or MPH, and that's essentially the same thing as Vmo vs Mmo.

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rwessel
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 5:08 pm

Quoting FredT (Reply 8):
Mmo and Vmo is the same speed, just expressed in two different ways. The speed limit on the roads stays the same, whether you choose to express it in km/h or MPH, and that's essentially the same thing as Vmo vs Mmo

Vmo is usually an indicated airspeed, Mmo is (more or less) a true airspeed. At low altitude Vmo is limiting, at high altitudes Mmo is. For high altitude jets, the coffin corner is usually where your stalling speed approaches the mach limit (usually the onset of mach buffet or tuck), although the term is applied in other situations where different limits converge.
 
TedTAce
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 9:04 pm

Quoting AirSpare (Thread starter):
So I U-2 pilot, would have just a couple of knots between stalling the wings or the compressor?

I saw something on Discovery wings were it was +/- 1 kt

Quoting Starlionblue (Reply 1):
stall and mach buffet

 checkmark 

Quoting Rwessel (Reply 7):
it's usually Mmo.

 checkmark 
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tlfd29
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RE: Altitude/supersonic Air/compressor Stall

Fri Mar 16, 2007 9:14 pm

Thanks for pointing that out FredT, it was a little late when I made that post  Smile
 
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Jetlagged
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RE: Altitude/supersonic Air/compressor Stall

Sat Mar 17, 2007 12:55 am

Quoting AirSpare (Thread starter):
a engine compressor stall due to supersonic air entering it's inlets

The compressor won't necessarily be on the verge of stalling. This is because supersonic air in the inlet won't inevitably lead to a stall and at "Coffin Corner" the U-2 Mach number will be less than 1 anyway.

As every other subsequent poster said, the upper limit on speed is loss of lift due to local shock waves (Mach buffet), which begins to occur well below Mach 1, especially with unswept wings.
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AirSpare
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RE: Altitude/supersonic Air/compressor Stall

Sat Mar 17, 2007 11:09 am

Thank you for your responses, you gave me some new terms and ideas to research.

Much obliged!
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Blackbird
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RE: Altitude/supersonic Air/compressor Stall

Sun Mar 18, 2007 4:57 am

The inlet of the U-2 was to some extent designed to milk as much ram-compression as they could in subsonic conditions. I would not be suprized if they also made measures to avoid transonic flow problems.

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