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Altitude/supersonic Air/compressor Stall  
User currently offlineAirSpare From United States of America, joined Jun 2006, 589 posts, RR: 6
Posted (7 years 9 months 1 week 4 days 16 hours ago) and read 3526 times:

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


Get someone else for your hero worship fetish
14 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17173 posts, RR: 66
Reply 1, posted (7 years 9 months 1 week 4 days 16 hours ago) and read 3518 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineFuturecaptain From , joined Dec 1969, posts, RR:
Reply 2, posted (7 years 9 months 1 week 4 days 15 hours ago) and read 3509 times:

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.


User currently offlineFuturecaptain From , joined Dec 1969, posts, RR:
Reply 3, posted (7 years 9 months 1 week 4 days 15 hours ago) and read 3509 times:

Oops, Starlionblue beat me to the post button. Cost of slow internet here.

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17173 posts, RR: 66
Reply 4, posted (7 years 9 months 1 week 4 days 15 hours ago) and read 3506 times:

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



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineTlfd29 From United States of America, joined Jul 2005, 81 posts, RR: 0
Reply 5, posted (7 years 9 months 1 week 4 days 6 hours ago) and read 3456 times:

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.

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 6, posted (7 years 9 months 1 week 4 days 3 hours ago) and read 3440 times:

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



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineRwessel From United States of America, joined Jan 2007, 2412 posts, RR: 2
Reply 7, posted (7 years 9 months 1 week 4 days 3 hours ago) and read 3436 times:
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Quoting FredT (Reply 6):
To be picky it is Vmo and not Vne.

To be really picky, it's usually Mmo.  Wink


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (7 years 9 months 1 week 4 days 3 hours ago) and read 3434 times:

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.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineRwessel From United States of America, joined Jan 2007, 2412 posts, RR: 2
Reply 9, posted (7 years 9 months 1 week 4 days 2 hours ago) and read 3431 times:
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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.


User currently offlineTedTAce From , joined Dec 1969, posts, RR:
Reply 10, posted (7 years 9 months 1 week 3 days 22 hours ago) and read 3394 times:

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 


User currently offlineTlfd29 From United States of America, joined Jul 2005, 81 posts, RR: 0
Reply 11, posted (7 years 9 months 1 week 3 days 22 hours ago) and read 3390 times:

Thanks for pointing that out FredT, it was a little late when I made that post  Smile

User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 12, posted (7 years 9 months 1 week 3 days 18 hours ago) and read 3363 times:

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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineAirSpare From United States of America, joined Jun 2006, 589 posts, RR: 6
Reply 13, posted (7 years 9 months 1 week 3 days 8 hours ago) and read 3322 times:

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

Much obliged!



Get someone else for your hero worship fetish
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 14, posted (7 years 9 months 1 week 2 days 14 hours ago) and read 3290 times:

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.

Andrea Kent


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