Blackbird
Topic Author
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Joined: Wed Oct 06, 1999 10:48 am

Fuel Consumption On Altitude & Mach-Number

Sat Sep 20, 2008 12:46 am

Most jet-engines, especially older ones seem to run on a fixed fuel-to-air ratio regardless of altitude, and as a result fuel consumption seems to fall off significantly as you fly higher and higher, as does thrust.

My question has to do with the fact that as Mach-number increases, so does ram-air pressure, which in turn increases thrust: Does fuel consumption increase as ram-compression increases (as air pressure in the duct goes up as well)?


Blackbird
 
OldAeroGuy
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RE: Fuel Consumption On Altitude & Mach-Number

Sun Sep 21, 2008 3:33 pm



Quoting Blackbird (Thread starter):
My question has to do with the fact that as Mach-number increases, so does ram-air pressure, which in turn increases thrust

While gross thrust can increase with air speed, so does ram drag. If you subtract one from the other, you'll find that net thrust dereases with airspeed for a constant throttle setting.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
Blackbird
Topic Author
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Joined: Wed Oct 06, 1999 10:48 am

RE: Fuel Consumption On Altitude & Mach-Number

Sun Sep 21, 2008 10:25 pm

OldAeroGuy,

I wasn't just talking about airspeeds, I was also talking about mach numbers too, technically at a high enough altitude I could be doing Mach 25 and read only 7 kts (upper edge of atmosphere)...


Blackbird
 
tdscanuck
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Joined: Wed Jan 11, 2006 7:25 am

RE: Fuel Consumption On Altitude & Mach-Number

Mon Sep 22, 2008 6:15 am



Quoting Blackbird (Reply 2):
I wasn't just talking about airspeeds, I was also talking about mach numbers too, technically at a high enough altitude I could be doing Mach 25 and read only 7 kts (upper edge of atmosphere)...

You need to be a little careful when you get out in the outer edges of the atmosphere...when the air gets thin enough for rarified gas dynamics to start applying (i.e. the individual atoms have almost no impact on each other) most of your fluid equations (which assume a continuum) go out the window.

Tom.
 
Blackbird
Topic Author
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Joined: Wed Oct 06, 1999 10:48 am

RE: Fuel Consumption On Altitude & Mach-Number

Mon Sep 22, 2008 5:34 pm

Tdscanuck,

I was trying to make a point that I was largely focused on the ram-recovery due to mach numbers.

I wasn't actually serious about what kind of performance you'd actually get out of an air-breathing aircraft at that height...


Blackbird

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