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Blackbird
Topic Author
Posts: 3384
Joined: Wed Oct 06, 1999 10:48 am

Ram Compression's Increase On Thrust

Wed Jul 04, 2007 8:31 am

I'm just curious how much of an effect subsonic ram-compression has on jet engine thrust.

If I recall the GE CF-6 produced around 50,000 lbf at sea level and 11,000 at altitude, I assume these figures are both static... how much is gained through ram-compression?

Also, which profit more from ram-compression
-Centrifugal Flow Turbojet
-Axial Flow Turbojet
-Turbofan (Low Bypass)
-Turbofan (High Bypass)

Andrea Kent
 
113312
Posts: 673
Joined: Sat Apr 16, 2005 9:09 am

RE: Ram Compression's Increase On Thrust

Wed Jul 04, 2007 11:28 am

Actually, it is less. The thrust equation is related to the net change of velocity between the inlet and tail pipe so static thrust will always be greater than thrust achieved with the engine inflight.
 
Blackbird
Topic Author
Posts: 3384
Joined: Wed Oct 06, 1999 10:48 am

RE: Ram Compression's Increase On Thrust

Wed Jul 04, 2007 12:39 pm

Well how does a 300,000 pound plane manage with just 3 x 10,000 lbs of engine power at altitude?

Andrea K
 
eelonghorn
Posts: 43
Joined: Wed Apr 19, 2006 4:52 am

RE: Ram Compression's Increase On Thrust

Wed Jul 04, 2007 1:31 pm

Weight does not matter. If 30,000 lbs of thrust is enough to overcome drag to produce enough forward airspeed to make 300,000 lbs of lift, that is all you need.
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Ram Compression's Increase On Thrust

Wed Jul 04, 2007 10:09 pm

Going even further, and looking back aways in earlier times, when the first fan engines began to appear on airline jets, it was found by many pilots that they could not cruise at the same altitudes/weights as they could before with the turbojets...it was lower with the fan powered airplanes.
These early fan engines (JT3D is a perfect example) lost a good bit of thrust up high versus their straightpipe counterparts, and this surprised many...it shouldn't have of course, but it did.
Conversely, if on the other hand, the straightpipe powered airplane was kept low by ATC, expected fuel consumption was a lot higher, and often a tech stop needed to be made as a result.
 
Blackbird
Topic Author
Posts: 3384
Joined: Wed Oct 06, 1999 10:48 am

RE: Ram Compression's Increase On Thrust

Thu Jul 05, 2007 2:12 am

113312,

Yeah but doesn't the inlet lip slow the airflow down before it reaches the engine. Wouldn't that provide some degree of ram-compression?

Andrea K
 
grandtheftaero
Posts: 247
Joined: Sat Nov 29, 2003 1:05 pm

RE: Ram Compression's Increase On Thrust

Thu Jul 05, 2007 11:06 pm

Quoting Blackbird (Reply 5):
Yeah but doesn't the inlet lip slow the airflow down before it reaches the engine. Wouldn't that provide some degree of ram-compression?

In subsonic inlets, ram compression should not be confused with thrust benefit. Static pressure does increase as a result of the inlet slowing down the flow however this results in drag; the aptly named "ram drag". And to be precise the inlet lip does not slow the flow, that's the job of the diffuser, which is located just upstream of the fan/compressor face. The local flow near the inlet lip is actually accelerated.

--Shane
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Ram Compression's Increase On Thrust

Fri Jul 06, 2007 7:41 am

Quoting Blackbird (Reply 2):
Well how does a 300,000 pound plane manage with just 3 x 10,000 lbs of engine power at altitude?

If it has an L/D of at least 10:1, it will maintain altitude and airspeed. Most airliners can manage something closer to 20:1, but usually at something a bit slower than their usual cruising speed.

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