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
Moderators: richierich, ua900, PanAm_DC10, hOMSaR
RE: Ram Compression's Increase On Thrust
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.
RE: Ram Compression's Increase On Thrust
Well how does a 300,000 pound plane manage with just 3 x 10,000 lbs of engine power at altitude?
Andrea K
Andrea K
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- eelonghorn
- Posts: 43
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RE: Ram Compression's Increase On Thrust
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.
RE: Ram Compression's Increase On Thrust
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.
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.
RE: Ram Compression's Increase On Thrust
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
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
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- grandtheftaero
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RE: Ram Compression's Increase On Thrust
| 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
RE: Ram Compression's Increase On Thrust
| 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.