Jet Exhaust Velocity
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Jet Exhaust Velocity
Hi All
Just wondering if there was a formula for converting lbs of thrust into an actual velocity (either in m/s, km/hr, or whatever)
Got a piece of work that hinges on this, so don't let me down!
Many thanks!
Matty
Just wondering if there was a formula for converting lbs of thrust into an actual velocity (either in m/s, km/hr, or whatever)
Got a piece of work that hinges on this, so don't let me down!
Many thanks!
Matty
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An approximate equation for calculating the net thrust of a jet engine is:
Fnet = m(vjfe - va )
where:
m = intake mass flow
vjfe = fully expanded jet velocity (in the exhaust plume)
va = aircraft flight velocity
While the m·vjfe term represents the gross thrust of the nozzle, the m·va term represents the ram drag of the intake. Most types of jet engine have an air intake, which provides the bulk of the gas exiting the exhaust. There is, however, a penalty for picking this air up and this is known as the ram drag.
Obviously, the jet velocity must exceed that of the flight velocity, if there is to be a net forward thrust on the airframe.
However for calculation purposes you can assume thrust equal speed, ergo you can calculate the speed if you know the trust and the bypass ratio or the amount (Mass of the air) going through the engine.
Cheers.
Fnet = m(vjfe - va )
where:
m = intake mass flow
vjfe = fully expanded jet velocity (in the exhaust plume)
va = aircraft flight velocity
While the m·vjfe term represents the gross thrust of the nozzle, the m·va term represents the ram drag of the intake. Most types of jet engine have an air intake, which provides the bulk of the gas exiting the exhaust. There is, however, a penalty for picking this air up and this is known as the ram drag.
Obviously, the jet velocity must exceed that of the flight velocity, if there is to be a net forward thrust on the airframe.
However for calculation purposes you can assume thrust equal speed, ergo you can calculate the speed if you know the trust and the bypass ratio or the amount (Mass of the air) going through the engine.
Cheers.
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It's related to the flow temp. and Mach no. - so the fan nozzle with its colder flow has a lower velocity than the hot core exhaust.
Trying to work it backwards from mass flow and thrust seems the wrong way around. A small engine with reheat will certainly have a greater exhaust velocity than a large high-bypass commercial fan.
Trying to work it backwards from mass flow and thrust seems the wrong way around. A small engine with reheat will certainly have a greater exhaust velocity than a large high-bypass commercial fan.
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Cheers everyone, that is great help, especially ORAC. I have googled such things but not found that site, so cheers for that.
Now all i have to do is work it into my CFD grid, and perform a lot of crazy calculations on it!
Matty
Now all i have to do is work it into my CFD grid, and perform a lot of crazy calculations on it!
Matty
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