View Full Version : Jet Exhaust Velocity
MMEMatty
16th Nov 2005, 16:04
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
popay
16th Nov 2005, 17:43
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.
:8
barit1
16th Nov 2005, 18:51
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.
Does this (http://www.allstar.fiu.edu/aero/Propulsion2.htm) help?
MMEMatty
18th Nov 2005, 09:06
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
PPRuNeUser0215
18th Nov 2005, 15:05
Check this link... (http://www.abrutis-videos.com/video.php?id=987)
:ok: :p ;)