Expert: Mark Janus - 6/11/2007

Question
hi, do you know what percentage of the forward thrust of a modern commercial jet's turbofan engine comes from the core exhaust exiting the nozzle and what percentage comes from the fan exhaust which bypasses the core turbine?  does the throttle or size of the nozzle affect this ratio?

thanks!

Answer
Had to look this up.....   (see Mattingly - Elements of Gas Turbine Propulsion)...

The formula for this rather simple...  FR = Fc/mdotC  / Ff/mdotf....

but it depends on a lot of variables...  bypass ratio, fan pressure ratio, compressor pressure ratio   

The other formulae dealing with this are much more involved.

Now for your real question...  theoretically for optimum fuel comsumption (which I imagine airlines value greatly) the FR =1/2  (i.e. the core thrust to mass flow is half that of the fan thrust to mass)

Check out Mattingly's book  (just don't get dizzy with all the equations)... :)

Hope this helps and sorry for the delay,
mj