Hawk,
Thats the overall total efficency;
n = (speed)/(TSFC * Specific energy fuel)
As for the TFE731-2 engine (not -20) I have some data on it.
SLS SFC = 0,493
Cruise SFC = 0,87
With
alatriste's FF of 817, that should give the TFE731
about 940 lb thrust in cruise - or about 25% of rated like you suggested
I’m used to is TSFC as in “Thrust specific fuel consumption”. That is, lbs of fuel burned, per lb of thrust produced, per hour.
Small typo there;
TSFC = lb/hr/lbf or kg/hr/kN
Cheers,
M