Originally Posted by
Fursty Ferret
To add to the thrust measurement topic, I think you can also assume that an EPR of less than 1 is an indication that the engine is generating more drag than thrust (would be good to get confirmation of this point).
I can say that an EPR less than 1.0 almost
never means 'negative net thrust'. With an EPR less than 1.0, yes the pressure of the core exhaust is less than the inlet, but it is much, much hotter - and hot air expands, so it can still be producing thrust. Further, since the vast majority of the thrust comes from the fan, and it's almost unheard of for the Fan Pressure Ratio ("FPR" - pronounced 'Feeper') to be less than 1.0.
It is
extremely rare that an engine that is still running (idle or above) is not providing some positive net thrust.