This section defines various terms used in jet-engine performance analysis. References  through  may be consulted for derivations of the expressions.
SFC: The fuel-flow rate required to produce one unit of thrust, or shaft horsepower (SHP):
SFC = (fuel flow rate)/(thrust or power) (10.1)
Units of SFC are in lb/hr per pound of thrust produced (in SI units, gm/s/N) – the lower the better. More precisely, reaction-type engines use TSFC and propeller – driven engines use PSFC, where T and P denote thrust and power, respectively.
For turbofan engines (see Section 10.4.2):
Following are the definitions of various types of jet engine efficiencies. The subscripts indicate the gas turbine component station numbers, as shown in Figure 10.4 (in the figure, 5 represents e).
mechanical energy produced by the engine
heat energy of (air + fuel)
V2 – V2 / 1-Y
e ^ = 1 – PR~
2Cp(T — T2) V )
For a particular aircraft speed, VTO, the higher the exhaust velocity Ve, the better is the nt of the engine. Heat addition at the combustion chamber, q2-3 = Cp(T3 — T1) « Cp(Tt3 — Tt 1).
useful work done on airplane mechanical energy produced by the engine Wa = 2VTO
We Ve + VTO
For subsonic aircraft, Ve > VTO. Clearly, for a given engine exhaust velocity, Ve, the higher the aircraft speed, the better is the propulsion efficiency, np. A jet aircraft
flying below Mach 0.5 is not preferred – it is better to use a propeller-driven aircraft flying at or below Mach 0.5.
It can be shown (see  through ) that, ideally, for nonafterburning engines, the best overall efficiency, no, is when the engine-exhaust velocity, Ve, is twice the aircraft velocity, VO. Bypassed turbofans provide this efficiency at high-subsonic aircraft speeds.