Definitions

This section defines various terms used in jet-engine performance analysis. Refer­ences [3] through [6] 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 [2] through [4]) 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.

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