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D

Thrust Data forPerformanceCalculations

TURBOJETS

Engine data, for several sample engines, are given for J-60, J52, JT9D-3, JT8D-9, TF-30, TFE731-2, GE F404-400, FJ-44, Allison T-56 turbo-prop, and AVCO Lycoming I0-540 reciprocating engines. Although allthese engines are somewhat dated and all have been superceded to givehigher thrust and better fuel consumption, they do represent typicalperformance trends. As is clearly seen, the thrust and specific fuelconsumption (SFC) curves vary widely with speed and altitude. Thus,there are no general-duty expressions available that would permit car-rying out easy and simple performance calculations. However, severalapproximate expressions, shown below, have been used with some en-gineering success. As a first-order rough approximation, engine thrustcan be scaled linearly (for similar engines) and the fuel consumptioncan be decreased by at least 5 percent.

The process consists of curve-fitting the thrust equation as a functionof altitude, velocity, or both. Each engine may require its own specialcurve-fitting expression and may have to be accomplished in a piece-wise fashion over the velocity and/or altitude range.

Jet Engines

For subsonic flight the simplest correlation is

Aircraft Performance. Maido Saarlas© 2007 John Wiley & Sons, Inc. ISBN: 978-0-470-04416-2

268 THRUST DATA FOR PERFORMANCE CALCULATIONS

Figure D.1 Pratt and Whitney J-60 Turbojet Engine

Figure D.2 AVCO Lycoming IO-540 Reciprocating Engine

T � T � (D.1)ref

where Tref may be taken as To, the sea level thrust value.A somewhat improved expression is

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Figure D.3 AVCO Lycoming IO-540 Reciprocating Engine

nT � T � (D.2)ref

which is often written as

T n� � h � 36,089 ftTo

� � h � 36,089 ft

A better correlation, but more cumbersome to curve fit and to use, is

T 2� (A � BV )� (D.3)To

The advantage of Eq. D.3 lies in taking into account the realisticthrust variation with velocity. Usually this is not very significant athigher altitudes and velocities but may be 10 percent or more during

270 THRUST DATA FOR PERFORMANCE CALCULATIONS

Figure D.4 Pratt and Whitney TF-30 Turbofan Engine

the take-off portion of flight (see TF-30 and JT9D data). At higherspeeds, another correlation that has been used is

T� (1 � cM)� (D.4)

To

where, typically, .25 � c � .5.Specific fuel consumption varies with both altitude and velocity and

defies generalization with both of those parameters. It has been foundthat the velocity effect can be correlated for some engines, very ap-proximately, by

TSFC� 1 � .5M (D.5)

TSFCref

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Figure D.5 Williams/Rolls FJ-44 Turbofan Engine

Reciprocating Engines

Reciprocating engines admit more generalizations:

• BHP is independent of velocity V.• SFC tends to be independent of both velocity and altitude.

For engine brake horsepower, the commonly accepted altitude vari-ation is

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Figure D.6 Garrett TFE-731-2 Turbofan Engine

BHP� 1.132� � .132 (D.6)

BHPo

where subscript o refers to the sea-level value.For supercharged engines, it is assumed that BHP remains constant

to at least 25,000 ft altitude. Correlations used for higher altitude su-percharged engines are:

BHP .765� � , h � 36,089 ftBHPo

� 1.331�, h � 36,089 ft (D.7)

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Figure D.7 GE F404-400 Installed Performance

274 THRUST DATA FOR PERFORMANCE CALCULATIONS

Figure D.8 Pratt and Whitney JT8D-9 Turbofan Engine

Figure D.9 Pratt and Whitney JT9D-3 Turbofan Engine

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Figure D.10 Pratt and Whitney J52 Turbojet Engine

Figure D.11 Allison T-56-A Turboprop Engine, Horsepower

276 THRUST DATA FOR PERFORMANCE CALCULATIONS

Figure D.12 Allison T-56-A Turboprop Engine, Thrust

Figure D.13 Allison T-56-A Turboprop Engine, Specific Fuel Consumption