# Jet engine performance

In fixed-wing aircraft driven by one or more jet engines, the performance of the jet engine is important to the operation of the aircraft and includes measurement of thrust, fuel consumption, noise and engine emissions.

The thrust, noise and emission elements of the performance of a jet engine are of vital importance in the takeoff phase of operation of the aircraft. The thrust and fuel consumption elements, and their variation with altitude and airspeed, are of vital importance in the climb and cruise phases of operation of the aircraft.

## Thrust

The thrust from a jet engine is determined by the velocity of exhaust gases measured relative to the engine.

The velocity of the exhaust gas as it leaves a jet engine is $V_j\;$:[1]

$V_j^2=2(h_4-h_5)$

where

 $h_4\;$ is the specific enthalpy of the gas leaving the turbine and entering the propelling nozzle $h_5\;$ is the specific enthalpy of the gas after it has left the propelling nozzle

Alternatively, $V_j\;$ is related to the temperatures of the exhaust gas:[1]

$V_j^2=2 C_{p0} (T_4-T_5)$

where

 $C_{p0}\;$ is the specific heat of exhaust gas at constant pressure $T_4\;$ is the temperature of the gas leaving the turbine and entering the propelling nozzle $T_5\;$ is the temperature of the gas after it has left the propelling nozzle

## Notes

1. ^ a b G. Van Wylen and R. Sonntag, Fundamentals of Classical Thermodynamics, Section 9.18

## References

• Kerrebrock, Jack L. (1992), Aircraft Engines and Gas Turbines, The MIT Press, Cambridge, Massachusetts USA. ISBN 0-262-11162-4
• G. Van Wylen and R. Sonntag (1985), Fundamentals of Classical Thermodynamics, 3rd edition, John Wiley & Sons, New York NY ISBN 04718293311