Page 225 - The Jet Engine

21: Performance

Contents Page

Introduction 215
Engine thrust on the test

bench 217

Comparison between thrust and
horse-power
Engine thrust in flight 218
Effect of forward speed
Effect of afterburning on engine
thrust
Effect of altitude
Effect of temperature
Propulsive efficiency 223

Fuel consumption and

power-to-weight relationship 225

2. Since the thrust or s.h.p. developed is dependent
on the mass of air entering the engine and the accel-
eration imparted to it during the engine cycle, it is
obviously influenced, as subsequently described, by
such variables as the forward speed of the aircraft,
altitude and climatic conditions, These variables
influence the efficiency of the air intake, the
compressor, the turbine and the jet pipe; conse-
quently, the gas energy available for the production
INTRODUCTION of thrust or s.h.p. also varies.
1. The performance requirements of an engine are 3. In the interest of fuel economy and aircraft range,
obviously dictated to a large extent by the type of the ratio of fuel consumption to thrust or s.h.p. should
operation for which the engine is designed. The be as low as possible. This ratio, known as the
power of the turbo-jet engine is measured in thrust, specific fuel consumption (s.f.c.), is expressed in
produced at the propelling nozzle or nozzles, and pounds of fuel per hour per pound of net thrust or
that of the turbo-propeller engine is measured in s.h.p. and is determined by the thermal and
shaft horse-power (s.h.p.) produced at the propeller propulsive efficiency of the engine. In recent years
shaft. However, both types are in the main assessed considerable progress has been made in reducing
on the amount of thrust or s.h.p. they develop for a s.f.c. and weight. These factors are further explained
given weight, fuel consumption and frontal area. in para. 46.

215