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National Advisory Committee for Aeronautics, Report - Method and Graphs for the Evaluation of Air Induction Systems

Graphs have been developed for rapid evaluation of air-
induction systems from considerations of their aerodynamic—
performance parameters in combination with power-plant
characteristics. The graphs cover the range of supersonic Mach
numbers up to 3.0. Examples are presented for an air-induction
system and engine combination at two Mach numbers and two
altitudes in order to illustrate the method and application of the
graphs. The examples show that jet-engine characteristics im-
pose restrictions on the use of fixed inlets if the maximum net
thrusts are to be realized at all flight conditions.

In order to obtain a true indication of the worth of a given
air-induction system as a component of a propulsive unit,
it is necessary to employ an evaluation parameter that repre-
sents a summation of all the gains and penalties resulting
from the use of that particular system. Such a parameter
should consider not only the aerodynamics of the entire
installation but also such factors as the weight, mechanical
complexity, purpose of the aircraft, and manv others.

Obviously, such a universal parameter is difficult to derive
and even more diflicult to apply. For this reason, it is con-
venient to make a partial evaluation based on the aero—
dynamic considerations before attempting a general evalua-
tion. In such a case, the net thrust or the net thermal
efliciency can be used as figures of merit because they pro-
vide a measure of the aerodynamic and thermodynamic
qualities of the installation. The net thrust represents the
force remaining after subtraction of the drag chargeable to
the propulsive system from the thrust that it develops. The
net thermal efliciency may be obtained from the net thrust,
the flight velocity, and the rate of fuel consumption.

The maximum net thrust and thermal efliciency attainable
with a jet—engine installation depend greatly on the per-
formance of the air-induction system employed. The char-
acteristics of air-induction systems are usually presented in
terms of total-pressure recovery, external drag coefficient,
and mass-flow ratio. Unless all three of these parameters
for one system excel those for another at supersonic speeds,
it is difficult to choose the better system because of, the
interdependence of the engine and induction-system para—
meters.