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National Advisory Committee for Aeronautics, Technical Notes - 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 impose restric-
tions 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 airy
induction system as a component of a propulsive unit, it is necessary to
employ an evaluation parameter that represents 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,
tactical purpose of the aircraft, and.many others. Obviously, such a
universal parameter is difficult to derive and even more difficult to
apply. For this reason, it is convenient to make a partial evaluation
based on the aerodynamic considerations before attempting a general eval-
uation. In such a case, the net thrust or the net thermal efficiency can
be used as figures of merit because they provide a measure of the aero—
dynamic 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 efficiency may'be obtained from the net thrust, the flight veloc-
ity, and the rate of fuel consumption.

The maximum net thrust and thermal efficiency attainable with a jet-
engine installation depend greatly on the performance of the air—induction
system employed. The characteristics 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 parameters. Because of this interdependence, it is
necessary to combine the induction system and power-plant characteristics
so as to obtain a. single figure of merit for the complete installation.
By comparing the figures of merit, it is possible to establish the rela-
tive aerodynamic worth of each of the air-induction systems considered
when they are used with a given engine.