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National Advisory Committee for Aeronautics, Report - Theoretical and Experimental Analysis of Low Drag Supersonic Inlets having a Circular Cross Section and a Central Body at Mach Numbers of 3.30, 2.75, and 2.45

A discussion of inlets having a circular cross section and a
central body, designed for high Mach numbers, has been made.
The optimum relationship between external and internal super-
sonic compression has been discussed with respect to the 62716771111
drag and the maximum pressure recovery. Practical design
criteria have been given and tests of inlet configurations designed
from these criteria have been presented for Mach numbers of
3.30, 2.75, and 2.45 and for corresponding Reynolds numbers
ranging from 3.5 X10“ to 3.0X10“. Values of maximum
pressure recovery and shadmvgraphs for difl'erent combinations
of central body and cowling shape and for different positions of
central body relative to cowling have been given. The'results of
the tests have been analyzed and show that, with the proper
selection of geometric and aerodynamic parameters, high pres-
sure recovery and low drag can be obtained. Pressure recoveries
of 0.57, 0.67, and 0.78 at Mach numbers of 8.30, 2.75, and
2.46, respectively, have been obtained with very low external drag.

In the design of supersonic inlets for ram jets and turbojets,
two parameters are of fundamental importance: the maxi—
mum pressure recovery obtainable from the inlet for a given
Mach number and the drag due to the aerodynamics]
phenomena of the inlet. The relative importance of the
two parameters depends on the practical application being
considered; therefore, an exact discussion of the optimum
inlet design 'is not possible. However, in general, it is
possible to show that the drag due to the deceleration of
flow from high speeds to low speeds becomes more important
when the free-stream Mach niunber increases.

Usually for ram-jet burners and turbojet compressors, the
flow must be decelerated to speeds of the order of 0.2 to 0.3
of the speed of sound. Therefore, at low free-stream super-
sonic Mach numbers of 1.4 to 2.0, the maximum cross-
sectional area is at the burner or the compressor. '(For the
turbojet the actual Mach number in front of the compressor
is usually higher than the values of 0.2 and 0.3 previously
mentioned, but the total cross section, if the central part
of the compressor is also considered, corresponds to Mach
numbers of that order.) For this reason, an external shock
drag always exists regardless of the type of inlet considered.