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National Advisory Committee for Aeronautics, Report - The Development and Application of High Critical Speed Nose Inlets

An analysis of the nose-inlet shapes dereloped in previous
investigations to represent the optimum from the standpoint of
critical speed has shown that marked similafity exists between
the nondimensional profiles of inlets which have unidely ,difler—
ent proportions and, critical speeds. With the nondimensibnal
similarity of such profiles established, the large diflerences
in the critical speeds of these nose inlets must be a function of
their proportions.

An investigation was undertaken in the Langley 8 foot
high-speed tunnel to establish the efects of nose-inlet propor-
tions on critical hfach number and to develop a rational method
for the design of higMnIical—speed nose inlets to meet desired
requirements. The nondimensional ordinates of the B nose
inlet, which were developed in a previous investigation to be
optimum from the standpoint of critical speed, were extended
and modified slightly to improve the fairing. These ordinates,
now designated the NACA I—series, were then applied to a
group of nose inlets involving a systematic variation of pro-
portions. Wind-tunnel tests of these nose inlets were made
through wide ranges of inlet—velocity ratio and angle of attack
at Mach numbers of 0.8 and 0.4. Tests of representative
nose inlets were carried to high speed (a maximum hfach
number of 0.7’). Pressure distributions and critical M'ach
number characteristics are presented for each of the nose inlets
tested. The results of these tests show that the length ratio
(ratio of length to maximum diameter) of the nose inlet is the
primary factor governing the maximum critical speed. The
efiect of inlet-diameter ratio (ratio of inlet diameter to maxi-
mum diameter) on critical speed is, in general, secondary;
but this ratio has an important function in governing the extent
of the inlet-oelocity—ratio range for marimum critical speed.
The highest critical Mach number attained for any of the nose
inlets tested was 0.89.