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National Advisory Committee for Aeronautics, Research Memorandum - An Experimental Investigation of NACA Submerged Air Inlets on a Scale Model of a Fighter Airplane

The results of an experimental investigation of an NASA submerged—
air-inlet system on a l/5-scale model of a fighter airplane are pre-
sented. Preliminary developmental tests were conducted to select the
Optimum entrance configuration. Duct-system total-pressure los'ses and
pressure distributions over the lip and ramp of this air intake were
obtained. An estimate of the dynamic pressure recovery at the entrance
to the Jet engine and critical Mach number of the inlet for the fighter
airplane is made. It is shown that the inlet location investigated is
unsatisfactory.

In conjunction with the general investigation being conducted by
the EAGA on jet—engine air inlets the develOpment of a submerged—type
inlet has been undertaken. The initial experimental work on this inlet
can be considered as having consisted of two interdependent phases:

(1) basic experimental investigations which were conducted on an
isolated inlet mounted in a small wind channel (reference 1), and

(2) wind-tunnel studies of complete submerged-inlet systems on scale
models of two suitable aircraft. The results from the second phase have
been published, in part, as reference 2, but due to the exigencies of
wartime wind-tunnel Operation, the remaining data, obtained from a
l/5-scale model of a fighter airplane, never progressed beyond preliminary
form. Because of the considerable interest now existing in NASA sub-
merged air inlets, the results of the l/5—scale-model investigation are
presented herein.

It will be noted that the plan-form shape of the approach (ramp)
to the submerged entrance used for this investigation is not the shape
recommended as optimum in reference 1. The submerged-air—inlet system
for the l/5—scale model of the fighter airplane was designed prior to
the completion of the first phase, and the data upon which the recom—
mendations of reference 1 are based were obtained subsequent to the wind—
tunnel investigation of this inlet application. The difference in ramp
plan forms, which probably decreased the dynamic pressure recovery 2 to
6 percent in the low-inlet—velocity ratio range (Vs/v0 < 0.7) in no way
reduces the value of these data as a guide for future submerged—inlet
applications.