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National Advisory Committee for Aeronautics, Research Memorandum - An Experimental Investigation of the Design Variables for NACA Submerged Duct Entrances

Information concerning the parameters and design variables
affecting an NAGA submerged duct design is presented. The prin—
cipal variables investigated include entrance‘width—to—depth ratio,
rampawall divergence, ramp angle, and deflector size. Tests were
also made to show the effect of variation of boundary—layer thick—
ness and ramp—floor contour.

Pressure recovery at the duct entrance and after slight
diffusion, pressure distribution over the lip and ramp, and drag
are given as functions of the inlet velocity ratio of the entrance.
An evaluation of the NACA submerged entries indicates that satis—
factory duct characteristics may be found for a range of the test
variables. It appears that an optimum NACA submerged inlet design
should employ curved diverging ramp walls, a 5° to 7° ramp angle,
and a width-to—depth ratio of from 3-to 5. The boundary—layer
thickness of the surface into which the inlet is placed was found
to have a large effect on the pressure recovery.

Possible applications,of this type of inlet and their
particular advantages are discussed.

For the development of a satisfactory air—induction system of
an aircraft, several aerodynamic criteria must be evaluated in con—
Junction with those involving structural design and installation.
Aerodynamically, the system should not reduce the available energy
of the entering air, the drag of the body into which it is placed
should not be increased, and the high—speed characteristics of the
body or aircraft should not deteriorate. Although, in practice, an
airhdnduction system possibly does not meet all these requirements,
the merits of a system can be determined by the degree to which its
characteristics approach the optimum.

A previous investigation of an air intake submerged below the
body surface (reference 1) was exploratory in nature and was meant
to indicate the trend for future research of this type inlet. This
present report gives the results of more extensive investigations of
NACA submerged duct entrances conducted at the Ames Aeronautical
Laboratory. The Work includes further developnent of certain con-
figurations found to be desirable from preliminary tests and the
investigation of other design parameters-not previously considered.