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National Advisory Committee for Aeronautics, Report - Preliminary Investigation of a New Type of Supersonic Inlet

The supersonic inlet with supersonic deceleration of the flow
entirely outside of the inlet is considered. A particular arrange—
ment withfired geometry having a central body with a circular
annular intalce is analyzed, and it is shown theoretically that
this arrangement gives high pressure recorery for a large range
of fllach number and mass flow and therefore is practical for
use on supersonic airplanes and missiles. For some fifach
numbers the drag coeficient for this type of inlet is larger than
the drag coefiicient for the type of inlet with supersonic com—
pression entirely inside, but the pressure recovery is larger for
all flight conditions. The diferences in drag can be eliminated
for the design Illa-ch number.

Experimental results confirm. the results of the theoretical
analysis and show that pressure recoveries of 95 percent for
mach numbers of 1.83 and 1.52, 92 percent for a Jlach number
of 1.72, and 86 percent for a fifach number of 2.10 are possible
with the configurations considered. If the mass flow decreases,
the total drag coefficient increases gradually and the pressure
recovery does not change appreciably. The results of this work
were first presented in a classified document issued in 1946.

The deceleration of air from supersonic to subsonic Mach
numbers is an important. problem that is encountered in the
design of supersonic ram jets and turbojets. The possibility
of the. practical use of the ram-jet or turbojet systems for
airplanes depends to a large extent on the high pressure
recovery of the air inlet.

An inlet designed so that all the supersonic part of the
compression was internal was analyzed in reference 1_, which
showed that. high pressure recovery is possible if the Mach
number-corresponds to the design Mach number and the
mass flow corresponds to the design mass flow. Reference 1
also showed that. the pressure recovery decreases notably
and abruptly if the Mach number and the mass flow which
must go into the inlet decrease from the values fixed for
the design conditions. The external drag which can be
very small for the design conditions (fig. 1 (3.)) changes
suddenly, also, and becomes very large when the hIach
number or the mass flow decreases from the. values fixed for
the design conditions because in this case a strong shock
occurs in front of the inlet (fig. 1 (b)). The pressure recovery;
also decreases rapidly if the hlach number is increased in
comparison With the design Mach number.