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National Advisory Committee for Aeronautics, Technical Notes - Wind Tunnel Tests of Carburetor Intake Rams

A carburetor—intake ram is a device used to increase
the carburetor~intake pressure through the use of the dyn
namic pressure due to forward motion. This increase in
carburetor—intake pressure results in several increases in
engine performance, namely, increa_se_d fu_1_l- throttle pewer
at the same altitude, increased critical altitude, and pos—
sible increased cruising altitude.

The use of carburetor intake rams is not new for many
installations may be observed. The rams in use fall into
two classes: the internal constantuarea type and the exe
ternal constant~area type. In general, the internal con-
stant—area type consists—cf one or more ducts passing be—
twsen tho engine cylinders to the carburetor and having an
entrance so located as to obtain air for the carburetor
from within the forward pertion of the engine cowling.
Several entrance locations are in.use, some near the nose
of the cowling, some near the eng_ine cylinders, some pro~
jecting forward, and some at an angle to the axis of the
engine and cowling. The external constant- area type con~
sists, in general, of a duct leading to the carburetor
with an entrance located somewhere on_ _the outsid_e_ of the
cewling. Here again, severa1 e_ntran_ce locations are used.
Although most entrances are on the engine cow1in_g itself,
others are to be found on the for_ward portion of the fusea
lage or nacelle and a few in the leading edge of the wing,
close to the fuselage or nacelle.

From the variety of installations observed and from
the lack of published information, it is appa_rent_ that
there is very little informati_en available concerning the
proper entrance location and the rammin_g eff;ect of the
various types _of carburetor—intake rams.

The present investigation was made in conjunction
with propeller tests conducted in the N. A. C. A. 2Q-foot
wind tunnel.

If the effect—of the propeller is neglected, it is
apparent that the maximum pressune obtainable from the
air stream is the dynamic press ure duo t_o the forward ve*
locity of. the airplane. Not all of this dynamic pressure
is available for increasing the carburetor—intake pres—
sure, however. since losses occur in the carburetor— intake
ram. Friction losses are preser nt in _every ram and cannot
be eliminated. Another loss exists in the form of an en-
trance loss, part _of which is due to the entrance shape
and location and part to. the fact. that the ve1ocity of
the air entering the ram must oe reduced from that of the
air stream to the velocity required the carburetor.