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Unformatted text preview: rough World War II, although some air-cooled engines were used in
b ombers and transports, and there was one excellent air-cooled European
fighter, the Focke-Wulf w ith the BMW 2-row radial, developed from a
P ratt & Whitney license. Japanese fighter aircraft also used air-cooled
r adials copied from Wright and Pratt & Whitney designs. Their other
m ilitary aircraft used these and copies of the German Daimler-Benz liquidcooled engine (fig. 33).
I n the United States, the Navy made a commitment to air cooling in
1921 which has held for reciprocating engines to this day. It was chiefly
N avy support that underwrote early Pratt & Whitney and Wright aircooling developments. The reason for this choice lay in the limitations of
t he aircraft carrier, which imposed such design criteria as short takeoff,
c ompact size, and minimum maintenance. Commander Bruce Leighton
was probably the individual most responsible for this well-considered
T he most intense controversy on this subject took place in the United
S tates Army Air Service, whose support for air-cooled engine development
in the 1920s a nd 1930s was never as enthusiastic as that of the Navy, because
of the assumed larger frontal area and greater drag of air-cooled radials,
especially for use in fighter airplanes. That cooling drag was a real problem
in the early days is illustrated by figures 50 and 51, showing typical
i nstallations of the 1920s.
T he drag of air-cooled engines was greatly reduced by the advent of
t he very effective cowling and cylinder baffing developed at Langley Field
by NACA, starting in 1929 (figs. 52 and 53). Further reductions in cooling
d rag were achieved by increased cooling-fin area, which reduced the air
velocity required for cooling (compare figs. 36a-e and 36f. These develop53 tPtu'ilion of
\.-aoseo/ Figure 53.—Cooling-air flow in tractor
i nstallation of a cowled radial engine. Only
the upper half of the installation is shown. Figure 54.—Installation of radial engine on a
Douglas DC-6, 1949, showing modern cowling
for radial engine, with controllable outlet flaps.
(Photo A-50822) Figure 55.—Comparison of radiator installations for water (left) and for ethylene glycol cooling, on
Curtiss Falcon airplanes, 1930. (From The Project Engineer, v ol. 13, no. 10, p. 9, 1954, p ubl. by
t he Thermix Corp.) Figure 56.—Liquid-cooled f ighter, North American P-51 Mustang of World War II. Coolant radiator
is housed under fuselage, below the star insignia. Inlet and outlet ducts are designed to minimize
d rag. (Photo A-45801) m ents p ut the air-cooled radial virtually on a par with the water-cooled
engines with regard to cooling drag, until the advent of high-temperature
l iquid cooling with glycol-water m ixtures. Figure 54 show's m odern cowling
for the air-cooled radial engines.
T he use of high-boiling liquids (mixtures of water and ethylene
glycol) for engines formerly water-cooled was an important forward step
in reducing the heat-transfer area, and thereby the drag, of radiators for
l iquid-cooled engines. At the suggestion of S. D. Heron, a 1-...
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This document was uploaded on 01/19/2014.
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