High thermal efficiency in airplane service Page: 4 of 8
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It will be noted that a decrease in the specific fuel con-
sumption of over 15 per cent is secured when the mixture
is leaned until there is a decrease of 10 horsepower in 150,
i.e., 7 per cent. Unquestionably then, so long as this type
of control has sufficient range, its proper handling will
result in a marked fuel saving. Will it receive such hand-
ling? To realize how unlikely this is, it must be remem-
bered that continuing the mixture impoverishment will ult-
imately result in a blowback in the carburetor, a likely
cause of fire. Knowing that safety depends on not reaching
this condition and lacking knowledge as to how close to it
a given carburetor setting is, the pilot has every incent-
ive to adjust away from, rather than toward, maximum effio-
iency. Even were it possible to eliminate the fire hazard,
the problem would be far from solved. In flight, the only
measure of performance ordinarily available is that of power
as indicated by the engine speed. In spite of all evidence
as to the benefit of the lean mixture from the standpoint
of efficiency, such an adjustment, inasmuch as it results
in lower power, the only gauge of performance available to
the pilot, is bound to be unnatural.
The explanation of the disadvantages of the first type
makes clear the merits of the second. With this, as the
mixture becomes.of poorer quality, the amount supplied is
increased. The natural adjustment, that for maximum power,
Nill be the one at which the decrease in quality ceases to
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Sparrow, S. W. High thermal efficiency in airplane service, report, December 1920; (digital.library.unt.edu/ark:/67531/metadc53714/m1/4/: accessed September 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.