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Quest for Performance: The Evolution
of Modern Aircraft
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- Part I: THE AGE OF
PROPELLERS
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- Chapter 7: Design Trends
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- Maximum Lift-Drag
Ratio
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- [161] The maximum
lift-drag ratio of the various aircraft was calculated according
to the methods described in appendix C and is shown as a function of years in figure 7.9.
The value of the maximum lift-drag ratio (L/D)max. is a measure
of the aerodynamic cruising efficiency of the aircraft. The upper
bound of (L/D)max varies from
values of about 9 in 1920 to a value of 16.8 for the World War II
Boeing B-29 and 16.0 for the Lockheed 1049G in 1952. The
(L/D)max upper-bound curve shows a sharp rise between 1920
and the early 1930's, which corresponds to the reduction in
zero-lift drag coefficient shown in figure 7.7 and to the
emergence of the monoplane with its higher aspect ratio as
compared with the biplane. Little change in maximum L/D has taken
place since the end of World War II. Any further increases in
maximum lift-drag ratio will require reductions in the value of
the zero-lift drag coefficient and/or increases in wing aspect
ratio that may be possible through the use of improved structural
materials.
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- Figure 7.9 - Trends in maximum
lift-drag ratio of propeller-driven aircraft.
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