Quest for Performance: The Evolution of Modern Aircraft
Chapter 7: Design Trends
Maximum Lift-Drag Ratio
[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.

chart illustrating trends in maximum lift-drag ratio from 1920 to 1980
Figure 7.9 - Trends in maximum lift-drag ratio of propeller-driven aircraft.