ch8-9

Quest for Performance: The Evolution of Modern Aircraft

Part I: THE AGE OF PROPELLERS

Chapter 8: Boats in the Sky


Comparative Aerodynamic Efficiency



[212] Because of the size and shape of the hull, the assertion has frequently been made that the aerodynamic efficiency of a flying boat, in any given time period, is inherently less than that of a landplane. To...




[213] Figure 8.25 - Flying-boat symbols used in figures 8.26 and 8.27.




Figure 8.26 - Trends in zero-lift drag coefficient for propeller-driven flying boats.


....provide some quantitative basis for the assessment of the comparative aerodynamic efficiencies of landplanes and flying boats, values of the zero-lift drag coefficient C_D,O and maximum lift-drag ratio (L/D) _max are [214] shown as a function of time in figures 8.26 and 8.27, respectively. The symbols used to identify the various flying boats in the figures are identified in figure 8.25. Values of C_D,0 and (L/D) _max are plotted on the figures for all the flying boats listed in table IV. The bound lines for landplanes shown in the figures were taken from the trends shown in chapter 7 of this volume, as were the data shown for four specific multiengine landplanes.

As was the case with landplanes during the same time period, the data in figure 8.26 show that the value of C_D,O for flying boats rapidly decreased in the years between 1930 and 1940. In comparison with the trend for landplanes, however, the lower bound of C_D,O values for flying boats is significantly higher. For example, the lower bounds of drag coefficient are separated by about 40 percent in the period of the early 1940's. Since some of the data used to form the lower bound for landplanes were for high-performance single-engine aircraft, specific data for four multiengine landplanes are also shown in figures 8.26 and 8.27. Two of the C_D,O points are seen to be close to the landplane lower bound, and two are near the flying-boat lower bound. In general,...




Figure 8.2 7 - Trend in maximum lift-drag ratio for propeller-driven flying boats.

[215]....the trends and data suggest that, in a given time period, zero-lift drag coefficients have been higher for flying boats than for landplanes.

In figure 8.27, the variation of the maximum lift-drag ratio with time shows a rapid increase, beginning in about 1930, followed by a leveling off in the 1940's. Again, the aerodynamic efficiency of the flying boat is seen to be lower than that of its landplane counterpart. Between 1930 and 1942, however, the difference between the two types of aircraft, in terms of (L/D) _max was significantly reduced. For example, the upper-bound line for landplanes was about 40 percent higher in 1930 than the best values of (L/D) _max for flying boats, but this difference had been lowered to about 14 percent by 1942. The high value of (L/D) _max. shown for the Martin JRM-1 resulted not only from its low value Of C_D,O but also from the high aspect ratio, nearly 11, of its wing.

If the large flying boat is ever revived, a major challenge will be to give it acceptable hydrodynamic characteristics while, at the same time, making its overall efficiency and cost effectiveness comparable to contemporary landplanes.