Quest for Performance: The Evolution of Modern Aircraft
Chapter 7: Design Trends
Zero&endash;Lift Drag Coefficient and Skin Friction Parameter
[158] The value of the zero-lift drag coefficient CD,0 is often used as an indicator of the aerodynamic cleanness or refinement of an aircraft. Values Of CD,O calculated according to the methods of appendix C are shown as a function of years in figure 7.7. The lower bound of CD,0 drops sharply from a value of about 0.040 in 1920 to a value of about 0.021 in the early 1930's. A smaller reduction in the lower bound values Of CD,0 took place in the years between the early 1930's and the years of World War II. The general aviation aircraft of today show a spread in the values Of CD,O from near the upper bound to near the lower bound. The lower bound curve shows the dramatic reduction in CD,0 that accompanied the basic change in airplane configuration from a strut-and-wire-braced biplane with a fixed landing gear to the highly streamlined, internally braced monoplane with retractable landing gear. As indicated in chapter 4, this transformation had largely taken place for high-performance operational aircraft by the early 1930's. Detailed aerodynamic refinements such as described in chapter 5 were responsible...

chart illustrating trends in zero liftdrag coefficient from 1920 to 1980
[159] Figure 7. 7 - Trends in zero-lift drag coefficient of propeller-driven aircraft.

....for further improvements in aerodynamic efficiency as indicated by the lower bound curve. The zero-lift drag coefficient, although useful as a measure of comparative aerodynamic refinement, has a basic limitation because the coefficient is based on wing area, and, for a given wing area, many different fuselage and tail sizes may be employed. Thus, differences in zerolift drag coefficients may be interpreted as a difference in aerodynamic refinement when the difference may result from a significant difference in the ratio of wetted area to wing area.
In order to remove the effect of variations in the ratio of wetted area to wing area, a zero-lift drag coefficient based on total wetted area rather than wing area was estimated in reference 90 for most of the aircraft for which drag data are given in figure 7.7. The reference area for this coefficient, termed the skin friction parameter CF (* read me) consisted of the total surface area of the fuselage, wings, and tail surfaces. The parameter CF (*) was obtained from multiplication of CD,O (**) by the ratio of wing area to total wetted area. Values of CF (*), taken from reference 90 are shown as a function of years in figure 7.8. The upper and lower [160] bounds of the data show the same trends as do those for the zero-lift drag coefficient shown in figure 7.7. The lower bounds of the skin friction parameter indicate that essentially no progress has been made in reducing CF (*) since World War II, and little progress has been made since the early 1930's. The data for the current general aviation aircraft fall generally between the upper and lower bounds but do not reach as low a value as that of the lower bound curve. This suggests that these aircraft can be refined to a value at least as low as that achieved during World War II. There is little likelihood, however, that values of CF (*) significantly lower than the lower bound shown in figure 7.8 can be achieved unless some breakthrough is made that permits the achievement of a significant extent of laminar flow on the aircraft. Other than reductions in the value of the skin friction parameter, future reductions in the airplane zero-lift drag coefficient CD,O (**) can perhaps be achieved through configuration design aimed at reducing the ratio of wetted area to wing area. The pure flying wing represents the ultimate improvement by this means.

chart illustrating trends in skin friction Parameter from 1920 to 1980
Figure 7.8 - Trends in skin friction Parameter CF (*), of propeller-driven aircraft. [ref. 90]


* CF should read skin friction parameter. ** CD,O should read zero lift drag coefficient [Chris Gamble, html editor]