Quest for Performance: The Evolution
of Modern Aircraft
Part I: THE AGE OF
PROPELLERS
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...
[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.
Figure 7.8 - Trends in skin friction Parameter
CF (*), of propeller-driven aircraft.
[ref. 90]
* CF should
read . **
CD,O should read [Chris Gamble, html editor]