NAVWEPS 00-801-80 
BASIC AERODYNAMICS 
as great a speed or one-fourth as much parasite 
drag at half the original speed. This fact may 
be appreciated by the relationship of dynamic 
pressure with speed-twice as much V, four 
times as much 4, and four times as much D,. 
This expressed variation of parasite drag with 
speed points out that parasite drag will be of 
greatest importance at high speeds and prac- 
tically insignificant in flight at low dynamic 
pressures. To illustrate this fact, an airplane 
in flight just above the stall speed could have a 
parasite drag which is only 25 percent of the 
total drag. However, this same airpfane at 
maximum level flight speed at low altitude 
would have a parasite drag which’ is very 
nearly 100 percent of the total drag. The 
predominance of parasite drag at high flight 
speeds emphasizes the necessity for great aero- 
dynamic cleanness (low j) to obtain high speed 
performance. 
In the subsonic regime of flight, the ordinary 
configuration of airplane has a very large por- 
tion of the equivalent parasite area determined 
by skin friction drag. As the wing contrib- 
utes nearly half of the total parasite drag, the 
profile drag of the wing can be minimized by 
the use of the airfoil sections which produce 
extensive laminar flow. A subtle effect on 
parasite drag occurs from the influence of the 
wing area. Since the wing area (S) appears 
directly in the parasite drag equation, a reduc- 
tion in wing area would reduce the parasite 
drag if all other factors were unchanged. 
While the exact relationship involves con- 
sideration of many factors, most optimum 
airplane configurations have a strong preference 
for the highest practical wing loading and 
minimum wing surface area. 
As the flight speeds of aircraft approach the 
speed of sound, great care must be taken to 
delay and alleviate compressibility effects. 
In order to delay and teduce the drag rise 
associated with compressibility effects, the 
components of the airplanes must be arranged 
to reduce the early formation of shock waves 
on the airplane. This will generally require 
fuselage and nacelles of high fineness ratio, 
well faired canopies, and thin wing sections 
which have very smooth uniform pressure dis- 
tributions. -Low aspect ratios and sweepback 
are favorable in delaying and reducing the 
compressibility drag rise. In addition, inter- 
ference effects are quite important in transonic 
and supersonic flight and the airplane cross 
section area distribution must be controlled 
to minimize local velocity peaks which could 
create premature strong shock wave formation. 
The modern configuration of airplane will 
illustrate the features required to effect very 
high speed performance-low aspect ratio, 
sweepback, thin low drag sections, etc. These 
same features produce flight characteristics at 
low airspeeds which necessitate .proper flying 
technique. 
AIRPLANE TOTAL DRAG 
I%,- rn+ql Jr,, nf ~ln eimlooe in fl.jght is the AI&C CYCYl Y Ye v YIL L”y’““c 
sum of the induced and parasite drag. Figure 
I.35 illustrates the variation of toral drag 
with speed for a given airplane in level flight 
at a particular weight, configuration, and alti- 
tude. The parasite drag increases with speed 
varying as the square of the velocity while the 
induced drag decreases with speed varying in- 
versely as the square of the velocity. The 
total drag of the airplane shows the predomi- 
nance of induced drag at low speed and parasite 
drag at high speed. Specific points of interest 
on the drag curve are as follows: 
(A) Stall of this particular airplane occurs 
at 100 knots and is indicated by a sharp rise 
in the actual drag. Since the generalized iqua- 
tions for induced and parasite do not account 
for conditions at stall, the actual drag of the 
airplane is depicted by the “hook” of the 
dotted line. 
(B) At a speed of 124 knots, the airplane 
would incur a minimum rate of descent in