NAVWEPS OO-BOT-80 
BASIC AE,RODYNAMlCS 
(C) The point of minimum total drag occurs 
at a speed of 163 knots. Since this speed in- 
curs the least total drag for lift-equal-weight 
flight, the airplane is operating at (L/D)ma,. 
Because of the particular manner in which 
parasite and induced drags vary with speed 
(parasite drag directly as the speed squared; 
induced drag inversely as the speed squared) 
the minimum total drag occurs when the in- 
duced and parasite drags are equal. The speed 
for minimum drag is an important reference for 
many items of airplane performance. One 
item previously ,presented related glide per- 
formance and lift-drag ratio. At the speed of 
163 knots this airplane incurs a total drag of 
778 lbs. while producing 12,000 lbs. of lift. 
These figures indicate a maximum lift-drag 
ratio of 15.4.and relate a glide ratio of 15.4.~ 
In addition, if this airplane were jet powered, 
the airplane would achieve maximum en- 
durance at this airspeed for ‘the specified alti- 
tude. If this airplane were propeller powered, 
the airplane would achieve maximum range at 
this airspeed for the specified altitude. 
(D) Point (D) is at an airspeed approxi- 
mately 32 percent greater than the speed for 
(L/D),.,. Note that the parasite drag com- 
prises 75 percent of the total drag at a speed of 
215 knots. This point on the drag curve pro- 
duces the highest proportion between velocity 
and drag and would be the point for maximum 
range if the airplane were jet powered. Be- 
cause of the high proportion of parasite drag 
at this point the long range jet airplane has 
great preference for great aerodynamic clean- 
ness and less demand for a high aspect ratio 
than the long range propeller powered airplane. 
(E) At a speed of 400 knots, the induced 
drag is an extremely small part of the total 
drag and parasite drag predominates. 
(P) As the airplane reaches very high flight 
speeds, the drag rises in a very rapid fashion 
due to compressibility. Since the generalized 
equation for parasite drag does not account for 
compressibility effects, the actual drag rise is 
typified by the dashed line. 
The airplane drag curve shown in figure 1.34 
is particular to one weight, configuration, and 
altitude in level flight. Any change in one of 
these variables will affect the specific drags at 
specific velocities. 
The airplane drag curve is a major factor in 
many items of airplane performance. Range, 
endurance, climb, maneuver, landing, takeoff, 
etc., performance are based on some relation- 
ship involving the airplane drag curve. 
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