control speeds” set by these factors rather than 
simple stall speeds based on C&,. 
When a wing of a given planform has various 
high lift devices added, the lift distribution and 
stall pattern can be greatly affected. Deflec- 
tion of trailing edge flaps increases the local 
lift coe5cients in the flapped areas and since 
the stall angle of the flapped section is de- 
creased, initial stall usually begins in the 
flapped area. The extension of slats simply 
allows the slatted areas to go to higher lift 
coe5cients and angles of attack and generally 
delays stall in that vicinity. Also, power 
effects may adversely affect the stall pattern of 
the propeller powered airplane. When the 
propeller powered airplane is at high power 
and low speed, the flow induced at the wing 
root by the slipstream may cause considerable 
delay in the stall of the root sections. Hence, 
the propeller powered airplane may have its 
most undesirable stall characteristics during the 
power-on stall rather than the power-off stall. 
PARASITE DRAG 
In addition to the drag caused by the de- 
velopment of lift (induced drag) there is the 
obvious drag which is nor due to the develop 
ment of lift. A wing surface even at zero lift 
will have “profile” drag due to skin friction 
and form. The other components of the air- 
plane such as the fuselage, tail, nacelles, etc., 
contribute to drag because of their own form 
and skin friction. Any loss of momentum of 
the airstream due to powerplant cooling, air 
conditioning, or leakage through construction 
or access gaps is, in effect, an additional drag. 
When the various components of the airplane 
are put together the total drag will be greater 
than the sum of the individual components 
because of “interference” of one surface on the 
other. 
The most usual interference of importance 
occurs at the wing-body intersection where the 
growth of boundary layer on the fuselage re- 
duces the boundary layer velocities on the wing 
root surface. This reduction in energy allows 
NAVWEPS OO-ROl-80 
BASIC AERODYNAMICS 
the wing root boundary layer to be more easily 
separated in the presence of an adverse pressure 
gradient. Since the upper wing surface has the 
more critical pressure gradients, a low wing 
position on a circular fuselage would create 
greater interference drag than a high wing 
position. Adequate filleting and control of 
local pressure gradients is necessary to mini- 
mize such additional drag due to interference. 
The sum of all the drags due to form, fric- 
tion, leakage and momentum losses, and inter- 
ference drag is termed “parasite” drag since 
it is not directly associated with the develop- 
ment of lift. While this parasite drag is not 
directly associated with the production of lift 
it is a variable with lift. The variation of 
parasite drag coefficient, C+, with lift coef- 
ficient, C,, is shown for a typical airplane in 
figure 1.34. The minimum parasite drag co- 
efficient, CDpmi,, usually occurs at or near zero 
lift and parasite drag coefficient increases 
above this point,in a smooth curve. The in- 
duced drag coefficient is shown on the same 
graph for purposes of comparison since the 
total drag of the airplane is a sum of the 
parasite and induced drag. 
In many parts of airplane performance it is 
necessary to completely distinguish between 
drag due to lift and drag not due to lift. The 
total drag of an airplane is the sum of the para- 
site and induced drags. 
G=c++cD; 
where 
C, = airplane drag coefficient 
C+=parasite drag coefficient 
C,,= induced drag coeaicient 
From inspection of figure 1.34 it is seen that 
both CD, and CD, vary with lift coefticient. 
However, the usual variation of parasite drag 
allows a simple correlation with the induced 
drag term. In effect, the part of parasite drag 
above the minimum at zero lift can be “lumped” 
a7