NAVWEPS 00-8OT-80 
BASIC AERODYNAMICS 
ure is not too accurate because of the sharper 
variation of parasite drag at high angles of 
attack. In a sense, the airplane efficiency fac- 
tor would change from the constant value and 
decrease. The deviation of the actual airplane 
drag from the approximating curve is quite 
noticeable for airplanes with low aspect ratio 
and sweepback. Another factor to consider is 
the effect of compressibility. Since compressi- 
bility effects would destroy this relationship, 
the greatest application is for subsonic perform- 
ance analysis. 
The total airplane drag is the sum of the 
parasite and induced drags. 
where 
D= D,+D< 
Di= induced drag 
in with the induced drag coefficient by a con- 
stant factor which is defined as the “airplane 
e5ciency factor”, c. By this method of ac- 
counting the airplane drag coe5cient is ex- 
pressed as : 
where 
C DPmB= 
minimum parasite drag 
coefficient 
CD;= induced drag coe5cient 
e = airplane e5ciency factor 
In this form, the airplane drag coefficient is 
expressed as the sum of drag not due to lift 
F%d” ) and drag due to lift (G). The air- 
plane efficiency factor is some co&ant (usually 
less than unity) which includes parasite drag 
due to lift with the drag induced by lift. 
C Dpmr” is invariant with lift and represents the 
parasite drag at zero lift. A typical value of 
C r,Pmin would be 0.020, of which the wing may 
account for 50 percent, the fuselage and nacelles 
40 percent, and the tail 10 percent. The term 
of ( 0.318 g > accounts for all drag due’ to 
lift-the drag induced by lift and the extra 
parasite drag due to lift. Typical values of 
the airplane efficiency factor range from 0.6 to 
0.9 depending on the airplane configuration 
and its characteristics. While the term of 
drag due to lift does include some parasite 
drag, it is still generally referred to as induced 
drag. 
The second graph of figure 1.34 shows that 
the sum of CD, and G can approximate the -mm e 
actual airplane CD through a large range of lift 
coefficients. For airplanes of moderate aspect 
ratio, this representation of the airplane total 
drag is quite accurate in the ordinary range of 
lift coefficients up to near 70 percent of CL,,. 
At high lift coefficients near CL-, the proced- 
and 
=(0.318 $+S 
D,= parasite drag 
When expressed in this form the induced drag, 
Di, includes all drags due to lift and is solely 
a function of lift. The parasite drag, D,, is 
the parasite drag and is completely independent 
of lift-it could be called the “barn door” 
drag of the airplane. 
An alternate expression for the parasite drag 
is: 
R=fq 
where 
f = equivalent parasite area, sq. ft. 
f = CDPmi,S 
q= dynamic pressure, psf 
UP =- 
295 
or 
DpEfg 
In this form, the equivalent parasite area, f, 
is the product of CDPml” and S and relates an 
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