impression of the “barn door” size. Hence, 
parasite drag can be appreciated as the result 
of the dynamic pressure, 4, acting on the 
equivalent parasite area, j. The “equivalent” 
parasite area is defmed by this relationship as 
a hypothetical surface with a C,=l.O which 
produces the same parasite drag as the air- 
plane. An analogy would be a barn door in 
the airstream which is equivalent to the air- 
plane. Typical values for the equivalent para- 
site area range from 4 sq. ft. for a clean fighter 
type airplane to 40 sq. ft. for a large transport 
type airplane. Of course, when any airplane 
is changed from the clean configuration to the 
landing configuration, the equivalent parasite 
area increases. 
EFFECT OF CONFIGURATION. The par- 
asite drag, D,, is unaffected by lift, but is 
variable with dynamic pressure and equivalent 
parasite area. This principle furnishes the 
basis for illustrating the variation of parasite 
drag with the various conditions of flight. 
If all other factors are held constant, the para- 
site drag varies directly with the equivalent 
parasite area. 
D,,= b C) D,, I 
where 
D,,= parasite drag corresponding to some orig- 
inal parasite area, fi 
D,,==parasite drag corresponding to some new 
parasite area, fi 
(V and (r are constant) 
As an example, the lowering of the landing 
gear and flaps may increase the parasite area 
80 percent. At any given speed and altitude 
this airplane would experience an 80 percent 
increase in parasite drag. 
EFFECT OF ALTITUDE. In a similar man- 
ner the effect of altitude on parasite drag may 
NAVWEK OD-BOT-BO 
BASIC AERODYNAMICS 
be appreciated. The general effect of altitude 
is expressed by: 
where 
D,, = parasite drag corresponding to some orig- 
inal altitude density ratio, 0, 
D,,=parasite drag corresponding to some new 
altitude density ratio, (ra 
(and f, V are constant) 
This relationship implies that parasite drag 
would decrease at altitude, e.g., a given air- 
plane in flight at a given T.4.Y at 40,COO ft. 
(e=O.29 would have one-fourth the parasite 
drag when at sea level (u=l.OO). This effect 
results when the lower air density produces 
less dynamic pressure. However, if the air- 
plane is flown at a constant EAS, the dynamic 
pressure and, thus, parasite drag do not vary. 
In this case, the TASwould be higher at altitude 
to provide the same EAS. 
EFFECT OF SPEED. The effect of speed 
alone on parasite drag is the most important. 
If all other factors are held constant, the effect 
of velocity on parasite drag is expressed as: 
&, V, * -=- (3 D,, V 
where 
D,,=parasite drag corresponding to some orig- 
inal speed, Vi 
D,,=parasite drag corresponding to some new 
speed, VS 
(j and o are constant) 
This relationship expresses a powerful effect 
of speed on parasite drag. As an example, a 
given airplane in flight at some altitude would 
have four times as much parasite drag at twice 
91