Thus, induced power required will vary with 
lift, aspect ratio, altitude, etc., in the same 
manner as the induced drag. The only differ- 
ence will be the variation with speed. If all 
other factors remain constant, the induced 
power required varies inversely with velocity 
while induced’drag varies inversely with the 
square of the velocity. 
where 
Pri,=induced power required corresponding to 
some original speed, Vi 
I+;,= induced power required corresponding to 
some different speed, V, 
For example, if an airplane in steady level flight 
is operated at.twice as great a speed, the in- 
duced drag is one-fourth the original value but 
the induced power required is one-half the 
original value. 
The parasite power required is a function 
of the parasite drag and velocity. 
where 
Pr,=parasite power required, h.p. 
D,=paraSite drag, lbs. 
V= true airspeed, knots 
Thus, parasite power required will vary with 
altitude and equivalent parasite area ( f) in the 
same manner as ‘the parasite drag. However, 
the variation with speed will be different. If 
all other factors are constant, the parasite drag 
varies as the square of velocity but parasite 
power varies as the cube of velocity. 
Pb% v* 3 
-=(-I Ph VI 
where 
Prpl= parasite power required corresponding to 
some original speed, Vi 
NAVWEPS 00-8OT-80 
AIRPLANE PERFORMAN:CE 
PrPs=parasite power required corresponding to 
some different speed, I’, 
For example, if an airplane in steady flight is 
operated at twice as great a speed, the parasite 
drag is four times as great but the parasite 
~;;zr required is eight times the original 
Figure 2.1 presents the thrust required and 
power required for a specific airplane configu- 
ration and altitude. The curves of figure 2.1 
are applicable for the following airplane data: 
gross weight, W= 15,000 Ibs. 
span, b=40 ft. 
equivalent parasite area, f=7.2 sq. ft. 
airplane efficiency factor, c= ,827 
sea level altitude, C= 1.000 
compressibility corrections neglected 
The curve of drag or thrust required versus 
velocity shows the variation of induced, para- 
site, and total drag. Induced drag predomi- 
nates at low speeds. When the airplane is 
operated at maximum lift-drag ratio, (L/D)-, 
the total drag is at a minimum and the induced 
and parasite drags are equal. For the specific 
airplane of figure 2.1, (,L/D),, and minimum 
total drag are obtained at a speed of 160 knots. 
The curve of power required versus velocity 
shows the variation of induced, parasite, and 
total power required. As before, induced 
power required predominates at low speeds and 
parasite power required predominates at high 
speeds and the induced and parasite power are 
equal at (L/D),,. However, the condition of 
(L/D&- defines only the point of minimum 
drag and does not define the point of minimum 
pozver required. Ordinarily, the point of mini- 
mum power required will occur at a speed 
which is 76 percent of the speed for minimum 
drag and, in the case of the airplane configura- 
tion of figure 2.1, the speed for minimum power 
required would be 122 knots. The total drag 
at the speed for minimum power required is 15 
percent higher than the drag at (L/D)- but the 
minimum power required is 12 percent lower 
than the power required at (L/D)-. 
97