in drag and there is a two-fold effect. A 50- 
percent increase in weight produces an increase 
of 83.8 percent in the power required to main- 
tain a specific CL. This is the result of a 50- 
percent increase in thrust required coupled with 
a 22.5-percent increase in speed. The effect of a 
weight change on thrust required, power re- 
quired, and airspeed at specific angles of attack 
and lift coefficients provides an important basis 
for various techniques of cruise and endurance 
conditions of flight. 
1 Figure 2.3 illustrates the effect on the curves 
of thrust and power required of a change in the 
equivalent parasite area,!, of the configuration. 
Since parasite drag predominates in the region 
of high flight speed, a change in f will produce 
the greatest change in thrust and power re- 
quired at high speed. Since parasite drag is 
relatively small in the region of low speed 
flight, a change in f will produce relatively 
small changes in thrust and power required at 
low speeds. The principal effect of a change in 
equivalent parasite area of the configuration is 
to change the parasite drag at any given air- 
speed. 
The curves of figure 2.3 depict the changes in 
the curves of thrust and power required due 
to a 50 percent increase in equivalent parasite 
area of the configuration. The minimum total 
drag is increased by an increase in f and the 
GWL is reduced. ‘Also, the increase in f 
will increase the CL for (L/D)- and require a 
reduction in speed at the new, but decreased, 
(L/D)-. The point of minimum power re- 
quired occurs at a lower airspeed and the value 
of the minimum power required is increased 
slightly. Generally, the effect on the mini- 
mum power required is slight because the para- 
site drag is only 25 percent of the total at this 
specific condition of flight. 
An increase in the equivalent parasite area 
of an airplane may he brought about by the 
deflection of flaps, extension of landing gear, 
extension of speed brakes, addition of external 
stores, etc. In such instances a decrease in the 
airplane efficiency factor, c, may accompany 
NAVWEPS 00-501-50 
AMPLANE PERFORMANCE 
an increase in f to account for the additional 
changes in parasite drag which may vary with 
C‘. 
A change in altitude can produce signifi- 
cant changes in the curves of thrust and power 
required. The effects of altitude on these 
curves providea great part of the explanation of 
the effect of altitude on range and endurance. 
Figure 2.4 illustrates the effect of a change in 
altitude on the curves of thrust and power re- 
quired for a specific airplane configuration and 
gross weight. As long as compressibility 
effects are negligible, the principal effect of 
increased altitude on the curve of thrust re- 
quired is that specific aerodynamic conditions 
occur at higher true airspeeds. For example, 
the subject airplane at sea level has a minimum 
drag of 1,250 lbs. at 160 knots. The same 
airplane would incur the same drag at altitude 
if operated at the same cqthdcnt airsprcd of 160 
knots. However, the equivalent airspeed of 
160 knots at 22,000 ft. altitude would produce 
a true airspeed of 227 knots. Thus, an in- 
crease in altitude will cause the curve of thrust 
required to flatten out and move to the direc- 
tion of higher velocity. Note that altitude 
alone will not alter the value of minimum drag. 
The effect of altitude on the curve of power 
required can best be considered from the effect 
on true airspeed to achieve a specific aero- 
dynamic condition. The sea level power re- 
quired curve of figure 2.4 indicates that 
CW>mz occurs at 160 knots and requires 615 
h.p. If this same airplane is operated at 
WD)ma at an altitude of 22,000 ft., the same 
drag is incurred at a higher velocity and re- 
quires a higher power. The increase in ve- 
locity to 227 knots accounts for the increase 
in power required to 872 hp. Actually, the 
various points on the curve of power required