NAVWEPS OO-.ROT-80 
AtRPlANE PERFORMANCE 
Induced drag predominates at speeds below 
the point of minimum total drag. When the 
airplane is operated at the condition of mini- 
mum power required, the total drag is 75 
percent induced drag and 25 percent parasite 
drag. Thus, the induced drag is three times as 
great as the parasite drag when at minimum 
power required. 
VARIATIONS OF THRUST REQUIRED AND 
POWER REQUIRED 
The curves of thrust required and power 
required versus velocity provide the basis for 
comprehensive analysis of all the major items 
of airplane performance. The changes in the 
drag and power curves with variations of air-- 
plane gross weight, configuration, and altitude 
furnish insight for the ‘variation of range, 
endurance, climb performance, etc., with these 
same items. 
The effect of a change in weight on the thrust 
and power required is illustrated by figure 2.2. 
1 The primary effect of a weight change is a 
change in the induced drag and induced power 
required at any given speed. Thus, the great- 
est changes in the curves of thrust and power 
required will take place in the range of low 
speed flight where the induced effects pre- 
dominate. The changes in thrust and power 
required in the range of high speed flight are 
relatively slight because parasite effects pre- 
dominate at high speed. The induced effects 
at high speed are relatively small and changes 
in these items produce a small effect on the 
total thrust or power required. 
In addition to the general effect on .the in- 
duced drag and power required at particular 
speeds, a change in weight will require that the 
airplane operate at different airspeeds to main- 
tain conditions of a specific lift coefficient and 
angle of attack. If the airplane is in steady 
flight at a particular C,,, the airpseed required 
for this CL will vary with weight in the fol- 
lowing manner : 
v, Tg -=J VI E 
where 
Vi = speed corresponding to a specific C, 
and weight, W, 
Va=speed corresponding to the same C, 
but a different weight, Ws 
For the example airplane of figure 2.2, a change 
of gross weight from 15,000 to 22,500 lbs. re- 
quires that the airplane operate at speeds which 
are 22.5 percent greater to maintain a specific 
lift coefficient. For example, if the 15,000-lb. 
airplane operates at 160 knots for (L/D)-, the 
speed for (L/D)mz at 22,500 lbs. is: 
v, = VI@ 
=I&) 22,500 
-\i- 15,000 
= (160) (1.225) 
= 196 knots 
The same situation exists with respect to the 
curves of power required where a change in 
weight requires a change of speed to maintain 
flight at a particular CL. For example, if the 
15,000-lb. airplane achieves minimum power 
required at 122 knots, an increase in weight to 
22,500 Ibs. increases the speed for minimum 
power required to 149 knots. 
0f course, the thrust and power required at 
specific lift coefficients are altered by changes in 
weight. At a specific C,, any change in weight 
causes a like change in thrust required, e.g., a 
50-percent increase in weight causes a 50-per- 
cent increase in thrust required at the same C,. 
The effect of a weight change on the power re- 
quired at a specific CL is a bit more complex be- 
cause a change in speed accompanies the change 
99 
Revised January 1965