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.  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.  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:  where  v,  Tg  -=J  VI  E  Vi = speed corresponding to a specific C,  and weight, W,  Va=speed corresponding to the same C,  but a different weight, Ws  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.  VARIATIONS OF THRUST REQUIRED AND  POWER REQUIRED  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  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 :  Revised January 1965  99 