NAVWEPS 00-501-50  AMPLANE PERFORMANCE  an increase in f to account for the additional  changes in parasite drag which may vary with  C ‘ .  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.  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 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.  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  can be considered  affected  in this same fashion.  At specific  lift coefficients  and angles of attack,  a change in altitude will alter the true airspeed  particular to these points and cause a change  in power required because of the change in  true airspeed. An increase in altitude will  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  Revised Januaty 1965  101 