NAVWEPS 00-8OT-80  AIRPLANE PERFORMAN:CE  PrPs=parasite  power required corresponding to  some different speed, I ’ ,  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.  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  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 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 parasite power required is a function  of the parasite drag and velocity.  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)-.  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.  where  Prpl= parasite power required corresponding to  some original speed, Vi  Pb% v* 3  -=(-I  Ph  VI  97 