NAVWEPS 00-801-80  BASIC AERODYNAMICS  as great a speed or one-fourth as much parasite  drag at half the original speed.  This fact may  be appreciated by the relationship of dynamic  pressure with speed-twice as much V, four  times as much 4, and four times as much D,.  This expressed  variation of parasite drag with  speed points out that parasite drag will be of  greatest importance at high speeds and prac-  tically insignificant in flight at low dynamic  pressures. To illustrate this fact, an airplane  in flight just above the stall speed could have a  parasite drag which is only 25 percent of the  total drag.  However, this same airpfane at  maximum level flight speed at low altitude  would have a parasite drag which’ is very  nearly 100 percent of the total drag. The  predominance of parasite drag at high flight  speeds  emphasizes  the necessity  for great aero-  dynamic cleanness  (low j) to obtain high speed  performance.  In the subsonic regime of flight, the ordinary  configuration of airplane has a very large por-  tion of the equivalent parasite area determined  by skin friction drag. As the wing contrib-  utes nearly half of the total parasite drag, the  profile drag of the wing can be minimized by  the use of the airfoil sections which produce  extensive laminar flow.  A subtle effect on  parasite drag occurs from the influence of the  wing area. Since the wing area (S) appears  directly in the parasite drag equation, a reduc-  tion in wing area would reduce the parasite  drag if all other factors were unchanged.  While the exact relationship involves con-  sideration of many factors, most optimum  airplane configurations have a strong preference  for the highest practical wing loading and  minimum wing surface area.  As the flight speeds  of aircraft approach the  speed of sound, great care must be taken to  delay and alleviate compressibility effects.  In order to delay and teduce the drag rise  associated with  compressibility effects, the  components of the airplanes must be arranged  to reduce the early formation of shock waves  on the airplane.  This will generally require  (B) At a speed of 124 knots, the airplane  would incur a minimum rate of descent in  power-off flight.  Note that at this speed the  induced drag comprises 75 percent of the total  drag. If this airplane were powered with a  reciprocating-propeller  type powerplant, maxi-  mum endurance would occur at this airspeed.  (A) Stall of this particular airplane occurs  at 100 knots and is indicated by a sharp rise  in the actual drag.  Since the generalized iqua-  tions for induced and parasite do not account  for conditions at stall, the actual drag of the  airplane is depicted by the “hook”  of the  dotted line.  I%,- rn+ql Jr,, nf ~ln eimlooe in fl.jght is the  AI&C  CYCYl  Y Ye v YIL  L”y’““c  sum of the induced and parasite drag. Figure  I.35 illustrates the variation of toral drag  with speed for a given airplane in level flight  at a particular weight, configuration, and alti-  tude. The parasite drag increases  with speed  varying as the square of the velocity while the  induced drag decreases  with speed varying in-  versely as the square of the velocity.  The  total drag of the airplane shows the predomi-  nance of induced drag at low speed  and parasite  drag at high speed.  Specific points of interest  on the drag curve are as follows:  fuselage and nacelles of high fineness ratio,  well faired canopies, and thin wing sections  which have very smooth uniform pressure  dis-  tributions. -Low aspect ratios and sweepback  are favorable in delaying and reducing the  compressibility drag rise. In addition, inter-  ference  effects  are quite important in transonic  and supersonic flight and the airplane cross  section area distribution must be controlled  to minimize local velocity peaks which could  create premature strong shock wave formation.  The modern configuration of airplane will  illustrate the features required to effect very  high  speed performance-low  aspect ratio,  sweepback,  thin low drag sections, etc.  These  same features produce flight characteristics at  low airspeeds which necessitate .proper flying  technique.  AIRPLANE TOTAL DRAG  92 