NAVWEPS OD-SOT-BO  BASIC AERODYNAMICS  greatest importance at low speeds and prac-  tically insignificant in flight at high dynamic  pressures. For example, a typical single en-  gine jet airplane at low altitude and maximum  level flight airspeed has an induced drag which  is less than 1 pcrccont  of the total drag.  How-  ever, this same airplane in steady flight just  above the stall speed could have an induced  drag which is approximately 75 pnrcnt of the  total drag.  EFFECT OF ASPECT RATIO.  The effect  of aspect ratio on the induced drag  is the principal effect of the wing planform.  The relationship for induced drag coefIicient  emphasizes the need of a high aspect ratio  for  the  airplane  which  is  continually  operated at high lift coefficients. In other  words, airplane configurations designed to  operate at high lift coefficients during the  major portion of their flight (sailplanes, cargo,  transport; patrol, and antisubmarine types)  demand a high aspect ratio wing to minimize  the induced drag. While the high  aspect  ratio wing will minimize induced drag, long,  thin wings increase  structural weight and have  relatively poor stiffness characteristics. This  fact will temper the preference  for a very high  aspect ratio.  Airplane configurations which  are developed for very high speed flight (es-  specially supersonic  flight) operate at relatively  low lift coefficients and demand great aero-  dynamic cleanness. These configurations of  airplanes do not have the same preference  for  high aspect ratio as the airplanes which op-  erate continually  at high  lift  coefficients.  This usually results in the development of low  aspect ratio planforms for these airplane con-  figurations.  The effect of aspect ratio on the lift and drag  characteristics is shown in figure 1.31 for  wings of a basic 9 percent symmetrical section.  The basic airfoil section properties are shown  on these curves and these properties would be  71  typical only of a wing planform of extremely  high (infinite) aspect ratio.  When a wing of  some finite aspect ratio is constructed of this  basic section, the principal differences  will be  in the lift and drag characteristics-the mo-  ment characteristics remain essentially the  same.  The effect of decreasing aspect ratio on  the lift curve is to increase the wing angle of  attack necessary  to produce a given lift co-  efficient. The difference between the wing  angle of attack and the section angle of attack  is the induced angle of attack, orit18.24 L  AR’  which increases  with decreasing aspect ratio.  The wing with the lower aspect ratio is less  sensitive to changes in angle of attack and re-  quires higher angles of attack for maximum  lift.  When the aspect ratio is very low (below  3 or 6) the induced angles of attack are not  accurately predicted by the elementary equa-  tion for 01~  and the graph of C, versus 01  develops  distinct curvature. This effect is especially  true at high lift coefhcients where the lift  curve for the very low aspect ratio wing is  very shallow and CL- and stall angle of attack  are less sharply defined.  The effect of aspect ratio on wing drag char-.  acteristics may be appreciated  from inspection of  figure 1.31. The basic section properties are  shown as the drag characteristics of an infinite  aspect ratio wing.  When a planform of some  finite aspect  ratio is constructed, the wing drag  coefficient is the rtlm of the induced drag coe&-  c,”  cient, C,,=O.318 AR,  and the section drag co-  efhcient.  Decreasing aspect ratio increases  the  wing drag coefficient at any lift coefficient since  the induced drag coefficient varies inversely  with aspect ratio.  When the aspect ratio is  very low, the induced drag varies greatly with  lift and at high lift coefficients, the induced  drag is very high and increases very rapidly  with lift coefficient.  While the effect of aspect ratio on lift curve  slope and drag due to lift is an important re-  lationship, it must be realized that design for 