(2) Sweepback  will reduce the magnitude  of change in the aerodynamic force coeffi-  cients due to compressibility. Any change  in drag, lift, or moment coefbcients  will be  reduced by the use of sweepback. Various  sweep angles applied to wings of moderate  aspect ratio will produce these approximate  effects  in transonic flight.  These advantages  of drag reduction and preser-  vation of the transonic maximum lift coefficient  are illustrated in figure 3.14.  Thus, the use of sweepback on a transonic  aircraft will reduce and delay the drag rise and  preserve the maneuverability of the aircraft  in transonic flight.  It should be noted that a  small amount of sweepback produces very  little benefit.  If sweepback  is to be used at all,  at least 30’ to 33’ must be used to produce any  significant benefit. Also note from figure 3.14  that the amount of sweepback required to  d&y drag rise in supersonic  flight is very large,  e.g., more than 60° necessary  at M=2.0.  By  comparison of the drag curves at high Mach  numbers it will be appreciated that extremely  high (and possibly impractical) sweepback is  necessary  to delay drag rise and that the lowest  drag is abtained with zero sweepback. There-  fore, the planform of a wing designed to operate  continuously at high Mach numbers will tend  to be very thin, low aspect ratio, and unswept.  An immediate conclusion is that sweepback is  a device of greatest application in the regime of  transonic flight.  A few of the less significant advantages of  sweepback  are as follows:  (1) The wing lift curve slope is reduced  for a given aspect ratio.  This is illustrated  by the lift curve comparison of figure 3.15  for the straight and swept wing.  Any  reduction of lift  curve slope implies the  wing is less sensitive to changes in angle of  attack. This is a beneficial effect only when  the effect of gusts and turbulence is con-  sidered. Since the swept wing has the  lower lift curve slope it will be less sensitive  to gusts and experience less “bump”  due  to gust for a given aspect ratio and wing  loading.  This is a consideration particular  to the aircraft whose structural design shows  a predominating effect of the gust load  spectrum, e.g., transport, cargo, and patrol  types.  (2) “Divergence” of a surface is an aero-  elastic problem which can occur at high  dynamic pressures. Combined bending and  twisting deflections interact with aerody-  namic forces to produce sudden failure of  the surface at high speeds. Sweep forward  will aggravate this situation by “leading”  the wing into the windstream and tends to  lower the divergence speed. On the other  hand, sweepback tends to  stabilize the  surface by “trailing”  and tends to raise the  divergence speed. By this tendency, sweep-  back may be beneficial in preventing di-  vergence  within the anticipated speed range.  (3) Sweepback  contributes slightly to the  static directional-or weathercock-stability  of an aircraft. This effect may be appre-  ciated by inspection of hgure 3.13 which  shows the swept wing in a yaw or sideslip.  The wing into the wind has less sweep and  a slight increase in drag; the wing away  from the wind has more sweep and less  drag.  The net effect of these force changes is  to produce a yawing moment tending to  retarn the nose into the relative wind.  This directional stability contribution is  usually small and of importance in tailless  aircraft only.  229  Revised January l%S  00 ...............................  0  150.  .............  ................  5  M” ..............................  15  45 ’ ..............................  35  600 ..............................  60  -_  NAVWEPS 00-801-80  HIGH SPEED AERODYNAMICS 