NAVWEPS OO-EOT-80  HIGH SPEED AERODYNAMICS  CD  DRAG  COEFFICIENT  FORCE DIVERGENCE  MACH NUMBER  CRITICAL  MACH NUMBER.  I  I  I  c  0.5  1.0  ht,MACH NUMBER  Figure 3ilO. Compressibility Drag Rise  shock wave formation, there will be a loss of  lift and subsequent loss of downwash aft of  the affected  area.  If the wings shock unevenly  due to physical shape differences  or sideslip,  a rolling  moment will  be created in the  direction of the initial loss of lift and con-  tribute to control difficulty (“wing  drop”).  If the shock induced separation occurs sym-  metrically near the wing root, a decrease  in  downwash behind this area is a corollary of  the loss of lift.  A decrease  in downwash on  the horizontal tail will create a diving moment  and the aircraft will “tuck under.”  If these  conditions occur on a swept wing. planform,  the wing center of pressure shift contributes  to the trim change-root  shock first moves  the wing center of pressure  aft and adds to the  diving moment; shock formation at the wing  tips first moves the center of pressure  forward  and the resulting climbing moment and tail  downwash change can contribute to “pitch  up.”  Since most of the dificulties of transonic  flight are associated  with shock wave induced  flow separation, any means of delaying or  alleviating the shock induced separation will  improve the aerodynamic characteristics. An  aircraft conhguration may utilize thin surfaces  of low aspect ratio with sweepback to delay  and reduce the magnitude of transonic force  divergence. In addition, various methods of  boundary layer control, high  lift  devices,  vortex generators, etc., may be applied to  improve transonic characteristics. For exam-  ple, the application of vortex generators to a  surface  can produce higher local surface  veloci-  ties and increase the kinetic energy of the  boundary layer. Thus, a more severe  pressure  gradient (stronger shock wave) will be neces-  sary to produce airflow separation.  219 