NAVWEPS 00-8OT-80  HIGH SPEED AERODYNAMICS  value all oblique portions of the waves incline  more greatly and the detached normal shock  portion of the bow wave moves closer to the  leading edge.  Of course, all components of the aircraft  are affected by compressibility in a manner  somewhat similar to that of basic airfoil.  The tail, fuselage, nacelles, canopy, etc. and  the efkct of the interference between the  various surfaces of the aircraft must be  considered.  shock wave forms as the boundary between  the supersonic flow and the subsonic flow on  the aft portion of the airfoil surface. The  acceleration of the airflow from subsonic to  supersonic is smooth and unaccompanied by  shock waves if the surface is smooth and the  transition gradual. However, the transition  of airflow from supersonic to subsonic is  always accompanied by a shock wave and,  when there is no change in direction of the  airflow, the wave form is a normal shock  wave.  FORCE DIVERGENCE.  The airflow sepa-  ration induced by shock wave formation can  create significant variations in the aerody-  namic force coefficients.  When the free stream  speed  is greater than critical Mach number some  typical effects on an airfoil section are as  follows :  Recall that one of the principal effects of  th,e normal shock wave is to produce a large  increase in the static pressure of the airstream  behind the wave.  If the shock wave is  strong, the boundary layer may not have  sufficient kinetic energy to withstand the  large, adverse  pressure  gradient and separation  will occur. At speeds only slightly beyond  critical Mach number the shock wave formed  is not strong enough to cause spearation or  any noticeable change in the aerodynamic  force coefficients. However, an increase in  speed above critical Mach number sufhcient  to form a strong shock wave can cause sepa-  ration of the boundary layer and produce  sudden changes in the aerodynamic force  coefficients. Such a flow condition is shown  in figure 3.9 by the flow pattern for M=O.n.  Notice that a further increase  in Mach number  to 0.82 can enlarge the supersonic area on the  upper surface and form an additional area of  supersonic  flow and normal shock wave on the  lower surface.  (1) An increase in the section drag coeffi-  cient for a given section lift coe5cient.  (2) A decrease in section lift coefficient  for a given section angle of attack.  (3) A change in section pitching moment  coe5cient.  A reference  point is usually taken by a plot  of drag coe5cient versus Mach number for  a constant lift coefficient. Such a graph is  shown in figure 3.10.  The Mach number  which produces a sharp change in the drag  coe5cient is termed the “force divergence”  Mach number and, for most airfoils, usually  exceeds the critical Mach number at least 5  to 10 percent.  This condition is also referred  to as the “drag divergence” or “drag rise.”  Associated with the “drag rise” are buffet,  trim and stability changes, and a decrease  in control surface effectiveness. Conventional  aileron, rudder, and elevator surfaces sub  jetted to this high  frequency buffet may  “buzz,”  and changes in hinge moments may  produce undesirable control forces.  Of course,  if the buffet is quite severe and prolonged,  structural damage may occur if this operation  is in violation of operating limitations.  When  airflow separation occurs on the wing due to  As the flight speed approaches the speed of  sound the areas of supersonic flow enlarge and  the shock waves move nearer the trailing  edge. The boundary layer may remain sepa-  rated or may reattach depending much upon  the airfoil shape and angle of attack. When  the flight speed exceeds the speed of sound  the “bow”  wave forms at the leading edge and  this typical flow pattern is illustrated in  figure 3.9 by the drawing for M= 1.05. If the  speed is increased to some higher supersonic  PHENOMENA OF TRANSONIC FLIGHT.  218 