text: Transonic and supersonic flight can cause a 
  the effectiveness of 
  in 
  noticeable reduction 
  trailing edge control surfaces. The deflection 
  of a trailing edge control surface at low sub- 
  sonic speeds alters the pressure distribution on 
  the fixed portion as well as the movable portion 
  of the surface. This is true to the extent that a 
  l-degree deflection of a 40 percent chord eleva- 
  tor produces a lift change very nearly the 
  equivalent of a l-degree change in stabilizer 
  setting. However, if supersonic flow exists on 
  the surface, a deflection of the trailing edge 
  control surface cannot influence the pressure 
  distribution 
  in the supersonic area ahead of the 
  movable control surface. This is especially 
  true in high supersonic flight where supersonic 
  flow exists over the entire chord and the change 
  in pressure distribution is limited to the area of 
  the control surface. The reduction in effective- 
  ness of the trailing edge control surface at tran- 
  sonic and supersonic speeds necessitates the use 
  of an all movable surface. Application of the 
  all movable control surface to the horizontal 
  tail is most usual since the increase in longi- 
  tudinal stability in supersonic flight requires a 
  high degree of control effectiveness to achieve 
  for supersonic maneu- 
  required controllability 
  vering. 