text: twisting deformation will be great enough to 
  the effect on aileron deflection and the 
  nullify 
  aileron effectiveness will-be zero. Since speeds 
  above this point create rolling moments op- 
  posite to the direction controlled, 
  this point 
  is termed the “aileron reversal speed.” Oper- 
  ation beyond the reversal speed would create 
  an obvious control difficulty. Also, the ex- 
  tremely large twisting moments which produce 
  loss of aileron effectiveness create large twist- 
  ing moments capable of structural damage. 
  
  In order to prevent loss of aileron effective- 
  ness at high airspeeds, the wing must have 
  high torsional stiffness. This may be a feature 
  difficult to accomplish in a wing of very thin 
  section and may favor the use of inboard ailer- 
  ons to reduce the twisted span length and 
  effectively increase torsional stiffness. The use 
  of spoilers for lateral control minimizes the 
  twisting moments and alleviates the reversal 
  problem. 
  
  Divergcm is another phenomenon common 
  flight at high dynamic pressures. Like 
  to 
  aileron reversal, it is an effect due to the inter- 
  action of aerodynamic forces and elastic deflec- 
  it differs 
  tions of the structure. However, 
  is a violent 
  from aileron reversal in that 
  instability which produces immediate failure. 
  Figure 5.5 illustrates the process of instability. 
  If the surface is above the divergence speed, 
  this sequence. 
  any disturbance precipitates 
  Any change in lift 
  takes place at the aerody- 
  namic center of the section. The change in 
  lift ahead of the elastic axis produces a twist- 
  ing moment and a consequent twisting deflec- 
  tion. The change in angle of attack creates 
  greater lift at the ac., greater twisting deflec- 
  tion, more lift, etc., until failure occurs. 