NAVWEPS 00-ROT-80  OPERATING STRENGTH LIMITitTIONS  buildup may overpower the resisting torsional  stiffness and “divergence”  will  occur.  The  divergence speed of the surfaces  must be suf-  ficiently high that the airplane does not en-  counter this phenomenon within the normal  operating envelope. Sweepback, short span,  and high taper help raise the divergence speed.  twisting deformation will be great enough to  nullify the effect on aileron deflection and the  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.  F/titter involves aerodynamic forces, inertia  forces and the elastic properties of a surface.  The distribution of mass and stiffness in a  structure determine certain natural frequencies  and modes of vibration. If the structure is sub-  ject to a forcing frequency near these natural  frequencies, a resonant condition can result  with an unstable oscillation.  The aircraft is  subject to many aerodynamic excitations while  in operation and the aerodynamic forces at  various speeds have characteristic properties  for rate of change of force and moment. The  aerodynamic forces may interact with  the  structure in a fashion which may excite or  negatively damp the natural modes of the  structure and allow flutter. Flutter must not  occur within the normal flight operating en-  velope and the natural modes must be damped  if possible or designed to occur beyond the  limit speed. A ’ typical  flutter mode is illus-  ‘ trated in figure 5.5.  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  to flight at high dynamic pressures.  Like  aileron reversal, it is an effect due to the inter-  action of aerodynamic forces and elastic deflec-  tions of the structure. However, it differs  from aileron reversal in that it is a violent  instability which produces immediate failure.  Figure 5.5 illustrates the process  of instability.  If the surface is above the divergence speed,  any disturbance precipitates this sequence.  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.  Since the problem is one of high speed  flight,  it is generally desirable to have  ‘ very high  natural frequencies and flutter speeds well  above the normal operating speeds.  Any  change of stiffness or mass distribution will  alter the modes and frequencies  and thus allow  a change in the flutter speeds. If the aircraft  is not properly maintained and excessive  play  and flexibility exist, flutter could occur at  flight speeds  below the limit airspeed.  Compres&ility pmblems  may define the limit  airspeed for an airplane in terms of Mach num-  ber. The supersonic airplane may experience  a great decay of stability at some high Mach  number or encounter critical structural or  engine inlet temperatures due to aerodynamic  heating. The transonic airplane at an excessive  At low flight speeds where the dynamic  pressure  is low, the relationship between aero-  dynamic force buildup and torsional deflection  is  ‘ stable.  However, the change in lift per  angle of attack is proportional to  ‘ vz but the  structural torsional stiffness of the wing re-  mains constant.  This relationship implies  that at some high speed, the aerodynamic force  342 