text: 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. 
  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 
  the 
  aerodynamic forces may interact with 
  structure in a fashion which may excite or 
  negatively damp the natural modes of 
  the 
  structure and allow flutter. Flutter must not 
  the normal flight operating en- 
  occur within 
  velope and the natural modes must be damped 
  if possible or designed to occur beyond the 
  flutter mode is illus- 
  limit speed. A’typical 
  ‘trated in figure 5.5. 