text: boundary layer. This smooth laminar flow 
  exists without 
  the air particles moving from 
  a given elevation. 
  
  An additional requirement of stability 
  is 
  that the airplane must stabilize and be trimmed 
  for flight at positive lift. When the c.g. is 
  located ahead of d.c., the weight acting at the 
  c.g. is supported by the lift developed by the 
  section. Negative camber is required to pro- 
  duce the positive moment about the aerody- 
  namic center which brings about equilibrium 
  ot balance at positive lift. 
  
  As the flow continues back from the leading 
  edge, friction 
  the boundary layer 
  continue to dissipate energy of the airstream 
  and the laminar boundary layer increases in 
  thickness with distance from the leading edge. 
  After some distance back from 
  the leading 
  edge, the laminar boundary layer begins an 
  oscillatory disturbance which is unstable. A 
  waviness occurs in the laminar boundary layer 
  which ultimately grows 
  larger and more 
  severe and destroys the smooth laminar flow. 
  the 
  Thus, a transition 
  laminar boundary layer decays into a “turbu- 
  lent” 
  layer. The same sort of 
  transition can be noticed inthe smoke from a 
  cigarette in still air. At, first, 
  the smoke 
  ribbon is smooth and laminar, then develops 
  a definite waviness, and decays into a random 
  turbulent smoke pattern. 
  
  Supersonic flow produces important changes 
  in the aerodynamic characteristics of sections. 
  The aerodynamic center of airfoils in subsonic 
  flow is located at the 25 percent chord point. 
  As the airfoil is subject to supersonic flow, the 
  aerodynamic center changes to the 50 percent 
  chord point. Thus, the airplane in transonic 
  flight can experience large changes in longitu- 
  dinal stability because of the large changes in 
  the position of the aerodynamic center. 