text: Of course, the wave released by decelerating 
  to subsonic speed can travel out ahead of the 
  airplane and traverse a populated area to cause 
  a sonic boom. The initial direction of the 
  released wave will be the flight path of the 
  airplane at the instant it decelerates to sub- 
  sonic speed. To be sure, the released wave 
  should not be aimed in the direction of a popu- 
  lated area, even if a considerable distance 
  away. There are instances where a released 
  wave has been of an audible magnitude as far 
  as 30 to 40 miles ahead of the point of release. 
  The released pressure wave will be of greatest 
  intensity when created by a large, high drag 
  configuration at low altitude. Since the wave 
  intensity decreases rapidly with distance away 
  from the source, the boom will be of strongest 
  audibility near the point of release. 
  
  HELICOPTER PROBLEMS 
  
  The main difference between helicopter and 
  The 
  an airplane is the main source of lift. 
  from a fixed airfoil 
  airplane derives its lift 
  from 
  surface while the helicopter derives lift 
  a rotating airfoil called the rotor. Hence, the 
  aircraft will be classified as either 
  “fixed- 
  The word “heli- 
  wing” or “rotating wing.” 
  copter” 
  the Greek words 
  is derived from 
  meaning “helical wing” or “rotating wing. ” 