NAVWEPS OD-ROT-80  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  HELICOPTER PROBLEMS  the ship.  When the airplane slows to sub-  sonic, the shock wave travels out ahead of the  airplane in a form which is somewhat spheri-  cal.  Because there are density variations  through the shock wave, the shock wave  moving ahead of the airplane can cause aber-  rations in light waves and it may appear.to  the pilot as if a large sheet of clear cellulose  or plastic were in front of the airplane. In  addition, the density variation and initial  shape of the wave leaving the airplane may  cause reflection of sunlight  which  would  appear as a sudden, brilliant  “flash”  to the  pilot.  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.  It should become apparent that sonic booms  are a byproduct of supersonic aviation and,  with supersonic  flight becoming more common-  place, the problem is more perplexing.  The  potential of sonic booms is mostly of the  audible nature and nuisance of the disturbance.  The damage potential of the ordinary sonic  boom is quite small and the principal effects  are confined to structures which are extremely  brittle, low strength, and have characteristic  high residual stresses. In other words, only  the extremes of pressure waves generated by  airplanes in flight could possibly cause cracked  plaster and window glass.  Such materials  Lift generation,  by a “rotating wing” enables  the helicopter to accomplish its unique mission  of hovering motionless in the air, taking off  and landing in a confined or restricted area,  and autorotating to a safe landing following  a power failure. Lift generation by “rotating  wing’ ’  is also responsible for some of the  unusual problems the helicopter can encounter.  Since the helicopter problems are due to par-  ticular nature of .the rotor aerodynamics, the  basic flow conditions within the rotor must be  considered in detail.  For simplicity,  the  initial discussion will consider only the hover-  ing  rotor.  Although  the term hovering  The main difference between helicopter and  an airplane is the main source of lift.  The  airplane derives its lift from a fixed airfoil  surface while the helicopter derives lift from  a rotating airfoil called the rotor. Hence, the  aircraft will  be classified as either “fixed-  wing”  or “rotating wing.”  The word “heli-  copter”  is derived from the Greek words  meaning “helical wing”  or “rotating wing. ”  The pressure  jump through the shock waves  in the immediate vicinity of the airplane is  much greater than those common to the  audible “booms”  at ground level.  Thus,  airplanes in close formation at supersonic  speeds  may encounter considerable interference  between airplanes. In addition, to eliminate  even the most remote possibility of structural  damage, a high speed airplane should not  make a supersonic pass close to a large air-  plane which may have low limit load factor  and be prone to be easily disturbed or damaged  by a strong pressure wave.  are quite prone to sharp dynamic stresses  and,  when .superimposed on the high  residual  stresses  common to the products and building  construction, slight but insignificant damage  may result. Actually, the most objectionable  feature of the sonic boom is the audibility and  the anxiety or apprehension caused by the  sharp, loud noise which resembles a blast. 