NAVWEPS oo-80~-80  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYl,NG  said about rotor aerodynamics is equally valid  for all types of rotor systems.  If the rotor could operate within  a wide  range of rotor speed, the efficiency and per-  formance could be improved.  With the previous relationships established  for the rotor in hovering flight, the effect of  forward flight br rotor translation can be con-  sidered. With forward flight, a third velocity  component, that tif the forward velocity of the  helicopter, must be considered in determining  the relative wind acting on each rotor blade  element. Since the entire rotor moves with the  helicopter, the velocity of air passing over  each of the elements on the advancfng blade is  increased by the forward speed of the heli-  copter and the velocity of the air passing over  each element of the retreating blade is de-  creased  by the same amount.  This is shown in  figure 6.16.  By analyzing the velocity components  acting  on the rotor blade sections from the blade  root to the tip on both advancing and retreat-  ing blades, a large variation of blade section  angle of attack is found.  Figure 6.16 illus-  trates a typical variation of the local blade  angle of attack for various spanwise positions  along the advancing and retreating blades of  a rotor at high forward speed. There is a  region of positive angles of attack resulting  in positive lift over the entire advancing blade.  Immediately next to the hub of the retreating  blade there is an area of reversed flow where  the velocity due to the forward motion of the  helicopter is greater than the rearward velocity  due to the blade rotation.  The next area is a  negative stall region where, although the  flow is in the proper direction relative the  blade, the angle of attack exceeds that for  negative stall.  Progressing out the retreating  blade, the blade angle of attack becomes  less  negative, resulting in an area of negative lift.  Then the blade angle becomes  positive again,  resulting in a positive lift region.  The blade  angle continues to increase,  until near the tip  of the retreating blade the positive stall angle  of attack is exceeded, resulting in stalling of  the tip section.  This wide variation in blade  section angles of attack results in a large  variation in blade section lift and drag coefK-  cients. The overall lift force on the left and  right sides of the rotor disc are equalized by  cyclically varying the blade pitch as explained  previously, but the drag variation is not  eliminated.  This drag variation causes a  shaking force on the rotor system and con-  tributes to the vibration of the helicopter.  If the blade angles of attack on both advanc-  ing and retreating blades remained the same  as in hovering flight, the higher velocity on  the advancing blade would cause a dissym-  metry of lift and the helicopter would tend to  roll to the left. It was this effect that created  great difficu!ty during many early helicopter  and autogiro projects. Juan De La Cierva was  the first to realize what caused this effect and  he solved the problem by mounting his auto-  giro blades individually  on flapping hinges,  thus allowing a flapping action to automati-  cally correct the dissymmetry of lift that re-  sulted from forward flight.  This is the method  still used in an articulated rotor system today.  The see-saw, or semi-rigid, rotor corrects the  lift dissymmetry by rocking the entire hub and  blades about a gimbal joint,  By rocking the  entire rotor system forward, the angle of at-  tack on the advancing blade is reduced and  the angle of attack on the retreating blade is  increased. The rigid rotor must produce cyclic  variation of the blade pitch mechanically as  the blade rotates to eliminate the lift dissym-  metry.  Irrespective of the method used to  correct the dissymmetry of lift, identical aero-  dynamic characteristics  result. Thus, what is  RETREATING BLADE STALL.  Retreat-  ing blade stall results whenever the angle of  attack of the blade exceeds  the stall angle of  attack of the blade section. This condition  occurs in high speed flight at the tip of the  retreating blade since, in order to develop the  same lift as the advancing blade, the retreating 