text: 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 
  The next area is a 
  due to the blade rotation. 
  negative stall region where, although 
  the 
  flow is in the proper direction relative the 
  for 
  blade, the angle of attack exceeds that 
  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 
  is not 
  eliminated. This drag variation causes a 
  shaking force on the rotor system and con- 
  tributes to the vibration of the helicopter. 