text: is upward through the rotor disc and there is 
  a vertical velocity component equal to the 
  rate of descent of the helicopter. 
  In addition, 
  there is a velocity component due to rotation 
  of the rotor. The vector sum of these two 
  velocities is the relative wind for the blade 
  element. The forces resulting from the relative 
  wind on each particular blade section will 
  provide the reason why the rotor will continue 
  to operate without power. First, consider, a 
  blade element near the tip of the blade as illus- 
  this point there is 
  trated in figure 6.17. At 
  a lift force acting perpendicular to the relative 
  wind and a drag force acting parallel to the 
  relative wind through the aerodynamic center. 
  Since the rotation of the rotor is affected only 
  by forces acting in the plane of rotation, 
  the 
  forces are components of the lift 
  important 
  and drag force in the plane of rotation. 
  In 
  this low angle of attack high speed tip section, 
  the net in-plane force is a drag force which 
  would tend to retard the rotor. Next, con- 
  sider a blade section at about the half-span 
  position as illustrated 
  In this 
  case, the same forces are present, but the iti- 
  plane component of lift 
  force is greater than 
  the drag force and this results in a net thrust 
  or forward force in the plane of rotation which 
  tends to drive the rotor. 