text: Rotor lift can be explained by either of two 
  methods. The first method, utilizing simple 
  momentum theory based on Newton’s Laws, 
  merely states that lift 
  results from the rotor 
  accelerating a mass of air downward in the 
  same way that the jet engine develops thrust 
  by accelerating a mass of air out the tailpipe. 
  The second method of viewing 
  lift 
  concerns the pressure forces acting on the 
  various sections of the blade from root to tip. 
  is useful in 
  The simple momentum theory 
  characteristics while 
  determining only 
  lift 
  the “blade element” 
  theory gives drag as 
  well as lift characteristics and is useful in 
  giving a picture of the forces at work on the 
  rotor. 
  the 
  blade is divided up into “blade elements” as 
  shown in figure 6.15. The forces acting on 
  each blade element are analyzed. Then the 
  forces on all elements are summed up to give 
  the characteristics of the whole rotor. The 
  relative wind acting on each segment is the 
  resultant of two velocity components: (1) the 
  velocity due CO the rotation of the blades 
  about the hub and (2) the induced velocity, 
  or downwash velocity caused by the rotor. 
  the velocity due to rotation at a particular 
  element is proportional to the rotor speed and 
  the distance of the element from the rotor hub. 