text: AIRCRAFT PROPELLERS 
  
  .The aircraft propeller functions to convert 
  the powerplant shaft horsepower into propul- 
  sive horsepower. The basic principles of pro- 
  pulsion apply to the propeller in that thrust is 
  produced by providing 
  the airstream a mo- 
  mentum change. The propeller achieves high 
  propulsive ef?iciency by processing a relatively 
  large mass flow of air and imparting a rela- 
  tively small velocity change. The momentum 
  change created by propeller is shown by the 
  illustration of figure 2.18. 