The Collaborative International Dictionary of English v.0.48:

Tractor screw \Tractor screw\ or Tractor propeller \Tractor
propeller\ (Aviation)
   A propeller screw placed in front of the supporting planes of
   an airplane instead of behind them, so that it exerts a pull
   instead of a push. Hence,

   Tractor monoplane,

   Tractor biplane, etc. This is the most common arrangement
      for propellers on modern airplanes.
      [Webster 1913 Suppl.]