Source : Webster's Revised Unabridged Dictionary (1913)

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

   {Tractor monoplane},

   {Tractor biplane}, etc.