Patent ID: 8286908

Claim:
A ducted fan for a helicopter, comprising: a transverse duct; and a counter-torque device supported within said duct, said counter-torque device including a rotor rotatably mounted within said duct and a stator fixedly mounted within said duct downstream from said rotor, said rotor including: a rotor hub having a rotor axis, and rotor blades extending from said hub, said rotor blades having a modulated angular distribution about said rotor axis, said angular distribution being defined by the equation: Î¸ i â€²=Î¸ i +Î”Î¸ i sin( mÎ¸ i ) where Î¸ i â€² is the modulated blade angle for the ith blade; Î¸ i is the nominal blade angle for the ith blade; Î”Î¸ i is the maximum modulation amplitude for a specific blade of said rotor blades, m is the modulation factor, and Î”Î¸ i is not a constant for said rotor as a whole, and wherein the resulting rotor is substantially balanced, and said stator including: a stator hub, and a plurality of stator vanes distributed around said stator hub, wherein said stator vanes are angularly modulated around said hub such that the angular spacing between adjacent vanes of said stator vanes varies around said stator hub for each pair of said adjacent vanes.