Abstract: A droop stop mechanism for a helicopter rotor blade includes a pair of opposing stops disposed on opposite sides of the blade's flap hinge. An interposer is mounted at the end of an arm on a rotatable shaft for rotation in a plane perpendicular to the hinge. A torsion spring disposed concentrically about the shaft has a first end that is fixed and a second end coupled to the shaft to bias the interposer to a position disposed between the stops, thereby limiting blade droop. A weight is attached to the shaft such that, with rotation of the rotor, centrifugal forces acting on the weight cause the shaft and arm to rotate so as to overcome the bias of the spring and urge the interposer out from between the stops, thereby allowing greater blade droop. The spring and shaft are sealed in a canister for protection from the elements.