581,797. Helicopters. UNITED AIRCRAFT CORPORATION. April 23, 1945, No. 10162. Convention date, Sept. 26, 1944. [Class 4] In helicopters, having rotor blades 26 hinged to a disc 12 for flapping movements and having drag hinges 22, mechanism is provided for maintaining a symmetrical arrangement of the blades about their drag hinges. A hydraulic cylinder 34 is pivoted to each blade shank 24, the piston-rod 32 with piston 40 being pivoted at 30 to the link 20 between the flapping axle 18 and drag hinge 22 ; each cylinder has its outer end connected by flexible pipes 52, 54 to the inner end of the preceding cylinder and the movement of any blade about its drag axis causes movement of the piston 40 so as to force liquid into or out of the preceding cylinder and hence its adjustment to keep pace with the first cylinder. The pistons 40 have restricted orifices 45 allowing communication between the two parts of the cylinder to admit of return after disturbance of the blades. A central reservoir 58 is provided to supply through a spring valve any additional liquid required ; there may be a small orifice in the valve to allow for expansion of liquid.