GB811211A - Improvements in or relating to cyclic pitch controls for helicopters - Google Patents

Abstract

811,211. Helicopters. McCUTCHEN, C. W. July 15, 1955 [April 15, 1954], No. 11236/54. Class 4. A helicopter has a cyclic pitch control in which the blade angle measured with reference to the tip path plane is sinusoidally variable, independently of movement of the fuselage. Fig. 1 shows a mast 15 supporting, on a gimbal mounting 17, two control blades 16 and two rotor blades (not shown). A clntrol rod 19 is universally pivoted to the mast at 20, and pivoted at 21 to two rods connected to pitch change horns on the blades 16. Altering the pitch of blades 16 tilts the outer ring of the gimbal, cyclically altering the pitch of the rotor blades. Since the pivot 21 is never far from the centre of the gimbal 17, the pitch of blades 16, and thus the pitch of the rotor blades, is substantially unaffected by fuselage movement relative to the tip path plane. A biasing force on rod 19 is provided by springs 22 connected between the rod 19 and arms 23 rigid with the control blades 16, and similar apparatus (not shown) between the rotor blades and rod 19. The biasing force tends to urge rod 19 to a position depending on the inclination of the tip path plane. In Figs. 2 and 3, the rotor mast 30 supports through a gimbal mounting (not shown) three radial arms 31 each bearing a blade 32 through a flapping hinge 33 and a pitch change bearing. A gimbal mounted swash plate 34 is connected by three arms 36 to three levers 37 each pivoted to an arm 31 at 39, and connected to the blade by a scissors link 38. Tilting of the swash plate imposes a cyclic oscillation on the levers 37 and thus the blade pitch is varied cyclically. Collective pitch change is achieved by rotating the swash plate 34 with respect to the rotor mast, which has the effect of lowering or raising the pivots 10 between arms 36 and levers 37 equally. A spring box 100 connected between the swash. plate and one arm 31 imposes a feel on the pilots' control member proportional to the tilt of the rotor head. The arrangement is such that movement of the fuselage and rotor mast about the rotor head gimbal has substantially no effect on the cyclic pitch of the blades, since the pivots 10 are very near the rotor axis. In Figs. 4 and 5, the blades 42 are again mounted on flapping hinges 33 to a three armed gimbal mounted (not shown) rotor head. The swash plate 55 has three rods 57 rigid therewith and pivoted to arcuate links 58, the other end of each link being pivoted at 59 to a rod 591 fixed to a sleeve 60 freely rotatable on an arm 62 extending from the rotor head. A second rod 601 fast to the sleeve 60 is connected by a rod 63 to a pitch changing horn on the blade. When the swash plate is tilted on its gimbal mounting, the rods 58 are oscillated in the general direction of their length, varying the blade pitch cyclically. Rotation of the swash plate on sleeve 56 moves each rod 58 equally for collective pitch control. If the fuselage and rotor mast move about the rotor head gimbal (not shown), the rods 58 are moved in a vertical plane, and this has substantially no rotational effect on sleeve 60. A spring box 100 is again included between the swash plate and rotor head. In Figs. 6 and 7, the blade mounting comprises a drag hinge and a flapping hinge in a universal joint 70. A radius arm 71 on each blade mounting is connected by a link 72 to a central shaft 73, so that the shaft 73 rises or falls depending on the coning angle of the blades. A member 79 is universally mounted on the rotor mast 30, and has a part 81 splined to shaft 73. Three levers 82 are each pivoted to member 79 and to a lever 76 pivoted on an arm fast to the rotor head and connected to a pitch change scissors linkage 78. A link 75 extends between the centre of lever 82 and the swash plate 74. Tilting of the swash plate thus effects cyclic pitch. If the tip path plane tilts, member 79 tilts equally, and adjusts rod 82, to maintain the blade angle with respect to the top path plane constant. A spring box 100 extends between the swash plate and an arm 100 on member 79. The shaft 73 is universally jointed, in the same plane as the universal joint of member 79, to a shaft 73<SP>1</SP> passing down through the interior of the rotor mast. The position of this shaft which reflects the coning angle of the blades is multiplied by the pilots' fore-and-aft cyclic pitch signal in a mechanical multiplier, Fig. 9 (not shown), and the product is used to control the swash plate. In this way, instability resulting in acceleration of the helicopter in a direction perpendicular to the tip path plane is stated to be overcome.