591,982. Helicopters. CIERVA AUTOGIRO CO., Ltd., and PULLIN, C. G. Aug. 1, 1945, No. 19758. [Class 4] In a helicopter having three lifting rotors disposed with their centres at the vertices of a triangle, the arrangement being symmetrical about the fore and aft vertical plane of symmetry, control of rolling is obtained by varying the collective pitch angles of the two side-by-side rotors by equal and opposite amounts, and control of pitching by varying the collective pitch angles of the sideby-side rotors by equal amounts in the same sense and the collective pitch angle of the third rotor in the opposite sense. As shown in Fig. 1, the helicopter comprises a body 20 provided with outriggers 21, 22 and 23 supporting rotors 24, 25 and 26, the inboard parts of the outriggers being of lattice girder construction and the outboard parts 21a, 22a and 23a of monocoque construction. The rotors are driven by a single liquid-cooled engine 28 through a bevel gearbox 29, provided with a brake for the driven shaft, universallyjointed transmission shafts 30, 31 and 32 and reduction gearboxes carried by the ends of the outriggers. The gearbox 29 comprises a compound clutch and freewheel coupling operated by a lever 294, Fig. 4, and drives a power take-off shaft for auxiliaries and a shaft 57 for a cooling fan 56 arranged in a vertical duct 55 containing the engine coolant radiator 54. A two-piece stream-lined fairing is arranged in the duct, the leading part 60 enclosing the fan gearbox 59 and the trailing part 61 forming the -core of the radiator. Each reduction gearbox comprises a bevel pinion driven by the shaft 30 (31 or 32) and meshing with a bevel gear driving the sun pinion of an epicyclic reduction gear, the latter comprising of a fixed annulus and planet wheels mounted in a cage driving the rotor hub. In the form shown, the reduction gearboxes are rigidly secured to the outriggers, but. according to a modification the gearboxes of the two side-by-side rotors may be mounted for rotation about the axes of the transmission shafts against the action of springs and locked in the same direction by a pulley and cable system coupled to a trimming control member. Each rotor hub has three flapping pivot pins 156, Fig. 12, on which are mounted drag links 157 having forked ends 161 slotted to accom. modate the forked ends of a blade root stub 166 mounted by co-axial drag pivot pins. The blade root, which consists of four elements 169-172, is rotatably mounted on the stub by bearings 174, 175, and the element 171 carries an inboard extension 177 connected by a universal joint 179 to a torque tube 178 supported in bearings in the drag link 157. The tube is integral with a lever 186 having a forked end ball-jointed to a link whose upper end is connected by a ball-joint 190 with one of three arms 147 of a swash-plate mechanism 146, Fig. 15. The ball-joint on the lever 186 is aligned with the axis of the flapping pivot pin and is located 90 degrees in advance of the blade axis. The collective pitch control mechanism of all three rotors is identical and comprises an axially-slidable hollow shaft 128, Fig. 15, rotatable by a pulley 104 (105 or 106) and formed with a Z crank 130 carrying an inclined ring 132 on a bearing 131. The ring 132 is prevented from turning by an integral arm 133 terminating in a ball 134 located in a radially-slidable socket 135. The axial displacement of the shaft caused by rotation of the pulley is transmitted through a bearing 137 to a non-rotatable .extension sleeve 138 which supports the swash-plate 146. In the two side-by-side rotors 24, 25, which are fitted for both collective and cyclic pitch control, the swashplate rotates on bearings 144, 145 carried by a hollow centre member 143 which is supported in bearings on trunnions 141 integral with a fork 140 on the sleeve 138. The swashplate is, therefore, free to rock about one horizontal axis only to provide cyclic pitch control of invariable phase, and this is effected by rotating a pulley 125 (126) to rotate a shaft 148 formed at its upper end with an inclined crank 151, which carries on bearings 152 an inclined ring 153 having trunnions 154 on which the member 143 is mounted by bearings 155. In the case of the rear rotor 26, which is not fitted, for cyclic pitch control, the member 143 is rigid with the sleeve 138, and the pulley 125 and shaft 148 are omitted. The control circuits for the pitch control mechanisms are shown in Figs. 4, 6, 7 and 8 and comprise a control column 62 mounted for fore-and-aft and lateral movement, a pitch-change lever 82 mounted on a rocking shaft 83, and a rudder bar 107. Fore-and-aft movement of the control column is transmitted by a rod 66, bell-crank lever 67 and rod 75 to one arm 77 of a universallymounted spider 79, and lateral movement by a rocking shaft 64, crank 68, link 69, bell-crank lever 70, rod 71, bell-crank lever 72 and rod 76 to a second arm 78 of the spider. Movement of the pitch-change lever 82 is transmitted by a rod 86 and bell-crank lever 87 to effect vertical movement of a pillar 74 on which the spider is mounted. The arm 77 and two further arms 89, 90 of the spider are connected by links 91, 92 and 93 to levers 94, 95 and 96 operating pulleys 97, 98 and 99 around which pass cables 100, 101, and 102. These cables are led over jockey pulleys 103 and operate the pulleys 104, 105 and 106 of the collective pitch control mechanisms of the three rotors. Operation of the lever 82 varies the pitch angles of all the rotors equally ; fore-and-aft movement of the column 62 decreases or increases the pitch of rotors 24, 25 and increases or decreases the pitch of rotor 26 to provide control of pitching of the helicopter ; and lateral movement of the column decreases or increases the pitch of rotor 24,and increases or decreases the pitch of rotor 25 to provide control of rolling of the helicopter. For any displacement of the.control column, the algebraic sum of the rotations imparted to pulleys 104, 105 and 106 is zero, so that the total lift of the rotors remains constant, at least in hovering flight. Movement of the rudder bar is transmitted by a rod 111, bell-crank lever 112, link 113, lever 114, links 116, 117, levers 118, 119, pulleys 120, 121, and cables 122, 123 to apply equal and opposite rotations to pulleys 125, 126 and effect differential cyclic pitch control of rotors 24, 25 to provide control in yaw. The rotors rotate in a counterclockwise direction and their axes are inclined in planes tangential to the circle containing the rotor centres to provide a counter-clockwise couple balancing the reaction torque. The rotor axes are also inclined inwardly in radial directions to improve the stability of the helicopter. A tricycle undercarriage is fitted and comprises three oleopneumatic wheel-carrying struts having a shock absorbing travel of about five feet, which are secured to the outriggers. According to the Provisional Specification, the cargo and passengers may be carried in a detachable container, if necessary of stream lined form, which can be secured to the underside of, or in a recess in the body ; the two side-by-side rotors may rotate in opposite directions and their axes inclined to compensate for the torque reaction of the rear rotor ; and control in yaw may be effected by cyclic pitch control of the rear rotor.