GB715119A - Improvements in or relating to rotorcraft - Google Patents

Abstract

715,119. Rotary wing aircraft. WESTLAND AIRCRAFT, Ltd. April 18, 1952 [April 30, 1951], No. 10102/51. Class 4 A rotary wing aircraft having a power driven rotor with an even number of blades exceeding two free to flap about horizontal pivots in a hub. said blades being coupled to permit rotation about their longitudinal axes for pitch variations and having mechanism producing pitch variations once per revolution of said hub, is provided with an eccentric pitch changing mechanism producing upon at least the tip portions ot the blades a pitch variation twice per revolution of said-hub. The rotor head comprises a tiltable swashplate having a non-rotary element 2 associated with a rotating member 3 connected to the pitch control arm 6 of each blade by a link 4. A pair of gear rings 8, 9 are mounted on the element 2, the ring 8 being fast therewith and the ring 9 being capable of a limited degree of rotation under the action of a screw jack Fig. 8, (not shown) controlled by the pilot. The number 3 carries, for each blade a radial extension each of which houses a pair of concentric rotatable shafts 16, 20 the shaft carrying a bevel pinion 15 meshing with the gear ring 8 and a spur pinion 17 meshing with a spur gear 18. The shaft 20 has corresponding gears 19 and 21 meshing respectively with the gear ring 9 and a further spur gear 22, the gears 18 and 22 being mounted by bearings 23 on a boss 24 for independent rotation. A lever 26 is pivoted at 25 to the gear 18 and is slotted at 27 to accommodate a block 28 engaged by a pin .29 attached to the gear 22 and projecting through a slot 30 in the gear 18 so that the lever 26 is capable of a 45 degrees swinging movement about the pin 25. Intermediate its length the lever 26 carries a pin 31 to which the lower end of the link 4 is pivotally attached. When the pin 31 is concentric with the boss 24 the rotor, when rotating, is subject only to the normal once-per-revolution pitch change movement imposed on the blades by the adjustment of the swashplate. If the pilot controls the jack to cause relative movement between the rings 8 and 9, the lever 26 will be displaced as shown in Fig. 3 so that an additional twice-per-revolution pitch change movement will be imparted to the rotor blades with a consequential twice-per-revolution flapping motion. This motion is generally selected by the pilot at low ratios of forward speed to top speed for at a high ratio the movement takes place automatically independent of the control mechanism described. The angular positions at which pitch change takes place may be varied by adjustment of both of the rings 8 and 9. Springs may be inserted in the pitch changing mechanism to impart a natural frequency of oscillation to the blades in resonance with the twice per revolution frequency imposed by the mechanism to reduce the forces on the control gear.