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Aviation History
1949
1949 - 0630.PDF
428 FLIGHT now go on to appraise the reasons for, and actions of, blade pitch-change. The angle of attack of a blade—or its pitch —if remaining unchanged during any one blade's sweep round the full circle, will result in a constant lift. And by altering the pitch of all three blades simultaneously and by a like amount, the lift of the rotor as a whole is appro- priately altered. This is referred to in rotary flight ter- minology as " collective '' pitch-change. There is, how- ever, another form of pitch alteration, and this is called '' cyclic '' pitch-change. Cyclic pitch is a self-explanatory term in that it implies the harmonic variation in pitch angle which a blade can make whilst rotating around the hub. For the sake of example, this can be illustrated as a blade pitch angle of, say, 5 deg when the blade is pointing north ; 10 deg when AIR HORSE the blade is pointing west; 15 deg when pointing south; 10 deg! when pointing east, and back to 5 deg at north. This supposes a rotor motion anti-clockwise in plan, as in the Air Horse. At this stage it might, perhaps, be as well to review briefly the method of blade articulation as given in detail in the previous issue. Each blade is carried off the hub by means of links pivoted horizontally at the inboard, and vertically at the outboard ends, in order to permit the blades both to flap up and down and also to vary their position in azimuth, i.e., to alter their angular dispositions relative one to each other. On the axis of the flapping link, and housed within the link, is a torque shaft incor- porating a universal joint coincident with the drag-hinge axis, the outboard portion of the torque shaft being formed as a semi-torous to engage with the twin-row, taper-roller bearing mounted on the drag hinge link. By this means, rotation of the torque shaft will effect blade pitch-change, any angularity due to movement of the blade about the drag hinge being accommodated by the universal joint in Above is a schematic illustration showing how the elbow crank of the cont gimbal-pivoting to tilt the head. At centre is a detail through the swash, tilted position, whilst at right is a schematic plan section of the head in showing how the swashplate spider rotates around the static the torque shaft. The twin-row, taper-roller bearing allows the blade pitch to be altered with quite low control forces despite the high centrifugal loading. Dual control is provided in the Air Horse for two pilots side-by-side, the primary flight controls being vested in a control column for "elevator" and "aileron" action, a pair of pedals for " rudder " action, and a sloping, pull-up/ push-down lever for ascent and descent action. There is no separate throttle control in the accepted sense: each "% common-collective pitch lever is mechanically linked to the engine throttle so that pull-up action of the lever to in- crease collective pitch and so obtain vertical lift also results in the simultaneous opening of the throttle. Con- versely, pushing the lever down to reduce collective pitch simultaneously closes the throttle. A refinement is added in the form of a spade-grip handle on the lever which can be turned to give a measure of over-ride in throttle control, in order to allow the engine power output to be matched accurately with the rotor power requirements. From the pilots' controls in the cockpit, a system of push-pull rods transmits control motions to the base of the In the diagram shown below, the port and starboard halves of the control assembly have been separated for clarity. Understanding of the action of the Servodyne assembly in translating pilot's control motions to pitch-change motion at the rotors is amplified by the detail drawing at right. FRONT ROTOR FRONT ROTOR STARBOARD ROTOR STARBOARD ROTOR COMMON COLLECTIVE - RICH LEVER ELEVATOR &. AILERON CONTROL COLUMN RUDDERPEDALS FORE/AFT PITCHING JACK . ROLLING N (DIFFERENTIAL COLLECTIVE) JACK \ THROTTLE CONTROL LINKAGE COMMONCOLLECTIVE JACK FORWARD FLIGHT (COLLECTIVE CYCLIC) JACK YAWING (DIFFERENTIAL CYCLIC) JACK PORT SERVODYNE ASSEMBLY (COLLECTIVE-PITCH) STARBOARD SERVODYNE , ASSEMBLY , (CYCLIC PITCH)
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