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Aviation History
1947
1947 - 1891.PDF
OCTOBER 30TH, 1947 FLIGHT 495 Bristol Helicopter climb, maintain altitude, or descend whilst, at the same time,moving forward, backward or sideways. We have not so far mentioned turning control. This isachieved by means of the tail rotor which is caused, by means of changing its blade pitch and therefore thrust, to overcome,exactly counterbalance, or succumb to, the torque reaction of the main rotor, in the Bristol helicopter, where the mainrotor turns in a clockwise direction viewed from above, torque reaction will naturally tend to swing the nose of the machineto port. The tail rotor, being mounted on the starboard side of the boom, is thus nominally a pusher airscrew. Having run through the power transmission and blade pitchchange motions, we can now consider the structural build-up of the rotor head. As we have seen, torque is transmitted fromthe inner shell of the upper reduction gear direct to the rotor hub proper. This forms the common anchorage for the rotorldes, each of which is pivotally mounted by a needle-bearing flapping-hinge. The "blade" half of the flapping-hinge is aforged link, which is bored at right angles to the flapping axis to accept a sleeve member around which the blade can swinga limited amount and so form a drag hinge. By this means, the at the head of which is carried each of the radial arms whichpick-up to the cantilevers off the blade sleeves. This forms the «spider, and a little below the stalk head is fashioned a ballwhich, housed in an appropriate socket, permits a stalk move- ment in the form of a double cone, 01 hour-glass, the " waist "pivotal point of which is the centre of the ball. It may be ap- preciated that displacement of the stalk from the vertical will,via the spider arms, set a given pitch an^le to each rotor blade and, as the blades rotate around the inclined axis of the stalk,their pitch-change will be varied cyclically. The socket in which the spider stalk ball is retained is itselfformed in the head of a conical sleeve which surrounds the stalk. This sleeve is capable of being moved axially and bythis means collective pitch control is achieved. Each rotor blade is 20ft 2.4in long from the anchorage pinsto the tip, and is built up on an improved-wood laminated spar with spruce ply-faced chordal ribs, spruce trailing edge stripand a special ply covering. The leading edge is sheathed with brass strip, the blade as a whole being covered with an acrylic-resin skin. Anchorage to the root arms is made with three steel plates top and bottom, sandwiched with two woodlaminae of the spar root, the whole being cramped up by bolts to give a "clutch" friction joint of large contact area. Asalready mentioned, the blade attachment is made with two pins to the end of the blade sleeve, andextraction of one pin allows the blade to be folded back for stowage in confinedspaces. At each blade element the centrifugalforce is arranged so as exactly to counter- balance the lift, and at the outer bladestations where the lift is very power- ful, lead weights are let into the spaito give the requisite centrifugal force. This design feature of maintaining abalance between centrifugal force an<J lift applies, of course, only at one par-ticular condition, but the variation throughout the whole rotor r.p.m. andpitch ranges is relatively slight. SIDEWAYS FLICHT TO STARBOARD Thess diagrams serve to show how the position of the rotor thrust axis relative to the " aircraft's centre of gravity produces a component force to give translationai flight. angular displacement of the blades is allowed to change slightlyin order to permit a degree of angular acceleration and decelera- tion during rotation. Each drag-hinge sleeve is surmounted bya friction damper, the compression of which is maintained by a low-rate spring-stack of rubber rings. The actual blade anchorage to the link is formed by an eye0rith which is integrally fashioned a sleeve: also incorporated with the eye is the root anchorage for a segmentally divided tie-bar which is housed axially within the sleeve, its opposite (blade) end being similarly anchored to the end of an outerenclosing sleeve. It may thus be appreciated that the trans- mission of all loads between blade and rotor is made throughthe tie-bar, which, in addition to acting as a tensile member catering for centrifugal loads, also functions as a torsion barto cater for blade movement in pitch. The outer extremities of the blade sleeves are fashioned withmultiple fork-ends with which lugs on the blade proper register for anchorage with two pins. At the inner ends of the bladesleeves are the cantilever arms whereby pitch-change motion is transmitted from the hub spider, but before going on to con-sider this, it should be stated that the inner blade-sleeve eye at the drag hinge embodies lugs on each side for attachment ofdrag struts which connect each blade with its fellows. These drag struts incorporate rubber compression/tension buffer-padswhereby the peak oscillations of angular movement are cushioned. Running up through the centre of the rotor hub is a stalk, Control Connections The run of primary controls transmis-sion between cockpit and rotor can be seen in the accompanying illustration andit is, perhaps, sufficient to state that the control columns for producing cyclicpitch change have their movement re- produced by the spider stalk. Like-wise, the collective pitch control lever is connected so as to produce a sympa-thetic movement of the cone sleeve carrying the stalk pivot bail. The col-lective pitch lever has a movement range of 51 deg which results in a blade pitch-change of plus 12 deg from zero or feathering pitch. We should here mention the inter-con-nection between.the collective pitch lever and the engine throttle. This is a Tele-flex cable, one end of which is anchored to the collective pitch lever, with the outer sheath terminating at a cam-lever linkedto-the throttle butterfly. The core of the cable passes straight through the cam-lever to terminate at a power control lever inthe cockpit. On the Bowden principle of curvature deforma- tion, movement of either end of the cable independently (i.e.,collective pitch lever or power control lever) will result in an appropriate movement of the cam controlling he throttle but-terfly. The cam is necessary to correlate throttle opening to blade pitch, so that rotor speed and engine boost are kept inthe appropriate relationship: thus, rotor speed is independent of blade pitch. Normal rotor speed range is from 230 r.p.m.to 287 r.p.m. and for any giv^rf flight condition the combina- tion can be either high rotor speed with low blade pitch, orlow rotor speed with higjr blade pitch the rotor s^eej^ being CENTRIFUGAL FORCE CENTRIFUGALFORCE A simple illustration of the way in which the balance between lifi and centrifugal force relieves the blades of bending loads and maintains the cone angle largely constant
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