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Aviation History
1951
1951 - 0513.PDF
16 March 1951 the ease of blade folding, and the outer cover of the blade shank is now carried on needle-roller bearings instead of the caged-roller units previously used. Another innovation is that the basic pitch setting of the rotor blades is made by adjusting a screwed pin in the fork-end of the blade shank, the root-arm of the drag-link from the spider being carried on the screwed pin. The blades themselves have been changed to a sym- metrical-section, chiefly because the original reflex trailing- edge cambered blades were both difficult to make and suffered from weathering effects which changed their aerodynamic characteristics. The section now used is an N.A.C.A. 0015 at the root, which tapers to a 0006 at the tip; the blades are not high-speed sections, and their performance is virtually the same as their cambered predecessors. Structurally, they comprise a Hydulignum leading-edge spar with four-ply skins top and bottom in five spanwise panels with a spruce trailing- edge fillet. Chordal ribs have ply webs with spruce booms glued to the skin. Each blade has a pair of ground-setting track-adjusting tabs centred on the 13OU1 and 258m radii; they are simply light-alloy strips, Redux-bonded and cold- glued to the trailing edge. At the leading edge, the blade skins are butt-faired to a brass leading-edge sheath, screwed to the spar and split diagonally into 4in lengths to permit blade flexure. Although at first sight forbiddingly complex, the control system in the 173 can fairly readily be sorted out into its component elements. Before attempting this, however, the definition should be given that cyclic pitch involves a regular (sinusoidal) variation of blade incidence (or pitch) with azimuthal position of the blade : collective pitch involves a simultaneous variation of incidence (pitch) on all blades. The basic control requirements are as follows : (i) that both rotors can simultaneously be controlled in cyclic pitch to give the aircraft direction in flight; (ii) that the cyclic pitch of the rotors can be imposed differentially in order to give the air- craft a turning moment; (iii) that collective pitch in both rotors can be controlled simultaneously for ascending, hover- ing and descending; (iv) that trimming means are provided to control the attitude of the aircraft under varying load conditions; (v) that biasing means are provided to trim-out operating loads in the cyclic pitch lever; (vi) that the power of both engines is increased simultaneously with collective pitch (with an overriding control over a small range); (vii) that independent "ground-running" control of the engines is provided. Cyclic pitch control is effected by the swan-neck column annotated No. 1 in the accompanying diagram. To the base of the column is attached a triangulated articulating mechanism (2) which operates by means of bell-crank levers and push-pull rods similar triangulated articulations (3 and 4) which pick up to the heels of the rotor spider stalks (5 and 6). Movement of the rotor spider stalks is sympathetic both in degree and sense with movement of the control column. Cyclic pitch control provides a blade incidence change of ~r5 degrees fore and aft, and _+4 degrees laterally. Yawing control is effected through the "rudder" pedals to impose differential lateral movement of the spider stalks. This is accomplished by mounting on eccentric shafts (7) the bell-cranks which actuate the articulation assemblies (3 and 4). The eccentric shafts are integral with chain sprockets (8), and thus by operating the yaw control pedals, the sprockets are rotated in opposite directions, so displacing the articulating mechanisms laterally but in opposite senses. Collective pitch-change is effected through movement of the collective pitch lever (9), from the cross-shaft of which an irreversible unit (10) is connected to a T-lever system, mounted on an eccentric shaft (11). The forward arm of the T-lever is coupled direct by a push-pull tube to the front rotor control-sleeve in which is carried the control spider (5). The rearward arm of the T-lever is coupled indirectly through levers and push-pull rods to the rear rotor control- sleeve. Operation of the pitch lever will thus raise or lower both rotor spider assemblies simultaneously and by an equal amount, the range covered being 10 degrees of blade incidence change. The purpose of the irreversible link (10) is to arrest any rotor vibration loads which may be transmitted into the system. Similar vibration loads in the rear rotor might lead 319 Detail of rotor blade construction showing method of anchorage to the tie-bar-and-sleeve blade shank. to resonance in the lengthy control run back to the irreversiblelink, and therefore an inertia damper (12) is embodied to absorb any such vibrations near the source. The ratio of front to rear rotor pitch-change provided by movement of the collective pitch lever(s) is nominally 1:1. Should flight-testing, however, reveal a nose-up or nose- down tendency in the aircraft when the pitch lever is suddenly raised or lowered, some adjustment to this basic ratio might be desirable. Provision for this is made by mounting the bell-crank (13) eccentrically, whereby the effective lever arm may be lengthened or shortened by ±2 per cent. Whilst simultaneous operation of collective pitch will affect only the height at which the aircraft is flying, a differential change of collective pitch will affect fore and aft attitude by producing more lift at one rotor and less at the other. This feature is used to provide longitudinal trim. Operation of the trim wheel (14) will, through cam assembly (15) and load- limiting strut (16), raise the front and lower the rear rotor control-sleeves (and therefore the spiders) or vice versa, by virtue of the fact that shaft (11) is eccentrically mounted. As the attitude of the aircraft is affected, differential collective pitch will produce a forward or rearward thrust component which can be used to provide a powerful force either to accelerate or retard the aircraft. It is obviously advantageous to make use of this feature, and it is for this reason that differential-collective pitch change is inter- connected to the fore and aft movement of the cyclic pitch control column. This cyclic/collective mixing is effected by means of the cam assembly (15) attached to the torque tube on which the cyclic pitch levers are mounted, and thus movement of the cyclic control column in, say, a forward direction will operate the eccentric shaft (11) in the same manner as the forward trim. Rotation of hand-wheels (17 and 18) displaces the spring- bias unit (19) to trim-out the operating loads in the cyclic pitch control lever(s). The engine controls (20 and 21) are normally operated through the bell-crank assemblies (22) by rotation of the twist-grip (23) or by movement of the collective-pitch lever. In this manner, a synchronized control over both engines is obtained. It is essential that either engine may be isolated during ground-running, and this is achieved by mounting bell-crank assemblies (22) on an eccentric shaft (24) and operating them via push-pull rods from levers (25) mounted in the control console. The levers are provided with a locating gate in the "flight" position only, and will return to it automatically, if not already so located, when the pitch lever is brought into operation. For ground-running, the collective pitch lever is locked in its minimum-pitch position with the twist-grip against its slow-running stop. Manipula- tion of the ground-running levers then permits either engine to be run at 67 per cent power, driving both rotors at minimum pitch and maximum r.p.m. C.B.B-W.
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