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Aviation History
1949
1949 - 0675.PDF
April 21st, 1949 459 Epicyclic reduction gear with upper stage displaced < the blades axe quite flexible in span wise bending, but are extremely stiflE both in torsion and in drag-induced bending. Another somewhat interesting aspect of structure is that the blades are built up on a completely unperforated D.T.D.254 hollow steel spar, which is of circular section for the inboard 8oin of its length, thence changing to an elliptical section of constant chord and depth out to the tip. The consonant aerofoil sections are N.A.C.A. 23022 inboard and N.A.C.A. 23015 outboard, the chord remain- ing constant throughout blade span. Spruce, birch-faced plywood-sandwich chordal ribs are spaced at 4in centres, each being attached to the spar with an enclosing clip, bolt-tensioned in the case of the circular section spar root, and wedge-tightened for the elliptical section span. Blade covering is made with a rVin three-ply birch skin, attached to a spruce trailing edge block and a mahogany leading edge block which is armed with a flush-fitting brass sheath. Control and Blade Action Reverting to the rotor head, on the underside of the spider is bolted the swashplate carrier with a two-row peri- pheral ball bearing to the cast D.T.D. 133 light alloy swash- plate annulus. This latter does not rotate, being held, through a ball/socket-ended connecting strut, to the anchor stay off the rotor-brake back plate. On the fore-and-aft centre-line of the aircraft and at 90 deg to port, the swashplate is double-universally pivoted to the heads of a pair of actuating jacks, the forward unit giving plus and minus 6 deg fore/aft tilt, and the port unit giving plus and minus 5 deg lateral tilt. The bases of the jacks are double-universally pivoted to brackets carried at the base of the rotor supporting cone through to the pylon ring, and at the head attached to the brake back-plate. Motion at the instigation of individual or combined jack movement is imposed on the swashplate to tilt the whole head about the pivotal centre of the trunnion, and so bring about pitch-change of the rotor blades. Before examining the manner in which this occurs, however, the physical control system employed should be appraised. From the control columns in the cockpit, push-pull rods transmit motion to a pair of swing links, each one of which serves a head jack. From the swing link, a con- necting strut is universally pivoted at the foot and ball/ socket jointed at the head to the rod of the jack actuating valve; the latter is an integral element with the jack itself. Thus, movement of the cockpit control columns results in movement of the control valves, so directing appropriate oil flow to the jacks to give the required movement. The fact that the valve body is integral with the jack cylinder results in automatic follow-up. So many designers have followed the example of Detail of hydraulic expanding twin-shoe rotor brake Sikorsky in using direct application of collective and cyclic pitch that this system has come to be regarded virtually as orthodox helicopter control practice. Perhaps due to his background experience with the Cierva Autogiro, however, Dr. J. A. J. Bennett, who heads the Fairey helicopter design team, has favoured a tilting head system for the Gyrodyne, in many ways reminiscent of that used for the various Autogiros. There is no direct application of cyclic pitch control to the Gyrodyne rotor blades, the pitch being altered cyclically purely as a function of head tilt. That is to say, as each blade is attached to the head at a fixed angle of incidence, the fact of tilting the head in any one direction will lead to the blade angle of attack being reduced as the blade rotates towards the direction of tilt, or " downhill," and increased as the blade rotates away from the direction of tilt, or "uphill." The tendency is, of course, for the blade tip-path plane to remain parallel with the plane of the head; but when the head is tilted, the blade tips will lag and will follow a roughly helical path until they once more rotate in a plane parallel to the tilted head. This they do by virtue of the flapping linkage. The collective pitch effect is, perhaps, slightly more diffi- cult to understand. As mentioned, each blade is set at a given angle of incidence in the static case, the blade axis being disposed approximately at an angle of 60 deg to the axis of its flapping hinge, this angle being termed the delta angle. A moment's consideration will make it clear that, as the blade varies its position in azimuth, the blade pitch must necessarily change accordingly as the delta angle increases or decreases. Thus, if the aircraft is imagined as hovering, and the pilot desires to increase height, he merely raises the throttle lever which increases the boost, and thus <the power output of the engine, so applying greater torque to the rotor. When this takes place, the blades tend naturally to lag (swinging about their drag hinges) so increasing the delta angle, and thus their pitch, to bring about an increase in lift. The cone angle of the rotor disc is, of course, a. function of the lift and centrifugal forces imposed on the blades. To balance the forward thrust of the airscrew at zero forward speed, the rotor tip-path plane is so arranged as to be inclined backward, so that the rearward component of the thrust vector affords compensation. Forward flight is achieved by decreasing the rearward inclination of the rotor tip-path plane and, in point of fact, it is not until a fairly high forward speed has been reached that the tip-path plane becomes horizontal. The normal operating condition of the Gyrodyne is, therefore, with both the fuselage and tip-path plane substantially level; a material advantage not only in rotor aerodynamics but also in pilot and passenger comfort. ' C. B. B-W.
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