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Aviation History
1959
1959 - 1394.PDF
15 May 1959 «77 Rotor BladeConstruction A DESIGN NOTEBOOK OF BRITISH, AMERICAN AND CONTINENTAL PRACTICE HELICOPTER rotor blades offer as great a challenge to theengineer as any airframe component. Weight is critical,stiffness particularly important, protective finish essential and research and fatigue analysis most necessary. Much worth-while development has been done in recent years and fatigue problems have been overcome to an extent where blade lives of6,000 hr are now reasonably common and full interchangeability is possible. While the construction of wooden rotor blades islargely based on extensions of wing or propeller manufacturing techniques, unique methods have been developed for the produc-tion of metal rotor blades. Until recently, much of this informa- tion was secret, but in January Sikorsky work in this field wasrevealed in a lecture to the I.A.S. in New York, and last year Bell published information on its bonded metal blades. The experienceof these companies has been drawn upon in compiling these notes. Whatever constructional techniques are employed—andmethods are legion—the same problems have first to be solved before detail design can begin. A basic parameter is blade discdiameter, because upon this will depend the amount of thrust that can be obtained for a given power, the relationship of disc topower loading (which determines the hovering performance and power available for climb) and, quite simply, the overall size ofche machine. A limit on blade diameter is set by the structure weight requiredto give sufficient static strength to the blade, and rotational speed is limited by the power available. Stalling of the retreat-ing blade under conditions of high disc loading and high forward speed can be avoided by increasing rotational speed, but only upto the point where compressibility effects are encountered at the tips of the advancing blades. Other ways of delaying ihe stall ofthe retreating blade are to increase the solidity factor by increasing the number of blades or the blade width (structural factors set thepractical limits) or to use negative blade twist. Although "exotic" aerofoils with high lift characteristics havebeen used experimentally, close control of tolerances is necessary and the high control loads which may occur outweigh theoreticaladvantages over the more widely used symmetrical sections such as the NACA 0012 or 0015 series. Nevertheless, wind tunneltests continue and some promising results—particularly at high Mach numbers—have been obtainedThe first Sikorsky metal blades consisted of an integral spar with web-like partitions faired by sheet metal trailing edge sectionand with reinforcing plates bonded to the root to take the localized stresses. Subsequently, an extruded spar was used with an upsetend at the root, but fabricating difficulties were encountered and Vertol 44 the full length of extruded spar is now machined to leave a heavyroot attachment. The thickness of the machined metal is checked with ultrasonic test equipment and many production spar assem-blies are fatigue tested. Individual pockets forming a segmented trailing edge are bonded to this extruded spar and the gap betweenadjacent pockets is sealed by resilient material fitted into the cavity formed by the ends of adjoining pockets. The pockets arethus sealed, but not rigidly fastened together. Two particular advantages are claimed for this type of con-struction. Firstly the segments will resist the high transient edge- wise loads along the trailing edge which tended to buckle thecontinuous-strip trailing edges of earlier metal blades, and secondly, the individual pockets can easily be removed for re-placement or repair. This feature is also to be found on the blades of the Fairey Rotodyne, which, although almost entirelystainless steel, have an articulated trailing edge of aluminium alloy skin-rib boxes. Sikorsky and Bell blades are fabricated principally in alumi-nium alloy. Sikorsky use 6061-T6 material for its good extrusion properties, good corrosion-resistance and low notch sensitivity,and they say that after many years of experience with this alloy there is no real advantage to be gained by changing to anythingelse, although titanium, steel, fibreglass and plastics have all been investigated. The basis of the Bell bonded metal blades is a heat-treated2014-T6 aluminium alloy spar to which .020in thick sheets of stretcher-levelled 2024-T81 skins are attached—the latter areselected to preclude susceptibility to inter-granular corrosion and to give the best resistance to notch sensitivity. The nose blockextrusion at the inboard end (see photograph on p. 678) is 2024-T6 aluminium alloy, but outboard hard drawn naval brass is usedof nearly the same coefficient of expansion as the aluminium. The trailing edge is a continuous extrusion; to improve theforward mass balance in the tip area it is tapered from mid-span. The leading edge is sheathed in stainless steel. On completion of all machining and cleaning operations thecomponents of Bell blades are bonded together in an autoclave, a holding fixture preventing sagging and controlling the twist ofthe bonded blade. Rubber bags used within the blade, as well as outside it, maintain an even pressure on all the joints and allowheat and pressure to be applied simultaneously. In operation the autoclave is pressurized to 100 Ib/sq in before heat is taken above150 deg F, so that relative movement of the various components can take place while the adhesive softens. Final bonding tem-perature is 400 deg F. After ultrasonic testing the finished bonded blade is neat, strong Three examples of bonded metal blades. Some 20,000 Sikorsky blades (left) have been built and millions of blade hours accumulated. The basis of the blade is an extruded aluminium alloy D-section nose, to which are attached individual trailing-edge sections. N.H.I. Kolibrie helicopter blades (centre) are stiffened by Redux-bonded plates where the root is attached to the laminated-spring hinges. Hiller blades (right) are similarly reinforced by a multitude of doubters; these blades are designed and built for Hiller by the Parsons Corporation
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