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Aviation History
1959
1959 - 1395.PDF
678 Bell are another advocate of all-bonded construction. The section shown is the Bell H-40 blade which has a chord width of about ISWm. The backbone of this blade is the extruded aluminium alloy spar. This is faced with a brass and aluminium nose block and a stainless steel leading edge; the light trailing edge section, thin sheet skins and I-spar are all aluminium alloy Vertol blades are an assembly of individual light alloy boxes forming the trailing edge section bonded to a steel D-section spar. A C.A.A.- approved service life of 6,000 hr has been granted for these blades on the Vertol 44 and they are also used on H-2h. The manufacturers claim that they have a vibration level equal to or better than that of wooden blades in current operational use FLIGHT, 15 May 1959 Rotor Blade Construction... and free from stress-raising bolt and rivet holes that limit fatiguelife—-even the root-end doublers are bonded in place; they are held in position before curing by spot-tacking with tape adhesive. Sikorsky found that bonding the trailing-edge pockets to thespar was absolutely necessary to obtain good fatigue character- istics. A similar technique is adopted by Vertol, who bond atrailing-edge section fabricated from a series of aluminium boxes on to a D-section steel spar. The light alloy boxes, consisting ofribs and skins, are also assembled by bonding. All of these bonded blades have baen cleared for service lives of 6,000 hr. The rotor blades of the ramjet tip-drive Dutch Kolibrie are avariation of American practice. The nose section is a light alloy extrusion (through which are ducted the fuel and ignition lines tothe tip ramjets) and this is bonded to Alclad sheets of U-shaped cross section to complete the profile. Surprisingly, there are noribs and no filling in the trailing-edge section at all, although the blade is stiffened and thickened at the inner end by Redux-bonded covering sheets. In contrast, the big Rotodyne blades are riveted assemblies of steel spars, skins and pressed ribs, the useof stainless steel on the main load-bearing structure v reducingsusceptibility to fatigue. To complete the picture, the accom- panying sketches show two methods of wooden blade construc-tion, the bonded Hordern-Richmond blades for the Bell 47 being noteworthy for die number of different materials used. Blades forproduction Saunders-Roe P.531s are manufactured by the same firm and construction—a spruce spar sheathed in stainless steelwith a spruce and balsa body—follows similar lines to the Bell blades. Prototype P.531 blades are based on Skeeter practice. Before a new blade is ready for service, balancing and an exten- Load-bearing portions of the Rotodyne blade are manufactured from steel. A machined D.T.D.730 front spar is slotted to carp/ an adjustable chordwise balance bar and the complete nose section is covered with 20 s.w.g. stainless steel skin. Aft of the rear spar construction is light alloy. Very light gauge material (28 s.w.g. ribs, 30 s.w.g. skins) is used and the trailing edge is Hycar foam rubber sive programme of fatigue tests must be carried out. Adjustablecounterweights are used to balance the blade against a master and the weight is checked at three positions so that the centre ofgravity of all blades is identical. Finally, static balancing is followed by aerodynamic balancing on a whirl-test stand. As many as 200 blades may be subjected to fatigue tests toreduce the scatter of random results and a typical programme will involve strain-gauge testing both under laboratory conditions andlater in flight so that results can be compared. Resonance testing ensures that the natural frequency does not coincide with amultiple of rotor r.p.m. and several thousand hours whirl-rig testing may be devoted to the development of a particular rotorhead and set of blades. Although a number of new methods are being employed inreducing design and development time, few short cuts are possible in the lengthy process of producing a new rotor. Computers helpto reduce the time needed to resolve conflicting design para- meters, and blade-mounted cameras have proved a useful flight-testing tool for measuring actual blade deflections. Perhaps the most promising next steps are new techniques for testing scalemodels of blades in a wind runnel, where there are possibilities of forecasting accurately the stresses likely to be encountered.Such a method of eliminating trouble at a very early stage could reduce development time by months or years. A.T.P. GLASS CLOTH COVERING PLASTIC TAPE SPRUCE TRAILING EDGE BALSA SRUCE BODY BONDED STAINLESS STEEL SHEATH BIRCH SPAR MILD STEELCORE Two types of wooden construction. Left, Hordern-Richmond''s composite birch-balsa blade used on the Bell 47. The root end fitting is bolted through the mild steel core that is used solely for mass balancing. The Sounders Roe Skeeter blade (right) is based on an Accles and Pollock steel-tube spar. Covering is madapollam, doped and painted
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