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Aviation History
1981
1981 - 0087.PDF
Aerospatiale Twin Squirrel IAN PARKER reports from Marignane Cutaway by JOHN MARSDEN AEROSPATIALE has developed the iAS.355 Twin Squirrel from the single-engined AS.350 for no other reason than safety. Flight over in habited areas and the sea requires high engine reliability regardless of aircraft type. But despite the ex tremely high reliability of modern gas turbine engines failures do occur, and the consequences of helicopter engine failure can be more serious than for a fixed-wing aircraft. This is not to suggest that helicopter autorotation presents any more intrinsic danger than a fixed-wing forced landing, but the choice of touchdown area is more limited. Two engines are much safer than one, but a few years ago it was not clear whether the market existed for a light-weight twin. In 1977 Aerospatiale surveyed the North American and European mar kets and was surprised by the demand for an economical, light, twin-engine helicopter. Petroleum Helicopters, for example, expressed a need for a light twin for VFR flying to its oil rigs in the Gulf of Mexico. Aerospatiale car ried out feasibility studies for install ing two engines on the AS.350 air frame without making major modifica tions to the power train, while increas ing maximum weight by minor rein forcements to the airframe. The idea was to keep costs down by incorporat ing a large number of AS.350 parts and benefiting from proven reliability. The decision to launch the programme was taken in November 1978 and announced at the HAA Convention in Las Vegas in February 1979. Orders for 107 aircraft were taken at this juncture, and the orderbook now exceeds 270. Certification for VFR flight was obtained on October 24 last year, and IFR certification should be gained at the end of 1982. This will require doubling-up of certain elec trical and hydraulic systems. Apart from a few minor reinforce ments, the cabin and fuselage arrange ments are the same as the AS.350. Main dimensions, cabin space and baggage holds remain unchanged, but visibility has been improved by en larged chin windows and optional lower windows in the main door. Twin Squirrel's Starflex rotorhead is identical to that fitted on the single, and has been in existence for several years. But it is still worthy of close consideration. Aerospatiale has its own glassfibre reinforced plastic Above Twin Squirrel's large chin windows improve visibility for landing (GFRP) production shop at Marig nane, where the basic Starflex com ponents are made. About 400 layers of glassfibre matting are cut by hand and heat pressed to form the basic star. Experiments with laser cutting are in progress, and this should enable large quantities of material to be cut at once. The basic star forms the principle link with the blades via sleeves allowing blade flap, drag and pitch movements without bearings (see cutaway). Centrifugal loads are transmitted to the non-flexible central area of the star by the sleeves, with flexible links provided between the sleeve and the star arm. A laminated spherical bearing sandwich made of steel cups and thin elastomeric sheets is located through a hole behind the flapping area of the star and two more elastomeric flanges, which distort under shear loads, are located at the outboard end of the sleeve securing it to the star arm. The whole system is practically maintenance-free and the fail-safe characteristics are extremely good. Any crack damage progresses very slowly and is easily visible. All parts are bolted and re placement of any part is simple. Total weight of the main rotor head is 1211b. By comparison, an Alouette III main
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