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Aviation History
1995
1995 - 1335.PDF
HOVERSPEED Fit for survival Hovercraft are thriving — against all the odds — but maintenance can be a challenge. ALLAN WINN/LONDON Only a few years ago, it seemed that the large passenger-carrying hovercraft was doomed to extinction. It had only ever achieved a foothold in one major mar ket — across the Channel between England and France, and even then only a few of the biggest Westland/British Hovercraft SR-N4 craft ever entered service. The promising technology of the 1960s had, it seemed, been overtaken by improvements to traditional fer ries, and the increasing cost competitiveness of the air carriers. The building of the cross- channel rail tunnel seemed to be the final straw, as it would offer the same short journey times as die hovercraft, without its vulnerabil ity to weather and sea conditions. In the event, the hovercraft has survived, and it has been the Channel Tunnel which has attracted the troubles and die headlines. There is now just one cross-channel hover craft operator, running just two stretched SR- N4s — and even it is now running high-speed catamaran ferry services alongside the hover craft. According to Hoverspeed, bookings on its services this year are 9% up on those of 1994, despite the increased competition as tunnel services are expanded in its first full year of operation. The near-death of the hovercraft, and its subsequent reprieve have, however, produced some interesting challenges for the operator. Power for many more hours — a rebuilt Proteus The company's two surviving machines are now 25 years old — and some of the technol ogy used in them is much older still. In par ticular, the craft are driven by Bristol (now Rolls-Royce) Proteus gas-turbine engines which were originally developed in the early 1950s for the Saunders Roe Princess flying boat and the Bristol Britannia airliner, and its Rover auxiliary power units (APUs) are from the same era. Each SR-N4 has four Marine Proteus, each of which drives a 3.9m-diameter horizontal lift fan and a 6.9m-diameter pylon-mounted propeller, and two Rover (later Lucas) IS-90 APUs which provide electricity to operate the bow ramps and stern doors of the craft. Both die Proteus and the IS-90 are now, effectively, orphans — long out of production and with factory spares scarce or non-existent. As Clive Hunt, technical manager of Hoverspeed, puts it: "Rolls-Royce now has no parts in store, and we live off the second-hand market." R-R is, however, still supportive of Hoverspeed, says Hunt, and there is still one R-R-approved overhaul base for the Proteus — Volvo Aero Engines in Sweden. SECOND-HAND PARTS To make up for the lack of factory spares, Hoverspeed has two options: to buy second hand engines from elsewhere; or to have new parts made itself. In recent months, Hunt has managed to buy some 30 Proteus, including a large number from the private collection of Wensley Haydon-Baillie. Most of these engines are marine versions once used by the British, German and Greek navies, but a cou ple are ex-Britannia aero-engines. "The marine engines are best, because they are cor rosion-treated," says Hunt. "We have used aero-engine turbines on the backs of marine engines, and we could use complete aero-engines, but they are to a lower modification status than the marine engines, especially in blades, etc." Marinisation of the Proteus has taken place in stages over the years. Originally, the first stage of the aero turbine had Nimonic 105 blades, which lasted only l,000h. That material was replaced by X40, which was "great for corrosion, but it stretched and cracked", says propulsion man ager David Pashley. Later still, the blade material was changed to Inconel 738, which does not have the best life, but does not cor rode or creep. Because it has less chrome and nickel, it is sprayed widi a Sermatec Sermalloy J coating. The alternative to buying second-hand is to repair existing components or build new ones. The company is exploiting the sophisticated repair techniques which are now on the mar ket, and sending some parts out to turbine repair stations. After that, the choice is to have new parts made: "For instance, R-R had to have die disc material reforged in Glasgow, so we can make new bladed discs," says Hunt. The ability to repair the Proteus is helped by the fact that it was — almost by accident — the first modular gas turbine. "Back in 1973, they were costing £340,000 each to overhaul, and David Pashley was the first to suggest "Why not treat it as modular?'", says Hunt. R-R responded with a service bulletin saying (in effect) "This is now a modular engine" but, as Pashley points out, "...to change a turbine, you have to remove the whole of the combustion system". That does mean that a combustion chamber can be changed with the engine in situ, but: "...it's an extremely old engine, and some parts you would never design that way now." The alternative to all this would be, of course, to re-engine the hovercraft, but Hunt says that this has been rejected. "We came rea sonably close in the mid-1980s, when we looked at the Allison 570 and also the Lycoming TF40. The TF40 gave only a mar ginal increase in power over the Proteus — but we really need more power. Had the Proteus not been available, the only thing we could have used was the [R-R] Tyne — which is far too sophisticated," he says. The mean time between overhauls of the 56 FLIGHT INTERNATIONAL 10 - 16 May 1995
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