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 market - 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 ferries, 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 the hovercraft, without its vulnerability 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 the headlines. There is now just one cross-channel hovercraft operator, running just two stretched SR-N4s - and even it is now running high-speed catamaran ferry services alongside the hovercraft. 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. The company's two surviving machines are now 25 years old - and some of the technology used in them is much older still. In particular, 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 the 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 couple are ex-Britannia aero-engines. "The marine engines are best, because they are corrosion-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 1,000h. That material was replaced by X40, which was "great for corrosion, but it stretched and cracked", says propulsion manager David Pashley. Later still, the blade material was changed to Inconel 738, which does not have the best life, but does not corrode or creep. Because it has less chrome and nickel, it is sprayed with 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 market, 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 the 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 reasonably close in the mid-1980s, when we looked at the Allison 570 and also the Lycoming TF40. The TF40 gave only a marginal 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 Proteus is now around 6,000h, but on average the engines are removed every 1,500h. (Each craft is taken out of service for three months every year for a major refit, after which it is cleared for a further 1,800h.) Hunt says that, if there are problems, the company can end up changing 20 engines a year. "We did 43 changes in 1986, when we had five craft running," he says. "Over the last ten years, we have only had to remove three engines because they had too many problems to leave in, and our shut-down rate is well below 1%," says Hunt. The engines are monitored continuously by the flight engineer in flight, and are given borescope inspections every 150h.
All four engines live at the rear of the craft, with access hatches in the roof of each engine room. Engine-change takes only 4h from beginning to test running, which Hoverspeed is cleared to do on the craft itself. The craft is certificated to run fully loaded - but not to depart - on three engines.
APU PROSPECTS
The outlook is not quite so positive for the APU. Lucas can no longer support the Rover unit, which has a particularly demanding life on the hovercraft. "They're loaded four times in 5.5min, as they start each engine," says Hunt - the Proteus has a 110V electric starter which "...does very well". It is not that the Rover is particularly rare - many were used by several navies. "The Danish navy has thrown a lot out, but there's no paperwork with them, so they're worthless," says Hunt. That applies not only to the gas-turbine part of the APU: there are no usable spares left for the generators either, but Hoverspeed tries to keep a stock of serviceable repaired units.
The APU runs at 46,000RPM and its generator at 8,000RPM. It generates 200V, three-phase, 400-cycle electricity, and uses 73litres (16gal) of fuel an hour. In contrast, each Proteus consumes just over 900litres/h. The power section of a Proteus runs normally at 10,200RPM, driving the lift fans through reduction gearing at 517RPM in the cruise, or 575RPM at full throttle. The two-stage power turbine of the Proteus is, in any event, governed to 10,720RPM, as the propellers have a 667RPM maximum permissible speed.
The propellers are dismantled and inspected every 1,500h, or 12 months, and non-destructive tests carried out on all components. The nickel leading-edge strip can last only half a season, however, so blades may have to be changed. "We can change blades in pairs, but can't balance them," says Pashley, "so we try to keep two fully assembled props in stock at all times."
The blade has a Dural aluminium alloy H-section spar, with two glassfibre-reinforced- plastic shells, with a foam in-fill and a nickel-plated leading edge on a rubber backing. Experience has shown that the shell-to-spar bond is critical. A "tap" test with a hammer is used to check for shell-spar bond failures, for which the only cure is to strip off the shell and install a new one. British Aerospace did devise a quarter-shell repair - Hunt says that it is sometimes successful, sometimes not. The blades are protected by a urethane paint, which yields and recovers with a capillary action, which is difficult to apply but "excellent", says Hunt.
The UK Civil Aviation Authority remains the regulatory authority for hovercraft operations in the UK, and pays a "huge" amount of attention to items like the propellers - and with good reason. According to Pashley, in the cruise, the 6.9m-diameter propeller is turning at just 630RPM - but delivering 5t of thrust, and imparting a centrifugal load on the barrel (hub) of some 60t. (To underline the importance of their integrity, there is no armouring beneath the propellers - nor around the 46,000RPM APU.)
The environment in which they operate is particularly harsh - especially at the French end of the crossing, where there is flying sand and, as Hunt says, "...in rough weather, they're almost operating under water, too". The propellers are fully reversible, but they are also mounted on pylons and move around their axes, so there are precessional loads as well. They suffer a lot of stress from the short-cycle nature of the operation: on an annual average, the engines are started every 45min. The life of rear propellers is much lower than that of front ones, because of the slipstream from the front: "The thing we fear most is a 'FO' [front only] sticker," says Hunt.
"The prop is far and away the most difficult component we have to maintain," says Pashley. The propellers were originally designed by de Havilland, which became a part of British Aerospace Dynamics. BAe eventually sold the design and the business to H + S Aviation. Hunt says that, although the sale to H + S left the hovercraft operation without some of the previous boffins, the propeller situation is under control.
"CREATIVE" REPAIRS
"They're requiring more and more creative repair and support schemes without endangering life," says Hunt, "but where the prop had a 7,500h design life, it's now up to 14,000h." He says that before the BAe propeller shop closed, a massive validation exercise was run in the fatigue laboratory in which the barrel was taken to 15,000h, and the blade to 20,000h.
At the same time, Hoverspeed bought from BAe all the remaining forgings and spares, for both blades and barrels. As a result of this, and some new manufacturing, in three months' time it will get its first set of new blades. Where the previous ones had been made at BAe Lostock, the new blades have been made at Westland, and the barrels by Ratier in France. Such manufacture does not come cheaply: each blade costs £60,000, and a barrel costs £35,000 ("...and that's with us supplying the forging, too", says Hunt).
The important thing is that, through such schemes, Hunt and an engineering staff of up to 72 during refits can keep these now-unique craft flying, to the very demanding standards laid down by the CAA.
"It won't be through a lack of bits that this show stops," says Hunt.
Source: Flight International
