Rolls Royce Tay Set for Success

A third Tay application replacement engines for BAe One-Elevens has recently been under consideration. San Antonio-based aircraft outfitter Dee Howard has identified more than 20 corporate operators interested in converting their One-Elevens to Tay power. The conversion, costing around US$6. 5 million, would add 25% to the aircraft’s range and reduce fuel consumption by 15% Dee Howard sees corporate One-Elevens as being the only type worth refitting with Tays, owing to the considerable airframe life remaining in these aircraft. Nevertheless, the company is hoping to complete a deal with Romanian One-Eleven licensee Rombac, which is building new aircraft.

In seeking a successor to the Spey, Rolls Royce was primarily concerned with meeting the latest International Civil Aviation Organisation (ICAO) noise regulations, due for introduction in January 1988. It also wanted to improve fuel efficiency without compromising reliability.

To begin with Rolls Royce studied changes that might be made to the Spey itself. A fundamental problem with this engine when considering its use on a new aircraft is its high jet velocity. The characteristic roar at takeoff makes it unpopular with communities near the airport and incapable of meeting ICAO’S noise requirements.

The most effective way to reduce significantly the jet velocity while retaining the thrust was to increase the mass flow of air through the engine by raising its bypass ratio- the ratio of the amount of air flowing around the core to that flowing through it. This can be accomplished by increasing the performance of the fan at the front of the engine, which in turn entails extra work for the low-pressure turbine driving it.

After examining ways of improving the existing fan and low pressure turbine, Rolls Royce decided to develop new units, taking advantage of the best current technology. For the fan, it was able to adopt a scaled-down version of the advanced wide-chord turbine blade developed for the RB211-535E4.

This fan, which does not require the mid-span snubbers normally needed to prevent vibration, employs much wider blades than conventional fans. As a result, the aerodynamic design of the blades can be optimised, improving performance and fuel efficiency. The Tay retains the Spey Mk555 high pressure compressor and turbine because the Spey core is extremely well-proven and achieves very high reliability. Designed specifically for the Fokker F28, the Mk555 entered service in 1969, with a thrust of 9850lbs. The core is of conventional design, consisting of a 12 stage high pressure compressor driven by a two-stage high-pressure turbine, the first stage of which is cooled. Inlet and outlet conditions are identical for both Spey and Tay so that, in effect, there is no difference in the core of either engine. Therefore, to match the core to the low pressure system, a three-stage intermediate-pressure compressor has been mounted behind the fan of the Tay and on the same shaft. Besides providing a pressure rise, it converts the radially varying pressure from the fan to the radially constant pressure required by the high pressure compressor.

A newly designed combustion chamber has been introduced for the Tay to improve cooling and increase combustion efficiency and chamber life. The chamber’s walls are fabricated from a new material called Transply, which uses one third of the cooling air to achieve the same reduction in metal temperature as the current material. At idling power, where emissions are measured, combustor efficiency is increased from 87 to 99%, bringing with it an extra 1% reduction on fuel burn.

Other features of the Tay include a new exhaust mixer at the rear and an all-compostite bypass duct. The mixer contains six deep and six shallow lobes, which mix the hot exhaust from the core with cool bypass air, reducing overall exhaust velocity and noise. The mixer also improves the propulsive efficiency of the Tay. A carbon fibre bypass duct has been developed which is 25% lighter than the conventional titanium item, as well as being cheaper to manufacture and easier to repair. The Tay will be the first civil turbofan to be equipped with such a duct.

The Tay has the potential for up to 20% more thrust than the Spey and a 5% to 10% reduction in specific fuel burn. Approval has already been given for a 10% growth engine for late 1988 certification, to be achieved by minor changes to the core and a throttle push modification.

The Tay has also met, or exceeded, all handling, starting and emission specifications. For the Fokker 100 the Tay will produce 13,320lb thrust – 35% more takeoff power and 17% more at cruise than the Spey Mk555 powering the F28. For this aircraft only, an additional 5% thrust will be available from an automatic reserve thrust system (ARTS). The engine is derated to 12,430lb thrust for the Gulfstream IV.

Production of Tays will take place at Rolls Royce’s East Kilbride factory, the first time the Scottish plant has been responsible for a major engine programme. At current order levels, up to 25 engines a month will be built, equalling the highest rate of production seen during the quarter century of e-spey production. Rolls-Royce says it is in a position to increase the rate if market conditions demand. It is clearly optimistic that the engine’s future will be as successful as its birth.