Rolls-Royce commits to geared turbofan

Written by: Justin Cunningham | Published:

The gas turbine has been the staple combustion engine for large aircraft since the 1950s. That's unlikely to change any time soon, however, advances in materials have enabled step change improvements in reliability with other advances, such as the more precise control of the combustion process, offering further enhancement.

However, the current engine architecture used by the major engine manufacturers is reaching its limit. As bypass ratios are pushed higher, and the consequential diameter of the fan becomes larger and one primary limiting factor comes in to play.

Efficiency and speed

Larger fan blades mean the tip speed of the blades begin to create shockwaves as they near the speed of sound. Consequently a shockwave begins to form and this massively hinders efficiency.

The result of all of this is that having larger diameter engines, and longer fan blades, will require the overall fan speed – so the speed of rotation – to be slower than the shaft speed of the engine transferring the mechanical power.

Consensus is narrowing in the aircraft industry that engines need to, and will get, larger and that means a fundamental need for the integration of some kind of reduction gearbox between the main shaft of the engine and the fan.

The concept is to allow the main core of the engine to rotate as normal and at its most efficient, with the fan rotating more slowly and therefore at its most efficient.

US engine manufacturer Pratt and Whitney has long been developing a geared turbofan that is due to enter service next year. Its planetary gear has been 20 years in the making and will transmit some 25MW of power at over 99% efficiency.

While Roll Royce has been able to push the fan diameter of Trent XWB, which has an impressive bypass ratio of 9.3:1, and fan diameter of 3m, it is reaching its viable limit.

Gear development

It's inevitable that to develop the 15-25% more fuel efficient engines the civil aircraft industry is demanding, without dramatically affecting the speed that aircraft can fly, will necessitate the need for the inclusion of a gear.

Professor Ric Parker, director of research and technology at Rolls Royce, said: "Our basic engine architecture is quite different to Pratt and Whitney's. They have got a 2-shaft engine and we have got a 3-shaft, so the tipping point in terms of fan size and weight, where it gets so big and heavy that we need to put a gearbox in there, is quite different."

It does appear that Pratt and Whitney has stolen a march on other aircraft engine manufacturers. However, its inherent 2-shaft architecture has meant that its tipping point came perhaps a decade or more earlier that the 3-shaft architecture used by Rolls.

A reduction gear becomes advantageous, and perhaps a necessity, for Pratt around bypass ratios of 11:1 or more. If they didn't use a gear on larger future engines, they would quickly suffer from diminishing returns.

However, Rolls Royce is likely to be able to comfortably push its bypass ratio to 13:1 and perhaps even as much as 15:1 before a gear becomes necessary.

It means while there is still a fundamental need for a gear in the longer term as engine fans get larger, its distinct 3-shaft engine has enabled it to stave off the costly and technologically demanding development of an appropriate gear until recently. And there is good reason for that.

"Our first gear application is going to be right at the top end of the thrust range," said Professor Parker. "Whereas Pratt and Whitney has a gearbox that transmits about 25MW, we are talking about a 50MW gearbox. So even when we achieve 99% efficiency in that gear, we have got to find somewhere for that 500kW to go. That will probably be lost as heat in the oil system, but that is still a lot of heat for the oil. So we have to be sure we are going to get a very efficient gearbox, more than 99%."

Daunting development

While the prospect of developing a gearbox for any future aircraft engine with this efficiency and reliability is a daunting task, this is Rolls Royce. It has immense and world-class experience in designing and producing gearboxes with one of the most recent examples being the contra-rotating lift fan for the Joint Strike Fighter (JSF) combat aircraft.

That gearbox is able to transmit 21MW from the main engine to the vertical lift fan and is incredibly well engineered, being both compact and highly efficient.

It's unclear at the moment what type of gear configuration Rolls will opt for, as that's part of the development work currently being undertaken. And the company remain tight lipped about divulging more details.

However, any gear is likely to be a planetary set up that's been speculated could have a ratio of around 3:1.

Efficiency is at the heart of making the gear viable, so precise manufacture and assembly will be vital, as will the development of the oil cooling system.

While Pratt and Whitney has taken 20 years to develop a gear, Rolls has ambitious plans to have a geared turbofan flying by 2025 in what is being called its UltraFan,

which will house numerous advanced components and technologies, and yield the 25% fuel efficiency improvement over its existing baseline engine model, the Trent 700.

"I'm confident that the gas turbine can be developed much further, and will remain the basis of aircraft power for the next 30 or 40 years. In terms of sheer power density, nothing yet has come close to it," he concluded.

Jargon Buster – Bypass ratio

The modern turbofan can be thought of as a gas turbine driving a ducted fan. While the turbine itself provides some propulsion, it's the ducted fan that provides most of the propulsion. Air passing through the ducted fan is not combusted, and bypasses the core of the engine. The amount of air that passes through the fan relative to the air that passes through the core of the engine is known as the bypass ratio. A larger bypass ratio is advantageous as it means the core of engines is made smaller to aid thermal efficiency, and the fan that creates the forward thrust, can be made larger.

Different configuration helps Rolls Royce develop later

Pratt and Whitney have long been advocates of the geared turbofan, developing its planetary gear for the turbofan over the last 20 years. Its 2-shaft engine architecture has meant it has had the 'balancing point' where it needs to incorporate a gear much earlier. To keep up with the trend of higher and higher bypass ratios (larger more efficient fans and smaller more efficient engine cores), it has had use a gear to match the efficiencies being offered by competitors.

In a 2-shaft engine there is the ducted fan and then a compressor booster stage that turns the fan at the same speed. At the back of the engine a low pressure turbine is attached to the same shaft that turns the fan. The problem is as the fan becomes bigger, the fan needs to rotate more slowly. This means the LP turbine at the back becomes getting heavier as additional compressor stages are added to recover work.

In Rolls Royce's 3-shaft engine, it still has a large LP turbine turning the fan, but there is also a second intermediate pressure shaft that has its own compressor and own speed. So the tipping point in terms of fan size and weight, where it gets too big and heavy that a gearbox is needed, is at a much larger size.

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