Spinning tops

Shape memory alloys and piezo- electric devices could be used to improve aircraft engine fuel efficiency by controlling the gap between spinning turbine blades and their casing, following research for Rolls-Royce.

The research, being undertaken at the newly created Rolls-Royce University Technology Centre in advanced electrical machines and drives at Sheffield University, will investigate the use of a range of actuation technologies to adjust the geometry of components within engines to actively control the clearance between the turbine blade tips and the stationary casing. The components to be adjusted will include the ring surrounding the edge of the casing, guide veins and the front air intake.

In existing jet engines, clearance varies according to the temperature of the blades. This means the casing must be designed to accommodate the blades at their largest, leaving an unnecessary gap that reduces the engine’s overall efficiency at all other times, said Geraint Jewell, advanced research fellow in the department of electronic and electrical engineering at the university, speaking at the launch of the UTC last week.

‘You can get efficiency savings by being able to control the clearance between the blade tips and the casing. If you can control that quite closely you can start to talk about a 0.25 to 0.5 per cent fuel saving. This doesn’t sound a lot but if you multiply it by the number of aircraft out there, it is quite significant,’ he said.

As well as shape memory alloys and piezoelectric actuators, the researchers will study the use of micro electronic mechanical systems (MEMS) to control the surface roughness of components, reducing turbulence and thereby increasing efficiency, said Jewell.

‘The project is very much focused on the engines because they are still fairly fixed mechanical lumps, albeit with a lot of electronic controls,’ he added.

The project is part of the EU-funded c100m Power Optimised Aircraft (POA) programme which has been designed to investigate the use of engine systems and electrical power control technologies to reduce the amount of power used by aircraft for non-propulsion purposes. The four-year programme involves 43 organisations including Rolls-Royce, Airbus, Messier-Dowty, Snecma and Thales.

Increasing the level of electrical and electronic control within aircraft engines will improve their efficiency, reduce their cost and weight and lengthen their life, according to Dr Mike Howse, director of engineering and technology at Rolls-Royce. ‘More electric systems mean more efficient systems so they are cheaper, they are more intelligent and they give improvements in performance, either because they are lighter or because they can be optimised more effectively,’ he said.

Installing generators directly around the engine rather than attaching them through a series of gearboxes and cables, as is the case in existing aircraft, would simplify the system and reduce weight.

Ultimately, using control systems to alter the ratio in which power is taken off each shaft within an engine would allow that engine to be optimised for the particular flight envelope in which it was being used, he said. ‘But that is a long way off yet.’ The Sheffield centre will work with a sister UTC in electrical systems for extreme environments soon to be established at UMIST, and with an existing electrical power systems UTC at Strathclyde University.

The researchers will be looking at materials capable of operating within the harsh environment of an aircraft engine where the temperature can reach 800 degrees C, said Professor David Howe, director of the Sheffield UTC. ‘We are looking at insulation and other materials with the appropriate combination of mechanical, magnetic and resistance properties,’ he said.

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