Hydraulics: so last century

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With the rise in lightweight and reliable electric gadgets throughout our homes - and even on our persons - the idea that machines should need heavy pumps and fluids applying great forces to move metal seems Dickensian, as anachronistic as steam engines.

But for aircraft engineers, replacing hydraulic control surface and landing gear actuation systems with electric ones is turning out to be a significant challenge.

While hydraulic systems are a tried and tested means of delivering great forces, the appeal of a move to electrics seems obvious. Hydraulic systems are compact and powerful, but demand lots of maintenance.

As far back as 1979 NASA examined whether electric systems could replace hydraulics in general aviation aircraft, where the power needed to move control surfaces is far less than for airliners. Electrical technology has come a long way since then and for the past 20 years there have been US and European studies into more-electric airliners.

electric landing gear extension and retraction 
 

The University of Nottingham's Philip Mawby has worked on such studies and is enthusiastic. "There is no problem for electrically actuated systems like landing gear. It is a reliability and regulatory issue." And, he adds, electric alternatives will have to show that their advantages outweigh the cost of certification.

BUSINESS CASE

However, there is a business case for electric landing gear. The turnaround time of an aircraft is in part determined by the time it takes for brake systems to cool. This is important where hydraulics are used, because oil dripping on to hot brakes is a fire hazard. Remove the hydraulics and you remove the fire hazard and can turn around the aircraft faster.

Elgear, an £11 million ($17.1 million) UK government-supported project with Goodrich that ended in the latter half of 2009, took the technology to a technology readiness level of four, a working prototype. Carl Maxwell, Goodrich's Wolverhampton facility technology manager, says: "Elgear had some major challenges. You must get the gear down even when there is a power failure and you can't have multiple gear boxes to circumvent a jam." Solutions were found and the prototype was enabled.

 

 Electromechanical systems: a brief history

The use of electromechanical systems as an alternative to hydraulics was identified as far back as 1979 by a NASA study. More recently the UK government has funded, in part with industry, an £11 million ($17 million), three-and-a-half year study that ran until mid-2009 called Elgear, or Electric Landing Gear Extension and Retraction. The use of electromechanical systems for control surfaces and landing gears has been addressed through more expensive European Union projects. One project was called Power optimised aircraft and lasted for four years, ending in December 2006, and cost €100 million ($136 million). Another was called More open electric technologies. This ended in December 2009 after three years and €63 million.

Other challenges for electrical systems that cannot have too much redundancy for cost reasons is coping with failure in an aircraft that is 200h flying time from base. Avoiding the cost of a round trip to the workshop can mean flying for days with a non-critical failure.

POWER SYSTEM RETHINK

Moreover, the introduction of electrics to replace hydraulic power requires the wholesale redesign of the aircraft's electrical power architecture. Future aeroengines are expected to generate up to 1MW of electrical power during flight and that needs to be distributed and controlled.

This new electrical system will not be complementary or secondary to hydraulic or pneumatic power - it becomes the core of a fully integrated "power-optimised aircraft". This concept also envisages substantial increases in the overall energy efficiency of the airframe. Power-optimised aircraft was the name of a €100 million ($137 million) European Union project that examined exactly that. Despite its hefty price tag that project was only one more step in this decades-long quest to replace hydraulics.

Last December the EU's More Open Electric Technologies study concluded that electric systems still have a mass penalty. Having used as the basis for study a single aisle, 165-passenger 6,475km (3,500nm) range twinjet, the study says: "The updated weight assessment shows that the 'more electric' short-range aircraft is heavier than the reference short-range aircraft for the considered systems part."

Maxwell and his Elgear project partners are under no illusion about what needs to be done to realise the more- or all-electric airliner. For example, he says there will have to be substantial use of composites and other technologies elsewhere in the aircraft to save weight.

But, he says, hydraulics' days are numbered: "For the Airbus A3OX the electric solution is likely to be real."