Distributed propulsion ‘may be the only means’ for small electric flight progress

Article Source: IMechE

Distributed propulsion could bring many benefits – and some new challenges (Credit: DLR E-Fliegen Hub)

Much has been written about electric planes, but it remains to be seen whether they will ever be commercially viable.

Amid the strain of the Covid-19 pandemic, Rolls-Royce and Airbus cancelled flight tests of their E-Fan X airliner, a promising project that could have provided vital data on issues such as thrust management and electric systems at altitude. 

The converted BAe-146 plane would initially have had one electric motor alongside three conventional jet engines. Airbus chief technology officer Grazia Vittadini said E-Fan X had “shattered preconceived notions of what is possible in future flight” and had offered insights into hybrid-electric propulsion, other avenues for carbon emission reduction – such as hydrogen – and regulation. 

“Among the many great achievements from E-Fan X has been the generator – about the same size as a beer keg – but producing a staggering 2.5MW,” said Vittadini’s Rolls-Royce counterpart Paul Stein. “That’s enough power to supply 2,500 homes and fully represents the pioneering spirit on this project.” 

Electric flight has seen other setbacks in recent months, such as the battery system fire that damaged the prototype of the Eviation Alice regional aeroplane in January. There is a pressing need for alternative methods of flight, but a single approach has yet to take the lead.

Snowballing improvements 

One promising candidate is distributed electric propulsion. Instead of one jet engine on each wing, why not six electric motors, each powering propellers? Electric flight is expected to only be suitable for regional routes without massive improvements to energy storage, so distributed propellers could be a solution. 

“I think for smaller aeroplanes it is maybe the only means to really make a step in the order of 10% forward,” said Dr Martin Hepperle, researcher at the German Aerospace Centre (DLR). 

The approach is so promising thanks to the “snowballing” improvements that are possible when flying with many independent propellers, said Hepperle. First off is extra lift. 

Each propeller leaves a wake behind it as air is accelerated to generate thrust. The wake blows over the wing, generating extra lift. This enables a reduction in wing size, roughly 10% narrower than would otherwise be required. This in turn brings its own benefits. “This means 10% less friction drag, which is a big amount for aircraft. You are normally talking about half a per cent,” said Hepperle. 

Thanks to their independent nature, thrust can be distributed between the propellers to provide direction. This allows a size reduction for the vertical tail-plane at the rear, perhaps by half. “This means less drag and weight,” said Hepperle. “It’s like a small snowball rolling down the hill and it gets bigger and bigger. The aim is to find as many positive effects as possible.”

Other advantages of distributed propulsion include extra redundancy – and therefore safety in the event of a motor failure – and potentially increased efficiency for each propeller. 

Of course there are challenges when planning such a radical departure from the status quo. Electric aircraft using distributed systems are not immune to the inherent fire risks, as demonstrated by the blaze that destroyed one of two prototypes of the 36-engine Lilium Jet in February. Noise from the propellers could be another issue.


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