TU Delft research pushes the boundaries of electric flying

Battery-powered flying will go further than previously thought. That is the conclusion of Elysian researchers Rob Wolleswinkel and Reynard de Vries. They and TU Delft researchers Maurice Hoogreef and Roelof Vos of the Faculty of Aerospace Engineering wrote two thick papers on this. They presented the papers earlier this month at the American Institute of Aeronautics and Astronautics’ SciTech Forum.

The first paper is a review of the range for electric flight. The authors take a close look at two parameters: the ratio of battery weight to maximum take-off weight, and the maximum ratio of lift to drag. These show that there is more potential than an electric two-seater reaching a maximum of 70 kilometres.

The second publication follows this up by sketching an electric rechargeable aircraft for 90 people with batteries in the large wings and propulsion through several electrically driven propellers. Special features include folding wingtips (because of the huge wingspan) and a gas turbine that drives a generator to assist should the aircraft need to fly further and the batteries are empty.

From Amsterdam to Milan

An aircraft like this would allow emission-free flights of up to 800 kilometres, from Amsterdam to Milan. Half of all flights wordwide cover less than 2,000 kilometres, Elysian writes in a press release, and these account for 20% of aviation’s CO₂ emissions. Elysian concludes that: ‘Electric aircraft have the potential to significantly accelerate the future-proofing of aviation’.

Not all short flights can be electric, so I assume the reduction in CO2 emissions from electric flying is limited to a few percent?

Melkert: “Indeed, that doesn’t add much. For emission reduction, the most gain would be from more efficient aircraft. Every year, the global air fleet becomes 1.5% more efficient through more fuel-efficient engines, better aerodynamics, lighter materials and also by limiting detours. If this continued to 2030, you will have gained 20% in efficiency. But on the other hand, the number of flights is increasing by 3.5% every year. So all emission reductions are more than offset by growth.”

What then is the value of these studies?

“We know that a small aircraft can fly electric for short distances. And we also know that a large aircraft crossing the ocean cannot fly electric. But what is the limit? Wolleswinkel and De Vries have fundamentally figured that out and have shown what an aircraft that pushes the limit of electric flight looks like.”

So what does it look like?

“There are quite a few clever things in the design, which means that the aircraft will look a bit different than our standard airliners. For example, it’ll have a pretty big and straight wing with several propeller engines. They put the batteries in the wings so the aircraft actually looks a lot like a flying-V.”

The Elysian will have propeller drive. Is that slower than jet engines?

“That depends a bit on how you shape the propellers. Six or seven hundred kilometres per hour should be achievable. But you don’t reach the 850 or 900 like jet planes. Incidentally, it doesn’t matter that much because these are short flights, and the flight time is never more than two hours. The speed probably doesn’t affect deployability that much, but charging does.”

How do you mean?

“Recharging takes more time than refuelling. That affects the economic viability of electric flying, since you can do fewer flying hours in a day. Elysian also says something about that, which is that the fuel price would have to double to make electric flying economically viable.”

So they are open about the obstacles?

“Yes, and I think that’s actually the beauty of it. They started by considering the facts. Unlike many other start-ups they don’t promise ‘we are going to fly next year, and that will solve all the problems in the world’. These guys give themselves 10 years’ time and they also list 10 ‘hot potatoes’ that all need to be solved before electric flight becomes feasible at this size.”