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Head of HY4 project optimistic about the future of zero-emission flying

Credit: DLR

ElecTrans speaks to University of Ulm Professor Josef Kallo about the fuel cell aircraft project

In October, ElecTrans covered the successful test flight of the HY4 hydrogen plane, which has been one of the most exciting developments in fuel cell technology this year. The flight marked a major milestone on the path to zero-emission aircraft, lasting 11 minutes and taking place in almost complete silence.

Recently, we spoke to Josef Kallo, a professor of aeronautical engineering at the University of Ulm and head of the HY4 project, to get his thoughts on the recent tests, his hopes for the future of the technology and how close they are to being realised

The project is entirely Kallo’s brainchild from the fuel cell design to the organising of the team, although he is quick to point out how vital the other key players are in the success of the project. “The team is perfect. They are very, very skilled on the technical side but also on the documentation side. I think this is the reason we are out in front.” he says.

His enthusiasm is justified; the HY4 is, so far, the first successful hydrogen fuel cell plane of its size and power. The 4-seater aircraft has an output of around 80 kW, a top speed of around 230kph and can in theory travel up to 1500 km – although according to Kallo, such a distance is well beyond the necessary range for his ambitions.

Rather than envisioning a long-haul zero-emission aircraft, he sees the HY4 as ideal for providing very short city-hop services which could help relieve congestion coming in and out of large cities, or even crossing especially big metropolises. “What I see as the next step would be to have a 40-seater which flies regularly between San Jose and San Francisco with an electrical motor,” only be a 50 mile hop across San Francisco Bay.

He envisions a combination between a Li-ion battery, which would enable rapid climbs to a high enough altitude, and a fuel cell to maintain the aircraft for the short flight and also to recharge the battery en route, enabling a quick turnaround at the other end. “We are heading for something like 600 metres of runway, which is enough for a 40-seater to take off due to the very high torque of the electrical motor”. The speed of such a flight would also be desirable; according to Kallo; whilst it would ordinarily take him two-and-a-half hours to drive from Stuttgart to the south of Germany, an HY4-style plane could get him there in just over half-an-hour.

Josef Kallo, HY4 project leader.
Josef Kallo, HY4 project leader.

Another opportunity for the aircraft is in the rapid expansion of cities in countries like China. Kallo specifically cites Shenyang in the north, which has nearly tripled its population from 6 million in 2000 to 16 million today: “At the end of the day we are not talking about cities, we are talking about landscapes with a lot of people on them. These aren’t cities anymore, not as we know them”. The fact that it takes over three hours to cross the city seems to bear this out. Kallo sees short-hop, zero-emission aircraft as a clean, efficient solution. He also points out that it would work well even in regions where the infrastructure for land transport is shoddy or non-existent.

Worth the weight

One of the major difficulties with batteries or fuel cells in a general sense is the power-to-weight ratio, and this is especially true in aircraft where the balance must be spot on to be both efficient and safe. At the moment, the HY4’s powertrain already makes up at least half of the maximum take-off weight of the aircraft, but Kallo dismisses the problem.

“In the HY4 we used very heavy systems because the first step is to learn and understand the functionality of the plane”. He says that using a lighter, more efficient engine at this stage would have introduced unnecessary problems to the test that had nothing to do with the actual functionality of the plane, corrupting the results. “It would be like taking the motor for a Formula 1 and using for a truck race”, he observes.

Now that the first test has been conducted, the HY4 can and will be getting a lighter powertrain: “Our next step will be to increase the power density by a factor of two because we know the functionality, we know how to use it and we know where the critical points are”.

Kallo’s designs are also rivaled, at least in terms of investment, by vertical take-off and landing aircraft. VTOLs, as they are known, are championed by many as they would not need a runway to start and finish each flight, which would certainly make it more efficient spatially and in terms of maneuverability. He, however, is not so keen on the idea, suggesting that his aircraft design represents “the maximum efficiency which you can get, because you have a fixed wing which has a high lift possibility”. He suggests that a VTOL has to waste so much energy getting into the air that its range would only end up being 50 or 60 km, whereas he hopes his aircraft will be capable of flying legs of up to 300 km.

Kallo appears to be sceptical of VTOLs in general, and when I bring it up again, he suggests that startups dealing in such technology may be more impressive for their marketing than for their engineering.

The HY4 team
The HY4 team

The transport of tomorrow

A running joke in EV circles is that hydrogen is the fuel of the future and always will be. Kallo himself is more than aware of how difficult it is going to be to get to the stage where his aircraft are used regularly. “It will cost; it’s not cheap. It will be expensive, but there is a chance to be emission free”.

He is optimistic about the timescale for the 4-seater plane, predicting them to be in the skies in 3 to 5 years, but he worries that it will be some time before we see a 40-seater aircraft flying across San Francisco Bay. The reason, he says, is legislative rather than technological: “There are no rules and regulations for such kinds of plane which means we will have to do a first iteration, a first plane which will have to be tested in daily operations, and based on that we will develop the rules and regulations for the next generation”.

When pressed on how long this might take he offers at least 10 to 15 years down the line, calling the legislative procedure a huge problem for the plane. So would it be quicker without all the paperwork? “Realistically, it will be no sooner than 10 years. But if you are interested in marketing I can say 7 years!”

The technology is still in its early stages, and there will be a number of roadblocks along the way, not least the current lack of infrastructure for hydrogen production for aircraft. But 10 years is a long time, and by the time the 40-seaters arrive, the groundwork for hydrogen-powered planes may well have been laid. In the meantime, though, Kallo is hoping to continue testing the aircraft and improving the powertrain. Whatever the future holds, 2017 should be an exciting year for the project.


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