Volocopter 2X in urban area.

Dr. Juergen Rauleder at the Technical University of Munich teaches rotorcraft aerodynamics to students excited about the Electric VTOL Revolution.

By Richard Whittle

Vertiflite, November/December 2019

Whether electric vertical takeoff and landing (eVTOL) aircraft will soon produce clean, quiet, autonomous air taxis for urban air mobility (UAM) is uncertain, but Dr. Juergen Rauleder says the drive to develop such aircraft is inspiring aerospace engineering students — about half of his class from Germany and the other half from across Europe — like nothing since the Space Race of the 1960s.

“The guys that come in my class say, ‘Why don’t we have more problems on urban air mobility vehicles? It’s much more sexy,’” said Rauleder, assistant professor for rotorcraft and vertical takeoff-and-landing aeromechanics at the Technical University of Munich (TUM).

Rauleder — a member of the Vertical Flight Society’s Board of Directors and deputy chair of the VFS Aerodynamics Technical Committee — earned his PhD at the University of Maryland in 2014. Advised by Prof. Gordon Leishman, Rauleder did experimental and analytical work on the turbulent vortex flow environment produced by rotors. He still focuses on rotorcraft aeromechanics at TUM. A research group of six PhD candidates he leads are doing projects that cover “lots of things in rotorcraft aeromechanics — ship-deck landing research for the US Navy, for example,” Rauleder said. His group also collaborates on projects with the University of Maryland, the University of Texas at Austin, and the US Naval Academy. They are also doing work on coaxial rotors with the US Army as well as a European Unionfunded project on morphing, shape-adaptive rotors for improving rotorcraft efficiency, noise and greenhouse emissions.

Dr. Juergen Rauleder, TUM

In addition to that research, Rauleder teaches two graduate courses at TUM on rotorcraft aerodynamics, one on fundamentals and the other an advanced class. In the classroom, he said, is where he sees students’ enthusiasm for tackling the problems of eVTOL and UAM.

“Most of the [assigned] problems are on a traditional helicopter, single main rotor, tail rotor configuration, maybe a Chinook-like kind of thing, maybe a tiltrotor,” Rauleder said. “That’s usually the things we calculate, kind of legacy platforms, because that’s still what’s flying out there.” Students learn to calculate rotor lift, thrust, performance, drag, flight dynamics and such. “Basically the aerodynamics of a rotor from first principles up to marching through the fidelity levels of methods that you can apply, from simple momentum theory-based calculations to more advanced computational models,” Rauleder explained. “It’s mostly focusing on getting a theoretical background delivered to the students. And that’s kind of where urban air mobility comes in as an interesting topic for the students. They’re excited about this. They ask, ‘Can I use the same methods?’” Yes, you can, he tells them, to which they reply: “Then why not do the same problems, homework problems, on the urban air mobility vehicle with ten rotors and a wing, instead of a single rotor?”

“At some point you need to excite the students about what they are doing,” Rauleder noted, and eVTOL is a topic that does that. “If they’re excited, they are willing to learn, and they absorb things. More and more students say, ‘I want to do this. This is exciting.’”

The enthusiasm UAM is generating is a welcome development for the aerospace industry as well as academia, because it may help counter what has been a worrisome trend in recent years.

“Where are the best and brightest [engineering graduates] going now? They join IT,” Rauleder said. “They can make more money. It’s also exciting. ‘I can do what I want. I can work with Google,’ or whatever. They do not join aerospace. ‘It’s hard to study, it’s difficult and what’s the payoff? I make less than an IT guy.’” Actually, Rauleder said, many TUM graduates go into the German automotive industry, a major employer in Munich and nearby. But UAM can help change that, he thinks, and aerospace companies are catching onto that fact.

“I know of a few companies, big aerospace companies, that do not necessarily believe in the market of UAM to be a soon major source of revenue, but they believe in it as an attraction to get new engineering material, aerospace engineers and innovations,” Rauleder said.

With more than 200 eVTOL aircraft projects listed on the VFS website, www.eVTOL.news, most of the money flowing into eVTOL development is venture capital, but the two biggest commercial aircraft companies, Airbus and Boeing, both have established new units in recent years to pursue eVTOL, electric conventional takeoff and landing (eCTOL) aircraft and propulsion research projects. Bell, formerly Bell Helicopter, has done the same. But those big manufacturers are also pursuing UAM to make sure they aren’t left behind if this does prove to be the next aviation revolution. They also want to prove that, like the dozens of startups in the UAM space, they are innovative. Even so, the amounts of money those big companies are investing in UAM, while significant, is about the price of a small commercial airliner or a couple of V-22 Ospreys.

“There are companies that have been producing three or four major types of helicopters for the last 50 years,” Rauleder said. “Do people view them as innovative? No. What can you do about it? You come out with something new. Here’s your chance with urban air mobility. Some companies have done that approach. Why not?”

Upper left is PhD candidate Jakob Bludau flying a Bo 105 in the TUM flight simulator, with the real-time visualization of the same flight condition from an outside perspective and with visualized streamlines. (Credit: Jakob Bludau)

The excitement that engineering students are showing for UAM also springs from a motive different from that of the Silicon Valley entrepreneurs and others who were inspired to develop eVTOL aircraft by frustration with daily commutes through heavy traffic by car.

“They’re not thinking about, ‘I’m spending so much time in traffic,’” Rauleder said of his students, most of whom take the U-Bahn — the subway — to get around congested Munich. No, UAM gets their adrenaline flowing because, “it looks exotic, just the idea of having something like in The Fifth Element,” he said, referring to the 1997 Bruce Willis science fiction movie in which air taxis and flying cars flood the skies between skyscrapers, traveling in air lanes.

The thought of air taxis zipping among skyscrapers brings to mind another thought about UAM that Rauleder is interested in — the turbulent flow caused by tall buildings in urban environments. With all the many issues and challenges facing UAM developers, figuring out how eVTOL aircraft — especially autonomous eVTOL aircraft — will cope with the fickle winds created by urban environments and skyscrapers has so far gotten little attention. And, of course, it doesn’t show up at all in the many slick video representations of future UAM traffic shown at conferences and online. But it is real.

“The atmospheric turbulence and the atmospheric boundary layer there is complicated,” Rauleder said of urban canyons. “There are basically accelerated and decelerated flow areas behind these big skyscrapers. There’s going to be lots of vortices shed off from these big structures, and they will influence these fairly small vehicles much more than bigger ones. They will influence them a lot. It’s very complex. I think that we as a community do not yet understand this problem well enough.”

Perhaps Rauleder and his team can help, for urban winds can hardly be as complicated as the shipboard-landing environment they already are studying for the US Navy.

“That is basically the most challenging landing situation for any helicopter pilot,” Rauleder said. “You have very strong winds, because you’re at sea. You have a moving ship deck, so a moving landing ground, and the ship goes fast to be stable in the water. You have a moving platform … and you have to land this thing, and you have heavy winds and waves and everything. Then on top of that you have this ship air wake, as we call it, so basically slow and high velocities behind the hangar and the ship structures, changing winds because we’re offshore. Then you have the rotor downwash and everything interacting. And the same thing minus the moving deck is the city environment.”

For UAM to really work as envisioned by its most passionate advocates, eVTOL flight control computers are going to have to be programmed to deal with these challenges as well, he said.

“I sometimes compare these, our ship deck landing research, with the challenges these vehicles will have,” Rauleder said. “Whenever there is aerodynamics interacting with structures, when there’s a complex flow field working on the vehicle — and the propulsive system, because you have lots of propellers, and they interact not with a free stream but with all these gigantic vortices — it will not have the same performance, it will not have the same flight dynamics. You need to know all of these influences a priori or you calculate them in real-time, otherwise your autonomous vehicle is not going to react to these things appropriately. We’re not there yet.”

But for young aerospace engineers at TUM, challenges such as these are what make eVTOL exciting.

About the Author

Richard Whittle is the author of The Dream Machine: The Untold History of the Notorious V-22 Osprey (Simon & Schuster, 2010) and Predator: The Secret Origins of the Drone Revolution (Henry Holt and Company, 2014), and a past contributor to Vertiflite.

Caption for opening photo: Aeromechanics principals of traditional rotorcraft are largely transferable to modeling unconventional eVTOL aircraft, such as the Volocopter 2X shown here, which are exciting the next generation of aerospace engineers. (© 2017 The Foreign Offce Collective, Hamburg, Germany.)

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