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  • 04.24.17

Uber’s Flying Car Chief On Noise Pollution And The Future Of Sky Taxis

Ex-NASA engineer says that slower-spinning electric motors will keep noise to a hum—with only a couple of planes overhead, even at rush hour.

Uber’s Flying Car Chief On Noise Pollution And The Future Of Sky Taxis
[Image: courtesy of Uber]

Flying cars still seem like one of those futuristic technologies that only exists for now in the realm of science fiction and old episodes of The Jetsons. But Uber is taking the technology seriously and this week it takes another step forward with a summit meeting that lays out its vision.

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In October, the ride-hailing giant published a 97-page white paper laying out all the challenges for setting up an urban flying taxi system to link with its on-demand car service. Since then, it’s hired NASA veteran Mark Moore as director of aviation engineering for its Uber Elevate initiative. Moore headed the space agency’s research on electric propulsion, autonomous control, and personal craft until February.

On Tuesday, Uber is convening its three-day Uber Elevate Summit in Dallas to lay out its plans for urban air travel. Given that the San Francisco-based company is flying everyone out to the Texan city, there’s a good chance that Dallas will be one of the cities with which Uber has promised to announce “collaborations.”

“What were looking at is, in the next several years, being able to bring experimental aircraft into and test them in the relevant environment of the city,” says Moore.

Uber will provide a live stream of the event from the conference home page, beginning Tuesday at 11 a.m. Eastern time.

Uber will also announce the companies that will supply these electric taxi planes. Note the word “planes.” Several companies, such as Germany’s E-Volo and China’s EHang, have introduced electric copters—known as VTOL (vertical take-off and landing) craft that look a bit like upscale toy drones. (EHang, in fact is, a drone maker). But that’s not the way Uber is going, says Moore. (Dubai’s transport agency will start a flying taxi program using EHang’s copters this summer.)


Related link: Airbus Is About To Build A Self-Flying Robo-Taxi

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Instead, Uber plans to use electric VTOL planes that briefly tilt their wings and propellers up to take off vertically like drones, then tilt them forward to fly forward. Such planes—as well as electric propeller systems—were being developed by Moore before he left NASA. He isn’t saying yet what companies will make planes for Uber, but Airbus provides a good example with its Vahana autonomous electric plane, announced in February and set for flight tests in the fall.

What other craft might Uber fly? Also in attendance at the conference will be Slovenian electric plane maker Pipistrel and German “electric jet startup” Lilium (it actually uses small, high-speed propellers). Big aviation players such as Embraer and Bell Helicopter will also attend.

Airbus Vahana concept. [Image: courtesy of Airbus]

Keeping Quiet

Noise is one major reason why Uber is going with planes instead of helicopters or oversize quadcopter drones like EHang’s. “One of the reasons helicopters haven’t gained traction in cities as a transportation solution is because they are so noisy,” says Moore. “They have a [low-pitched] noise characteristic that just travels forever, and it’s quite annoying.”

Brien Seeley, founder of the Sustainable Aviation Foundation, agrees. I spoke with him over the weekend at the organization’s 2017 SA Symposium. “Ladies and gentlemen, if you think you are going to come into to a quite residential, serene community and land close by the houses, with a noisy [helicopter] rotor downwash vehicle, you’re crazy,” he says. (Seeley isn’t affiliated with Uber.)

Moore claims the sound from Uber’s planes will be higher-pitched, as well, blending into the hum of car traffic in cities rather than rumbling on over a longer distance and rattling windows.

Switching from piston engines or turbines to electric motors cuts down noise, but what really makes a difference is that plane propellers can spin slower. Moore gives an example of a plane’s propeller tips slicing through the air at half the speed of a helicopter’s. Based on the physics of flight, that makes the plane propeller as much as 32 times quieter. “That’s where the magic happens,” says Moore.

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In case you’re wondering why, it’s in part because helicopter blades are essentially spinning wings. The faster air flows over a wing the more lift it generates. As a blade swings forward toward the front of a helicopter, it’s moving in the same direction the helicopter is traveling. Airflow speed is a combination of how fast the rotor is turning, plus how fast the helicopter is moving forward. It’s kind of like walking up an escalator that’s already heading up. As the same blade spins toward the back, it’s heading in the opposite direction the helicopter is flying—like trying to walk down that same “up” escalator—hitting the air more slowly and generating a lot less lift. Adjusting the tilt of the blades and spinning the rotors very fast are the tricks helicopters use to even out lift, but faster-spinning rotors make more noise.


Related link: Here’s Why South America Is Getting Helicopter Ride-Sharing Before The U.S.


Outsiders say Uber may be understating the noise challenge. “Noise is going to be a big issue, that I don’t think anyone’s addressing appropriately,” says Tyler MacCready, the CEO of Apium, which is developing swarm technologies so that craft like drones and sky taxis can fly in tight formation. “And that’s one that even Uber in their Elevate report—they say hey, don’t worry this is going to be quiet. That’s wrong,” says MacCready.

Brien Seeley reckons that the sound of a plane or helicopter has to be below 50 decibels, about the volume of a conversation at home, at a distance of 40 meters from its landing area at a small airport. (Here’s a good decibel guide to the noise level of different real-life sounds.) Otherwise either the noise will annoy neighbors or the airport will have to be too big to create a buffer. About VTOL, Seeley says that, “It’s appealing because of its perceived small landing pad; however, again, its noise signature will dictate the true acreage and thereby its proximity.” He’s proposed an XPrize competition to develop air taxis that meet the 50-dB at 40 meters target, which he calls a “Herculean challenge.”

Uber talks about putting its mini-airports, called vertiports (complete with fast battery charging), on top of buildings to minimize the noise. “You would think so,” says Seeley, “but those people going out of the skyscrapers want to go to their suburban McMansions, whose serene community won’t allow them to land there.” Dallas, for instance, is a very flat city.

The type of craft is very important to noise, says Seeley. He agrees that planes are better than helicopters, but the type of plane matters. A tilt-wing craft is essentially a helicopter when it’s taking off. Also, Moore is a longtime advocate and developer of what’s called distributed electric propulsion—spreading a bunch of small motors and propellers across the airplane wing. One of Moore’s last projects at NASA was the X-57, a research plane with 14 electric motors and propellers. Covering the wing in small propellers is more efficient than using a few large props, but it’s generally a lot noisier, as they have to spin faster. Perhaps this won’t be as noisy as a helicopter, but it could still be too loud.

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“The great spectrum that pushes and pulls against itself is, extremely tiny little rotors, and 30 of them, all blowing; and they’re screaming like banshees,” Seeley says, “or one extremely large [propeller], slow-turning like a Danish windmill that moves the same amount of air silently.”

Sky Gridlock?

Even if robo-taxi planes are virtually silent, how will people feel about a sky full of them? “You’re never going to blacken the skies,” says Moore. “It’s never going to look like Star Wars.” Even with a thousand air taxis per city, Moore says someone would see only “a couple aircraft” when they look up. Others tend to agree, saying that the promised reduction in street traffic will be worth it. “There’s a lot more room in the sky. I think we’re way off from the day when the skies get too crowded,” says Tyler MacCready. (He recommends using systems like his to help aircraft fly in tight formation so that traffic is kept to minimal areas.)

One reason the skies will stay clear, says Moore, is because Uber will use planes and not helicopters. To keep noise manageable, electric choppers would have to fly slower—around 50 miles per hour. (E-Volo projects a max speed of 100 kilometers per hour, about 62 mph. EHang lists an average cruising speed of 60km/hour, about 37mph.) Moore says that Uber’s taxis will fly at around 150mph. “So they get to where they’re going very quickly. They don’t stay up there,” he says.

About the author

Sean Captain is a technology journalist and editor. Follow him on Twitter @seancaptain.

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