All aboard
By Keith Button|May 2019
It’s a dream shared by everyone who’s ever been stuck in a traffic jam, from the bored child in the back seat to the late-to-work-again commuter: If only I could fly. That day could be coming. Keith Button spoke to experts in the urban air mobility field about how this might work.
For Davis Hackenberg, NASA’s urban air mobility strategic adviser, rolling on wheels is oh so primitive. “You’ve got to build a road that goes from A to B, and everybody sits on it,” he laments.
Hackenberg is one of those working to grow urban air mobility, or UAM, from today’s collection of experimental conceptual aircraft into a U.S. economy of 130 million passenger trips a year, in one estimation.
Entrepreneurs, from airplane builders to ride-sharing companies, are working on at least 100 UAM vehicle designs, and NASA has organized a series of demonstrations of UAM technologies scheduled to start in 2020 to inspire public interest and display everything from aircraft designs to airspace management software.
Let’s assume that the market takes off as Hackenberg and many others hope. What will the commute look like? No one knows for certain, of course, but based on my discussions with experts, here are some possibilities for how things could work in a decade or so when UAM reaches an advanced stage:
1. There won’t be a human pilot aboard.
Two reasons: Cost and safety, says Ella Atkins, an aerospace engineering professor at the University of Michigan. On the cost side, UAM will have to win over a large number of riders who will pay for regularly scheduled flights — 130 million trips per year, industrywide, before the market becomes profitable, according to an analysis by the New York-based McKinsey & Co. management consulting firm. The study is included in the 2018 “Urban Air Mobility (UAM) Market Study” commissioned by NASA from a team led by Crown Consulting Inc., an engineering and analytics consulting firm in Arlington, Virginia. To attract that many riders, the cost per ride will have to be low. Paying skilled pilots would make the rides too expensive, and finding skilled pilots to fly so many flights would be too difficult, Atkins says. Preliminary results from a Georgia Tech survey show that commuters with annual incomes of more than $100,000 generally would be willing to pay $25 per ride in a piloted UAM vehicle.
On the safety side, riders will demand safer flights than today’s mode of urban air travel, human-piloted helicopters. Otherwise, UAM won’t become popular enough for millions of scheduled flights per year.
“The average passenger who wants to live to a nice old age is not going to feel really happy climbing onboard a helicopter every day if they know that the risk is greater than if they drove in a car or flew in a commercial [passenger] airplane,” Atkins says.
UAM will likely go through a transition period from today’s human-piloted aircraft “to gradually get to the point where we are comfortable treating [urban air mobility] like a train at the airport that talks to us in a computerized voice,” Atkins says.
UAM aircraft may employ “safety pilots,” or human pilots who fly with the aircraft even after technology has advanced to the point where the vehicles are capable of autonomous flight, to help build consumer buy-in, says Scott Winter, an assistant professor and consumer perceptions researcher at Embry-Riddle Aeronautical University in Daytona Beach, Florida. Research shows that people are less willing to fly when a human pilot is absent, because they are more comfortable with what they know. Over time, as consumer confidence increases in autonomous flight, the safety pilots could be phased out, Winter says.
2. Passengers will fly between “vertiports” on scheduled flights, like passenger trains running between stations.
UAM commuters will probably fly between vertiports, which would be airports specifically built for vertical-takeoff-and-landing aircraft. The high cost per ride for on-demand air taxis will make them impractical, outside of areas with high concentrations of potential riders who are extremely wealthy. The Crown Consulting UAM market study for NASA found that UAM flights organized like passenger train systems, with scheduled routes among 100 to 300 vertiports in a metropolitan region, might become profitable after 11 years of operating at a loss. At that point — the study marked the first profitable year at 2028 — the price per trip would be $50. But the study found that an air taxi system with on-demand single-passenger rides for “door-to-door” service, in which a passenger would walk 2.5 minutes or less for single-passenger rides, would likely cost nearly $2,000 per ride. Uber, which started holding annual “Elevate Summits” about UAM in 2017, says it plans to offer an “Uber Air” service starting in 2023 in Dallas and Los Angeles. Passengers would order shared rides by air, taking off from vertiports and not, initially at least, with door-to-door flights. Uber Air service would cost riders $1.85 per mile “within a few years of launch,” according to Uber’s website.
Vertiports could be built on train or bus stations, or at highway interchanges, so a passenger could access ground transportation for the next leg of their journey, Hackenberg says. Positioning a vertiport at a ground transportation hub would also ensure that people could move out of the area efficiently. In downtown areas, vertiports could be located on top of tall buildings.
3. Their aircraft will need to be quieter than today’s helicopters.
Aircraft noise will be a critical issue in determining whether UAM succeeds or fails. If UAM aircraft are too noisy, then local communities could oppose flights over their neighborhoods or the construction of vertiports in their neighborhoods.
If UAM aircraft designers don’t fix “the noise problem,” then UAM flights will generate a lot of complaints early on, which will make it tough for UAM to gain traction, Hackenberg says. This is one factor that has prevented urban helicopter flights from growing more popular.
Noise levels will also help determine where and when the UAM aircraft will fly. One idea is to mix the noise of the aircraft in with ambient noise sources that already exist. For example, UAM operators could fly them above highways so the ground traffic covers up the aircraft noise.
Communities will be more likely to buy into the concept of UAM serving their neighborhoods if the flight paths are planned to stay as far as possible from schools, hospitals, parks and residential areas, says Raymond Bea, a noise analyst for ATAC Corp., a Santa Clara, California, consultant on air traffic issues. In general, the main factor determining how noise will propagate is distance, he says.
In addition to the cost, noise levels could also discourage UAM concepts in which individuals catch flights from their homes, instead of vertiports, says Winter of Embry-Riddle. “If you think it’s bad when somebody fires up their Mustang in the morning to leave your neighborhood, just imagine if he fires up the helicopter.”
But aerospace engineers will probably solve the noise problem with good designs, says Brian German, an aerospace engineering associate professor at Georgia Tech. Propellers could be positioned on the airframe to limit noise; rotor blades could be widened or more blades added so tip speeds could be lowered; long ducts could be added to reduce noise at certain frequencies, German says.
4. Their aircraft will be battery powered.
Electric propulsion with its energy and aerodynamic lift efficiencies will be a key to UAM success. With electric propulsion, designers can create aircraft with propellers placed where they are most aerodynamically effective for maximum lift and fuel efficiency. Today’s electric aircraft are divided between battery-powered concepts and hybrid-electric versions, in which a gas-powered turbine generates the electricity.
Eventually, the hybrid-electric designs likely will be abandoned in favor of the battery-powered propulsion, says German of Georgia Tech. The reason is cost: Hybrid-electric propulsion will be more expensive to maintain. “A gas turbine in general is a high-maintenance-cost piece of equipment, and if you now have a gas turbine plus a whole bunch of electric motors and generators and batteries, you actually have a potentially higher level of system complexity than you do with current aircraft,” he says. Battery-powered propulsion has fewer moving parts, so less wear and tear, and it can draw its charging electricity from renewable energy sources, with no need to maintain combustible fuel supplies for the aircraft.
In five to 10 years, advances in battery technology could produce batteries with enough energy density, or energy stored per unit of space, for the length of flights that UAM would require — up to 80 kilometers, German says.
But even when battery technology has matured, there still could be a role for hybrid-electric aircraft with longer UAM flights, says Parimal Kopardekar, acting director of the NASA Aeronautics Research Institute. Even when battery-powered aircraft are reaching 130-kilometer flights, hybrid-electric aircraft will be capable of flying 450 kilometers, he says.
5. Their aircraft may still need a wing.
UAM aircraft designs will probably include a wing, which will make the aircraft more aerodynamically efficient in forward flight and faster, to reduce travel time, German says. For flights of 50 to 60 kilometers, a winged aircraft has a big speed advantage. The exception might be when flying only over highly congested space for short distances — 8 to 16 kilometers, for example. A wing might not be necessary, he says.
6. Passengers will share their ride with three to five other passengers.
The vertical-takeoff-and-landing aircraft typically envisioned for UAM would carry four to six passengers. UAM operators will want to fly as many passengers as possible per flight to make the flights less expensive, but the amount of energy and power required for taking off and hovering limits the size of the aircraft, German says. Also, as the aircraft gets heavier relative to its combined rotor size, the noise produced by the rotors increases dramatically. And, it increases the downwash field, or the intensity of the wind kicked up to the rotors.
To accommodate more passengers per vehicle, aircraft designers would have to create aircraft with larger rotors, which would require larger landing pads. “There’s many operational advantages to having a greater number of people in the aircraft, but in my opinion that puts some more technical challenges on the vehicle design up against some fundamental physics,” he says.
7. Their flight routes may not follow any predictable pattern.
When UAM operations and technology reach their most advanced stage, the software and data links that control where the aircraft fly and keep them from crashing into each other will be autonomous. Human air traffic controllers will not be required, so there will be no need to maintain air traffic in prescribed corridors in the sky, Atkins of the University of Michigan says. Other than avoiding obstacles and not flying over sensitive areas, the software-controlled flight paths may appear random to humans.
“If you imagine standing on a city street and looking up into the sky, the efficient urban air mobility set of operations is going to be like taking a pencil and drawing a whole bunch of lines, but they’re not all going to be in the same direction,” she says. “I don’t think it’s going to be as orderly as a human mind would like it to be, unless we really dial back the availability of those operations and never have them be very dense or very varied.”
Because conserving energy will be key for battery-powered aircraft, traffic control will also ensure that the aircraft won’t be held in holding patterns waiting to land, Kopardekar says. “You don’t want to hover around and waste the energy. You want to schedule them so that everybody gets a slot, so to speak.”
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Economics
“If you think it’s bad when somebody fires up their Mustang in the morning to leave your neighborhood, just imagine if he fires up the helicopter.”
Scott Winter, Embry-Riddle Aeronautical University, on UAM’s potential noise problem
