The eVTOL is in the Details
The AHS International Transformative Vertical Flight meeting looks at the
payoffs and challenges of electric vertical takeoff and landing.
By Frank Colucci
All 61 technical papers from the TVF conference are now available in the AHS Vertical Flight Library and can be purchased individually or as part of the proceedings CD-ROM. The conference and workshop plenary presentation videos and PDFs can also be downloaded for free at www.vtol.org/TVF-2018.
A packed house at January’s AHS International Transformative Vertical Flight Technical Meeting and Fifth Annual Workshop learned about bold visions for urban air taxis and the challenges of electric vertical takeoff and landing (eVTOL). Four days of presentations hosted by the AHS San Francisco Bay Area Chapter covered aeromechanics, propulsion, acoustics, infrastructure and certification advances aimed largely at airborne mass transit. The new rotorcraft suggested for moving people over clogged streets include multicopters, tiltrotors, tilt wings and ducted fans, nearly all designed around Distributed Electric Propulsion (DEP). AHS International executive director and workshop program chair Mike Hirschberg observed, “Really, transformative flight is [largely] about DEP and the design freedom that it enables.”
Technical papers addressed analytical tools applicable to the new vehicles and their missions, and the aeromechanics of coaxial rotors, optimized tilt rotors, cycloidal rotors, and other configurations. With the full title of the conference being the “Aeromechanics Design for Transformative Vertical Flight Technical Meeting,” not all the technical papers dealt with eVTOL platforms. Byung-Young Min from Sikorsky presented information about propeller performance in the wake of the S-97 Raider. The integrated pusher propeller at the tail of the high-speed compound helicopter ingests both the main rotor wake and fuselage boundary layer. Wind tunnel tests showed the Boundary Layer Ingested (BLI) propeller increased body drag but enhanced propeller efficiency. The net benefit is about 10% more thrust — important for the agile armed scout helicopter.
Other San Francisco technical papers included tools to optimize the length of shrouded rotor ducts and the relative performance of variable-pitch versus variable-speed propellers for quadcopter control.
Distributed Electric Propulsion nevertheless plays heavily in the Uber Elevate ecosystem of urban air taxis, vertiports (or “SkyPorts” in Uber parlance) and supporting infrastructure that dominated the eVTOL discussion. Uber Chief Product Officer Jeff Holden noted that urban centers will be home to six billion people in 20 years and told his AHS dinner audience, “The way people move in and around cities is not keeping up.” Aviation specialists working for the ground ride-sharing giant told the AHS audience how hundreds of thousands of flights a day by eVTOL aircraft will unload choked highways and make cities more productive.
Their vision centers on battery-powered, four-passenger aircraft with multiple small propellers to transition from vertical flight to horizontal flight. Networked air taxis will follow sky corridors from one rooftop vertiport to another at altitudes less than 1,500 ft (450 m) and speeds of 150 mph (240 km/h) and greater. Human pilots eventually give way to autonomous flight controls, and another paying passenger will displace the paid pilot. “You can put these piloted aircraft in the air and build a huge amount of experience with the autonomy running in parallel,” said Holden.
Uber head of vehicle engineering Dr. Rob McDonald told the AHS audience, “Uber’s not building an aircraft. We’re trying to get other people to build aircraft we want.” Whatever the shape and source of future air taxis, the company wants to make sure all the elements of the ecosystem come together as an effective transportation ecosystem solution. It expects first flight of its eVTOL aircraft in 2020 and initial service in Dallas and Los Angeles in 2023.
Big cities offer high-density markets where travel time is money. Recently, Uber collaborator Bell Helicopter flew Holden and his team from Dallas/Fort Worth Airport to Frisco Station, making the 1-hour 10-minute drive in just eight minutes. With four- or five-passenger air vehicles that are six times faster than single-commuter cars crawling in traffic, Uber expects its vision to be productive and ultimately affordable. “We actually believe we’ll get the cost-per-mile-per-passenger below the cost of car ownership,” said Holden.
The global ground mobility integrator was just one of many visionaries at the AHS meeting. Airbus urban mobility general manager Mathias Thomsen showed animations and photos of its concepts for urban air mobility taking shape. The modular Pop.Up concept includes an autonomous ground vehicle that drives a passenger to an air park, latches on to an eVTOL wing and flies away. Another four-seat, quad-ducted rotor vehicle, CityAirbus, will fly before end of 2018. And the full-scale singleseat Vahana demonstrator was showed undergoing ground tests in preparation to fly (see pg. 12). Workhorse Group chief executive officer Steve Burns stated his robust, easy-to-fly SureFly hybrid electric octocopter will fly soon and quoted a price tag of $200,000.
By the time of the AHS technical meeting, Bell Helicopter had already unveiled its Uber-applicable Bell Air Taxi at the Consumer Technology Association’s CES expo in Las Vegas (see pg. 14). The air taxi cabin mock-up revealed nothing about propulsion or flight dynamics, but according to Bell director of innovation Scott Drennan, “It’s about reaching a new set of customers… This is about feeling like you got inside a car or an aircraft you are familiar with.”
Drennan gave more details on its circular-wing HYDRA (Hybrid Drive Train Research Aircraft), used to test advanced distributed propulsion systems, as well as its Autonomous Pod Transport, both first revealed in December. The helicopter maker is looking at intra-city and longer inter-city markets for eVTOL. Drennan estimated direct operating costs for an eVTOL platform operating 2,000 to 3,000 hours a year can be one-half those of small helicopters. Bell studies also indicate a fourth-generation electric tiltrotor has special appeal. “When you get wing-borne, the speed and efficiency start to make sense.”
John Piasecki of Piasecki Aircraft provided an update on the Defense Advanced Research Projects Agency (DARPA) autonomous Aerial Reconfigurable Embedded System (ARES) with its tilting, turboshaft-driven ducted fans. He said his company has also conducted design studies for an electric VTOL for high-value transport missions. They considered 25 different configurations, but “safety and time to certification are significant discriminators.”
Francesco Giannini of Aurora Flight Sciences told how DARPA’s 12,000 lb (5.4 t) XV-24A VTOL X-Plane uses hybrid turboshaft-electric power to turn ducted fans in its tilting wing and canard. He also gave details of Aurora’s battery-powered eVTOL demonstrator. The subscale air taxi model has eight lift motors for takeoff, plus a single pusher propeller and fixed wing for cruise.
Uber targets the cost of an eVTOL air taxi around $2 million initially and around $700,000 in large production quantities. “We have to design for low complexity, but in an elegant way,” said Mark Moore, Uber’s engineering director of aviation. Moore told the largely Silicon Valley audience, “We know we can’t do it all alone. That’s why we’ve gone to the experts in the industry to build the vehicles.” Uber’s experienced aerospace partners include Aurora, Bell, Embraer, Mooney, and Pipistrel, but Aurora’s Giannini acknowledged, “As an industry as a whole, we’re not used to producing thousands.”
Uber’s polished pitch is nevertheless data-driven. With 65 million ground passengers a month worldwide, the company’s origin and destination data helps define air-taxi markets and performance. Reference models revealed in San Francisco are meant to give industry examples for developing analytical tools for eVTOL aircraft. “Part of what makes this possible is the very limited scope of the mission,” said McDonald. The battery-powered reference models with brushless, direct current motors will fly about 60 statute miles (100 km) on a charge. Based on ground passenger data, Uber expects most air-taxi trips will be just 25 miles (40 km) in day-night VFR (visual flight rules) conditions with four or five people aboard.
UberCHOPPER’s on-call experimentation with piston-engine Robinson R44s at special events quickly showed helicopters too expensive for large-scale air taxis. “Helicopters won’t do the trick,” said Jeff Holden. “One [issue] is the noise… Communities just reject them.” Holden added, “To me, DEP is really the paradigm shift that enables what we’re talking about.”
Carbon emissions nevertheless rule out hybrid turbine-electric powertrains in intra-city service. Uber’s own engineering priorities focus on batteries with 300 Wh/kg pack energy density and 3C discharge and recharge ratings, aircraft noise levels at least 15 dB quieter than edgewise rotor helicopters, and tools to model the transition from vertical takeoff to horizontal cruise. “Edgewise flight in cruise — that era is over,” said Mark Moore. “It really is time to get on a fixed wing in cruise [which] lets us go to really low tip speeds and again do things fundamentally different that are absolutely enabling to that ultra-low noise signature.”
The choice of pure electric or hybrid electric VTOL propulsion is driven by mission profile. Jason Schug, from strategic engineering consultant megafirm Ricardo, acknowledged longer endurance possible with hybrid propulsion and the difficulty of getting fuel on top of a building. Paul Debitetto, vice president of hybrid UAV maker Top Flite Technologies, said battery energy density is not yet sufficient for air-taxi operations and noted batteries do not get lighter as they discharge — the Breguet Range Equation doesn’t help, as it does for conventional aircraft. “The hybrid systems have a much smaller [disk] loading… You kind of get the best of both worlds.” Schug cautioned hybrid design with software for smart battery charging is not easy, and hybrids are still subject to heat, noise, and vibration problems.
eVTOL configurations are likewise open to discussion. Uber’s first reference model integrates tilting proprotors on a fixed wing with coaxial, co-rotating lift propellers that retract into fairings behind the wing and in the rear fuselage, cutting drag in cruising flight. Vanes beneath the boom rotors help provide transition, single rotor failure, and hover control. With advanced controllers, the compact lift propellers use variable azimuth spacing to reduce noise and increase mass flow for a 4-6% better hover Figure of Merit, while at the same time achieving reduced harmonic and broadband noise. Mark Moore stated, “We are going to bludgeon the industry about community noise, because that’s the doorway to the market.” Without achieving dramatically lower noise than helicopters, eVTOL air taxis won’t be able to operate in urban centers, making them unprofitable.
Silicon Valley enthusiasm was moderated with realistic insights. “Certainly, eVTOL is going up the hype curve,” acknowledged Air Force Lt. Col. Nate Diller, responsible for air and space requirements and technology development in the Office of the Joint Chiefs of Staff. While military and civil transformative vertical lift requirements may overlap, he noted, “We don’t necessarily know what the flying qualities will look like with these [aircraft].” Col. Diller added that testing aims to manage risk and said, “You achieve that with multiple prototypes flying quickly.”
DARPA has a long history of rapid prototyping and the agency’s Tactical Technology Office program manager Dr. Graham Drozeski observed, “You are starting to see more urban [requirements] come in the military side.” He noted that while the civil market is looking for revolutionary air mobility, defense buyers want to multiply their tactical reach. “The contrasting use cases drive common technologies.” With limited payload and volume in small eVTOL concepts, autonomy becomes a critical enabler. Drozeski stated even man-rated aircraft need autonomy designed-in from the beginning. “We need to pull them through certification. Don’t do the one-off demonstration.”
NASA Ames aerodynamicist Dr. Wayne Johnson reviewed his own concept studies for eVTOL aircraft. His team modeled a single-passenger quad-rotor with a 49 kWh battery and 50 nm (93 km) range, a six-passenger, hybrid tilt rotor with 200 nm (370 km) range, and a 15-passenger, turbo-electric tilt-wing airliner with a 5,000 shp (3,700 kW) turboshaft and four electric motors to reduce the size of the storage battery. NASA analytical tools now permit manufacturing cost calculations accurate within 20%, and Johnson said, “We’re not trying to pick [a] winner in the race. We’re in this to identify critical technologies for all these aircraft.”
Andrew Gibson from ESAero (Empirical System Aerospace) explained how the prime contractor for NASA’s X-57 Maxwell aims to use enabling technologies from the fixed-wing, battery-powered DEP aircraft on a 19-passenger, parallel hybrid electric conventional takeoff and landing (eCTOL) transport around 2025. The same supply chain making motors, generators, and batteries for the Maxwell can make refined hybrid propulsion for bigger vehicles. “For electric propulsion, it’s all about payload-range,” observed Gibson. He expressed eVTOL concerns, “The battery scares me, and the noise scares me. If you’ve got 1,000 aircraft, is 15 dB less than a helicopter good enough?”
Dr. Ken Brentner from Penn State University described ongoing rotor noise modeling work. He explained that rotorcraft noise has multiple components, including thickness noise that radiates forward and loading noise that radiates down. Researchers are measuring acoustic force, physical response, and non-acoustic response generated by helicopters and other rotorcraft. They calculate increasing the number of blades decreases noise significantly while multiple propellers can increase noise significantly with unsteady loading.
Acoustic experts are also looking at operational noise reduction with technology to give pilots real-time noise avoidance feedback based on airspeed, flight path, acceleration or deceleration, and bank angle. “There are a lot of things for the pilot to handle at once,” noted Brentner. He concluded that noise reduction logic may have to be built into transformative flight controls. “As we develop a proto-autonomy sort of system, all these things get baked into it.”
Georgia Tech professor Dan Schrage raised an important aeromechanical safety consideration when he reminded designers that agility and maneuverability will be critical in turbulent urban canyons and degraded visual environments. “I think the challenge is going to be [that] people want you to pick them up [at places] other than rooftops… You’re not just going to hover in ground effect for 30 seconds.”
On the urban air mobility panel discussion during the first day of presentations, Chris Van Buiten, vice president of Sikorsky Innovations stated, “This is absolutely going to happen.” He noted Sikorsky progress in rotorcraft autonomy and said, “It’s now time to certify and qualify it. We intend to transition that technology to existing helicopters.” Nevertheless, the engineer and executive mixed his eVTOL enthusiasm with a cautionary tale. In 1977, a New York Airways S-61 rolled over atop the Pan Am building, killing passengers waiting to board and a pedestrian in the street below. The accident crushed city-center helicopter airlines for decades to come. Van Buiten reminded the transformative flight audience, “There have been promising industries wiped out by early mistakes.”
Van Buiten also noted that the S-92 was, in his words, the safest helicopter in the world, with flight critical components designed for only one failure per billion flight hours; this integrates to one aircraft loss per million flight hours. Even this level of safety, however, will not be sufficient for the types of widespread societal penetration expected by electric VTOL proponents: if 50,000 aircraft are each flying 3,000 hours per year — as has been suggested by several studies — that would result in 150 accidents per year. At least an order of magnitude improvement is needed, the Sikorsky executive asserted, and cited a quote by Boeing Technical Fellow Dan Newman: “Until you’re ready to fly a person in it, it is a toy. When you are ready to fly your family in it every day, it is an aircraft.”
Certify and Operate
In a later panel, Aurora’s Francesco Giannini discussion summed up the inescapable relationship of eVTOL safety and air taxi business: “The day after one of these vehicles crashes, no one is going to want to get into another one.” Bringing safe, legal transformative vertical flight to the market will require legal consensus between industry and government regulators on ways to certify and operate new kinds of aircraft, propulsion, and operations. It will also require public consensus that air taxis with near zero-zero parachutes, airborne collision avoidance systems and other innovations are safe to fly. Mark Moore from Uber told the conference audience, “It’s about … safety and winning over the user base.” He added, “All the [operational] uncertainties surrounding helicopters, we remove them” and enhance safety through operations occurring only at controlled SkyPorts.
Unusual DEP configurations, high power-density batteries and autonomous flight controls pose entirely new safety questions for regulators. Tom Gunnarson, regulatory affairs lead at eVTOL developer Zee Aero and secretary of the light sport aircraft committee for ASTM International told the eVTOL audience in San Francisco, “If we were to try to certify today with things as they are, none of us would get very far.”
General Aviation Manufacturers Association (GAMA) vice president for global innovation and policy Greg Bowles summed up eVTOL certification efforts when he said, “A video is wonderful. An experimental prototype is fantastic. That’s just where the work begins.” According to Bowles, the new Part 23 Federal Aviation Regulations applied to fixed-wing aircraft under 19,000 lb (8.6 t) and Part 27 rules applied to helicopters are sound starting points for eVTOL certification. Industry and regulators have begun exploring consensus-based standards that recognize different approaches to prove safety. “What we want to do is field actual safety in the eyes of the public,” said Bowles. “What we don’t want to do is build piles of paperwork.”
How the new kinds of vehicles fit in with existing concepts of aviation operations and rotorcraft crashworthiness is also to be determined. Uber reference models separate the air taxi pilot from passengers for security and include crumple zones in cabin designs for energy dissipation. However, Dr. Tom Prevot, Uber’s director of engineering for airspace systems conceded, “If you build them to current aviation standards, the vehicles are not going to be affordable.”
eVTOL safety concerns go beyond the air vehicle to include new air traffic control models and vertiports optimized for fast departures and returns. Prevot said, “We don’t want to blacken the sky. We have to make it acceptable to the public…. We want to be very predictable flying in known corridors and vertiports.”
Dr. Parimal Kopardekar manages NASA’s Safe Autonomous System Operations Project and VTOL 2.0 studies of large-scale operations in the National Airspace System (NAS). He noted that the NAS currently already supports nearly 50,000 operations a day. eVTOL air taxis, in principal, will have to share the airspace with other users and place no additional burden on Air Traffic Control resources. Dr. Kopardekar called for “flexibility where possible and structure where necessary.” Demand capacity coordination using new ways to track autonomous aircraft will have to take into account noise, traffic volume and the availability of vertiport charging spots. “Connectivity is key,” said Kopardekar. “Autonomy alone won’t do it.”
Karthik Balakrishnan of Airbus Group talked about the Altiscope analysis of manned and unmanned traffic and explained that local noise and safety effects are dictated by broader safety, security, and economic considerations. Aircraft Collision Avoidance Systems on unmanned aircraft, for example, could increase risk of collisions by commanding avoidance maneuvers in high-density airspace. “What works in one place doesn’t necessarily work in others.” Balakrishnan added, “In a single airspace, you’ll have to support a variety of architectures,” Such as SATCOM, 5G, etc. “It’s not everybody speaking the same language.”
Uber director of airspace Tom Prevot reminded the AHS audience that pilots today talk to air traffic controllers when attempting to enter controlled airspace. The same communications, command and control, and controller- and pilot-workload conventions won’t suffice with lots of eVTOL taxis negotiating low-altitude urban airspace. “These aircraft are going to be operating in areas where conventional radars won’t work,” said Prevot. Vertiport landing spots also limit airspace throughput. Uber wants seamless connections between transportation nodes — ground vehicles to air taxis to trains or airport flights. “We’re going to manage the fleet in a very different way than it’s managed today. We want to be very predictable, flying in known corridors and vertiports.”
The NASA Aeronautics Research Institute has been supporting the AHS-led Transformative Vertical Flight working groups that are road-mapping private/recreational, short-range commercial intra-city, longer-range inter-city and public service eVTOL applications. Yolanka Wulff, Comparative Aircraft Flight Efficiency (CAFE) Foundation executive director, explained the potential difference inter-city aircraft could make in traditional regional airline systems. Free of runways and big airports, flexible eVTOL aircraft can fly point-to-point rather than hub-and-spoke. “What we’re looking to do is turn that hub-and-spoke into a spider web — eVTOL can do that.”
Important insights into future air taxi operations came from the Uma Subramanian, CEO of Airbus Voom Flight, the on-demand helicopter service in São Paulo, Brazil. São Paulo has around 400 heliports, a fleet of contract helicopters and a population willing to pay for rapid air transport. Subramanian told the AHS audience, “One thing we’re learning is the market is there today.” Turbulent weather and rooftop heliports are present-day complications that will impact future air taxis. “Weather is a buzz-kill,” acknowledged Subramanian. “Bumpy, rocky rides are not a pleasant experience.” She also noted “Landing on a pinnacle is hard. Everything has to work perfectly.” Voom experience shows about 60% of passengers have luggage, another complication for small, weight-sensitive air taxis. Subramanian also concluded, “In order to make this a viable business, we need to bring down the design and manufacturing costs.”
Infrastructure Inside and Out
John Badalamenti, Uber head of design for SkyPorts and cabin experiences gave the AHS International audience a glimpse of what it would be like to enter the Elevate ecosystem. “You can’t just design a cabin and ignore all the other parts,” he said. Passengers arriving at a conceptual rooftop vertiport are identified by biometrics and weighed discreetly on the way to their aircraft. Tilt propeller reference models seat four passengers in a staggered configuration isolated from the pilot and provide a measure of connectivity on the short flight. When passengers disembark at the next busy vertiport, facilities are designed to keep them away from moving parts on aircraft.
To identify sites for the vertiport network, Uber’s head of operations Stan Swaintek explained, “We’re starting that network with data.” The company’s urban database includes physical spaces and constraints and identifies structures that can support 5,000 lb (2.3 t) aircraft. Vertiport buildings also have to accommodate cables and cooling provisions for electric chargers, and the space to isolate high-powered equipment. Alan Dowdell from battery charger maker ChargePoint suggested candidate buildings could be configured to provide high-speed charging stations for air taxis on the roof and slower chargers for cars down below.
Bringing utility power to rooftop recharging stations poses its own challenges. The Uber model of five to 10 vertiports in each launch city calls for 2 to 10 MW power at each site. Paul Stith from engineering, procurement, construction and consulting company Black and Veatch explained, “By and large, there’s enough power; it’s just not in the right places.” New York City, for example, draws 2 GW per day to charge its electric bus fleet. Stith observed, “Doing this in a parking lot is one thing. Doing it on top of a building is a whole ‘nother thing.” Massive conductors and related charging equipment will require major renovations in selected buildings. In addition, while buses can be recharged at night to avoid utility peak demand charges, Uber probably won’t ground air taxis waiting for a better electric rate. Large storage batteries at rooftop verti-ports may help recharge air taxis at busy times and return stored power to the utility grid in slow periods.
David Sawaya of California utility Pacific Gas & Electric likewise talked of load scheduling and said the real solution was a new power grid. “I do not think there is a magic bullet to solving those types of loads.” He added, “This is complicated stuff compared to what we’ve done in the past.” The cost of grid improvements to taxpayers again impacts public perception of air taxis. Sawaya suggested the key is getting public support: “It’s how do you frame the benefits to the average citizen.”
About the Author
Senior contributing editor Frank Colucci has written for Vertiflite for the past 20+ years on a range of subjects, including rotorcraft design, civil and military operations, testing, advanced materials, and systems integration. He can be emailed at firstname.lastname@example.org.