Nearly a quarter-century ago, the Vertical Flight Society, a helicopter-industry group founded in 1943 as the American Helicopter Society, published a graphic dubbed “V/STOL Aircraft and Propulsion Concepts.” The 45 vertical/short-take-off-and-landing (V/STOL) vehicles depicted in the 1997 document showed a diversity of designs, including tilt duct, tilt-wing, deflected slipstream, tail sitters, tip jets, ejectors, rotors and the lift-plus-cruise design that is currently gaining favor.
The graphic now is respectfully referred to as the “Wheel of Misfortune” because only three of those V/STOLs (all military)– the BAE Systems Harrier Jump Jet, Bell Boeing V-22 Osprey, and Yakovlev Yak-38 Forger ¬ entered service. The 42 others languished. That’s how aviation (and automotive) history marches forward – in fits and starts. Technology breakthroughs foster numerous new projects that arise with a flurry of enthusiasm. But eventually, only a slim few achieve any degree of commercial success.
Fast-forward to 2020. Advances in electric and autonomous vehicles have emboldened a new group of aeronautic engineers to fit motors, propulsors and sensors to aircraft and launch R&D programs. The wheel of fortune for electric and hybrid VTOLs keeps spinning. The Vertical Flight Society now keeps track of more than 285 eletric VTOL concepts. It's a richly rewarding environment for engineers.
Start with the right use case
Technologist David Merrill, chief executive of San Francisco-based Elroy Air, and his co-founder Clint Cope became excited about the potential for air taxis in 2016. But they quickly realized that societal acceptance could forestall the launch of a commercially successful air taxi business. “Carrying people is the highest possible bar. Nobody’s going to want to take a ride until those systems have been thoroughly proven,” said Merrill. So Elroy Air, while still in its infancy, pivoted to a more immediate use case: cargo.
Merrill and Cope set the goal of building “an autonomous pickup truck of the sky,” as they describe their VTOL-based logistics business. Nearly five years later, Elroy Air is continuing to iterate on its lift-plus-cruise VTOL aircraft with a 28-foot wingspan. The vehicle, dubbed Chaparral, uses six vertical-lift propellers and one pusher prop to autonomously fly 300-lb (136-kg) payloads up to 300 miles (483 km).
On August 14, 2019, the Elroy Air Chaparral completed a test of its vertical-flight powertrain and control laws at an airstrip in Camp Roberts, Calif. The full-scale demonstrator aircraft successfully reached a height of 10 feet, hovering for 64 seconds. For that flight, Elroy Air’s lead flight-test engineer was in the line of sight, using a stick-and-rudder remote control to manage the throttle, pitch and roll of the demonstrator vehicle. Other inputs and internal states, including yaw, were software-controlled. Six weeks later, the team completed an autonomous hovering flight.
When the Chaparral arrives at a destination, it's designed to autonomously release its canoe-shaped cargo pod, which measures 105 x 24 x 17 inches (2670 x 610 x 432 mm), by lowering it on two cables. Equipped with a lidar sensor, the aircraft navigates on the ground directly above another pod loaded for the return flight. It then uses the same two cables to grasp and lift the new pod, and then latch it to the underbelly of the fuselage before embarking on the next flight. The company plans to continue building and testing through 2022, scaling production for real-world deployment in about 2023.
Wings and rotors
The wings on a VTOL are designed for efficiency and range. Adding a suite of fixed-pitch propellers to a wing or crossbeam allows the vehicle to first be lifted into the air. Then, a separate propeller pushes the aircraft forward, relying on the wing to do the work of maintaining the desired altitude. This lift-plus-cruise design also is employed by the Wisk Cora autonomous air taxi. Wisk was established in 2019. It’s an independent company backed by The Boeing Company and Kitty Hawk Corporation. The company’s roots go back to 2010 when Kitty Hawk was first established by Google co-founder Larry Page.
Competing air taxis, such as the Volocopter and EHang, are wingless, multi-propeller vehicles. Other designs include Toyota-backed Joby Aviation’s tilting propellers and the eight-prop Airbus Vahana that uses a tilt-wing configuration. The X-22-inspired Bell 40EX, shown at CES 2019, uses tilting ducted fans. Hyundai also displayed a VTOL aircraft concept at CES 2020. Hyundai's four-passenger S-A1 uses four tilting propellers for both vertical and horizontal flight, as well as four fixed fans for hovering.
Pure multi-copter wingless designs can unleash more power to carry bigger payloads. The tilting strategy saves some overall mass by eliminating the wing’s weight and uses fewer propellers that do double-duty for both taking off vertically and cruising. But the tilting mechanism can add weight back into the system, while increasing complexity.
Nearly every contemporary VTOL design uses a half-dozen or more redundant rotors as a safety measure. If one rotor fails, the remaining units continue to serve the mission. Elroy Air decided on a hybrid-electric powertrain, rather a pure-electric system, because it gives the Chaparral about 10 times greater range, “The heart of our system is a fuel-burning engine and a generator,” said Merrill. “So, it’s like a Prius.”
Elroy Air mounts a 180-kW turboshaft engine and electric generator within the fuselage. A turboshaft engine produces shaft power rather than jet thrust. The powertrain also uses a small battery to supplement the engine and to act as an energy buffer in case the six lift propellers need a burst of electrical power, for example, to respond to a gust of wind.
Purely electric, completely autonomous
Meanwhile, the Wisk Cora is an all-electric VTOL air taxi, also employing a lift-plus-cruise design. However, it uses 12 lift propellers for vertical takeoff before transitioning to a single push prop for cruise flight. Because the Cora aircraft is self-flying, there’s no need for an onboard pilot. Its cockpit provides space for just two passengers. Flights are monitored remotely via a ground-control team. Wisk’s strategy is to lower the direct operating costs of an air-taxi service the same way that Uber and Lyft plan to remove drivers from ride-hail vehicles.
Wisk is confident about going all-electric because it targets initial flight routes of about 25 miles (40 km)—the distance from the heart of Silicon Valley to downtown San Francisco, for example. The energy-taxing vertical-takeoff segment of a flight typically is brief, only a minute or two. The cruising phase is more efficient, especially for a winged aircraft.
Electric VTOLs use batteries sized not only to travel the distance of an intended route but to ensure that the aircraft can reach a secondary landing site in case of an emergency. A significant energy reserve is needed for longer-than-expected hover durations or if the rotors need a burst of power.
Reducing noise and offering a less bumpy ride are primary design goals. People who live and work near a vertiport would be subjected to the whir of propellers only until the aircraft transitions from lifting to cruising. “When the aircraft flies overhead, it’s really not making any noise,” said Jim Tighe, chief technical officer at Wisk.
VTOLs will fly above 400 feet and below 10,000 feet (122 to 3048 m) ¬– higher than small drones and lower than commercial passenger jetliners. “We expect to be sharing the airspace with helicopters and small planes,” said Merrill. Wisk’s Tighe said the company, much like automotive NVH specialists, carefully examines how acceleration and noise impact ride quality and comfort. He believes a flight in the Cora will be “much more comfortable than your average general-aviation airplane but probably not as nice as a Lexus.”
The Cora air taxi cruises at 100 mph (161 km/h). Increasing that velocity would drain the battery faster. But considering the roughly 25 miles (40 km) on a typical route, doubling the cruising speed would not substantially reduce the overall trip duration, including getting to and from the vehicle and boarding.
Wisk has conducted more than 1,200 test flights using prototype aircraft configurations. The tests have evaluated the flight envelope, including hover, the transition from the lift-propellers to the wing and transition from the wing back to the propellers. Wisk said it also evaluated the aircraft’s ballistic recovery system, testing parachute deployment on a full-scale vehicle. The company plans to conduct more flight tests at its Lake Tekapo facility in New Zealand and the San Francisco Bay Area.
Automotive and VTOL supply chains
The low latency of electric motors and the fast response of control logic are critical to VTOLs. “The same reason that people love how fast a Tesla gets off the line with electric motors that are torquey, you need that to drive the rotors in a multi-rotor aircraft,” said Merrill, Elroy Air’s chief.
“Multi-rotor flight dynamics require a rapid response,” he said. “You spool one motor up and spool another down in what’s called a mixing of the commanded vertical thrust across the collection of upward-facing propulsors,” he said. Makers of VTOLs envision a future when they can fully leverage the automotive supply chain. Auto suppliers produce parts by the millions, thereby reducing cost compared to low-volume aerospace component companies.
However, there’s enough specialization for VTOLs that it could create challenges for using automotive motors, batteries, chargers and sensors in aerospace applications. Weight is a significant issue. “A lighter motor is good for a car. But it makes the difference between an airplane working at all or not,” said Merrill.
Despite the obstacles, the two supply chains are beginning to overlap. Elroy Air worked with an aerospace OEM to produce a battery for its first aircraft. Non-recurring engineering costs and long lead times resulted in a battery system that cost roughly a half-million dollars. Elroy then shifted to a supplier that primarily provides automotive battery systems. Elroy Air thereby reduced the cost of its battery system to about $60,000. As production ramps up, the price will come down further. VTOL companies eventually want to manufacture aircraft by the tens of thousands, much higher volumes than traditional planes and helicopters.
Elroy Air uses radar for its detect-and-avoid system, primarily serving as a backup for off-board air-traffic management systems. But the radar system for a VTOL aircraft needs to see from several kilometers, detecting fast-moving aircraft much further than an autonomous vehicle senses the road ahead. Besides, the Federal Aviation Administration requires the use of a transponder under its Automatic Dependent Surveillance-Broadcast (ADS-B) protocols. All vehicles sharing the same airspace are required to broadcast altitude, latitude and longitude data.
The Chaparral also uses a lidar unit to navigate on the ground as part of its drop-freight strategy. Its lidar gets a full 360-degree view, with a multi-beam sensor looking forward, and a single beam providing location data for the sides and back. “To adapt an automotive lidar and radar systems for our purposes, where we might need a little more range or software developed for aerospace standards, tends to be very difficult,” said Tighe, Wisk’s chief technologist.
There are fundamental differences between the needs of an autonomous car and a self-flying VTOL. A child might run in front of a car, which can pull to the side of the road when it encounters an unfamiliar condition. That’s not an option for an autonomous aircraft.
“We would love to use more automotive products,” Tighe explained. “But there are challenges with that. Typically, automotive products are designed to a very specific spec and manufactured a very, very low cost, which is awesome. But those specs are usually very hard to change, right? To adapt an automotive lidar or radar system for our purposes, where we might need a little more range, or a different environmental spec, or software developed with aerospace standards tends to be very difficult. So that's a challenge for us.
“The technologies that [SAE] readers are familiar with in the automotive world, though, are very similar to what we're doing,” he said. “But we're usually trying to see a little bit further down the road,” with typical closing speeds between aircraft of up to 200 knots (230 mph) or more putting a premium on sensor range.
The lack of mature systems and supply chains means that VTOL companies currently develop in-house expertise. “Most of our employees come from a prime aerospace or aviation background, so have a lot of institutional knowledge,” said Tighe.
Like much of autonomous-vehicle testing, hours of simulation far exceed real-world evaluations. During the pandemic lockdown, Elroy Air’s engineers can continue to make progress using digital tools. Control laws are being developed in MATLAB and validated using aircraft simulations of various fidelities. Computational fluid dynamics (CFD) modeling is used to study such factors as turbulent air-rotor wash that could penalize some aspect of lift or cruise flight operations.
Many thousands of flight hours lie ahead. That extensive testing of mature airframe designs, hardware and controls will be required, mostly in remote areas, before flights are commonly allowed over dense urban areas. As successful real-world testing ensues, it will stoke more excitement about the potential of a new era of VTOL flight. But a handful or even hundreds of test flights, no matter where they occur or how many miles they cover, doesn’t mean that the public will be commuting to work the following week in an air taxi. Years will likely pass between “world-first” headline-grabbing flights and the launch of widespread commercial operations.
Nonetheless, each successful trip of a self-flying VTOL will demonstrate the impact of high-performance electric motors, lithium-ion batteries, perception-oriented sensors and high-speed computers on aviation. Aviation is inexorably moving closer to achieving a decades-old dream: widespread use of agile lightweight aircraft that takes off and lands without a runway. “When you have the right enabling technology, amazing things become possible,” said Merrill.