As he hovered 50 feet above the runway in Plattsburgh, New York, Kyle Clark suddenly had a distinct sinking feeling. A literal one. He had full control just moments before, but his electric, eight-rotor aircraft was dropping fast.
Clark did, however, know this was coming. The founder and chief test pilot of flying car developer Beta Technologies had deliberately put the aircraft into a tricky aerodynamic situation called vortex ring state, or “settling with power.” That’s when the rotors lose lift and the aircraft descends rapidly into air made turbulent by the transition from horizontal flight into a hover. No amount of additional power will allow the aircraft to climb out of it. In fact, adding power can often accelerate the descent. It’s the problem that caused the fatal April 2000 crash of a V-22 Osprey tiltrotor during a test flight, as well as the loss of a Black Hawk helicopter in the 2011 raid that resulted in the death of Osama bin Laden.
It’s also just one of the many often mystifying aerodynamic challenges that neither pilots nor engineers can see but which they can absolutely feel, thanks to the complex forces swirling around clusters of fast-moving blades and all the spindly bits protruding from the fuselage to support motors, generate lift or control, or support landing skids or wheels. It’s a soupy yet fast-moving environment that will require high-level understanding and eventually virtually infallible management if the aviation world’s vision of a new way of flying has any hope of succeeding.
Aircraft makers can minimize the likelihood of vortex ring state with multi-rotor designs that distribute the downwash over a wider area, and pilots learn how to respond to the threat when it happens. As Clark dropped, he sidestepped to the right, moving into cleaner air, where the rotors’ power can be put to use. “I got out just 3 feet above the ground,” he says. “Just because you have a bunch of powerful electric motors and rotors, it doesn’t mean you can simply accelerate out of it.”
That test—one of about 200 so far—counted as a success for the aircraft, called Ava. The new kind of flying machine retained control despite the loss of lift, much like a conventional helicopter. But managing vortex ring state is just one of many challenges facing the development of this entirely new class of electric, multi-rotor, vertical-lift aircraft, dubbed eVTOL but more popularly known as air taxis or flying cars (for their ease of use, not because they also drive).
As companies like Lilium, Joby, and Kitty Hawk explore new configurations—with pivoting rotors, wings, moving control surfaces, and more—they must crack the devilish problem of keeping heavier-than-air machines aloft.
Making an eVTOL aircraft hop off the ground and transition to forward flight is the most pressing challenge here. “We want simplicity in our design and predictable behavior across a wide transition envelope,” Clark says. By that he means making the transition at different speeds and altitudes. “We want it to maintain nice, even responses—what we call control harmony—no matter what configuration it’s in or what the conditions are. We don’t want it to feel firm and precise in one direction but mushy in another.”
For Beta’s Ava aircraft, Clark worked toward the simplest system possible. That meant, for starters, avoiding variable-pitch propellers, which adjust their blade angle to regulate speed. They’re common in turboprop aircraft because they allow for single-speed engines. But they’re also complicated, heavy, and maintenance-intensive, with many moving parts that would be spread out, in Ava’s case, over eight props. The alternative is a propeller that sits somewhere in the middle between being efficient while hovering—where low propeller speed is more efficient—and cruising, where higher speeds carry the day. Clark’s aerodynamics team designed a large wing that would work well in slow flight, aiding in the transition. It also uses a larger-than-normal retractable flap to increase its surface area at low speed, improving lift and efficiency.
The team also avoided a tilting-wing configuration, another common VTOL strategy that mounts the propellers on the wing and pitches the whole assembly up and down. The problem with such designs is that going from horizontal to vertical flight becomes much less stable, since the wings tend to stall asymmetrically, Clark says. In other words, as the wing loses lift while slowing to transition to vertical descent, one wing tends to dip before the other. A tilting wing also exposes the aircraft to the risk of being pushed around by wind gusts when tilted up. Instead, Beta used tilting motors on their own outriggers, since it’s not combining the wing and the motor supports into single assemblies that have to do several jobs.
Not that this configuration is challenge-free. For one thing, Clark wants the control system to be immune to pilot mistakes during the transition, without relying on computer controls. It has to be inherently stable. Though computer simulations suggested that might be the case, with “symmetrical and benign” reactions to such factors as gusting winds, real-life testing showed that as conditions change, pilot reactions can produce inconsistent results, and thus instability. So Beta made the motor-supported outriggers aerodynamic in both vertical and horizontal flight. They made the wing thicker and stronger to help it resist turbulence generated by the ever-changing motor angles. That helped Ava better manage loads across all the aerodynamic forces in play, whether from winds, rotor downwash, or the shifting forces as it moves through the air.
Ava is not bound for commercial service. It’s a control and aerodynamic test mule for Beta’s real product, which will use a different propulsion configuration. That aircraft will push its lift-over-drag ratio—a key indicator of aerodynamic efficiency—considerably and ensure minimal disruption of airflow across all structural elements and in all phases of flight. “With an aircraft like this, we have a problem with interfaces—where the motor outriggers and the wing mount to the fuselage, how the tail assembly and landing gear affect the aerodynamics, et cetera,” says Mark Page, Beta’s aerodynamics lead. “So my job is smoothing out those points and generally ‘deconflicting’ the wakes of the airframe. We use computer simulation to see where the air is moving, and that gives us this 3D jigsaw puzzle that allows us to make everything fit, to determine where the pieces can and can’t be. In the end, we have clean airflow.”
Maximizing efficiency also maximizes battery power, Clark says, essential to reaching the 200-plus-mile range he’s targeting for the final aircraft. Ava, the prototype, will be good for a 150-mile range at 172 miles per hour. Those numbers could be validated—or discredited—by this summer when Clark will attempt to fly it across the country, both to log more test hours and to expose the broader challenges of electric aviation. Things like charging infrastructure, integration of the aircraft into public airspace, and the challenges of flying the radical new machines. Before that, though, there are still dozens of test flights in order—though hopefully with fewer and fewer of those sinking feelings.
Story updated at 13:55 ET on Tuesday, March 12, to clarify that Clark sidestepped to the right to move the Ava into cleaner air.