Source: Graham Warwick May 31, 2023
U.S. startup MagLev Aero has emerged from stealth mode with plans to develop a radically different propulsion system for electric vertical-takeoff-and-landing aircraft. Leveraging the magnetic levitation principle used in trains, the system promises high efficiency with low noise and increased safety, the company says.
In rail transportation, magnetic levitation (maglev) propulsion uses electromagnets to lift the train, then push and guide it along its track with no friction. MagLev Aero has adapted the principle from a linear rail to a circular rotor.
- Multiblade rotor promises high efficiency with low noise
- Integrated drive and suspension provide redundancy
- Passive magnetic bearing improves safety
Boston-based MagLev was co-founded by aerospace engineer Ian Randall and his father, Rod Randall, a former telecom industry executive who is now executive partner at Siris Capital and a board member at electric vehicle manufacturer Fisker.
MagLev was founded to develop a novel propulsion system with noise and safety levels that could unlock the mass-market potential of electric vertical-takeoff-and-landing (eVTOL) aircraft. “Penetrating into the neighborhoods is going to require being ultraquiet and ultrasafe,” Rod Randall, MagLev’s board chairman, says.
The startup’s HyperDrive system is a many-bladed annular rotor suspended, guided and rim-driven by a circular maglev system. The multiple thin, swept blades are shaped to increase efficiency and reduce noise compared with a conventional helicopter rotor.
“This is distributed electric propulsion, but the propulsion is distributed circumferentially around the ring in a highly fault-tolerant manner and enables ultrahigh efficiency and ultralow noise,” Rod Randall says.
Conventional shaft-driven rotors produce lift from a small number of blades with relatively high thickness, tip speed and blade loading. Rotor blades are long, but airspeed renders the inner sections ineffective, and most of the lift is generated on the outer spans. Lift can be increased by adding blades, but that increases the power required.
“HyperDrive uses only the most effective outer blade sections and enables dramatically higher blade count and lower tip speeds,” Rod Randall says.
A conventional rotor’s high tip speed and blade loading, as well as the concentration of that load on the outer span, are the core mechanisms of helicopter noise—steady and unsteady blade loading noise and thickness noise from the speed at which the blade is moving through the air.
“These are the three mechanisms that produce the various forms of noise that make helicopter rotors intrinsically loud,” says Ian Randall, MagLev’s CEO, noting that multirotors can break up the loud sources into a larger number of small sources but still have the load concentrated at the tips.
“By only using the outer blade span, we would normally suffer from using only the most heavily loaded part,” he says. “But with our architecture, we are able to dramatically increase the blade count, which dramatically reduces the blade load. And we can shape the blades to further smooth the loading so it has no peaks.”
To minimize noise, the blade is designed to have the least change in loading as it rotates. “We have this large-diameter, ultrahigh-solidity, ultraslow rotor where the blades can be lightly and uniformly loaded across their span and have a very small, unsteady loading as they rotate,” Ian Randall says.
Blade thickness is reduced, “but because we have so many more blades, we get the same lift at much lower rpm and tip speeds and so dramatically reduce the noise,” he notes.
Blade cyclic and collective pitch also are controlled, but MagLev is not revealing how. “Essentially, we have a software-defined swashplate,” Ian Randall says.
The multiblade rim is supported by a magnetic bearing. Permanent magnets in the rotor make this suspension passive. Even if there is a total power failure, “all we need is rotor speed, and we have the authority and stiffness to perform a gentle autorotation using the massive amount of inertia in the spinning rotor,” he says.
The dynamics of the magnetic bearing can be tuned to damp vibration and improve ride quality. This electrodynamic suspension is integrated with electric propulsion in the HyperDrive. “All electric motors are wrapped linear motors, and all linear motors are unraveled electric motors. In our case, it’s a very-high-aspect-ratio electric motor that is also providing our bearing function,” Ian Randall says.
“What we do get to take advantage of is the ability to segment the rotor into a highly redundant, distributed electric propulsion. If you lose an inverter or two, it does not reduce the number of blades providing lift and control for the vehicle,” Rod Randall says, noting that a rotor failure in some eVTOL designs requires the balancing rotor to be shut down.
MagLev has secured early support from prominent technology investors. The startup is working toward flying a prototype HyperDrive system 1-3 m (3-10 ft.) in diameter. “Our time frame is that we’ll have aircraft flying in the next few years,” Ian Randall says.
“Our focus now is on the many-bladed rotor and motor/bearing system that enables the propulsion system,” he says. “We have subscale rigs where we’re spinning rotors and measuring thrust, proving out the individual components and making sure they match our models. Then we have some integration tests that will continue through the end of the year.”
Anyone challenging the propulsion paradigm of shaft-driven propellers, rotors and fans can expect appropriate skepticism. Doubly so when MagLev’s own concept artwork, intended to illustrate the possibilities, also highlights the challenges of integrating the HyperDrive into a practical design.
But MagLev is a propulsion system developer, not an aircraft designer. “We believe this can enable industrial designs for aircraft that have both performance and consumer appeal,” Rod Randall says. “That’s why we’re excited to work with other OEMs. We’ve begun speaking with a few—and some of the concepts and missions they are pursuing are fascinating and unique.”