The Karem Aircraft spinoff Overair has revealed the commercial design for its Butterfly eVTOL: a six-seat, fully electric air taxi that the company claims will be the quietest and most efficient aircraft in its class.
The original concept for Butterfly was unveiled at the second Uber Elevate Summit in 2018. The vectored thrust aircraft is based on Karem’s proprietary optimum speed propulsion technology, which incorporates more than 20 years and $150 million of research and development in the context of military rotorcraft programs.
Optimum speed propulsion uses relatively large, slow-turning propellers with blades that are light and rigid enough to vary their speed of rotation in different flight regimes without encountering unacceptable vibration. According to Overair, compared to eVTOL designs with smaller propellers, Butterfly will require less power at all airspeeds, but especially in a hover.
Meanwhile, the large total blade area and low disc loading will allow Butterfly’s propellers to spin at slower speeds than competing designs, enabling the low noise that will be critical for operating in urban environments. Overair projects that Butterfly will generate just 55 dBA in a hover (measured at a distance of 100 meters/330 feet) and 30 dBA in cruise (at a distance of 500 meters/1,640 feet) — at least as quiet as any eVTOL demonstrated to date.
“What we’ve done over the last 20 years . . . is develop a very rich tapestry of interlinked technologies having to do with vertical-lift propulsion that provide extraordinary performance [with] very, very low noise and very, very low levels of vibration,” Overair CEO Ben Tigner said during a Vertical Flight Society briefing for media on Aug. 11. “And it’s that interlinking set of technologies that really builds the magic of what is embedded in Butterfly.”
With a $25 million Series A investment from Korean industrial conglomerate Hanwha Systems, Overair began the process of splitting off from Karem in 2019, leaving the parent company — founded by legendary aircraft designer Abe Karem — free to focus on its military contracts.
According to Tigner, “Karem Aircraft provided to Overair all the technology, all the design data, and all of the technical expertise in terms of personnel that would be needed to commercialize Butterfly, and since then, we’ve been growing the team as an independent company, and . . . making tremendous strides.”
The new Butterfly retains the same general concept as the original version, with two tilting propellers on a forward wing and two on a rear lifting surface. However, the size of the rear props has been increased, giving the aircraft the ability to land safely with any one prop inoperative. With redundancy across motors, inverters, batteries, and flight control systems, “there’s no single point of failure anywhere that can undo the capability of the aircraft, and in most cases dual and triple failures are things that we can tolerate,” Tigner said.
The piloted aircraft has a target payload of 1,100 pounds (500 kilograms) and will be able to accommodate up to five passengers and/or cargo. Butterfly should have a top speed of 200 miles per hour (320 kilometers per hour) and a range of over 100 miles (160 km). According to Butterfly program manager Jim Orbon, those figures are based on state-of-the-art batteries that are commercially available today, but the company — which is targeting Federal Aviation Administration certification in 2025 — doesn’t expect to lock in cell chemistry for its production aircraft until around 2023.
Orbon noted that, compared to eVTOL designs with high disc loading and high power consumption in a hover, Butterfly’s specific power profile is “a lot more consistent with automotive uses that are a little bit more biased towards energy density versus power density.” That should give Overair the opportunity to leverage more of the investment that is pouring into R&D for electric vehicle batteries, he suggested.
According to Tigner, Butterfly will have a unified flight control system like the one found on the Lockheed Martin F-35B, which keeps control actions consistent between hovering and forward flight (an approach also being taken by Joby). Meanwhile, the fly-by-wire flight control system will enable envelope protections to prevent many accidents caused by pilot error, in keeping with the eVTOL industry’s vision for simplified vehicle operations, he said.
Also similar to Joby, Overair is “interested in capturing the synergies on both the financial and engineering side that result from operating the aircraft in addition to building it,” according to head of business development Josh Aronoff. But the company is keeping its options open, added product manager Craig Smith: “There’s certain aspects that we are looking at and other aspects that we’re considering other partnerships for handling certain parts of the ride service.”
The Santa Ana, California-based company expects to fly an uncrewed, full-scale demonstrator aircraft next year with an emphasis on proving out the Butterfly’s propulsion technologies. That will be followed by design of a conforming prototype, probably by late 2023, Orbon said.
With respect to funding, Overair expects that Hanwha Systems will continue to be an important financial as well as strategic partner, as Hanwha “has very large and clear aspirations to be a global leader in urban air mobility, beyond [its] participation with Overair,” Aronoff said. For example, Hanwha and the advanced air mobility (AAM) infrastructure provider Skyports recently announced a memorandum of understanding to collaborate on AAM infrastructure, flight services, and a mobility platform.
“But obviously, [the] certification of a clean-slate aircraft will require additional financing,” Aronoff continued. “So safe to say you can expect to see additional financial rounds, but TBD on the details.”