By Tamara Botting

Tamara Botting is an award-winning journalist and freelance writer in Ontario, Canada. She has more than a decade of editorial experience and is a published author of several books. Tamara is looking forward to covering this emerging industry.

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Overair tests full-scale propulsion system for Butterfly eVTOL

California-based Overair has officially started ground testing the full-scale propulsion system for its Butterfly eVTOL aircraft.  

Overair Butterfly air taxi
Overair began conducting full-scale propulsion system testing in the Southern California desert, which according to Overair, is “an environment that simulates some of the most demanding aerodynamic conditions anticipated in real-world operations.” Overair Image

“The propulsion system test rig is an asset we will use on a continuing basis to continually validate new design ideas as they get incorporated into the propulsion system,” Ben Tigner, Overair CEO, and Jim Orbon, Butterfly program manager, wrote in an email to eVTOL.com. “Much of the initial data gathering from the propulsion system testbed will be completed this spring.”  

The company anticipates unveiling the full-scale prototype of the Butterfly by the end of 2022, and is targeting a first flight by 2023.  

Tigner and Orbon said, “We are currently fabricating the blades, hub and retention components, and also have airframe components underway,” as part of getting their long-lead time parts ready for the project.  

Overair is using a vectored thrust design with the Butterfly. The aircraft has very large propellers relative to other eVTOLs — “approximately double the disk area to the powerplant than our closest eVTOL competitor,” Tigner and Orbon said.  

A recent press release from Overair stated that the Butterfly’s larger size propellers allows it to spin slower during various phases of operation, including hovering, transitioning, and cruising. Because of this, the company said Butterfly’s propellers would be quieter and draw less power from the aircraft’s batteries. The Butterfly will also rely on wing-borne flights for cruising over longer distances.  

“Thanks to substantial power and control margins, these large propellers provide unmatched efficiency — an incredibly important attribute when flying high-tempo urban routes in varied environments,” the press release stated.  

While larger propellers might seem counter-intuitive when it comes to discussing efficiency — with larger propellers adding more weight on the aircraft — Tigner and Orbon said the underlying physics make it a better choice.  

“Lifting devices normally decrease in overall efficiency when they are subdivided into smaller blocks,” they said, giving the example that an airplane with one large wing is generally more efficient than an airplane with many small wings.  

“Even though the single large wing might be heavier than smaller wings, the additional aerodynamic efficiency is such an advantage that the trade-off pays for itself. This is also the case here, where we use a small number of large propellers instead of a large number of small propellers.”  

They said that helicopters have been a mainstay of vertical flight for so many years because their large disk area allows for more efficient hovering — that’s why Overair incorporated a large disk area into its aircraft, too.  

Tigner and Orbon noted that Overair’s staff have spent decades perfecting the technology while working in collaboration with partners at NASA, the Defense Advanced Research Projects Agency (DARPA), and the United States Department of Defence (DoD).  

“This design choice is something that makes Butterfly a very different aircraft from the rest of the field and gives us outsized advantage in payload capacity, power and control margins while undertaking vertical flight operations to safely and robustly operate Butterfly in a wide-array of real-world conditions,” Tigner and Orbon said.  

This video captures the Overair team subjecting the propeller blades and hub components to forces that the company says will be experienced during flight operations, as they validate Overair’s flight dynamics simulation software tools.

The full-scale propulsion system testing is being done in the Southern California desert, which according to Overair’s press release, is “an environment that simulates some of the most demanding aerodynamic conditions anticipated in real-world operations.”  

Tigner and Orbon said they will be testing the other extremes of freezing temperatures, snow and ice as “part of subsequent weatherization tests on the complete vehicle [and] certification to operate in those conditions.”  

Overair is working toward getting the Butterfly certified with the U.S. Federal Aviation Administration (FAA), and has a goal of entering commercial service in 2026.  

The Butterfly is being designed to carry a target payload of 1,100 pounds (500 kilograms) and be able to carry a pilot and up to five passengers and/or cargo. Basing his calculations on state-of-the-art batteries that are commercially available today, eVTOL.com previously reported that Orbon expected the Butterfly to have a top speed of 200 miles per hour (320 kilometers per hour) and a range of over 100 mi (160 km).  

Overair was founded in 2019 as a spin-off venture from Karem Aircraft. Its headquarters is located near John Wayne Airport in Orange County. Interest in the Butterfly started strong and has stayed that way. In 2019, South Korea’s Hanwha Systems invested $25 million into the company, and Bristow Group pre-ordered up to 50 aircraft from Overair at the start of 2022.  

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1 Comment

  1. Interesting that the video of the test had an ‘epic’ soundtrack so we couldn’t hear the noise of the engine. Hiding something overair?

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