Bell Engineer Talks Hybrid-Electric Propulsion for Air Taxi

By Nick Zazulia | July 22, 2018
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Bell Helicopter Air Taxi Cabin

The air taxi cabin revealed by Bell Helicopter at CES 2018. Image courtesy of Bell

Kyle Heironimus is the electric-propulsion development lead for Bell’s air taxi — part of the innovation team in charge of exploring new technology. As a partner for Uber Elevate, who is bullish on a 2023 operations launch, Bell needs to have a vehicle ready for testing in two years.

A Florida native, Heironimus followed a childhood of disassembling things and learning to do the opposite during two college summer internships at Bell’s Texas assembly center, which is where he got hooked on vertical-takeoff-and-landing (VTOL) aircraft.


As a high schooler, he built a dune buggy in his parents’ garage, the first vehicle that would propel him toward a career in mechanical engineering in the aerospace industry. Hieronimus has a master’s degree in systems engineering because he believes in taking a “holistic view of an aircraft’s lifecycle.”

Before the air taxi Heironimus worked on such helicopters as the UH-1 Iroquois, V-22 Osprey and the 505 Jet Ranger X, on which he was responsible for controls and hydraulics.

Heironimus spoke with sister publication Avionics International to discuss his work at Bell and the planned air taxi.

Bell Kyle Heironimus

Kyle Heironimus

Walk me through your day-to-day at Bell.

I currently function as the Bell air taxi electric-propulsion lead. My typical day is spent interfacing with our multi-disciplinary team within Bell and with a wide range of typical and non-typical aviation suppliers. Right now, I see my job as helping to pull together all of the various technical experts required to bring a hybrid electric propulsion system to life.

When did you switch from traditional-propulsion helicopters to eVTOL, and how has the move to electric propulsion changed things for you?

After my time on the Bell 505, I joined Bell’s innovation team in 2017 with the goal of influencing the conceptual design of Bell’s future products and the technology to support them. When I saw what the innovation team is developing, it became quickly apparent that electric propulsion is a key to unlocking next-generation VTOL aircraft configurations.

I’m a mechanical and systems engineer by training and experience; there’s been a learning curve for me to come up to speed on electrical propulsion design. I’ve spent the past year studying, researching and collaborating with the experts in batteries, electrical machines, power electronics and electrical distribution. Interestingly I have found many mechanical considerations for electric propulsion design — thermal management, for example — that are directly applicable to my background.

What stands out as the most fun or interesting project you’ve worked on? What about that still sticks out to you?

I’m extremely proud of the Bell 505 and what our design team achieved with the design of that aircraft. I’m lucky to have had the opportunity to go through a clean-sheet design and certification of a new aircraft type so early in my career. The 505 will always stand out for me. That said, my role now on the innovation team is certainly one of the most fun, interesting and challenging endeavors I’ve been involved with.

Explain the strategy behind the plan to put hybrid-electric propulsion on the air taxi.

We see the flexibility that a hybrid-electric system provides as necessary to build an aircraft that meets the full range of missions that we are going to fly. Of course, battery electric VTOL aircraft are already flying; however, their capabilities are significantly limited by the batteries available today. Simply put, today’s batteries are too heavy for a meaningful VTOL mission. We believe that demonstrating a hybrid-electric propulsion system positions us and our partners to take advantage of the best of current propulsion technology as well as new emerging electric motor and control technology.

Can you tell me anything about how the propulsion will be integrated with and by the flight control systems?

Electric propulsion systems allow us to re-think our traditional approach to propulsion and control. The capability to control an aircraft with rpm of the rotor system(s) means the propulsion system is essentially now a flight control and must be treated as such. It also means that these aircraft must inherently be electronically controlled. Starting with a fly-by-wire aircraft should allow a direct step into autonomy. Without downplaying the significant technical challenges of autonomous flight control, we at least have the opportunity to design our systems from the ground up with this end goal in mind, instead of having to adapt some legacy mechanical and analog systems to do the job.

Is Bell developing all of the avionics in house or working with vendors?

Bell is working with a variety of industry partners, including our own in-house experts to evaluate the best path forward for the platform. We certainly aren’t the only ones who want to shape this new market.

Project five years from now: What will be the most significant change?

From my standpoint, increased power and energy density of electric propulsion and control components will be key to unlocking eVTOL. I certainly don’t hold the crystal ball, but in five years I hope to see components and systems reaching weights that are not only economically viable for specific missions, but are so good that they supplant legacy solutions in cost, weight and performance. Bell is well on its way to designing and certifying eVTOL aircraft.

Does your job bleed into your day-to-day life? For example, do you drive an electric car?

I’d like to think my background and inclinations make me well-suited to be an engineer; it’s me that bleeds into engineering, not the other way around. Luckily, my fiancée is a special person to put up with the day-to-day of living with me. It’s a mix of detailed organization (i.e. spreadsheets to track everything) and barely managed chaos (where do I park the five cars I own?). I think anyone who has worked on aircraft design and test understands this dichotomy. After everything I’ve said about my excitement for the rise of electric aviation, I don’t actually own an electric car. I probably ought to go buy one.

This interview was edited for length and clarity.

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