Electric or hydrogen? Group Editor Marty Kauchak surveys rapidly evolving developments in the eVTOL propulsion sector.   

  • Majority of flights in the US are less than 500 mi
  • Hybrid-electric may be feasible today
  • Hydrogen offers range and aircraft turnaround advantages
  • Military sees eVTOL logistics benefits versus helicopters, tiltrotors

 As a traditional objective journalism publication, CAT hesitates to use the word “frenetic” in the commercial aviation sector, but that descriptor is even an understatement of the pace of developments and activities in the eVTOL propulsion market. Industry teams are doing nothing less than expanding the technology envelopes in chemistry, physics, metallurgy and other disciplines to provide effective, efficient and safe subsystems for the first tranche of these new aircraft preparing to enter service. 

This survey highlights trends and developments in this propulsion market through the insights of eVTOL OEMs, a propulsion materiel supplier, and the FAA.

New Breed of Propulsion Systems                    

While OEMs and their commercial sector end-users eye the dual-track evolution of two primary propulsion sources – electric and hydrogen – there is common ground that it is no longer business as usual in terms of fielding propulsion subsystems for these future aircraft.   

A representative perspective was provided by Eviation CEO Omer Bar-Yohay, who pointed out electric aviation is a competitive and sustainable solution for the regional mobility of people and goods. More to point, “Electric propulsion produces no carbon emissions, significantly reduces noise and is anywhere from 40-80% lower cost per flight hour. As it relates to commercial aviation, it also means operators will be able to fly more planes into smaller airports closer to homes and destinations, meaning shorter and more convenient door-to-door experiences.” 

In fact, more than 200 airports in the US with noise restrictions will be able to offer more flights at more times for improved choice and connectivity. The Eviation leader noted, “Additionally, the majority of flights in the US are less than 500 mi (805 km) range, which means there is high demand for regional passenger and commuter travel alike. In short, electric aviation will open up new possibilities for affordable, sustainable regional travel around the world.”

Electric Solutions in Play  

Spoiler alert. The main competition in this sector is between the suppliers of electric powertrains of various designs and their counterparts developing hydrogen fuel cells. 

One use case in the battery market was described by Eviation’s Bar-Yohay, who observed his ‘Alice’ nine-passenger, two-crew member aircraft requires 30 minutes or less to charge per flight hour for a maximum range of up to 440 nm (506 mi). “The aircraft is powered by two magni650 electric propulsion units from magniX, the only flight-proven electric propulsion systems at this scale,” he said, and noted the advanced fly-by-wire system is made by Honeywell, “the market leader in such systems.” The single-volume, high-energy density Alice battery system is made from currently available battery cells and is claimed not to be reliant on future advancements. “These proven technologies and design elements make it easy and reliable for pilots to seamlessly transition to flying the Alice and will create a superior passenger flying experience, accelerating the aircraft's path to market.”

Eviation’s CEO reported, “Alice is on track for first flight later this year, and we are working closely with the FAA on certification and entry into service expected in 2024.”

Helena Treeck, head of public relations at Volocopter, said their company has chosen to use the battery technology available to create its electric aircraft. “This gives Volocopter a competitive advantage,” she declared, and added, “The business case around our urban air mobility solutions is based on certified technology that is available for commercial use today. By taking this approach with our electric aircraft, Volocopter utilizes technology available today and reduces extra risks in the timeline that other new technologies may have during the certification process.” 

Treek then briefly delved into battery chemistry, which she said plays an important role in electrically powered UAM [urban air mobility] aircraft like the company’s VoloCity or VoloDrone, with Volocopter always following the latest developments in battery technology very closely. “While developing an eVTOL commercial aircraft, we must ensure the batteries we use meet our key parameters like high-power densities, low internal resistances, and of course, high capacities. Keep in mind that we are in a highly regulated environment and that our supplier landscape must comply with the standards of safety in aviation and must be certified by the respective authorities.”

Volocopter expects “to launch our 100% electric solutions in the next 2-3 years; we have committed to launching in Paris and Singapore already – places where we have already conducted public flights with our electric aircraft.”

Electric for Conventionals, Too

While this article is primarily focused on eVTOLs, it’s important to briefly glance into the adjacent eCTOL (electric conventional takeoff) aircraft space, which is seeking similar propulsion solutions. Kevin Noertker, CEO of Ampaire, first noted airlines always have an imperative to reduce operating cost, and added, “Our first commercial product, a hybrid-electric powertrain for the Cessna Grand Caravan, will reduce fuel consumption by up to 25%, while retrofit costs will be in line with overhaul costs. Also, airlines increasingly recognize the need to reduce their carbon footprint.”  

Noertker pointed out his eCTOL powertrain, a series hybrid drive, with a right-sized turbogenerator to recharge batteries, is independent of ground-charging infrastructure, and this seems the right place to start – similar to the Prius hybrid-electric car some years ago. “This is the first step to fully electric aircraft. The transition will take some time due to limited battery capacity today and lack of ground infrastructure for charging,” he said, and significantly added, “The technical and infrastructure challenges of hydrogen mean a longer likely timeline for that possible avenue. We plan to have hybrid electric aircraft in service by 2025.”

The CEO returned to the technology challenges facing the electric-propulsion market to validate his team’s solution. While Ampaire is in the eCTOL space, he offered the limitations of today’s battery technology apply universally. “To preserve utility – payload and range – battery weight must be minimized. Except in some very specialized cases where routes are extremely short, fully electric aircraft will not have satisfactory range or payload carrying capability. Not until battery density at least doubles and that is likely many years away. We think the way to transition to a carbon-neutral market is to start with feasible solutions today, and that means hybrid electric.”

Accent on Hydrogen

At first glance, hydrogen should be more widely embraced in the eVTOL and adjacent parts of the aviation ecosystem, with the fuel’s main benefit being in the combustion process – its emissions are decarbonized. Space in this article precludes a detailed review of current technology impediments and related challenges, which should prevent industry-air carrier teams from more widely embracing hydrogen to power these next-generation aircraft. A short list of reasons not to embrace the fuel includes: the need to use hydrogen in liquid form to achieve sufficient density for aviation use – and its subsequent storage challenges; and the uncertainty of a future, adequate global supply of this new fuel. 

Enter Dr. Alex Ivanenko, founder/CEO of HyPoint, one company addressing the challenges of using hydrogen power trains and helping to launch a tectonic shift in the commercial aviation market toward hydrogen.

The HyPoint leader initially responded to CAT’s question: Why not use electric systems for eVTOLs and other aviation assets? Ivanenko pointed out the core problem with current electric systems for aviation/eVTOLs/etc. is specific power and energy density. “There are several limitations to Li [lithium]-ion batteries, especially for massive vehicles, long-distance hauls, or high-frequency use cases (e.g., air taxis that can't spend hours per day charging), all of which have specific power and energy density requirements that Li-ion cannot deliver. However, we believe that Li-ion (such as Tesla batteries) makes sense for some short-distance urban air mobility and/or private use cases, where long-distance or multiple trips per day are not needed.”

Of more relevance, HyPoint’s approach to high-temperature PEM [polymer electrolyte membrane] hydrogen fuel cells, based on a new membrane chemistry, is radically different from any hydrogen fuel cell system on the market, because it utilizes an industry-first air-cooling method for circulating pressurized air — rather than heavy liquid — which dramatically reduces its overall weight, according to Ivanenko. “As a result, HyPoint's hydrogen fuel cells can deliver significantly greater specific power and energy density, which means it can support larger vehicles across longer distances, which will be critical for hydrogen airplanes and eVTOLs.” 

The CEO discussed more of his portfolio’s current ‘secret sauce,’ adding, “The reason HyPoint is exciting – why it's caught the attention of so many – is because its air-cooled high-temperature fuel cell system is three times lighter than traditional (liquid-cooled low-temperature) fuel cells, representing a 61% reduction in total weight. This is important because HyPoint is the only company able to deliver the necessary combination of energy density and specific power that airborne vehicles require for regular/long-distance use.” The CEO also emphasized, “Testing has shown that HyPoint's turbo air-cooled hydrogen fuel cell system will be able to achieve up to 2,000 watts per kilogram of specific power, which is more than triple the power-to-weight ratio of traditional hydrogen fuel cell systems. It will also boast up to 1,500 watt-hours per kilogram of energy density, enabling longer-distance journeys.” 

HyPoint reports existing purchase orders with Piasecki and ZeroAvia (and others that cannot be named), which the company “expects to see in production in the next couple of years – as well as development agreements with companies like Urban Aeronautics and others.” 

Ivanenko concluded the much-anticipated arrival of eVTOLs, air taxis, and like platforms, makers have been hindered by the inadequacies of existing power systems, namely Li-ion batteries. “We expect that as eVTOL makers begin to realize that batteries will not be sufficient to bring their vehicles to market, they will begin exploring hydrogen as a suitable alternative. Of course, hydrogen is not the panacea for all eVTOLs, but for those that are heavy (many passengers/ goods), expect to make many regular trips where battery charging between trips would severely impact usage; or expect to make long-haul flights where battery charging is impossible. We are ready to help them incorporate hydrogen into their powertrains.”

FAA: Existing Regs Apply

A new aircraft model’s path to operation must pass through the FAA’s (and other nations’) certification processes, and so it will also be with the new eVTOLs queuing up to enter revenue service in US airspace. In response to CAT’s queries on the FAA’s decision-making and regulatory processes to approve Li batteries and hydrogen as eVTOL propulsion sources, and related topics, an agency spokesperson responded, “The FAA can certify new technologies such as eVTOLs through its existing regulations. When tailoring existing rules to an applicant’s new concept, the FAA determines the certification requirements for the eVTOL’s design, production, airworthiness, and operation. Some certifications could require the FAA to issue special conditions or additional airworthiness criteria, depending on the type of project. Determining qualifications for these aircraft is an ongoing process.”

Earlier this year, the FAA and the UK Civil Aviation Authority announced they are engaged in a range of bilateral and multilateral discussions focused on facilitating certification and validating new eVTOL aircraft, production, continued airworthiness, operations, and personnel licensing.


Not for Civil Aviation Only 

The military services are quickening their pace to embrace electric-based solutions for VTOL and like designs in their programs of record. Such is the case with PteroDynamics, an aircraft design and manufacturing company that develops innovative VTOL aircraft, which recently secured a contract with Naval Air Warfare Center Aircraft Division (NAWCAD) to deliver three VTOL prototypes for the Blue Water Maritime Logistics UAS (BWUAS) program. 

Tim Whitehand, VP of Engineering at PteroDynamics, pointed out under the BWUAS contract his company will deliver three all-electric aircraft to the Navy, with the firm’s Transwing design, which “will exceed all of the Navy's requirements with electric motors and common off-the-shelf batteries.”

Whitehand presented the business case, noting a 2018 Military Sealift Command study which found that 90% of all sea deliveries of critical repair cargo were less than 50 pounds [22.7 kg], and 80% were less than 10 lbs. “These deliveries are conducted using expensive manned vertical lift platforms, particularly MH-60S and V-22. By offering a flexible, low-cost delivery option, we can save the Navy millions of dollars in operational costs and free up those manned assets to focus on tactical missions vice logistics.”

Asked to briefly discuss the Transwing design, the executive offered it has significant advantages over other hybrid multi-rotor designs because it uses the same propulsors and energy source for vertical lift and cruise flight. “There is no need to fly a mission with the weight and drag penalty of aircraft structure, propulsion, and energy that is used only during the takeoff and landing phases. This advantage can be realized in increased range, endurance, and/or payload, based on mission needs. The Transwing design offers a payload/weight fraction of upwards of 20%, far above comparably sized hybrid multi-rotor aircraft.” 

Whitehand pointed out the benefits of shifting away from hydrocarbon-based energy sources are clear; however, the majority of commercially viable use cases his company can service now demand range, speed and endurance that is unattainable with current battery technology. “That being said one of the differentiating features of the Transwing versus other VTOL configurations is a shifting to a low-energy state quickly, resulting in a more energy-efficient system. With a modest improvement to battery energy density (2x) this would open up dramatically. In the near-term, expansion of small, modular multi-fuels hybrid gen-sets is a clear market opportunity.” 

The executive emphasized Transwing designs scale very well from the current project at 73 lbs MTOW (maximum takeoff weight) to aircraft in the thousands of pounds. He concluded, “Current battery technology will need to be augmented as we scale this technology. PteroDynamics is exploring options that could include an onboard generator to recharge the batteries and a hybrid system with two internal combustion engines and two electric motors.”