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The founders of VerdeGo Aero have long believed that hybrid-electric propulsion was going to be a key enabling technology for a new generation of electric vertical takeoff and landing (eVTOL), electric conventional takeoff and landing (eCTOL) and electric short takeoff and landing (eSTOL) aircraft to be commercially successful.
Seventeen years ago, Executive Chairman Erik Lindbergh, Chief Executive Officer (CEO) Eric Bartsch and Chief Technology Officer (CTO) Dr. Pat Anderson started designing, building, promoting and piloting sub-scale and full-scale electric aircraft in their previous careers, but battery makers couldn’t deliver the energy density and power density increases they regularly promised. That triggered an interest in hybrid-electric propulsion, which could provide the payload, range and safety reserves the new electric aircraft required.
Fast forward to 2025, and VerdeGo’s portfolio of three hybrid-electric propulsion systems — the VH-3-185, VH-4T and VH-5, spanning the 185-kW to 1.5-MW output range — are generating plenty of commercial and military interest for different mission applications.
Wake-Up Call
When Pat Anderson started designing his first electric aircraft in 2009 as director of the Embry-Riddle Aeronautical University (ERAU) Eagle Flight Research Center (EFRC), he said it was already abundantly clear that by “doing some simple algebra … that batteries were going to be difficult as your method of storing energy” especially since battery energy densities were only increasing by about 2–4% a year.
“I always thought that people would come around to this conclusion [about the limits of batteries] much faster than they have. But I really think that 2024 is the year that the industry takes a turn towards hybrids,” he said during a VFS interview at the Experimental Aircraft Association’s (EAA) AirVenture convention in Oshkosh, Wisconsin, in July 2024.
Personal Air Taxi 200 eVTOL
When Anderson, Bartsch and Lindbergh formed VerdeGo Aero, their first project was to design the two-seat Personal Air Taxi 200 (PAT200) eVTOL aircraft with hybrid-electric propulsion, which was announced at EAA AirVenture in July 2017 (see www.evtol.news/verdego).
They wanted to be a leader in the “eVTOL revolution” and conceived of a hybrid system that combined a liquid-fuel combustion engine and an electric generator that would power eight electric motors on a novel tandem-tiltwing aircraft. One of the more distinguishing features of the PAT200 was that the rear wing was fitted with pusher propellers that tilted downward, and the forward wing had tractor propellers that tilted upward for VTOL flight.
A hybrid-electric energy source was strongly favored by the team, since it would allow the PAT200 to fly multiple revenue flights between refueling stops and have the required energy reserves, without adding a lot of weight.
VerdeGo opened an office in the Embry‑Riddle Research Park in the MicaPlex Aerospace Innovation Complex in Daytona Beach, Florida.
However, a year later, work on the PAT200 airframe stopped. “We pivoted into a propulsion company because people didn’t take us seriously because we were not reaching the holy grail,” said Bartsch, meaning a battery-electric aircraft design.
“We looked around and saw all the companies selling fiction. The more fiction they wanted to sell, the more money they were getting from investors,” said Anderson, a little more strongly in the VFS Leadership Profile published in Vertiflite in 2021.
Instead of competing head-to-head with dozens (and later, hundreds) of eVTOL aircraft developers, VerdeGo decided to develop hybrid systems that its former competitors could use.
Challenge of Batteries
In October 2016, Uber, the multibillion-dollar Silicon Valley mobility company (with inputs from NASA, the Vertical Flight Society and other leading vertical flight experts), published its groundbreaking Elevate white paper, “Fast-Forwarding to a Future of On-Demand Urban Air Transportation” in October 2016 (see www.evtol.news/uber). A key tenet of this vision was quick, cheap, urban transportation that would take cars off the road and carbon (and lead) out of the air.
The idea that millions of people could one day commute to work on emissions-free, battery-electric VTOL aircraft that would fly over gridlocked highways seemed like an idea out of science fiction, but it energized the industry and developed a huge following.
The Elevate white paper noted that “electric batteries are an obvious energy source” for emissions-free eVTOL aircraft. One of the key assumptions, however, was that “our analysis tells us this design mission range can likely be met within the next five years [with] battery cell specific energy of 400 Wh/kg.” This optimistic forecast was repeated at the first Uber Elevate Summit in Dallas, Texas, in April 2017 and many subsequent advanced air mobility (AAM) events.
But it set the stage for “one of the conflicts that’s happened many times in this industry,” said Bartsch, which is “the collision between what I will call ‘aircraft physics’ and ‘money physics.’”
Bartsch believes that investors decided that emissions-free, battery-electric aircraft was “a cool concept to fund … whether the real physics behind it worked out the way people wanted or not.”
However, it should be noted that Uber was looking at shorter range for intracity urban air mobility (UAM) missions, where low noise and zero emissions were paramount. Several companies, like Archer, Beta and Joby in the US, plan to begin UAM flights within the next two years. Although hybrid-electric propulsion systems produce fewer emissions than conventional engines and can run on lower-carbon sustainable aviation fuel (SAF) for a further reduction in pollution, hybrids still produce carbon and noise emissions, which are important for extensive operations within cities.
Integrated Distributed Electric Propulsion
In 2018, VerdeGo announced its Integrated Distributed Electric Propulsion (IDEP) system with the key elements including an engine, generator, power distribution system, batteries, battery management system (BMS), attitude controllers and electric motors powering rotors or propellers.
The pivot took the company from being an airframer with the PAT200 to being a propulsion company delivering both hybrid powertrain hardware (the main focus) and engineering support for the complex integration of hybrid systems into new types of aircraft. This work included helping customers efficiently determine how to best leverage the transformational capabilities of VerdeGo’s hybrid powerplants.
At the time, Anderson said, “everyone has their crystal ball for the timing of new battery technologies. We have been working for more than a decade with the global battery experts who project it will be 15–20 years before commercial levels of performance are achieved from purely battery-operated VTOL aircraft.”
The hybrid-electric system VerdeGo had intended for the PAT200 became the foundation for its propulsion business.
In 2018–2020, several aircraft developers partnered with VerdeGo to explore hybrid-electric technology, including Transcend Air for future versions of its Vy 400 aircraft, XTI Aircraft for its TriFan 200, Jaunt Air Mobility for a cargo version of the Journey slowed-rotor compound rotorcraft and Airflow for its eSTOL aircraft (note that all of these models have been superseded today).
In August 2020, the company began running its iron bird prototype Jet-A-fueled hybrid generator system at their facility in Daytona Beach. The ground-based development hardware was built around the Continental CD-265 compression-ignition diesel engine and was used for testing to refine the weight, power output and cooling systems, as well as their integration and reliability.
The iron bird was used to validate the operating economics of its diesel-hybrid power generation system and perform hardware-in-the-loop simulations using mission profiles of its customers.
First VH-1 and VH-2 Sales
During testing of the iron bird, VerdeGo received an unexpected order to deliver hybrid-electric propulsion systems to an undisclosed US customer, later revealed to be Moog, Inc., of East Aurora, New York, which is a worldwide designer, manufacturer and integrator of precision control components and systems.
In December 2019, Moog purchased the assets associated with the SureFly eVTOL aircraft from its parent company, Workhorse Group, Inc., and then set about developing an eVTOL aircraft for military use. Workhorse planned to develop a hybrid-electric propulsion system, using a Honda motorcycle engine as a generator, but all of its flights used batteries.
The sale of the VH-1 iron bird provided Moog engineers with a tool they could use to evaluate various hybrid-electric propulsion systems in-house for their proposed HE350 Recluse uncrewed aircraft systems (UAS) for autonomous resupply (see “Moog: Growing with New Strengths,” Vertiflite, July/Aug 2023), though this was later cancelled.
Three second-generation VH-2-150 hybrid-electric propulsion systems (producing 150 kW of power) were delivered to an undisclosed customer in January 2022, apparently with two hybrid powerplants installed in a single aircraft.
In the push to deliver flying hardware, VerdeGo engineers were able to significantly reduce the weight and the size of the VH-2 compared to its first-generation iron bird.
Powerplant Sizing
Sizing a hybrid powerplant for an electric aircraft requires a different approach than sizing a powerplant for a helicopter or general aviation aircraft, said David Eichstedt, vice president of product management at VerdeGo. “We really want to size the hybrid system based on the cruise power required. You need to establish what the steady-state cruise power is and then you will have peaks for the takeoff and landing. This is called a ‘peaks and valleys mission profile,’ and this is where the battery is used in parallel with the hybrid powerplant to deliver a lot of power for a short period of time for those spikes.”
“Then you size the hybrid powerplant to have more power than needed for cruise so you can recharge the batteries in flight,” he added.
VH-3 Propulsion System
In March 2021, VerdeGo announced its first production hybrid product, the thirdgeneration VH-3-185 (producing 185 kW of power), designed to provide sufficient onboard energy for useful missions of hybrid eVTOL, eCTOL and lighter-thanair (LTA) aircraft, with extended range, payload and energy reserves.
The VH-3-185 has been designed to power 2–6-seat aircraft for maximum efficiency in cruise, and UAS with 500– 1,500-lb (225–680-kg) payloads.
The VH-3 combines a Jet-A ignition-compression engine with a 185-kW hybrid generator, where batteries can be used to provide additional bursts of power for takeoff and landing, with the amount of power available dependent on the specific battery configuration. From the beginning, VerdeGo designed the VH-3 to be capable of both parallelhybrid and series-hybrid operation.
In parallel-hybrid configuration, the powerplant can provide both mechanical power for driving propellers, rotors, fans or gearboxes and electrical power for driving electric motors or charging batteries at the same time. The VH-3-185 is also capable of maximum burst shaft power, where battery power is used to turn the generator into a motor to provide up to 500 hp (370 kW) of combined blended power on the output shaft.
In a series-hybrid configuration, the VH-3 provides only electrical power to the high-voltage bus for distribution to any combination of electric motors and batteries. The powerplant is also capable of operating in quiet mode with the engine turned off and battery power driving the output shaft.
The built-in flexibility of the hybrid system also gives aircraft designers several different installation options. For example, the VH-3 could be installed as a single unit, in pairs to power an eVTOL aircraft with both tractor and pusher propellers, and/or stacked with battery packs to provide burst power.
At the heart of the VH-3 powerplant is a four-cylinder, four-stroke SMA SR305 Jet-A engine originally developed by SMA Motors of France (but part of Germany’s Röder Group since 2020). The SR305 was developed in the late 1990s for the general aviation market and is certificated by both the European Union Aviation Safety Agency (EASA) and the US Federal Aviation Administration (FAA). VerdeGo’s iron bird utilized the Continental CD-265, but when the Chinese-owned US engine maker suspended development of the CD-265, VerdeGo easily substituted the SMA powerplant, upon which the CD-265 was based.
The specifications of the VH-3-185 include a powerplant weight of 650 lb (295 kg), including engine, generator, invertor and thermal systems. The overall 35-in (89-cm) width and 33-in (84-cm) height of the hybrid propulsion system is similar to that of a stock SR305 engine; the complete propulsion system is 61-in (15-cm) long, once the electric generator/motor, shafting and integrated cooling system are added.
The VH-3 boasts an industry-leading fuel burn of about 10 gallons per hour at full throttle, and the SMA engine can run on sustainable aviation fuel (SAF), which reduces its carbon footprint.
In September 2022, the VH-3-185 development benefited from a Small Business Innovation Research (SBIR) funding award of $1.2M via the US Air Force’s AFWERX innovation unit to support powerplant development. This was followed by a Tactical Funding Increase (TACFI) award in 2024 of $1.9M to develop a conformal prototype system.
Then, in June 2024, VerdeGo received a second contract supporting the development of VH-3-185 under NASA’s SBIR Ignite program for conceptual design studies of potential applications.
VH-4T Turbine
In July 2024, VerdeGo unveiled its 400-kW VH-4T-415 turbine hybrid-electric powerplant at the AFWERX booth at AirVenture.
Eichstedt said, “400 kW seems to be the sweet spot for 5–7 passenger [eVTOL] air taxis, or eSTOL aircraft that carry up to nine passengers, and cargo drones that need to carry over 1,000 lb [550 kW] of payload.”
In mid-2022, RTX Ventures, a unit of RTX (which owns the aerospace brands of Raytheon, Pratt & Whitney, Collins and others) made an investment in VerdeGo, opening the door to the use of the Pratt & Whitney Canada PW206/207 turboshaft in an integrated hybrid system. All the PW200-series engines to date have been used in pairs for twin-engine helicopter applications, so the selection of a single PW206/207 gas turbine to power the VT-4T is an interesting product milestone.
Full-scale iron-bird testing of the VH-4T began in late 2023 and the development of a flightworthy prototype was the subject of a second AFWERX SBIR Phase II grant. More recently, VerdeGo completed 150 hours of durability testing on the VH-4T prototype under an AFWERX Phase II grant.
During the development of the VH-4T, VerdeGo engineers leaned heavily on lessons learned developing the VH-3, including cooling and energy management.
VH-5 Revealed
At the VFS Forum 80 in Montreal in May 2024, VerdeGo revealed it was also developing the VH-5 propulsion system to address an emerging defense requirement for a vertical takeoff and landing (VTOL) aircraft that can cruise at very high speeds.
The idea of a convertible jet engine that can power a VTOL aircraft has been around for more than a half century — and has been the subject of many VFS technical papers over the years — but a jet engine that can produce electric power when it’s not producing thrust is something new. The company believes it can leverage hybridization of a jet and electric motors to meet the high-speed VTOL (HSVTOL) requirements proposed by DARPA for future military requirements.
“The VH-5 is intended to enable a single powerplant to power both the vertical and forward flight modes of an HSVTOL aircraft,” said Eichstedt, prior to the official launch of the VH-5. “And there are multiple other applications, not just HSVTOL, such as smaller unmanned aircraft.”
The company’s focus is getting to the point “where we can prototype and prove out the architecture of the system at a relevant and useful power range.”
The company sees an opportunity for a range of powerplants to meet a HSVTOL requirement, with 1.5 MW as a good starting point to support prototype aircraft development, and final sizing driven by customer requirements.
Eichstedt said that the electronic components required to support larger power outputs will eventually be available, “but there will be a frontier where you can’t go that easily.” VerdeGo recently completed a study under a Phase I AFWERX effort that explored the scalability of the VH-5 blended turbofan architecture. Eichstedt said, “we were pleasantly surprised through this study that the blended turbofan concept scales well across a range of potential power outputs. We now have conceptual designs as small as 15 kW and as large as 9 MW.”
VerdeGo is currently engaged with a number of aerospace primes, gathering information on requirements with a view toward selecting the right size system to build as the first implementation of the blended turbofan concept.
One of the characteristics of hybrids is that electric motors can also be generators, said Eichstedt, adding that it’s somewhat analogous to antennas, which can be used for both transmitting and receiving.
There may be occasions when a designer can optimize the hardware to be a generator and other times it’s optimized to be a motor with the desired RPM and torque levels, but it could be used interchangeably. VerdeGo sometime refers to this part of the hybrid hardware as an “e-machine,” because it’s dual use and may play both roles in a parallel design. In a military mission, for example, this dual-use function can help produce the energy required to power a large sensor package that is only utilized during the “cruise” portion of a particular HSVTOL mission.
Tipping Point
It’s been almost 15 years since Pat Anderson built and flew the first piloted hybrid-electric gasoline/battery aircraft for the NASA Green Flight Challenge. As more aircraft developers and potential customers recognize the current trajectory of battery development, the frequency of announcements regarding interest and investments in hybrid-electric and hydrogen-electric propulsion systems has increased, as has support from agencies such as NASA and AFWERX, which has become a champion of hybrid-electric propulsion for military missions.
VerdeGo has signed up customers for its hybrid-propulsion systems but can’t reveal whom at this point. The AAM industry has entered a new phase of “stealth” activity, where some aircraft developers are holding their cards close when it comes to a pivot to alternate energy sources.
About the Author
Ken Swartz is a senior aerospace marketing and communications strategist, running Aeromedia Consultants. A long-time consultant to the aviation, aerospace and vertical flight industry, he’s held management positions in the regional airline, helicopter and aircraft manufacturing industries for 30+ years, and has reported on vertical flight since 1978. In 2010, he received the Vertical Aviation International “Communicator of the Year” award.
