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The Potential of eCTOL
  • 29 Aug 2019 09:15 AM
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The Potential of eCTOL

Electric Conventional Take-Off and Landing Aircraft Take Off!

By Nicolas Zart

Vertiflite, September/October 2019

While essentially all of the news on electric aviation in Vertiflite has focused on electric vertical take-off and landing (eVTOL) aircraft, developments of electric conventional take-off and landing (eCTOL) airplanes have recently accelerated.

Airbus and Boeing Electric Airplanes

Big aviation players have already struck strategic alliances with aerospace startups in addition to developing their aircraft. Boeing and Airbus, for instance, are hedging their urban air mobility (UAM) bets with both eVTOL and eCTOL projects.

At larger scales, electrifying airplanes is a complex topic that can only be achieved slowly, progressively, until battery energy density improves enough to allow for greater payloads and range. In the meantime, electric power is picking up ancillary systems and removing heavier hydraulic, pneumatic, and other traditional systems in “more-electric aircraft.” The Boeing 787, for example, pioneered many electric systems for commercial airline use, including its lithium-ion battery system.

As far as eCTOL, Boeing — with JetBlue and much fanfare — invested millions of dollars into Zunum Aero in 2017 to build a 12-seat, hybrid-electric aircraft for short-haul passenger service. However, today Zunum has all-but folded. Boeing has Uber Elevate partner Aurora Flight Sciences as a subsidiary and teamed with Kitty Hawk for its Cora eVTOL program, but does not appear to have an active eCTOL program underway.

On July 10, 2015, Airbus Group’s E-Fan technology demonstrator crossed the English Channel, landing here in Calais, France. (Airbus photo by Gilles Bassignac)

The Airbus E-Fan  also has a history that puts the electric air-mobility race in perspective. After teaming with Siemens on early demonstration projects, Airbus developed its all-electric, all-composite E-Fan two-seater. The Airbus E-Fan first flew in 2014 and the company began designing production versions. Airbus also planned a 90-seat regional jet, the E-Thrust, but each of these projects were abandoned.

Instead, Airbus started the E-Fan X project in November 2017 as an eCTOL testbed for the European Commission’s Flightpath 2050 initiative. Airbus, along with partners Rolls-Royce and Siemens, are converting a British Aerospace BAe 146 regional jet to hybrid-electric power. However, in June, Siemens sold its electric aircraft propulsion unit to Rolls-Royce, thus signaling a shift in the electric aviation Tier 1 supply chain.

The Airbus EcoPulse, announced at the Paris Air Show, adds six electric motors to the improve lift over the wing of the Daher TBM. From left to right: Nicolas Orance, Daher; Stéphane Cueille, Safran; and Jean-Brice Dumont, EVP Engineering, Airbus — and past VFS Chair of the Board. (Airbus photo by Hervé Goussé)

Airbus is also working on a smaller hybrid design. The Airbus Ecopulse  is a French team effort with aircraft manufacturer Daher and turbine producer Safran. Safran will develop the distributed hybrid system and converting the single-engine Daher TBM into a hybrid platform. The system consists of a turbine and power generator (together, a “turbogenerator”), an electric power management system, and integrated electric propellers. The aircraft itself will be modified with reduced wing surface area and drag. Airbus will handle aerodynamic optimization of the propulsion system and high-energy-density batteries installation. Daher will handle the installation of components and systems, flight testing, regulatory approvals and construction.

Airbus also unveiled its “Bird of Prey” concept at the Paris Air Show in June — but it is just that, a concept. The company said that conceptual airliner design aims to motivate and challenge the next generation of aeronautical engineers and designers.

With all this in mind, Vertiflite asked a few electric aircraft stalwarts and startups how they view the very active eCTOL market.


When it comes to electric aviation, Ajdovščina, Slovenia-based Pipistrel has 15 years of eCTOL experience under its belt. The company was established by Ivo Boscarol in 1989 in the former Yugoslavia; it was the first private aircraft producer in the country. At the time, alternative and ultralight flying was prohibited. The triangular, powered hang gliders were flown at night, looking like bats — the name, “Pipistrel,” comes from the Latin word for bat.

Pipistrel’s electric propulsion systems work was begun in 2004 and the world’s first two-seat electric aircraft, the Taurus Electro, first flew in 2007. The Taurus Electro had strong sales and the company released its second-generation electric propulsion system in 2012 with the Taurus Electro G2. The latest version, the Taurus Electro G2.5, was released in 2018 and features additional improvements.

In 2011, Pipistrel won the NASA Green Flight Challenge — sponsored by Google and managed by the Comparative Aircraft Flight Efficiency (CAFE) Foundation — with its Taurus G4, the world’s first fully electric, four-seat aircraft. Pipistrel showed the world that an eCTOL was more than twice as efficient as its piston-powered competitor in a cross-country, 200-mile (320-km) flight competition. Pipistrel USA — led by Dr. Jack Langelaan of the Pennsylvania State University — achieved a spectacular 403.5 equivalent passenger miles per gallon, according to CAFE Foundation’s measurements. For an automotive comparison, think of driving 200 miles at more than 100 mph (160 km/h), sipping fuel at less than a gallon per passenger.

Pipistrel’s Alpha Electro factory in Ajdovščina, Slovenia. (2017 Pipistrel photo)

In 2014, Pipistrel began converting its Alpha Trainer aircraft to electric propulsion; this model was released in 2017 as the Alpha Electro. It was the first all-electric aircraft to be approved for the Light Sport Aircraft (LSA) category and the company website says “in most areas it could be as little as $3/hour for electricity to operate the aircraft.” In parallel, the company designed the Hypstair, the first serial hybrid electric system for general aviation, as well as the Pipistrel HY4 hydrogen aircraft — both successfully flown in 2017.

The company has more electric and hybrid-electric aircraft in development. Pipistrel’s Boscarol told Vertiflite: “We will be ready in February with the first flight of the four-seat, hybrid-electric Panthera. The fully electric model will follow soon, as well as a 19-seat, hybrid-electric concept and I hope the future of aviation will be really emission-free.”

Boscarol said he was very surprised to not see more electric fixed-wing aircraft on the market over the past 12 years, wondering why none of the few very promising projects were completed. Of course, going from a first flying prototype to serial mass production is a daunting task for startups. Most rely on financial resources and investments. He concluded: “there needs to be more producers and different products with electric propulsion.”

Dr. Tine Tomažič’s, head of research and development for Pipistrel and CEO of Pipistrel Vertical Solutions, at AirVenture with a subscale model of the Pipistrel 801 eVTOL for Uber Elevate. (VFS photo by Kenneth I. Swartz)

Beginning in April 2017, Pipistrel began teasing information about its eVTOL plans at the three annual Uber Elevate Summits. This June, the company finally revealed some details and a model of its Model 801 eVTOL , which was also on display at AirVenture. This puts Pipistrel in an enviable position of having both eVTOL and eCTOL aircraft in development.


Hawthorne, California-based Ampaire was founded in 2016 to develop “high performance, zero emission aircraft to transform the landscape of aviation.” The company has a vision for a high-performance, 19-passenger eCTOL concept called TailWind , powered by a single, rear-mounted ducted propeller.

Ampaire has a vision for its nine-passenger all-electric TailWind, which features a rear-thrusting ducted propeller and zero emissions. (Ampaire graphic)

Brice Nzeukou, Ampaire Product Manager explained that eCTOLs can carry more passengers with greater payloads and farther than eVTOLs because of the much greater power required for vertical flight.

The hybrid powertrain is a good first step toward a pure-electric aircraft, he said, and a traditional airplane fuselage is perfect for a hybrid conversion. In June, Ampaire flew a hybrid Cessna 337 Skymaster; Ampaire replaced one of the two centerline engines with an electric motor for its “Electric EEL.” Hawaii-based Mokulele Airlines wants to begin demonstrations on Maui in late-2019.

Ampaire’s hybrid-electric EEL is a Cessna Skymaster with a piston engine in the front and an electric pusher motor at the rear for half the fuel consumption. (Ampaire photo)

Ampaire CEO Kevin Noertker said there are varying eCTOL design solutions, but Ampaire’s “parallel hybrid” architecture is the most feasible practical platform with compelling performance and reduced maintenance and operating costs. “The current market stage for electric aviation is developing rapidly and we’re leading the charge in the industry. We’ve flown the largest hybrid-electric aircraft to date and are already working on the next iteration of our first product, the Electric EEL, which will operate in a demonstration program in Hawaii.

“The Hawaii demonstration, in partnership with Mokulele Airlines and [funds from] Elemental Excelerator, will be the first of its kind. It will be the first time an electrified aircraft is operated in a mock-commercial setting, giving all stakeholders an intimate understanding of how to incorporate electric aircraft into fleets.”

Retrofitting existing planes instead of the huge expense of developing and certifying clean-sheet designs means only a supplemental type certificate (STC) is required; Ampaire plans to have the Electric EEL certificated by the end of 2021. In addition to the Mokulele demonstration, Noertker said Ampaire has 50 EELs orders plus 50 options from Personal Airline Exchange (PAX) — for a backlog worth $70M. He sees the EELs as a necessary steppingstone to its purpose-designed TailWind eCTOL. “While others are still in development, we’re in the air,” their website states.

When asked about designing their own eVTOL, Noertker said the idea is certainly not off the table.


Eviation Aircraft  stunned the world with its pure-electric, 650-mile (1,050-km) range, nine-seat commuter called the Alice it showed at this year’s Paris Air Show.

The Eviation Alice prototype was unveiled at the Paris Air Show in June. (Eviation photo)

Kadima, Israel-based Eviation was founded in 2015 by CEO Omer Bar-Yohay and Chairman Aviv Tzidon. Comprised of 95% composite materials, the fly-by-wire Alice is powered by propellers at the wingtips and the rear fuselage. Eviation plans first flight this summer.

Cape Air, one of the largest independent regional airlines in the United States is considering a fleet of 92 Alices. Cape Air founder and CEO Dan Wolf said:

"Cape Air has never been just another airline. We are a company of firsts, and one with a deep sense of social responsibility. Seven years ago, we were recognized by the EPA [US Environmental Protection Agency] for our sustainability efforts. Today, we are stewards in what is the world's single most emissions-laden industries. We see tremendous opportunities to reduce the environmental impact of our operations and to help our employees and communities do that as well. Augmenting our fleet with the all-electric Alice aircraft is the next chapter in our future."

Short-haul regional services have been on the decline in the US for a variety of factors; the advent of low-operating-cost electric aircraft could reverse this trend.

Eviation's Alice can make regional trips cheaper, faster, more efficient, quieter and cleaner. It will reduce operating costs, produce fewer or no greenhouse gas emissions, and lower maintenance.

Bye Aerospace

Another big name in the industry and early leading player is Denver, Colorado-based Bye Aerospace. Bye recently announced 624 “customer commitments” for its electric eFlyer 2 training aircraft: 170 paid deposits, 318 memoranda of understanding (MOUs) and 136 MOU options. The new eFlyer orders include a commitment for 60 eFlyer 2 two-seat aircraft from Norway’s OSM Group and 100 units of the company’s planned eFlyer 4 from BlackBird air taxi.

The eFlyer 2 — originally called the Sun Flyer 2 — made its first flight in April 2018. In July 2019, Bye flew with a 20% more powerful Rolls-Royce (formerly Siemens) SP70D motor. It reached speeds of up to 90 kt (111 km/h), sipping just 35 kW for a 450-lb (205-kg) passenger payload, and a three-hour endurance. Company founder and CEO George Bye commented, “These important tests are validating the eFlyer’s incredible operating economy, efficiency, and performance while producing no CO₂.”

A Bye Aerospace eFlyer 2 in July with the more powerful Siemens (now Rolls-Royce) SP70D motor and three-bladed propeller. Bye has commitments for 624 electric aircraft. (Bye Aerospace photo)

Bye Aerospace and Ballistic Recovery Systems (BRS) are developing a ballistic parachute system with additional safety features. The company is focusing on pre‐crash sensing technologies, parachute ballistic recovery systems, landing gear‐airframe crashworthy structural concepts, high-energy absorbing seats and advanced restraints.

Bye Aerospace also teamed up with UK-based Oxis Energy to develop a proof-of-concept lithium-sulfur battery cell for the eFlyer 4. The five-year project aims to increase the Li-S cell energy density to 400 Wh/kg.


While not a startup, the US National Aeronautics and Space Administration (NASA) has been working on its own electric aircraft research for several years. The NASA X-57 Maxwell  project is an electric aircraft conversion of an Italian Tecnam P2006T fuselage being used to validate and demonstrate distributed electric propulsion benefits and inform electric propulsion certification standards for the future of electric aviation. It will be the agency’s first manned X-Plane in more than two decades.

NASA X-57 Maxwell image of the Modification IV final configuration, showing the propellers on the 12 high-lift motors folded back for cruise. (NASA graphic)

The project began in 2014 as LEAPTech when researchers from NASA’s Langley Research Center and Armstrong Flight Research Center funded San Luis Obispo, California-based Empirical Systems Aerospace, Inc. (ESAero) and Santa Cruz, California-based Joby Aviation. ESAero is the prime contractor responsible for system integration, instrumentation, qualification and testing, while Joby provided the unique, air-cooled cruise electric motors and cruise motor controllers. Electric Power Systems (EPS) of Logan, Utah, was later selected by ESAero and NASA to provide the battery system.

The X-57 “Maxwell” is partway through three modification phases. The final version will sport 14 electric motors and propellers — 12 high-lift motors along the leading edge of the wing designed by ESAero and partner Zone 5 Technologies, and the two large, wingtip Joby cruise motors.

Designed to show a fivefold reduction in energy use at high-speed cruise compared to traditional propulsion — as well as zero in-flight carbon emissions and quieter flights — the X-57 will also provide industry with data for certification criteria and standards. Maxwell is currently in its Modification II Integrated Test Phase that has replaced the two inboard combustion engines with the large Joby electric motors. In June, Sean Clarke, NASA’s principal investigator for X-57 said, “This is the first time we've had the electric motors installed with propellers and had them spinning.”

Later, Modification III will replace the wing with one of much higher aspect ratio and lower wing area for more efficient cruise flight and move the two larger electric motors to the wingtips for cruise. The carbon-composite wing, manufactured by Xperimental, LLC also in San Luis Obispo, has been delivered to NASA and is currently undergoing Loads Testing. The final version, Modification IV, will install the 12 smaller, high-lift electric motors under the wing. In flight, the ESAero-designed, high-lift propellers are planned to stop and fold to reduce drag and power consumption.

eCTOL, eVTOL and Hybrid Propulsion

Much of the technology for eCTOL is applicable to eVTOL and vice versa. At the propulsion level, for example, technology for batteries, electric motors and hybrid-propulsion systems can often be interchangeable.

For instance, electric motor company magniX is a major supplier for many eCTOL and eVTOL aircraft developers, with the magni250 producing 280 kW of continuous power and the magni500 delivering 560 kW. magniX has its motors on Eviation’s Alice and is working with several companies to retrofit electric motors on Cessna Caravans and de Havilland seaplanes (see “Oshkosh e-AirVenture”).

Meanwhile, VerdeGo Aero is developing its integrated distributed electric propulsion (IDEP) system as a Tier 1 supplier. IDEP is developing a hybrid-propulsion system that can be used for eVTOL or eCTOL applications that is, “balanced to optimize performing many functions for the aircraft.”

Eric Bartsch, VerdeGo Aero Chief Executive Officer, said the challenge when converting an aircraft to a hybrid is the risk of making it worse than before. Hybrids are not automatically “green,” but they can recover wasted energy, making it an efficient system. If the hybrid aircraft converts mechanical energy into electricity using a generator and a heavy battery to feed an electric motor, it’s not very efficient, he said. On the other hand, an eCTOL aircraft designed around a hybrid electric system and/or with a distributed electric propulsion from the beginning can potentially reach higher efficiency and redundancy not possible with conventional gas turbine propulsion.

eCTOL Progress

Far from exhaustive, this summary gives insights into the progress of eCTOL aircraft developments through July, with new announcements of demonstrators by Embraer, ZeroAvia and the German Aerospace Center (DLR) arriving by press time.

Although vertical flight is required for flight within cities to provide UAM service, horizontal takeoffs and landings greatly extend the short ranges that eVTOL aircraft are confined to today in cases where VTOL isn’t required.

Startups, established aviation original equipment manufacturers (OEMs) and new aerospace suppliers alike are very active on the eCTOL side of the electric aviation industry. The fate of eVTOL and eCTOL are intertwined and developments of each will benefit the other.

The world’s first production eCTOL aircraft: Pipistrel’s Taurus Electro & Alpha Electro. (Photo © Andrzej Rutkowski via Pipistrel. Used with permission. For more photos, see: www.photo-plane.com)

About the Author

Nicolas Zart has written on electric cars, autonomous cars, electric aircraft and other green mobility vehicles since 2007 for various outlets, including CleanTechnica.com.


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