The Potential of eCTOL (2)

EF Global VTOL

19 May 2023


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: