Published 06/26/2026

Helix and Zoerkler Develop Ultra-Compact, Composite-Supported EPU for EVTOLs

Geared electric propulsion unit (EPU) architecture that is more compact and low mass than direct-drive systems can support AAM designers in optimizing air mobility space, weight and system efficiency.

Edited by: Grace Stubbins

Senior Managing Editor, CompositesWorld and Products Finishing

Helix (Milton Keynes, U.K.), power-dense electric motors and inverters manufacturer, has collaborated with high performance transmission specialist Zoerkler Gears GmbH & Co KG (Jois, Austria) to develop and validate a geared electric propulsion unit (EPU) for advanced air mobility (AAM) applications.

Helix’s electric powertrains expertise is formed under its Scalable Core Technology: proprietary permanent magnet rotor, patented rotor-cooling, high pole-count, proprietary winding construction, carbon fiber composite magnet containment, carbon fiber composite coolant sleeve, high-velocity radial and axial water cooling. In addition to mobility applications, the company’s products have been used in marine, such as the Magic Carpet e high-performance sailing yacht, led by Helix’s expertise in elite electric motorsport.

Designed to support demanding eVTOL applications, the EPU developed with Zoerkler can be installed to support propellers in lift, cruise and tiltrotor configurations. Historically, these high-torque applications have required EPUs set out in a “direct drive” layout, which demanded wider and higher-mass motors. Thanks to the seamless integration between Helix’s motor and Zoerkler’s lightweight transmission (which achieves 98% efficiency), however, this latest EPU is reportedly able to deliver high torque in a package just 50% of the diameter (26 centimeters) of an equivalent direct drive unit.

Intended to be the first in a family of advanced air mobility EPUs, the initial collaboration has resulted in a 32.2-kilogram package, which can deliver:

100 kilowatts of power with up to 1,600 newton-meters of torque when used in a lift configuration.
250 kilowatts of continuous power — and 400-kilowatt peak — when used in a cruise configuration.

Combined with the low mass of the package, the EPU’s lighter and narrower format will enable reduced nacelle size and drag-in eVTOL applications to support greater payloads and ranges.

Spinning at the speed of carbon (fiber)

The use of carbon fiber composites in electric motors developed largely out of the aerospace and high-speed machinery sectors, where engineers sought to replace heavy metallic rotor containment sleeves — historically made from alloys like Inconel 718 or titanium — with lighter, stronger alternatives.

For many years, one type of electric vehicle (EV) motor, called a permanent magnet motor (PMM), has incorporated a carbon fiber-reinforced thermoset sleeve that wraps around the PMM's rotor, keeping the rotor assembly from flying apart at high rotational speeds. The physics driving this adoption are straightforward, explains Addcomposites: When a motor rotor spins at 20,000 rpm — now common in modern EVs — every component experiences centrifugal forces that follow a quadratic relationship with speed, meaning that doubling the rpm quadruples the forces trying to tear the rotor apart.

Carbon fiber composites address this problem with a compelling combination of properties: Replacing a metallic sleeve with a carbon fiber composite for retention yields a lighter weight motor assembly, lower rotational inertia, greater compressive pressure and a thinner air gap between the shaft assembly and stator, while also reducing self-load due to centrifugal forces — and though electrical losses in the sleeve persist, as carbon fiber is axially conductive, they are significantly lower than in the metallic sleeve it replaces.

Research from ORNL has further validated these advantages at the design level, with FEA confirming that maximum stress experienced by a carbon fiber sleeve in a surface permanent magnet outer rotor motor spinning at up to 20,000 rpm remained under the material’s yield strength — a finding that has helped open the door to carbon fiber sleeves in high-power applications ranging from EVs to turbogenerators and aerospace drives.