eVTOLs: The Connector Challenge

Electric vertical take-off and landing (eVTOL) is at the forefront of realizing the air taxi for advanced urban mobility. While this technology could be truly disruptive, only a few programs are on their way toward commercial launch and in the production phase. This is because the distributed electric propulsion (DEP) architecture often found within eVTOL vehicles must be optimized with redundancy to maximize safety, as this propulsion diverges greatly from conventional combustion aircraft. The safety and quality of the subsystems within the eVTOL are critical. This includes the connector and cable assemblies within the aircraft. 

 

A Brief History of eVTOL

Vertical take-off and landing vehicles (VTOLs), such as the popular helicopter, have long been utilized for military and civilian purposes. The lift is supplied by the rotors directly, so no runway is needed for takeoff and landing, making these aircraft a viable option for urban mobility. 

However, compared to fixed-wing aircraft such as airplanes, these aircraft are generally more complex, have less endurance, and are far less fuel efficient. These challenges have, so far, prevented them from permeating the urban transportation industry, relegating them to niche applications such as air medical services or scenic helicopter tours.

Advancements in electric propulsion (e.g., electric motors, motor controllers, quiet propellers, batteries, wide bandgap technologies in switched power converters, etc.) have steadily enabled eVTOLs to go from concept to reality. Murmurs of realizing the idea of the Jetsons’ “flying car” using an eVTOL began in the early 2010s when NASA engineer Mark Moore released a video of a striking eVTOL concept: the NASA puffin. The video went viral and popularized the eVTOL as a method to use urban airspace to mitigate congestion on the ground.  

 

NASA puffin eVTOL concept. Image used courtesy of NASA

 

Since then, large companies like Uber began investing in eVTOL technology with the “Uber Elevate” project. This was sold to Joby Aviation, a company now at the forefront of eVTOL development with an eVTOL that was already awarded military airworthiness approval in 2020 and established a new certification agreement with the notoriously stringent Federal Aviation Administration (FAA) for commercial entry. Other major players already have eVTOLs in development. In the United States, these companies include Boeing, Archer Aviation, Beta Technologies, Kittyhawk, and Wisk; in Europe, some major players include Airbus, Volocopter, Lilium, and Vertical Aerospace. China is also developing a massive market for eVTOLs with companies such as EHang, AutoFlight, XPeng HT Aero, MuYu Aero, and more. 

 

Evolving Standards of eVTOL Development 

The aircraft industry is rife with standards and regulatory constraints that govern the manufacturing and operation of aircraft. With these standards, there has been a general template for avionics, power distribution, infotainment, communication protocols, and the interconnects that come with them. The eVTOL diverges from conventional aircraft greatly. This is particularly true for their electrical power systems and distributed electric propulsion architectures. Airbus’ CityAirbus NextGen demonstrator, for example, includes eight propellers and 16 electrical power units for dual systems (motors, inverters, batteries, etc.) redundancy. This way, the aircraft automatically adjusts and continues flight if a motor, inverter, battery, or even a propeller is lost. 

The DEP approach is used in many major eVTOL programs, and the DOT/FAA are following closely by developing standards and regulations around this new architecture. A recent DOT/FAA report, “Investigation of Certification Considerations for Distributed Electric Propulsion Aircraft,” begins a multi-year effort in analyzing the algorithmic computation of remaining control power (RCP) as a metric for assessment of potential loss of control (LOC) events. The effort focuses on studying sensor data and algorithms scaled to various passenger-carrying eVTOL sizes that compute power margins in all mission conditions (e.g., local winds and gusts) to develop industry best practices.

 

The exhaust port behind the FAA Airflow Induction Test Facility’s wind tunnel for disturbance flight testing to measure vehicle response to local gusts and winds. Image used courtesy of FAA

 

eVTOLs are a relatively nascent technology and, compared to conventional combustion aircraft, feature a lower current performance. The DEP architecture addresses this with redundant safety control capabilities and feedback stabilization to improve flying quality, options for executing maneuvers, and in case of failure, trimmed flight (e.g., reduced acceleration, reduced maneuvering). 

 

eVTOL Connectors: The SWaP Challenge

Components within this power distribution must maintain safety and the consideration put into the system design. This is particularly true for the interconnect. Connectors and cable assemblies must maintain a high-integrity connection regardless of the environmental stressors of temperature extremes, mechanical shock, and vibration. While redundant systems bolster overall safety, it also adds to the size and weight of the system―a powerful tradeoff in all aircraft. Manufacturers must shed weight wherever possible within the system; this includes all the installed cable assemblies. Like many aircraft, the eVTOL is no exception in requiring a highly reliable size, weight, and power (SWaP)-optimized connector.  

 

Connector-based Standards

There are many connector-based aircraft standards in both military and commercial applications (e.g., ISO 461-1, MIL-DTL-38999, MIL-DTL-83513, ARINC standards, etc.). The MIL-DTL-38999 standard, for instance, specifies the construction, dimensions, and tests for high-performance cylindrical connectors that connect to one another or electronics, often in I/O applications. The ARINC connectors are rack and panel connectors primarily used for avionics. The MIL-DTL-83513 describes the requirements for ruggedized micro D-subminiature (micro-D) connectors, making them ideal in PCB I/O use cases for sensing, command, and control.  In an eVTOL’s DEP system, connectors, in general, must transmit signal, power, high-frequency signals, or a combination to and from a PCB. Combining power and signal, for instance, will save board space. Standards such as these provide a high-reliability connector backdrop to work from (e.g., shock and vibration tests, outgassing, humidity classification, etc.). 

Still, there are many instances where the overhead (cost and time) that comes with highly regulated parts such as military-grade equipment is unnecessary. Commercial off-the-shelf parts will satisfy the electrical, mechanical, and environmental performance requirements without carrying the additional burdens of MIL-SPEC components. Recent trends point in this direction where large organizations like the U.S. Department of Defense and NASA are seeking to adjust their tendency to purchase massive, long-term contracts to avoid the risks of project obsolescence and extended timelines. Instead, many of these agencies are taking advantage of the high-volume civilian market to increase competition and manage risk by vetting vendors and ensuring the part requirements are validated and qualified for end use. 

 

eVTOL Interconnect

Harwin provides precision, high-reliability connectors. The Gecko portfolio of 1.25 mm pitch connectors features a miniaturized (up to 45% and 75% lighter) alternative to the ruggedized micro-D connector with operability from -65oC to 150oC. The precision micro-turned multi-finger beryllium copper female contacts ensure a strong connection despite vibration and shock. The stackable Gecko connector’s relatively compact footprint (when compared to the conventional micro-D) makes it a strong contender for I/O connections from a PCB. These connectors come with power and signal pins for additional flexibility. Instead of a latch, these connector heads can be mounted with “screw-lok” fixing hardware to ensure a tight fit to the board. 

 

The Gecko connector offers an alternative to micro-D connectors with a miniaturized footprint and higher pin density. Image used courtesy of Harwin

 

When coax and higher current capable power and signal connections are required, the Datamate Power & Coax 4 mm pitch connectors offer 5 A to 40 A power contact design and 50 ohm coax for performance up to 6 GHz, giving designers higher power density in space-constrained applications.

The Datamate Power & Coax connectors are multiport ganged connectors with power ratings up to 40 A and 6 GHz coax. Image used courtesy of Harwin

 

The variety of rugged signal, power, and coax connectors allows designers of DEP systems to save board real estate and weight with increased pin density and power capability per pin. These precision connectors have been tested under EIA-364 to ensure component quality from the pin and connector head to the cable assembly. 

 

Takeaways of eVTOLs

The eVTOL redefines air transportation with the ability to make air travel more accessible to organizations (e.g., Uber, UPS, etc.) and people. The combination of advances in electric propulsion and machine learning enables a new level of autonomy for aircraft. These advances rely on redundancy to ensure safety is not overlooked in the chase for innovation. 

Redundancies, however, add weight. Therefore, shaving off as much weight as possible wherever possible in these battery-powered multi-copters is desirable. This is where ruggedized, high-pin-density connectors can be employed to save weight and space.