Air Taxi Interconnect Solutions

DISTRIBUTED Electric PROPULSION

GLENAIR

NOTE: This is an excerpt from a technical Glenair whitepaper on the topic of all-electric distributed power in eVTOL systems. For the complete dissertation, including, charts, tables, footnotes, etc., please consult the whitepaper available at glenair.com/eVTOL-air- taxi-interconnect-solutions. Distributed Electric Propulsion (DEP) is a key element of all eVTOL aircraft. A basic description of a DEP design is a power transmission system whose electrical energy sources are interconnected, via EWIS cabling, to multiple electric-motor-driven propellers or rotors. The native power sources in a DEP can be as simple as a battery-plus-inverter design, or as complex as a hybrid system made up of gas combustion engines, storage cells, electric generators, inverters, power feeder cables, interconnect harnessing, and more. The DEP is designed to feed aircraft “propulsors,” or thrust producing devices including propellers and fans, with adequate power for vertical takeoff, landing, and cruise operations. An all-electric DEP system may incorporate high- voltage elements (greater than 3kV) as well as high kW power for peak output to electric propulsion motors, inverters, controllers, and batteries during takeoff and landing. Shared components may be grouped together or distributed throughout the airframe for a redundant distributed thrust system. The safety hazards inherent in such distributed electric systems requires the platform be designed and configured with robust technologies qualified for high-voltage, high-current, and high-frequency aviation applications. The following guidelines explore these critical issues in greater detail. Working Voltage vs Dielectric Withstanding Voltage All DEP designs begin with a definition of the operational voltage, or maximum continuous working voltage, of the equipment. For all DEP applications, a certain safety factor is required between equipment operational voltage (OpV) and the proof-test voltage (DWV) of interconnects and other EWIS components. However, the magnitude of this safety factor varies greatly depending on the exact implementation of the distributed power system.

Here is a useful metaphor for why aviation systems take “derating” so seriously: if a

lifeline rope needs to support a 200lb individual, it would be unsafe to use a rope that has only been proof-tested to 200lb., as this would provide no margin for error, nor allow for any aging

or degradation over time. Instead, emergency teams use a 2000lb proof-tested rope, providing a 10x safety factor to guarantee performance in critical situations. However, if the goal is to hang a 20lb bicycle in a garage—where failure could hardly lead to loss of life—a 50lb proof-tested rope (2.5x) would likely be sufficient. For this reason, high safety derating factors are always used in aviation interconnect systems where failure could result in loss of life. FAA or other national agency qualification of eVTOL electric propulsion systems will absolutely require adherence to higher levels of safety and testing in electrical wire interconnect systems. This table illustrates the relationship between the actual Working Voltage of an aviation-grade system to the tested Dielectric Withstanding Voltage of its component parts: Suggested DWV Based on Referenced Industry Standard Working Voltage (OpV) Suggested Dielectric Withstanding Voltage (DWV) 250 1,500 500 2,000 750 2,500 1,000 3,000 1,250 3,500 1,500 4,000 1,750 4,500 2,000 5,000 By way of example, while it is common practice for airframe harnesses to operate at 115 VAC, these cables would need to perform and be tested at 1500 VAC DWV for flight qualification.

QwikConnect • July 2021