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The equipment required for an electric propulsion test is ready for research. (NASA/Lauren Hughes)

Testing the transition to low-carbon, low-noise, electric propulsion

As the industry’s appetite for electric propulsion continues to increase, so too does the need to understand the intricacies of the systems that are needed to make it work, particularly electric motor systems. To that end, researchers at NASA's Armstrong Flight Research have developed a unique 13.5-ft tall test stand as part of a multi-center approach to explore the use of electric propulsion on future aircraft.

Made of steel and aluminum, the Airvolt test stand was designed and fabricated at Armstrong to help researchers anticipate system integration challenges and verify and validate electric propulsion components.

"The test stand will help us to understand electric propulsion and the nuances of different systems," said Yohan Lin, Airvolt integration lead. "A lot of claims are made about the efficiency of electric motors and we want to verify that and gain experience with commercial off-the-shelf, or custom-designed systems."

Airvolt also permits researchers to evaluate early-stage technology and build confidence in its use for future systems.

One of the key items researchers need to know is if integrated electric propulsion can be used like traditional aircraft propulsion, Lin explained. If there are distinctions in how the systems work, researchers will find methods of managing the differences.

For example, Airvolt research has already confirmed at least one challenge: electro-magnetic interference (EMI). EMI occurs when an electric circuit is interrupted by an internal or external force or condition, which results in noise interference.

"EMI issues impacted data collection and real-time displays and gave us false indicators in the control room," said Lin. "It was caused by the propulsion system's noise."

The solution was to install a combination of hardware filters on the test instrumentation and use digital filters on the acquired data. With the challenges eliminated, researchers in the control room were able to safely monitor key parameters.

Test operations on the Airvolt begin with the installation of an electric motor with a propeller attached and the system affixed to the test stand. A number of high-fidelity sensors on the test stand provide critical measurements to a data acquisition unit that processes, records, and filters the data and sends it to a control room for monitoring.

During the test, a 50-ft area is cleared and most of the staff is in the control room monitoring the research results, Lin said. As the motor starts the sound is similar to that of a large window fan, only slightly louder and with the propeller blade turning much faster. He says it is much quieter than a typical conventional combustion piston engine of the same size.

The first tests on Airvolt occurred in late 2015 and focused on the energy-efficient Pipistrel Electro Taurus electric propulsion system, which Lin says is typically used for motor gliders.

The Pipistrel motor is powered by lithium-polymer batteries and produces 40 kW of power. The Airvolt is capable of accommodating systems that use up to 100 kW and can withstand a thrust of 500 lb.

Researchers using Airvolt are interested in determining voltage, current, power, torque, and thrust performance of the commercial off-the-shelf components and learning about the characteristics of such a system. In addition, researchers are looking to build competencies in electric propulsion system verification and validation.

"Preparatory work for developing in-house skills and knowledge can't wait until an X-plane comes," he said, in particular referring to NASA's recent announcement about the X-57 electric propulsion X-plane. "We want lessons learned early on so that we can apply them to upcoming designs and establish best practices for operating those systems."

Next up for the Airvolt are tests during late summer on the Joby Aviation JM-1 motor that will provide information for modeling simulations of the electric propulsion elements.

The data collected from the tests will include torque and thrust measurements, high-fidelity voltage analysis, power efficiency, and details on how the system behaves. A simulation model will be developed from that information to study flight controls, power management, and transition issues of a distributed electric aircraft.

To prepare for the possibility of distributed electrical propulsion, where multiple motors are used, "you want to understand the characteristics of one motor system first so variables can be reduced when trouble shooting the multi-motor configuration," said Lin. "Overall, we are getting excellent data. What we are learning will help us to understand this new technology, and be a starting point for complex challenges. Each system is different, but we will be ready." 

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