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2020-05-03

NASA Showcases an All-Electric Future with X-57

Electric aircraft present a wide array of potential benefits to aviation, including increased efficiency, reduced or eliminated in-flight carbon emissions, and flight that is quieter for communities on the ground.

For over 70 years, NASA’s Armstrong Flight Research Center, located at Edwards Air Force Base in California, U.S. has been home to many historic X-planes, or experimental aircraft, responsible for expanding the envelope.

The X-57, which is NASA’s first piloted X-plane in two decades, has an all-electric configuration, known as Modification IV, or Mod IV. This configuration will feature a skinny, high-aspect ratio wing, designed to boost efficiency by reducing drag in flight, and electric cruise motors with five-foot diameter propellers on the wingtips, to recover energy that would otherwise be lost to wingtip vortices. NASA’s X-57 will help set certification standards as these electric aircraft markets begin to emerge.

Also, the addition of 12 smaller high-lift motors and propellers on the wing’s leading edge will allow X-57 to be able to take off at standard speeds. These motors will activate during takeoff, spinning the propellers, and deactivate during cruise mode, at which point the propeller blades will fold into the nacelles, reducing drag.

The X-plane is being built by modifying a baseline Italian Tecnam P2006T to be powered by an electric propulsion system. The advantage of using an existing aircraft design is that data from the baseline model, powered by traditional combustion engines, can be compared to data produced by the same model powered by electric propulsion.

The project includes four configurations and stages of research called modifications.

Modification I

The first phase of investigating the potential for electric propulsion included defining the requirements of the research, along with systems analysis, design, and a number of tests, both in the air and on the ground.

LEAPTech: One of the earliest evaluations during the Modification I phase included ground validation of the distributed electric propulsion high-lift system, in 2015. The Leading Edge Asynchronous Propeller Technology testing (LEAPTech), performed this validation on the Rogers Dry Lakebed at Edwards Air Force Base in California. The project was a collaboration between NASA and California companies Empirical Systems Aerospace of San Luis Obispo, and Joby Aviation of Santa Cruz.

An experimental electric wing, named the Hybrid Electric Integrated Systems Testbed (HEIST), was hoisted atop a heavily modified big rig, which drove at speeds close to 80 mph to simulate the effects of a wind tunnel. The wing was outfitted with 18 electric motors and propellers, which ran simultaneously during the lakebed runs.

The tests showed that the motors produced a total of 300 horsepower. It validated that the airflow from the distributed 18 motors generated more than double the lift of the unblown wing.

Data Collection: In 2015, NASA pilots and engineers performed test flights of a production instrumented Tecnam P2006T in the skies above and around Edwards Air Force Base. The purpose of the flights was to collect data, such as lift, drag, cruise efficiency, energy usage and ride quality.

The X-57 is a P2006T integrated with an electric propulsion system, which will be flown to gather data from similar flights. The two sets of data will be analysed and compared, helping NASA’s X-57 team to meet its design driver of validating the ability of distributed electric propulsion to improve efficiency.

Modification II

The project’s second phase included several milestones for X-57. In addition to continued systems testing and validation, Mod II marked the beginning of physical integration of X-57 itself into its earliest stage as an electric aircraft.

Battery Redesign: As an all-electric experimental aircraft, X-57 will be powered through the use of lithium ion battery systems. In order to implement these systems securely, NASA first had to validate that the batteries could safely power an entire flight profile, and demonstrate the system’s ability to isolate any rapid temperature increase to prevent it from spreading, and escalating into a fire – an event known as a “thermal runaway”.

NASA engineers achieved this milestone following a system redesign in 2017, allowing the batteries to be fabricated, and ensuring a safe environment for the test programme. The successful battery redesign was shared publicly, providing an immediate benefit to the industry as a result of the research.

Interactive Simulator: Pilots and engineers developed and began flying an X-57 simulator at NASA’s Armstrong Flight Research Center in California, designed to feature the specifications of the electric propulsion system. The interactive simulator began preparing pilots for X-57’s future flight-testing phase.

Flight control engineers and technicians at NASA Armstrong developed the simulator to provide a virtual flight experience based on what the X-plane itself will actually feel like when it’s in the air. The system helps familiarise pilots with the system and makes them more adept with reaction times and manoeuvres.

Managing Differences: Before an experimental electric system could be integrated into an aircraft, it had to first be tested and validated. A 13.5-foot tall test stand helped NASA researchers better understand the intricacies of electric motor systems and provided validation for the thrust of the electric motors. The stand also helped to verify the components of electric propulsion.

As early-stage technology is further evaluated, confidence in its use for future systems is established. Managing the differences between traditional aircraft propulsion and distributed electric propulsion is made easier by finding methods of managing those differences.

Retrofit and Testing: The Modification II version of the X-57 looks similar to the baseline 2006T, except the two traditional inboard combustion engines are replaced by two inboard electric motors, essentially turning the aircraft’s propulsion system electric.

Ground taxi tests at NASA Armstrong help to examine and validate the safety and functionality of the electric system, while flight tests will seek to validate the X-57’s electric motors, battery, and instrumentation.

Modification III

Work on the third phase of the project began in tandem with Modification II, with the goal of taking the X-57 to a new level of experimental electric-powered flight.

Experimental Wing: One of the most noticeable difference from the X-57’s Modification II configuration is the development and integration of an experimental, high-aspect ratio wing. Designed by Xperimental of San Luis Obispo, California, the wing features a large reduction in area, with wing loading increasing from 17 pounds per square foot to 45 pounds per square foot.

The reduction in wing area also contributes to more efficient cruise flight through decreasing friction drag. The final Modification IV effort will demonstrate that the high aspect ratio wing with the integrated high lift motor system will allow the X-57 to take off and land at the same speed as the baseline P2006T. The aircraft will also be less sensitive to gusts and turbulence, leading to a smoother flight.

Relocation of Cruise Motors: One of the most critical elements of the high aspect ratio wing is the relocation of the large cruise motors to the wing tips. The replacement of 100-horsepower Rotax 912S engines with 60-kilowatt motors, developed by Joby Aviation, reduce the weight of each motor and propeller from approximately 125 pounds to about 57 pounds.

The much lighter-weight electric motors allow for their relocation outboard. By moving the cruise motors from their Mod II inboard position to the wingtips for Mod III, the cruise motors recover energy that would otherwise be lost in the wingtip vortices.

Nacelles, which are outer casings that can generally act as housing for an aircraft’s engine, are also installed along the leading edge of the wing where 12 high-lift motors will eventually be positioned.

Modification IV

The final configuration of the X-57 presents the X-plane in its full, experimental configuration.

High-Lift Motors: The X-57, in its final form of the project, features 12 high-lift motors along the leading edge of the distributed electric propulsion wing. Similar to the 18 small motors used during LEAPTech ground tests, the high-lift motors are electrically powered to generate enough lift for X-57 to be able to take off at standard Tecnam P2006T speeds, even with the high aspect ratio experimental wing.

The high-lift motors and propellers are designed to activate, along with the wingtip cruise motors, to get the X-plane airborne. When the plane levels out for cruise mode, the high-lift motors will deactivate, and the five propeller blades for each motor will stop rotating, and fold into the nacelles, so that they don’t create unwanted drag during cruise. The two wingtip cruise motors will maintain flight during this phase of the flight.

When the time comes to land, the motors will then reactivate, and centrifugal force will cause the propeller blades to unfold and create the appropriate lift for approach and landing.

Environmentally Friendly: The design driver for X-57 will also seek to reach the goal of zero carbon emissions in flight, which would surpass the 2035 N+3 efficiency goals. Electric propulsion provides not only a five-to-10 times reduction in greenhouse gas emissions, but it also provides a technology path for aircraft to eliminate 100 Low Lead AvGas, which is the leading contributor to current lead environmental emissions.

Reference Text/Photo:

www.nasa.gov

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