On Nov. 8, NASA unveiled the agency’s X-57 electric-powered research airplane for the first time in its initial all-electric configuration.
The aircraft is being prepared for ground testing that will pave the way for flight tests next year. The primary purpose of the X-57 is to validate and demonstrate the benefits of distributed electric propulsion for future aviation applications.
NASA aeronautics researchers are hoping to demonstrate how electric propulsion can make airplanes quieter, more energy efficient, and environmentally friendly.
X-57 project manager Tom Rigney hailed the beginning of this latest phase in an ongoing effort to study electric aircraft propulsion. “The X-57 team will soon conduct extensive ground testing of the integrated electric propulsion system to ensure the aircraft is airworthy,” he said.
Plans call for the X-57 to undergo several major configuration changes, the final one featuring 14 electric motors and propellers (12 small, wing-mounted high-lift motors and two, slightly larger, wingtip cruise motors). Researchers predict the X-57 will demonstrate a 500-percent increase in high-speed cruise efficiency over conventional gas-powered propulsion, zero in-flight carbon emissions, and quieter flight characteristics that will reduce impact to communities below the airplane’s flight path.
Planners divided the research effort into several phases, called modifications or mods. The first of these, dubbed Mod I, focused on investigating the overall potential for electric propulsion and on defining research requirements, conducting systems analysis, developing vehicle design parameters, and conducting a variety of ground-based and airborne tests. In 2014, researchers from NASA’s Langley Research Center and Armstrong Flight Research Center partnered with two California companies, Empirical Systems Aerospace (ESAero) in San Luis Obispo and Joby Aviation in Santa Cruz to perform ground validation of a high-lift, distributed electric propulsion system. In this initial effort, dubbed Leading Edge Asynchronous Propeller Technology, or LEAPTech, an experimental wing was mounted on a specially modified truck. The 31-foot-span, carbon composite airfoil was equipped with 18 electric motors powered by lithium ion batteries. Its design was highly representative of the full-scale wing that will eventually be installed on the X-57 research vehicle. Though fitted with propellers much smaller than those typically used on conventional aircraft, each motor generated significant lift by blowing air across the airfoil surfaces. Testing with the mobile ground rig in 2015 provided valuable data and risk-reduction that was applicable to future flight research. The test article was attached to load cells on a supporting truss while the truck was driven at speeds close to 80 miles per hour across a dry lakebed at Edwards Air Force Base. The electric motors demonstrated an energy output of 300 horsepower, and testing validated that airflow from the distributed motors generated more than double the lift of an unblown wing.
In preparation for flight-testing, NASA acquired a Tecnam P2600T four-seat, high-wing, propeller-driven light aircraft. Powered by two gasoline-fueled, 100-horsepower, four-cylinder, internal combustion engines, the airplane was capable of cruising at 150 knots with a range of 669 nautical miles. In addition to being representative of a typical modern civil aviation aircraft, the P2600T was selected because it would be easy to modify into a variety of planned electric-powered configurations. NASA research pilots Tim Williams and Wayne Ringelberg, and a team of engineers, conducted a series of test flights for baseline data collection on lift, drag, cruise efficiency, energy usage, and ride quality of the standard P2600T airplane.
For the current X-57 configuration, known as Mod II, the original fuel systems and engines were replaced with high-performance batteries and two 60-kilowatt electric motors developed by Joby Aviation. These reduced overall engine weight (including each propeller) from approximately 125 pounds to about 57 pounds, somewhat offsetting the weight of the batteries. For safety, and to provide a more direct comparison of performance and handling qualities, the original wing was retained and the motor locations unchanged. The propellers used with the electric motors are of comparable size and design to the original Tecnam configuration. “We wanted to demonstrate the new technologies on a safe configuration with known flight characteristics,” said X-57 principal investigator Sean Clarke.
Once all-electric flight-testing begins, Williams and Ringelberg will collect data for comparison to the baseline data set. This will help NASA’s X-57 team meet several milestones for systems testing as well as validation of the safety and functionality of the airplane’s electric motors, batteries, and instrumentation. The electric-powered X-57 is expected to cruise at about 150 knots, but flight duration will be limited to about 45 minutes due to battery charge limitations. “Battery life is one of our biggest technological hurdles right now,” Williams noted. “Some day we may have more efficient fuel cells.”
Flight control engineers and technicians at NASA Armstrong developed a simulator to provide a virtual flight experience that replicates X-57 handling qualities and failure modes. “A lot of what we do in the build-up to flight is done in the simulator,” said Williams. “I get to see any potential anomalies there before seeing them in flight, so I should have no surprises.”
The simulator helps familiarize pilots with aircraft systems and makes them more adept with reaction times and maneuvers. “It’s a great tool for learning how controllable the aircraft is in various situations,” Ringelberg added.
This will become increasingly crucial when the X-57 enters Mod III. The third phase of testing will be the first time the aircraft flies with an entirely new, high-aspect-ratio wing. Additionally, the cruise motors will be moved out to the wingtips. These changes will alter the airplane’s handling qualities and increase risk in the event of a single-engine failure. The new airfoil configuration features a large reduction in wing area, and stress loads are expected to increase from 17 pounds per square foot to 45 pounds per square foot. The airfoil’s high aspect ratio will contribute to more efficient cruise flight by decreasing friction drag. Moving the cruise motors from their Mod II inboard position to the wingtips for Mod III will allow recovery of energy that would otherwise be lost due to wingtip vortices. The Mod III wing will have 12 small nacelles (bullet-shaped outer casings) along the leading edge, where high-lift motors will eventually be placed for Mod IV testing. Lack of either high-lift motors or any sort of flaps in the Mod III configuration will necessitate relatively high landing speeds compared to previous and later configurations.
Mod IV, the final phase of X-57 flight research, will include installation of motors along the wing’s leading edge to fully demonstrate distributed electric propulsion and blown-wing lift. This will allow the X-57 to take off and land at standard Tecnam P2006T speeds, even with the high-aspect-ratio experimental wing. The Mod IV aircraft is also expected to be less sensitive to gusts and turbulence. To increase energy efficiency, the high-lift motors will deactivate during cruise mode, and the five propeller blades for each motor will stop rotating and fold into the nacelles to minimize unwanted drag. Flight will be sustained using only the two wingtip-mounted cruise motors. Prior to touchdown, the high-lift motors will reactivate to provide appropriate lift for approach and landing.
The X-57 project operates under the Integrated Aviation Systems Program’s Flight Demonstrations and Capabilities project within NASA’s Aeronautics Research Mission Directorate. Its primary goal is to share the aircraft’s electric-propulsion-focused design and airworthiness process with regulators in order to advance certification of distributed electric propulsion in emerging electric aircraft markets. Additionally, the X-57 team is focused on specific technical challenges to drive lessons learned and best practices.
All-electric-powered aircraft have the potential to enable new markets for aviation and improve existing markets. Among the most promising new markets is urban air mobility, which includes short-hop commuter flights and inter-urban air taxi services. Reduced operations and maintenance costs could change the economic model for short-haul aviation and make such flights more economically viable than has previously been the case. Additionally, the quieter flight characteristics of electric aircraft would go a long way toward mitigating public opposition that often comes with increased air operations over urban environments.
Currently, helicopters represent the most common platform for urban air mobility, but they have been historically both noisy and costly to operate. “Electric airplanes have the potential to solve those problems,” said Brent Cobleigh, NASA Armstrong project manager for Demonstrations and Capabilities. The new technologies offer not only noise reduction but also potentially lower ticket prices based on reduced operating costs. “Use of electric aircraft could also open up more small, feeder airports to commercial service,” Cobleigh added.
The biggest breakthrough, said Williams, is the use of scalable electric motor technology. “We can control each motor individually, using software,” he said. “We can, for example, modify the lift curve over the entire wingspan.” He acknowledged that the greatest remaining hurdle is battery technology. “Lithium ion batteries are very heavy,” he said, “and have a high rate of discharge.”
Clarke agreed that existing batteries pose serious challenges, particularly with regard to weight and energy storage. “The amount of energy we can store in a battery is not as much we could store in a gasoline or hybrid system,” he said. The X-57 weighs around 3,000 pounds, of which the batteries account for 850 pounds. Distributed electric propulsion, Clarke noted, offers lower maintenance costs and redundancy for increased safety compared to internal combustion engines.
Cobleigh said that the current state of the art in electric aircraft technology is most applicable to small aircraft in the near term. “Right now, battery technology limits potential applications,” he said. “There is no battery technology on the horizon for larger commercial aircraft configurations.” He suggested that some sort of combination of fuel and electric, similar to the concept of hybrid automobiles, would allow for design trades to improve efficiency in larger aircraft. “The short-term stuff we are doing now is very exciting,” he said, potentially leading to viable applications within five to ten years. Larger electric configurations may be 20 to 40 years in the future. “Even a few percent increase in efficiency has a huge benefit in the aviation world,” Cobleigh said, “and [with further development] there is potential for 10 percent or better improvement in large airplanes.”
If the X-57 performs as predicted, it will also help validate the concept of environmentally friendly aviation. The goal of zero carbon emissions in flight would surpass NASA’s 2035 efficiency goals. Electric propulsion provides not only a five-to-ten times reduction in greenhouse gas emissions, but also offers a technology path to eliminate the use of 100 Low Lead AvGas, which is the leading contributor to current lead-based environmental emissions.