While numerous electric vertical takeoff and landing (eVTOL) aircraft move toward certification, NASA Langley Research Center in Hampton, Virginia, is researching how these aircraft operate and interact with an electric-propulsion system.
The testbed for wind tunnel and free flight testing is the distributed electric-propulsion Langley Aerodrome No. 8 (LA-8) subscale eVTOL aircraft. The 65-lb (29.5-kg) LA-8, named in honor of aviation pioneer Samuel Pierpoint Langley, who coined the term “aerodrome” when naming his series of subscale test aircraft in the late 1890s, is a tandem tiltwing aircraft with four propellers on each tilting wing.
The LA-8 features a tricycle landing gear, with a vertical strut at the front of the aircraft and an inverted V-shaped structure for its rear landing gear. About 80% of the model is built using 3D printers using nylon and polycarbonate, which allows engineers to change the wings, the fuselage and add other sections of the model, such as propellers and computer hardware, rapidly. Flexibility in the aircraft’s main components allows for quick changes to the vehicle’s configuration to consider alternate designs.
The LA-8 is part of NASA’s Advanced Urban Air Mobility Test Beds project, initiated to provide a baseline for understanding the aerodynamics of present and future aircraft in the advanced air mobility (AAM) sector, which includes the gamut of electric aircraft from short-haul urban to regional flights.
Data from the study could be used to develop test processes for the US Federal Aviation Administration (FAA) airworthiness certification. NASA wants to share the LA-8 test data with the private sector to ensure eVTOL aircraft are safe for various roles, including package delivery and passenger service.
“This particular aircraft doubles as both a wind tunnel and flight dynamics model,” said David North, Unmanned Aerial Systems Section Lead at NASA Langley. “The idea is to obtain useful data that can be used to perfect modeling tools.”
Part of NASA’s work is to determine “how we model these new type of vehicles, where you have highly integrated propulsion and airframe interaction,” said North.
The LA-8 effort was announced by NASA in April 2019 after it had completed the first tests in NASA’s 12-ft (3.7-m) wide Low Speed Wind Tunnel. Since then, the agency has kept the research relatively quiet.
North updated Vertiflite on the ongoing testing.
“We’ve been through two years and four rounds of wind tunnel testing, which we may revisit. And completed three months of initial flight testing,” he said. “Those tests included pre-flight and check flights, which validated the airframe and controller.”
So far, the LA-8 has been through functional flight-testing, which established the basic airworthiness of the aircraft, and allowed the flight test team to become familiar with the craft. In early May, NASA posted a video of the second LA-8 transitioning to full wingborne flight at 130 ft (40 m) altitude and 32 kt (59 km/h) at full tilt. North expects the LA-8 program will move to more expansive flight tests later this year.
The LA-8 has an extensive data recording system that can record 148 flight parameters, including airspeed, altitude, pilot inputs, flight control outputs, Inertial Measurement Unit (IMU) sensor data, power use, and motor RPM. Upcoming research will focus on advanced flight vehicle modeling techniques that use flight data to build analytical models.
“Development of these techniques will help the eVTOL private sector build their certified flight controllers in a more efficient way,” said North.
NASA researchers also are trying to find the most effective way to use the flight controls and ensure they can tolerate and adapt to failures.
Part of NASA’s research is determining the best way to control and maneuver the LA-8 and, by extension, other eVTOL aircraft. Explained North: “We want these vehicles to be over-actuated. Meaning, you have several means of controlling the aircraft in the three axes of pitch, roll and yaw. You might use motors, elevators or wing tilting to control pitch.”
As with all research, mishaps happen. On one test fight, the aircraft lost a blade on one of the propellers and the motor shut down. “There wasn’t a significant change in the way the aircraft flew,” said North. “We were able to get the vehicle down safely. With a smart flight controller, you can sense those failures.”
North said researchers would soon venture into another test phase, integrating the Moog flight controller and “smart” autopilot.
Moog, Inc., a manufacturer of precision control components and systems, is a partner in the LA-8 program. So, too, is Auburn University and Flight Research, Inc., a flight testing and contract air support company. Both entities have done research on the LA-8 program under Small Business Innovation Research (SBIR) funding. SBIR grants are awarded to small businesses with innovative ideas for government research.
Moog has a history with NASA. The company’s radiation-protected avionics technology will control the space agency’s VIPER rover during next year’s 100-day mission to the moon’s south pole.
Several technical summaries have been published so far on the LA-8 program, including technical papers for American Institute of Aeronautics and Astronautics (AIAA) conferences, but also published on the NASA Technical Reports Server (NTRS). “Design and Fabrication of the Langley Aerodrome No. 8 Distributed Electric Propulsion VTOL Testbed” highlights the early development work on the drone.
“System Identification for eVTOL Aircraft Using Simulated Flight Data,” notes, “Propellers used for electric vertical takeoff and landing (eVTOL) aircraft propulsion systems experience a wide range of aerodynamic conditions, including large incidence angles relative to oncoming airflow…”
Another paper, “Wind Tunnel Testing Techniques for a Tandem Tilt-Wing, Distributed Electric Propulsion VTOL Aircraft,” notes that eVTOL “designs frequently include multiple distributed propulsors, complex wing-propulsor aerodynamics, significant airframe configuration changes during normal flight operations, and no historical database regarding the best ways to transition between vertical and horizontal flight.”
NASA Technical Memorandum TM-2020-220437, “An Experimental Approach to a Rapid Propulsion and Aeronautics Concepts Testbed,” described the design process, fabrication process, ground testing, and initial wind tunnel structural and thermal loading of the LA-8, as a proof-of-concept test bed to develop a database to improve the prediction of complex propulsion-airframe interactions.
When asked if NASA was late in offering research data for eVTOL aircraft that have been developed already, North said, “The vast majority of data from eVTOL developers is not publicly available.” He added, “Some companies don’t have the resources to generate the data necessary to build accurate models. Others, like Joby Aviation, Archer and Beta Technologies have the resources to hire analysts and modelers, but are not sharing that data.”
Consequently, NASA Langley is gathering data that is available to anyone, including eVTOL aircraft developers, regulators and the flying public. Despite profitability, regulatory and operational concerns, the prospective applications for eVTOL are numerous and varied.
“The emerging class of eVTOL aircraft has considerable, even transformative market potential,” said Ray Jaworowski, senior aerospace analyst, Forecast International. “That said, there are a number of issues that need to be addressed before the full market potential of eVTOL aircraft can be realized.”
He explained, “One involves infrastructure: a network of vertiports and support facilities needs to be developed. In addition, the FAA and other civil aviation authorities must continue to develop regulatory and policy frameworks within which these platforms can operate.”
Sharpening the Tools
The LA-8 program is the latest iteration of NASA’s AAM related projects. In 2014–2016, Langley tested the first distributed electric VTOL drone, the GL-10 Greased Lightning. The 62-lb (28.1-kg) aircraft had a 10-ft (3-m) wingspan and 10 electric propellers. That program garnered international attention for the potential of distributed electric propulsion and helped kick off the eVTOL revolution. The LA-8 flight control system is based on the successful flight control system used on the GL-10.
The idea of designing, fabricating and testing these new types of aircraft was part of NASA’s Transformative Aeronautics Concepts (TAC) Convergent Aeronautics Solutions (TAC-CAS). NASA’s Air Traffic Management — Exploration (ATM-X) and TAC Transformational Tools and Technologies (TAC-TTT) programs, are also supplying funding.
The TAC-TTT section is responsible for NASA’s controls research program titled, Learn to Fly, which deals with the rapid development of flight controllers and the ability to make quick changes to flight control algorithms.
At present, the burgeoning eVTOL market resembles a Gold Rush. More than 670 concepts for eVTOL aircraft have been proposed, according to the Vertical Flight Society’s World eVTOL Aircraft Directory (www.eVTOL.news). Industry observers believe only a handful of these aircraft are expected to be certified and marketable. Developing new aircraft is capital intensive. eVTOL developers with deep-pocketed partners and a balance between simplicity and performance, with manageable direct operating costs, have a better chance of surviving.
Despite the challenges, analysts concur on the potential for this new, clean form of air travel. The global eVTOL aircraft market in 2021 was valued at $5.4B, according to Fortune Business Insights. By 2028, the market is expected to climb to $23.21B.
If those projections hold up, eVTOL aircraft will become a permanent provider of global air transportation.
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