Somehow, the sight of an airplane wing perching incongruously atop a big-rig truck tractor that’s rumbling across a dry lake bed at dawn might not seem particularly significant to the future of aviation. But when the 18 small, pinwheel-like propellers on the wing’s leading-edge flash into the sun’s first rays like a swarm of crimson butterflies trying to lift the wing into the still, desert air, the odd spectacle actually provides a good indication of what may become a new propulsion paradigm for next-generation aircraft.
Those morning speed runs across Edwards Air Force Base in California represent some of NASA’s first real-world tests of distributed electric aircraft propulsion technology, a budding design concept that could revolutionize everything from light planes to regional airliners within a decade or so, said Mark D. Moore, Principal Researcher for NASA’s Convergent Electric Propulsion Technology Sub-Project at Langley Research Center.
In place of one, two, or several large propulsion engines as in conventional aircraft, the unusual “blown wing” that the Hybrid-Electric Integrated Systems Test-bed (HEIST) is evaluating features an array of small electric-powered propellers that send multiple streams of high-speed air over the upper surface of the wing to produce unprecedented lift capabilities, he said.
“If you distribute higher velocity air across the entire wing, you can raise the dynamic pressure over the wing and thus increase lift substantially at low flight speeds,” Moore explained. This novel arrangement allows use of a downsized wing that nonetheless generates greater lift during takeoffs and landings, which not only provides a greater safety margin for the pilot and shorter takeoff runs, but better overall ride quality as well. The design also can deliver less drag and fuel use in cruise operations and longer range, even lower noise levels.
The experimental HEIST test article, a 31-ft-span, carbon-composite wing section, is mounted on a supporting truss with load cells attached, all of which floats on a vibration-absorbing airbag, said NASA Armstrong Flight Research Center project engineer Sean Clark. Combined, the 18 propellers generate about 300 hp and the wing provides around 3500 lb of lift. The ground-test rig thus serves as a “mobile wind tunnel” at significantly lower cost than a large-scale wind tunnel. Testing at speeds up to 70-80 mph is providing valuable data at an affordable price.
A leap in aviation
The ground tests are part of NASA’s $15-million, three-year Leading Edge Asynchronous Propeller Technology (LEAPTech) program, which aims to evaluate the premise that the tighter propulsion-airframe integration that is enabled by electric power will yield improved efficiency and safety, as well as environmental and economic benefits.
To develop and build HEIST, NASA engineers at Langley and Armstrong partnered with specialists and engineers at “two small, nimble, enthusiastic firms,” Empirical Systems Aerospace of San Luis Obisbo, CA, the prime contractor, and Santa Cruz-based Joby Aviation, which built the test rig, wing, motors, and propellers.
Within a few years, the NASA team hopes to develop and test a LEAPTech flight demonstrator by replacing the wings and engines of a Tecnam P2006T light twin airplane with an improved version of the distributed-propulsion blown wing. Using an existing airframe will allow the researchers to compare the performance of the modified vehicle with that of the standard configuration. Moore says that the researchers are applying for “X-Plane” status for the re-winged test aircraft.
New transportation solutions
The research project got its start in 2011, when Moore and his colleagues began studying the possibility of “turning the small airplane into real mid-range transportation solution instead of a mostly recreational novelty,” Moore recalled. “An automobile works great up to about 100 miles, while a commercial airliner works great for 500 to 1000 miles, but for affordable, high-speed speed mobility between 100 to 500 miles, there’s no great transportation solution.
“Our studies say that distributed electric propulsion would be cost-effective for distances less than 600 miles,” he continued. The new technology plus improved autonomy (control/safety) systems could provide dramatically higher-speed, more affordable access than cars but with car-like ease of use, he contended. “It could create a whole new market for general aviation aircraft.
“As our analysis went forward, we realized that distributed propulsion is applicable to larger aircraft as well, even commercial transports flying stage lengths of around 600 miles. It could therefore also be a game-changer for turboprop and regional jets that the airlines fly today.
“Think of the Tecnam X-Plane as a subscale demonstrator,” he noted. “We want to incubate the concept at the GA level and then scale it up.”
The key to the entire approach is that the electric motor is “a scale-free technology,” Moore asserted. “Current propulsion engines just don’t scale well. A full-size turbine engine can be around 40% efficient, but if you take it down to 100 hp, it’s only 24% efficient—6 hp/lb vs. 0.5 hp/lb.”
In contrast, electric motors can be very compact, very reliable, and highly efficient, he said. “They provide extremely good power-to-weight ratio—two times better than turbine engines and three times better than any reciprocating engine.”
And not only is “electric propulsion happy to scale to any size, you can place them anywhere you want, for instance, along the entire leading edge of a wing to attain synergetic benefits from close-coupled control and lift surfaces.” This innovation, he stated, offers “exciting opportunities.”
A traditional light aircraft needs a large wing area to meet the low stall-speed requirement for FAA certification, but it is inefficient in cruise. The LEAPtech flight demonstrator would feature a wing that is one-third the size for reduced drag and have nearly three times the wing loading (more than 50 lb/ft² vs. 20 lb/ft² for a typical small aircraft) for improved ride quality and better response to gusts. Meanwhile, the propeller array should double the maximum lift coefficient at low speeds.
Optimized for low speed, the small-diameter propellers have low tip speeds for reduced noise. In addition, they all rotate at slightly different velocities to spread out the sound frequencies they emit, cutting community noise, it is hoped, by as much as 15 dB. All the props blow the wing for takeoff and landing operations, but some fold back to reduce drag in cruise, when wingtip propellers that are optimized for high velocities provide propulsion, operating inside the wingtip vortices to boost efficiency.
“Such changes are expected to deliver a 30% reduction in operating costs, not to mention zero in-flight emissions” if improved batteries replace hybrid power at some point, he said.
Moore concluded by noting that “the safety statistics for GA aircraft are not all that great, with most accidents happening during takeoffs and landings, when planes are flying low and slow.”
Distributed propulsion provides redundancy against engine failure and maximizes control authority. “With the blown wing, you have incredible lateral control. If one wing loses lift and stalls at low speed causing the plane to roll off to one side, you can just power out of it.”