Flying the Electric Skies

The sight of an airplane wing perched atop a big-rig truck—especially one that’s rumbling across a dry lake bed at dawn—might not seem significant to the future of aviation. But when the wing’s 18 small propellers flash into the sun’s first rays like a swarm of manic red butterflies, the spectacle actually provides a good indication of what may be in store for next-generation aircraft.

NASA's big rig barrels across the salt flat during a test run.

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 start to revolutionize everything from light planes to regional airliners within a decade or so, says Mark D. Moore, principal researcher for NASA’s Convergent Electric Propulsion Technology Sub-Project at Langley Research Center.

Today, when you see a prop plane or jet flying overhead, it is driven through the air by propellers or fan blades spun by powerful engines that burn fossil fuels. But if distributed electric propulsion (DEP) is successfully developed they could be quieter, more efficient, cleaner, and reliable than conventional piston and turbine engines. They would also make a significant dent in the aviation industry’s carbon footprint, which today is about 2% of all emissions.

The unusual “blown wing” that NASA’s Hybrid-Electric Integrated Systems Test-bed (HEIST) is evaluating features an array of small electrically powered propellers that send multiple streams of high-speed air over the upper surface of the wing to produce unprecedented lift capabilities.

Such battery- or hybrid-electric aero-propulsion technology could also enable very different-looking aircraft to take to the skies. “DEP could mean a fundamental shift in how we design aircraft,” Moore says. Imagine, for instance, a light airplane with a dozen small electric propellers up front rather than just one or two large props. Or picture an airliner that, instead of being powered by the typical engines that hang under the wings, is driven by a dozen smaller ducted fans lining the wings’ trailing edges. In time, DEP could make possible novel and potentially more capable airplane designs.

Everyday electric aircraft are still probably more than a decade or two away, but researchers and engineers from all corners of the aerospace industry have been making surprising progress, from airframe manufacturers including Airbus and Boeing to engine makers such as Siemens, Rolls Royce, and GE to small aero-engineering firms like California-based Joby Aviation, ES Aero, and Zee.Aero. University and government labs are also working the technology, says Ruben Del Rosario, manager of the Fixed Wing Project of the Fundamental Aeronautics Program at NASA’s Glenn Research Center.

“Of course, many other experts still strongly doubt that electric propulsion for larger aircraft will be available for decades to come,” he says. But there seems to have been a sea change recently. “Two or three years ago, there was little hope among experts that you could even build electric motors that were powerful and lightweight enough to do the job for anything but rather small airplanes, but recent advancements in materials and the development of new airframe and motor design and engineering concepts are changing that view.”

Efficient E-Aircraft

The main advantage of DEP is that the enhanced airflow over the wings can enable an airplane to fly more efficiently. “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 says. The array of small propellers would, for example, let planes take off and land on shorter runways.

With DEP, wings would only need to be a third of the width as on a conventional aircraft, saving weight and fuel costs, he says. On light aircraft, that’s particularly beneficial, since they are typically relatively large to prevent the plane from stalling, or losing lift at low speeds. But large-area wings are inefficient when an aircraft is cruising at speed because they create considerable aerodynamic drag.

A more slender, DEP-equipped wing could also be smoother flying and more fuel efficient, Moore says. It would also be safer—losing one engine mid-flight isn’t as much of a problem when you have a dozen or more. And they would even generate less noise because their tips slice through the air more slowly than standard propellers.

NASA's HEIST is studded with sensors that record data during distributed electric propulsion tests.

NASA’s experimental HEIST test article, a 31-foot-span, carbon-composite wing section, is mounted on a supporting truss that floats on a vibration-absorbing airbag, says Sean Clark, project engineer at Armstrong Flight Research Center. Combined, the 18 props produce about 300 horsepower, and the wing creates around 3,500 pounds of lift. The ground-test rig serves as a “mobile wind tunnel” that generates data at significantly less cost than a conventional large-scale wind tunnel.

A Leap Forward

The HEIST ground tests are part of NASA’s $15-million, three-year Leading Edge Asynchronous Propeller Technology (LEAPTech) program, which is evaluating how DEP can be integrated into airframes and what benefits that integration might produce.

To develop and build the rolling test bed, NASA engineers at Langley and Armstrong partnered with “two small, nimble, enthusiastic firms,” Moore says—Empirical Systems Aerospace, the prime contractor, and Joby Aviation, which built the test rig, wing, motors, and propellers.

DEP could change the way aircraft are designed and built.

NASA’s next step is a project called SCEPTOR in which the wing of a four-seat light aircraft—a twin-engine Italian-built Tecnam P2006T—will be replaced with a DEP wing that incorporates perhaps a dozen electrically driven propellers.

Using an existing airframe will allow researchers to compare the performance of the modified plane with the original. “We’re going to characterize the baseline aircraft while in parallel testing the design of the new DEP wing,” he says. “Then we’ll pull off the old wings and put on the new ones.”

“A very significant portion of this research effort, which is being performed by Sean Clark’s group at Armstrong, is to develop the distributed power systems, the many motors, motor controllers, propulsors and master power controller as well as to understand the complex architecture by which they can all talk to each other,” Moore says.

SCEPTOR’s wing will be optimized for high-speed cruise—around 170 mph—yet still supply enough lift to help prevent stalling on takeoff or landing. The DEP wing is to perform other tricks as well. The speed of each electric propeller can be controlled independently, which allows engineers to tweak the airflow over the wing to cope with fast-changing flight conditions such as wind gusts. When cruising, the propellers nearer the fuselage could be folded back to cut drag, leaving those on the wing tips to do the work.

The SCEPTOR demonstration aircraft will compare how DEP stacks up against traditional propulsion.

The re-winged Tecnam is scheduled to start test flights in 2017. “We hope to showcase that DEP can achieve a five-times reduction in energy consumption in high-speed cruise over the baseline aircraft,” he says, adding that the researchers are applying for “X-plane” status for the aircraft, a coveted designation.

“Think of the Tecnam X-plane as a subscale demonstrator. We want to incubate the concept at the general aviation [light plane] level and then scale it up,” Moore says. If the test flights are successful, DEP technology could be incorporated into small aircraft within a decade, especially if battery performance and weight improves as much as experts anticipate.

New Transportation Solutions

NASA’s DEP research efforts got their start in 2011, when Moore and his colleagues began studying the possibility of “turning the small airplane into a real mid-range transportation solution instead of a mostly recreational novelty,” he recalls. “An automobile works great up to about 100 miles, while a commercial airliner works fine for from 500 to 1,000 miles, but for affordable, high-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 of less than 600 miles.”

DEP coupled with self-driving auto-pilot systems that would handle takeoffs and landings could provide dramatically higher-speed, more affordable travel than cars but with car-like ease of use, he says. But only a fully “bulletproof-safe” flight-control system could ensure nearly accident-free operations.

Moore likens these autonomy systems to riding a horse. The rider directs and supervises the horse, but the horse is sufficiently intelligent and competent “to place one foot in front of the other, stay on the path and not jump off a cliff.” The researchers hope to piggy-back onto fast-improving self-driving car technology. “DEP plus autonomy could create a whole new market for general aviation aircraft,” he says.

His NASA colleague, Ken Goodrich, who is a senior research engineer at Langley and an expert on aircraft controls and automation, is somewhat less sanguine. “Human error is responsible for 90% of accidents both on the road and in the air, but nobody’s thinking about the new errors all this automation could produce,” Goodrich says. “So far, no one knows how to program common sense into machines.” Also needed is a new procedure that the U.S. Federal Aviation Administration could use to certify the safety of autonomy technology, he says.

Goodrich does acknowledge, however, that automating airplane control is, in many ways, easier than it is for cars. “The road is a very cluttered environment, and you operate much closer to other vehicles, so the necessary reaction times are much shorter.” In contrast, “the sky is wide open” with aircraft spaced much farther apart. And with the automotive industry and companies like Google devoting huge resources to develop self-driving cars, autonomy may happen much faster than many industry observers imagine.

But even without autonomy, DEP would be beneficial because electric motors are “a scale-free technology,” Moore says. “Current propulsion engines just don’t scale well. A full-size turbine engine can be around 40% energy efficient, but if you take it down to 100 horsepower, it’s only 24% efficient—6 horsepower per pound vs. 0.5 horsepower per pound.”

Unlike fossil-fuel turbines, electric motors can be compact, reliable, and efficient. “They provide extremely good power-to-weight ratios—two times better than turbine engines and three times better than any reciprocating engine,” he says. And not only is “electric propulsion happy to scale to any size, you can place them anywhere you want.” Clever placement of electric motors could, for example, improve a plane’s lift and control.

Today’s tiny, all-electric and hybrid-electric aircraft use electric motors that deliver several kilowatts of power, but they suffer from severely limited range because of battery constraints. To travel longer distances, next-generation battery-electric and hybrid-electric light aircraft and regional airliners that carry up to 50 passengers will need to produce, say, from 1 to 2 MW of power. Larger airliners will require 2- to 5-MW-class electric motors and power systems. While batteries have improved lately, they are still many years away from providing that sort of power at the light weight required for flight.

Hypercommuter Planes

That hasn’t tempered everyone’s enthusiasm, though. JoeBen Bevirt, serial entrepreneur and founder of Santa Cruz-based Joby Aviation, is one of e-aircraft’s true believers: “Six or seven years ago, everybody in the aviation industry thought we were completely nuts to be working on electric aircraft. Now that major companies like Airbus and Siemens have jumped into the field, everybody is taking notice.”

Bevirt hopes to build two-seat hypercommuter planes, personal air vehicles that resemble George Jetson’s flying cars. These pioneering personal aircraft would produce zero in-flight carbon dioxide emissions and generate less noise while cutting overall operating costs by perhaps as much as a third. “For me, this technology is a chance to make aviation much more relevant to people’s day-to-day lives,” Bevirt says.

Hypercommuters, which would be driven by arrays of small propellers, could initially operate as Uber-like “sky taxis” to speed the long-distance daily commutes of high-income workers who live in traffic-congested regions such as Silicon Valley. Such quieter, short-takeoff-and-landing aircraft could fly from a network of small public and private landing pads dotting urban areas.

Joby's S2 concept shows what a DEP hypercommuter might look like.

The hypercommuter concept’s small-diameter propellers are optimized for low rotational speeds, which means they produce less noise than larger props. In addition, each propeller can rotate at slightly different velocities to spread out the sound frequencies they emit, cutting “community noise,” researchers hope, by as much as 15 decibels. That means that a hypercommuter-type airplane could produce “100 times lower noise levels than those of helicopters,” Moore says.

The multiple propellers also enhance safety, he says. “The safety statistics for general aviation aircraft are not all that great,” with most accidents happening during takeoff and landing—“when planes are flying low and slow.” Not only could DEP provide redundancy against propeller failure, it could maximize control. “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.”

Larger Electric Aircraft

By 2018, the NASA team plans to apply DEP to “thin-haul” aircraft such as the nine-passenger, twin-engine Cessna 402 that airlines such as Cape Air in Massachusetts use for trips of less than 250 miles. “We think that DEP technology can deliver a 20–30% reduction in Cape Air’s total operating costs with zero in-flight emissions—the best of both worlds,” Moore says.

Moore and his colleagues think electric aircraft have potential at even larger scales. “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.”

Should smaller electric airplanes start to gain traction during the coming decade—and battery tech keep pace—aerospace engineers could then begin developing larger aircraft with advanced DEP systems. Banks of electric motors along the wings could boost propulsive efficiencies by as much as 10% by sucking in the slow-moving boundary layer of air nearest the wing, NASA’s Del Rosario says.

Future airliners employing DEP—electric fans span the rear of this concept aircraft's body—could look radically different from today's models.

That would help save the energy compared with traditional engines, which hang below the wing and generate their own drag. The smaller propellers would be immersed in the slower-moving boundary-layer that flows over the wing, accelerating the already moving air and not generating additional drag. Boosting efficiency by 10% would be a major advance for the airline industry in particular, which today fights for every fraction of a percentage point to lessen fuel costs.

The question is whether the fan can withstand such a turbulent environment without its efficiency being compromised. Early results say it’s possible. “First, can you build a fan that can withstand and operate in that distorted air flow?” Del Rosario asks. “Second, by ingesting the boundary layer, are we indeed cutting drag? Our latest research says yes.”

As the tests on NASA’s HEIST big-rig and other projects move forward, it’s looking more and more like electric aircraft will become an everyday fixture at airports, Moore says. “We think that electric motors and DEP technology will become available on airplanes a lot quicker than most people think. It’s going to fundamentally change what we can do and what we’ve wanted to do for so long in aviation.”