A version of this article appears in the July 7 edition of Aviation Week & Space Technology.
Aviation did not enter the Jet Age overnight, and a decades-long journey to the next propulsion paradigm may already be underway. At, the exploration has begun with plans for ground and flight tests to determine whether hybrid and distributed electric propulsion could be the next disruptive shift in civil aviation.
A wing carried high above a truck racing across the dry lakebed at Edwards AFB, California, in November could provide the first validated data to prove that distributed electric propulsion can offer the promised benefits. The 31-ft.-span wing will carry 18 small electrically driven propellers, and is a precursor to a small X-plane demonstrator proposed under’s new Transformative Aeronautics Concepts program.
In parallel, over the next five years, the agency wants to develop technology for compact, high-power-density electric motors generating 1-2 megawatts—sufficient to power an all-electric general-aviation aircraft or helicopter, a hybrid turbine-electric regional airliner or a large transport with many small engines distributed around the aircraft in ways that make it safer and more energy-efficient.
The sweet spot for a first generation of electric-powered aircraft seems to be between 1 and 2 megawatts, says Ruben Del Rosario, NASA Fixed Wing program manager. But the agency also sees an intersection of unmanned and personal air vehicles around electrical propulsion and increasing autonomy, beginning this spiral exploration with small unmanned aerial systems (UAS) (see page 20) and light aircraft.
“What problems are we trying to solve in general aviation?” Mark Moore, advanced concepts engineer atResearch Center, reflects. His answer is many, and they encompass the low efficiency; poor safety, emissions and ride quality; and high operating costs of some light aircraft and helicopters.
Distributed electric propulsion promises dramatic increases in aerodynamic and propulsive efficiency, and reductions in noise and energy costs. “It is not just about general aviation, but they are earlier adopters at a smaller scale, faster and cheaper,” Moore says.
Electric propulsion is not without its penalties. Energy-storage weights are far worse than those of aviation fuel, and battery-pack costs are high. But electric motors are more efficient than turbines or pistons across a wide rpm range, and power-to-weight ratios are higher; they are quiet, compact and reliable, with zero emissions and energy costs that are much lower than for aviation fuel. And, crucially for aircraft design, efficiency and power-to-weight are independent of size.
“You can have multiple small electric motors with the same output as a large one without much penalty. You can put them anywhere around the aircraft, versus heavy piston engines that can only go in one or two places,” says Joby Aviation’s Alex Stoll, chief designer of the Lotus small UAS and two-seat S2, both vertical-takeoff-and-landing designs using distributed electric propulsion. “You can use them to make a personal air vehicle practical, versus an expensive, noisy, unsafe helicopter.”
To test the premise that the tighter propulsion-airframe integration possible with electric power will deliver efficiency, safety and environmental and economic benefits, NASA has partnered with Empirical Systems Aerospace (ESAero) and Joby to propose the Leading Edge Asynchronous Propeller Technology (LEAPTech) demonstrator as an X-plane testbed for distributed electric propulsion.
A traditional light aircraft needs a large wing to meet the low stall-speed requirement for certification, but this is inefficient in cruise. LEAPTech replaces the big wing with one that is one-third the size for lower drag, and has three times the wing loading for better ride quality. Cruise lift-to-drag ratio at 200 mph is greater than 20, versus 11 for a comparable Cirrus SR22, NASA estimates.
To achieve the required 61-kt. stall speed with such a small wing, LEAPTech mounts an array of small propellers along the leading edge. These accelerate airflow over the wing, increasing dynamic pressure at the leading edge and more than doubling the maximum lift coefficient (CLmax) at low speed. “In computational fluid dynamics, we have seen lift coefficients of 5.5. We need 4.5 for a 61-knot stall,” says Stoll. Unblown, with full-span Fowler flaps deployed 40 deg., CLmax is 2.7, says Moore.
Optimized for low speed, the small-diameter, high-solidity propellers have low tip speeds, around 450 ft./sec. compared with an SR22’s 919 ft./sec., for reduced noise. In addition, they all operate at slightly different speeds to spread the frequencies and reduce the annoyance. The props blow the wing for takeoff and landing, but fold back to reduce drag in cruise, at which point wingtip propellers optimized for high speed provide propulsion, operating inside the wingtip vortices to increase efficiency.
The intent of LEAPTech is to modify a Tecnam P2006T light twin with the new wing, to provide a direct comparison between conventional and distributed-electric propulsion. But the first step is the Hybrid-Electric Integrated Systems Testbed (Heist), a truck-mounted rig that will enable NASA to ground-test a full-scale wing at the 61-kt. stall condition at lower cost than a wind-tunnel test.
“NFAC [National Full-Scale Aerodynamics Complex wind-tunnel facility] would have cost more than the entire budget [for Heist],” says Moore. “And we need to get to this scale to have reasonable data.” The wing will float on an airbag system in the truck to minimize vibration from the lakebed, and the remaining noise will be removed during post-processing to obtain lift measurements with less than 5% error, he says.
ESAero is the prime contractor for Heist. Joby Aviation is building the test rig, wing, motors and props. Combined, the 18 propellers will generate 300 hp and the wing will provide 3,500 lb. of lift. ESAero will conduct shakedown tests on a paved runway before the lakebed tests in November. “We will get ground vibration . . . [but] are confident we can get clean data on lift, drag and pitching moment,” says NASA Armstrong Flight Research Center project engineer Sean Clark.
In parallel, NASA is beginning fundamental research to understand and overcome the challenges of electric propulsion. Now taking shape at Armstrong, the Airvolt test stand will be used to characterize each element of a single-string propulsion system, from batteries to propeller. A year from now, this will be upgraded to the Airvolt Hybrid, with aM250 turboshaft, electric motor/generator and enlarged batteries. This will be arranged so that both the gas turbine and electric motor can drive the propeller, and will be used to look at power-transfer stability issues with parallel hybrid propulsion.
In February 2016, NASA Armstrong plans to begin the Heist power management and distribution (PMAD) ground demonstration. This will be a static propulsion test stand co-located with Airvolt and used as a long-term research platform. NASA plans to evaluate stability issues inherent in parallel-hybrid electric bus architectures, characterize aggregate thrust control of many motors, investigate algorithms for thrust-augmented yaw control and to assess power generation and consumption problems such as catastrophic load shedding, says Clark.
“We will take the wing off the truck, put it on a static test stand and start to do power distribution studies,” he says. These will include shifting between batteries and turbine-driven generator feeding the distributed power bus, and stabilizing the high-frequency electrical loads from the motors. “We will study how to schedule power for yaw control, and how to integrate prognostic health monitoring,” including sensors on the motors, Clark says.
A distributed propulsion electronic controller will translate thrust targets input by the pilot into individual thrust commands for each of the propulsors, while managing the balance between power generation, storage and consumption. A control algorithm will manage the loading of the generator, real-time capacity of the energy storage buffer and power demand from the collection of propulsors. Another algorithm will synthesize individual propulsor commands based on total thrust targets and generator- and stored-power availability.
Heist will use 100-120-volt electrical systems. “That’s not optimal, but it allows us to do [Heist] within a year, and we do not care about weight,” says Andrew Gibson, ESAero business-development president. NASA plans to move to a 600-volt system to reduce distribution wiring weight. “It’s the sweet spot; [there are] no arcing concerns and components are available today,” says Clark.
To follow PMAD, NASA plans hardware-in-the-loop tests to integrate flight-like electric propulsion hardware with simulated aircraft flight controls. Next up will be aircraft-in-the-loop “iron bird” tests with real controls surfaces, flight-ready control system and flight-like energy storage components. The iron bird would test systems for a kilowatt-class distributed electric propulsion X-plane, the LEAPTech, and a 1-2 megawatt hybrid-electric flight demonstrator, notionally one of NASA’sGlobal Hawk unmanned aircraft.
Clark says the iron bird will be used to assess actual electric-propulsion implementation effects such as real-world volume and weight constraints, to address failure modes and recovery strategies, identify and resolve interdependencies between propulsion and other systems, validate aero-propulsive efficiency gains and demonstrate realistic performance benefits from propulsion-airframe integration.
While it seems futuristic, distributed electric propulsion is on the near horizon for light aircraft. Companies such as Joby and secretive Zee.aero are designing personal aircraft around the concept, knowing they must wait for batteries and motors to improve. Joby plans to fly the S2 in 1-2 years and certificate it in “a few years,” says Stoll, who believes “by 2020 electric aircraft will fly 500 miles at 200 mph.”
Moore believes electric propulsion, coupled with autonomy, will spark a breakthrough in on-demand aviation and that the technology will move up to larger aircraft over time, because, just as “the PC came before the Internet, transformative vehicles lead transformative capabilities,” he says. “As you go up in size, the benefits decrease because larger aircraft are already more efficient. But it is still compelling.”