LightningStrike X-plane concept
While hybrid and electric engines are becoming commonplace for cars and trucks, that is not the case for aircraft.
However, basic and applied research on all-electric, turbo-electric and hybrid power sources for aircraft is ongoing in civilian agencies such as NASA, the private sector and at least in one Defense Department program.
“It is very similar to what is playing out in the automotive industry to some degree,” said Richard “Pat” Anderson, professor of aerospace engineering and director of the Eagle Flight Research Center at Embry-Riddle Aeronautical University’s Daytona Beach, Florida, campus.
“There is a desire to move toward lower direct operating costs … less dependence on fossil fuels and lower noise,” he said in an interview.
The military may at some point benefit from some of these new ideas to power aircraft, experts said. The Defense Advanced Research Projects Agency has one program looking into the technology.
Brian German, associate professor at the Daniel Guggenheim School of Aerospace Engineering at Georgia Tech, said there are no distinct lines in the sand yet for aircraft categories, but generally researchers are looking at all-electric, or battery only, systems for smaller aircraft and various hybrid or turbo-electric systems to power the larger ones.
“You’ve got to be a little bit of a futurist and be in it for the long haul and say, ‘I think 15 to 20 years from now, we might be able to do that,’” he said.
One of the ways the technology can be applied for larger aircraft is distributed power systems, German said. For example, the larger a gas-turbine engine is the more efficiency can be squeezed out of it. That’s why there tends to be only a few massive engines hanging off aircraft such as the C-17. Researchers have known for many years that putting many smaller propellers or engine fans, distributed at key areas would be even more efficient.
Every aircraft has a boundary layer, an area of dead air above the wing that builds up and creates drag as the plane flies. By placing several smaller fans along the aircraft, the boundary layer is “ingested” and almost disappears, making the aircraft faster or more energy efficient, German explained.
Engineers look at this one of two ways. It’s either making the engine fans more efficient, or creating less drag on the wing. Both effects are at play, German said.
This is the principle behind DARPA’s LightningStrike vertical takeoff and landing X-plane that it is developing with Aurora Flight Sciences. An artist’s rendering of the plane shows 26 hybrid-electric propulsion fans distributed on the aircraft. The program has flown a 325-pound scale model and expects to build a full-scale version within the next two years, the company said in a statement. Operating from austere landing zones is the requirement the program is seeking to fill, it said.
There are few efficiency penalties for doing this with electric motors, German said. “If you tried to do that around the plane with gas motors it would be terrible,” he added.
Another possibility for larger aircraft is simply a charged battery that gives the aircraft additional power when it takes off and lifts. This would allow the gas-turbine engine to be smaller, German said. Traditional engines are most efficient when traveling at one consistent speed, and a battery could provide auxiliary power as it increases thrust.
Anderson said another flight profile might be an unmanned aerial surveillance aircraft that uses the gas engine for the flight to a targeted area, then switches over to the much quieter battery in order to be more stealthy, or extend its range.
The issue now is weight, Anderson added. The automotive industry — as it moves toward a world with all-electric cars — is supremely concerned about the batteries’ price point. In the aviation world, it’s all about making the batteries lighter.
The nation’s center for battery research is currently Argonne National Laboratory in Chicago. When Congress mandated that the lab’s joint center for energy storage research pursue battery technology, it didn’t specify that aeronautics be part of the program. Embry-Riddle is working with the lab to create a consortium that would also look at aircraft batteries, which would free up some resources so research could be directed toward lighter technologies.
German said “specific energy” is the single most important metric for vehicle and aircraft batteries. That is defined as watt-hours per kilogram. The state of the art for automotive batteries is nearly 200 watt-hours per kilogram, about what it takes to power a Tesla.
Compare that to a typical gas-turbine aircraft engine that puts out some 10,000 to 12,000 watt-hours per kilogram. Compared to 200, that seems like a huge gulf, German noted.
However, the vast majority of energy in a typical aircraft engine is wasted. As much as 70 percent is heat exhaust. So the watt-hours-per-kilogram gulf is closer to 3,000.
Lithium-ion battery capacity is doubling about every seven to eight years, so 400 watt-hours per kilogram is foreseeable. Powering a general aviation, single-engine aircraft with current ranges can be practical at that density level, German said.
Anderson said: “There are those out there in the scientific community who think that the specific energy of batteries will never reach that of liquid fuels. And if that is the case, then probably the ultimate goal in airplanes is hybrid. …We will see some smaller, slower airplanes that are fully electric, but it is more likely that for larger aircraft, a hybrid system will be the end point.”
What everyone is trying to do is move up the food chain in terms of the aircraft size, he added. Small UAVs and small aircraft will be able to go all-electric.
However, “if you want to go supersonic on battery power that is not happening anytime soon,” he said.
German said the trend in the military is for “more electric.” That is, jets need more power to run its suites of energy-hungry electronics, radars, and so on. It wants less fuel being diverted to these systems.
The main player in the United States is NASA and its advanced air transport technology project.
Cheryl Bowman, technical lead of the hybrid-electric subproject for large planes at NASA’s Glenn Research Center in Ohio said, “we are looking at technologies that are relevant to the single-aisle passenger aircraft design.”
NASA’s overarching goal is to reduce the carbon footprint of the aviation industry as it grows in the coming decades and to either reduce or maintain emissions on a steady line.
It is tackling the problem in several ways.
The agency is looking at the distributed power system described earlier to make major aircraft reconfigurations as well as investing in the underlying technologies such as lighter materials and additive manufacturing techniques.
“We know that the basic technology needs to be improved so we are really looking at it from the top down from the vehicle configuration aspect, but also from the bottom up where we are investing in basic technology,” she said.
NASA is looking for a sweet spot in terms of size. It’s already a given that all-electric systems could power small UAVs. The agency’s X-57 demonstrator will attempt to fly an all-electric medium-sized aircraft, about the size of a two-seat general aviation plane that can fly at around 200 knots, Bowman said.
It has several universities assisting it, along with engine manufacturers Rolls-Royce America and United Technologies Research Center.
“The machine and power electronic and power architecture studies are informing our vehicle configuration designs, and the vehicle configuration designs are informing what we need for our technology development,” she said.
The X-57’s first flight may come in two to three years, she said with the equivalent of a single-aisle passenger aircraft coming in about 10 years.
Ralph Jansen, hybrid-electric technical integration manager at Glenn Research Center, said: “For this to close with a net benefit in fuel burn, and other parameters, we need to develop the technology to get the weight of the electrical system down a lot, and the efficiency up a lot.”
He agreed that larger aircraft will probably still have to burn some jet fuel. Any aircraft with a range of 2,000 nautical miles or more will probably be a turbo-electric system.
“We have research that shows we are in striking distance of doing that,” he said.
There is also private sector money going into all-electric aircraft power.
Europe’s Airbus has E-Fan, a hybrid-electric powered aircraft that it recently flew over the English Channel. It brought the aircraft to display at the EAA AirVenture Show in Oshkosh, Wisconsin, in July.
“Airbus is using it to evolve electric propulsion as a possible alternative to fossil fuels in the development and powering of its defense drones and satellites within the next few decades,” a statement prior to the show said. The company declined to make executives available to talk about the program.
Embry-Riddle’s Anderson also noted that there is a movement afoot in Silicon Valley to develop small, personal vertical-takeoff and landing aircraft that will whisk passengers across towns and over traffic jams.
Ride share company Uber is looking into the concept. Start-up Zee-Aero said on its website that it is “working at the intersection of aerodynamics, advanced manufacturing and electric propulsion” to find better ways to get from point A to point B. Joby Aviation is developing an “electric vertical takeoff aircraft to revolutionize personal mobility,” according to its website.
Outside California, a German company E-Volo is developing a similar “Volocopter” for personal transportation.
There are 100s of millions of dollars of private investment money going into these start-ups, and they all plan to employ electric propulsion, Anderson said. They will inevitably have some military applications, he added.
Art: Aurora, NASA