If hybrid-electric propulsion ever becomes a primary source of aviation thrust, the industry must invent a new kind of highly efficient electric generator small enough for an aircraft yet powerful enough to generate thrust.
Perhaps years ahead of its competitors, Honeywell believes it already has the answer.
The Aurora Flight Sciences LightningStrike is scheduled to fly in 2018, using three 1MW generators developed by the same Honeywell division that produces thousands of gas-fuelled auxiliary power units. The unmanned XV-24A LightningStrike, a high-speed, vertical take-off and landing X-plane funded by the US Defense Advanced Research Projects Agency (DARPA), will use electric power to generate enough thrust to hover like a helicopter, yet still approach the forward speed and fuel-efficiency of a similarly sized fixed-wing aircraft.
DARPA launched the programme to achieve a dramatic improvement in speed for an aircraft that can take off vertically without paying nearly as much of a penalty in range or payload performance as needed for other high-speed approaches, such as a tiltrotor aircraft. “This is trying to get to a vertical take-off aircraft that can fly like a business jet,” says Eric Blumer, a director of advanced technology at Honeywell Aerospace.
Unlike rivals that used tiltrotors or ducted fans, Aurora selected a hybrid-electric propulsion system. Although popular in trains, ships and cars, using a gas engine to generate electric power to provide thrust has yet to catch on in aviation, mainly because of an aircraft’s power requirements: at flying loads, electric generators are too big and produce too much heat to be practical.
To generate all of that electric power for a scheduled first flight of the LightningStrike in 2018, Aurora turned to two suppliers. Rolls-Royce will supply a single AE1107, the same turboshaft engine that powers the twin-engined Bell Boeing V-22 Osprey. The gas turbine is older, relatively low-risk technology. One of the most important innovations driving the DARPA programme is Honeywell’s 1MW generator.
Instead of using that gas turbine to generate thrust, the AE1107 on the LightningStrike produces energy, which Honeywell’s three generators convert into 3MW of electric power. A single megawatt of electric power roughly equates to about 1,400hp, meaning the XV-24A generates about 5,200hp of thrust split between 24 fans embedded into the wings and canards.
“The big enabler here for this all-electric propulsion is these three 1MW generators,” Blumer says.
Enabling hybrid propulsion is a popular theme in advanced aerospace technology circles. Several years ago, NASA released a concept of an N-3X airliner, showing a blended wing body design with wingtip gas turbines generating electric power for a row of fans buried along the trailing edge of the fuselage. If such an aircraft must develop 30,000lb of thrust, onboard electric generators must grow in capacity by an order of magnitude.
In the meantime, several governments and companies are experimenting with smaller aircraft using hybrid propulsion systems. In addition to DARPA’s LightningStrike, Airbus is flying the all-electric E-Fan, but the larger goal is a 90-seat regional airliner driven by hybrid-electric propulsion after 2030. Even smaller start-ups, such as Lillium Aviation, based in Germany, are developing prototypes for relatively long-range general aviation aircraft with electric-powered fans.
Although the LightningStrike’s engines are off-the-shelf, Honeywell’s generators are so far beyond the state-of the-art they have not yet been tested at full power. The absence of full-scale testing reflects the breakthrough nature of the generators themselves. Honeywell’s test stand can accommodate generators with up to 500kW capacity. Each of the three generators planned for LightningStrike is rated at 1MW continuous power.
To put that into perspective, the 250,000kg-class Boeing 787, which pioneered the use of electric power for wing de-icing and cabin pressurisation in commercial aviation, requires a total of 1.4MW of electric power for all onboard systems. The 5,000kg-class LightningStrike will be powered with 3MW of electric thrust. The 787 uses a total of six generators to produce 1.4MW, whereas the LightningStrike requires only three to make 3MW. The 787’s four 250kW generators each weigh 95kg. Honeywell’s new 1MW is 400% more powerful, but is only about 40% heavier. Honeywell packaged the high-horsepower generator in a roughly 60cm by 35cm package weighing 127kg.
Compared with the efficiency of even lower-power, aviation-grade generators, Honeywell’s new invention remains impressive. A standard electric generator in aviation is about 90% efficient; that is, only 10% of the power is wasted as heat, which must be removed from the onboard power system. If that standard generator is rated at 100kW, dissipating the waste heat is a manageable problem. But managing 10% energy loss from a 1MW generator is untenable. The waste heat would overwhelm the material properties of the generator, causing the whole system to melt or catch fire.
So Honeywell had to dramatically improve the efficiency of electric generators – a high-profile technical challenge. NASA has released a roadmap showing a progression of hybrid-electric vehicles over the next 20 years, leading to 10MW-class, 300-seat airliners after 2035. One of the critical enablers identified on the agency’s roadmap is more efficient electrical generators. To that end, NASA has sponsored research by Ohio State University on 1MW generators with 96% efficiency. Honeywell claims to have already surpassed that lofty objective, with a design that achieves 98% efficiency.
“The way we got there largely – which turns into a big benefit, but we didn’t do it for the benefit, it was the only way to get there – was to be 98% efficient,” says Blumer.
That means only 2% – or about 60kW – of the energy flowing from the gas engine into the generator is wasted as heat.
Bob Witwer, Honeywell’s vice-president of advanced technology, says: “If you’re starting with a baseline of 90% efficiency, if you get to 98 you didn’t get five times more efficient. But what you did do is you took those sources of inefficiency that [cost you] 10% and you reduced them by a factor of five.”
Another innovation in the electric power architecture for the LightningStrike is the absence of motor controllers. In most aviation applications, a motor controller sits between the source of the electric power and the actuated component, mitigating any swings in frequency output and providing full speed control. The Honeywell system for the high-efficiency generator uses a constant frequency, eliminating the need for additional electronic components for modulation. In addition to slightly reducing weight and cost, Honeywell also likes the reduced complexity.
“If you’re trying to turn 3MW of power over here into thrust over there, the fewer things that I have to go through, the better,” says Blumer.
That means LightningStrike’s 24 electric fans – nine on each wing and three on each canard – are connected directly to the power generator. Each of the fans features a set of variable-pitch blades, giving the pilot extraordinary flexibility to control the aircraft in all six axes of motion.