When we first looked into the then wistful subject of hybrid propulsion, the idea wasn’t getting much love. The thought of adding an additional power system that would serve to backup the primary one seemed to go against the less is more ethic of aircraft design. After all, battery tech would advance to the point where hybrid power was excessive and out of date. Well, maybe not. Just a few years later, where we are, looking at little progress in battery density or charging speed, at least not enough to support the idea that a breakthrough is coming, and light aviation needs a breakthrough to make all-electric planes a common reality.
For years our dreams of flight were powered by avgas. Lots and lots of it. For his generation, big, radial Wright Cyclone engines and Rolls-Royce Merlin V-12s were the ticket skyward—transforming about 100 gallons an hour into 300 knots or more of speed and an instantly recognizable percussion that those in any nearby zip codes would have felt to their core.
My own dreams aloft soar through the sensibility of a different century. I want silent speed, the quieter the better, so as not to disturb the neighbors. Touch screens for everything, please. A zero-emissions footprint and a fuel bill to match.
In other words, give me an Air Tesla with Ludicrous Mode at Flight Level 250, please.
Yet, any dreams of a purely electric future will wake to some harsh limitations, starting with the power aboard. The 60- to 100-kilowatt-hour battery that drives a terrestrial Tesla 300 miles or so weighs about 1,200 pounds. Strap that under a Skyhawk, and you’re anchored to the ramp.
In fact, Paul Robertson, a professor of engineering at the University of Cambridge, calculated that replacing an airliner’s jet engines with current-technology lithium-ion batteries would yield a plane able to power up for just 10 minutes.
So, where does that leave us evolved and eco-aware pilots?
One word: Hybrids.
It turns out, your future flights may look more like a Prius ride than a Model S. And, that future may not be as far away as you might think. As with pure electrics, the first hybrids experiments have already taken flight, and capabilities now are scaling up rapidly.
The hybrid idea makes a lot of sense when you think about it. A plane might taxi to the runway on electricity. During high-demand phases of flight such as takeoff and climb, it could supplement that with energy from a turbine generator to provide lots of power without lots of noise. Once at altitude, the plane could loaf along with just the generator driving electric fans, diverting excess power back to recharge onboard batteries. Then, the plane could descend and land again as a quiet electric.
In 2014, Professor Robertson and his team of researchers worked with Boeing to test a light, single-passenger motor glider rigged with just such a parallel hybrid propulsion system, the first of its kind to take to the air. Their flights consumed 30 percent less fuel than a traditionally powered plane. That’s promising—but with a motor glider, only in a theoretical, laboratory-of-the-sky way.
“A key value of the Honeywell/Rolls solution is that the translation from turbine energy to electric is 98 percent efficient, compared to 80 to 90 percent in previous-generation installations.”
Today, there are more ambitious—and commercially viable—projects in the works. Torey Davis, Director of Engineering, Advanced Technology, at Honeywell Aerospace, recently discussed his company’s power partnership with Rolls-Royce, in which megawatt generators are mated to turbines.
For those who don’t have a slide rule at the ready, a megawatt of power equals 1,341 horsepower, and it’s roughly 150 times more powerful than Professor Robertson’s demonstrator. Now, we’re talking!
A key value of the Honeywell/Rolls solution is that the translation from turbine energy to electric is 98 percent efficient, compared to 80 to 90 percent for previous-generation installations.
Once that generator does its work, it’s also much easier to distribute energy to multiple engines than it would be in a traditional aircraft. That enables designs like the Aurora LightningStrike, a project currently being built as a research platform for the U.S. government’s Defense Advanced Research Projects Agency (DARPA) X-Plane program.
The LightningStrike sends power from three of the Rolls/Honeywell megawatt generators (three—that’s more than 4,000 horsepower!) to 18 ducted, electric fans embedded inside a tilting wing and canard. You want vertical takeoff and landing (VTOL)? No problem; you can depart from the backyard. But, the LightningStrike also shines in cruise, where it’s projected to break 400 knots.
It turns out that distributed power makes for a much more efficient aircraft. Each fan can be much smaller than a conventional jet engine, which makes them quieter. And, integrating them into the wing design results in cleaner airflow and less drag across the wing surface.
Aurora has already flown a sub-scale model, and engine testing is underway. Expect a full-scale, operational aircraft in a couple of years, though a Cessna version you can park in your own jet-port is still just a dream. And, don’t even ask what kind of certificate you’d need to fly one of these. Semi-fixed wing, multi-multi-multi-engine?
Davis does point out that Honeywell is working on a similar distributed-electric hybrid application for XTI Aircraft, a crowd-funded startup company developing a tri-fan VTOL business aircraft. XTI’s TriFan 600, if it’s funded and built, would cruise at 300 knots, but save time by landing closer to the boss’ business meeting than faster jets that need an airport with a long runway somewhere nearby.
Still another hybrid propulsion joint project, between Pipistrel and Siemens, is underway. The team is testing a 200-kilowatt hybrid motor, a power level more akin to the engine on today’s typical piston four-seater. Called the Hypstair, the system uses multiple power inverters and power sources to bring the redundant safety of a multi-engine aircraft to a single propeller design. It’s capable of three modes: Battery power, generator power, or both. If either the generator or the batteries were to fail, the other system could kick in. Unlike a normally aspirated, piston-driven plane, it also would deliver full power at any altitude.
In February, the Hypstair engine ran for the first time in a ground test. Pipistrel CEO Ivo Boscarol said in a statement at the time, “We are proud of what Hypstair represents for the development of electric flight. It demonstrates the possibility for general aviation class aircraft to be electrically powered.”
“Known as E-Thrust, the Airbus/Rolls concept envisions an airliner with six electric fans driven by a single gas turbine.”
Even for those who have to buy tickets to board a plane, Airbus Group and Rolls-Royce have a hybrid plan for the future. Airbus Group CEO Tom Enders said in a statement that his company’s ultimate target is “zero emissions aviation,” following formal European guidelines that call for 65 percent noise reduction and 75 percent fewer carbon emissions by 2050, compared to a year 2000 baseline.
Known as E-Thrust, the Airbus/Rolls concept envisions an airliner with six electric fans driven by a single gas turbine.
As in the LightningStrike, the turbine wouldn’t provide thrust; its only job would be to power a generator that, in turn, would drive the fans. In a particularly nifty trick, the slipstream during descent would provide windmill power to the plane’s six fans. That energy would help to recharge the onboard batteries in much the same way that regenerative braking does on a Prius.
In the Airbus concept, an E-Thrust airliner would be sufficient to carry 50 to 100 passengers on a regional flight, say, from Dallas to Nashville. Both fuel consumption and carbon emissions would be reduced enroute, along with environmental noise.
“We believe that by 2030, passenger aircraft below 100 seats could be propelled by hybrid propulsion systems,” said Enders.
In the end, perhaps our swift and nearly silent flying dreams will come true. With luck, they’ll even take a form that’s sexier than a Prius. Said Siemens Head of eAircraft Frank Anton at the first Hypstair run-up, “The world is becoming electric, whether in the air, on land or at sea.”
Plug me in to that future, and turn me on.
A commercial pilot with instrument privileges, Grant Opperman is a writer and business strategy consultant who flies himself to more than 20 states across the U.S. for business and pleasure.