Aircraft are known for two things: noise and pollution. (You know, besides flying.) Planes get to where they're going by burning copious amounts of fuel—so much that aviation emissions are one of the biggest contributors to climate change. So for that, and many other reasons, NASA is building an all-electric test plane.
It will be called the X-57, and it'll look goofy as hell. Its wings—long, straight, and skinny as cowboy jeans—are lined with 12 small electric motors. And at each wing tip, two larger engines. The spec illustrations, revealed June 17 during NASA administrator Charles Bolden's keynote at the American Institute of Aeronautics and Astronautics' annual meeting, look like cover shots on the build-your-own sport plane brochures your dad keeps pining over. But apparently, this thing is going to change aviation.
"This all comes back to that traditional aircraft design process," says Sean Clarke, NASA engineer and co-principal investigator for electric propulsion and the X-57. "I'm given a certain performance envelope: Make this thing go fairly fast, using as little fuel as possible." But Clarke (and all aircraft engineers) also have to meet requirements from the FAA about safe takeoff and landing speeds—the slower the better. Planes with low aspect ratio wings can take off and land at lower speeds, but those wide wings create a lot of drag, wasting a lot of fuel to stay at cruising altitude. Long, narrow wings perform a lot better while cruising, but need more oomph to get aloft.
The wonky engine configuration Clarke and his team developed works kind of like an automobile transmission. During takeoff and landing, the 12 small motors kick on for about 30 seconds. Think of this as the low gear torque that your car needs to get moving after being stopped at a red light. Once it's moving, the X-57's version of shifting into high gear is shutting off those 12 engines (their propellers fold back to become flush with the wing) and letting the two larger end-wing propellers take over.
But the car gear thing isn't a perfect analogy. Because your car, no matter what gear it is in, uses the same motor. By contrast, the two types of motors on the X-57's wings are taking advantage of different aspects of how wings generate lift.
In case you need a refresher, the most basic way wings fly is by moving fast through the air. (If you want to get more specific, the air on top of the wing must move faster than the bottom—hence the wing's foil shape—in order to generate lift.) Aircraft accomplish this with engines that push the whole aircraft at the requisite speed, which depends on the wing's shape, the plane's size, and other geometric variables.
But as I pointed out before, different wing geometries beget different performance traits: long and thin, good for soaring; short and fat, good for takeoff; short, tapered and backswept, good for trips to the Danger Zone. Clarke and his team wanted wings long and thin enough for soaring, but those would require liftoff and landing speeds that violate FAA safety requirements. They get around this by using the 12 small motors to push wind across the wing at nearly twice the speed that the aircraft is moving. Essentially, tricking the wing into thinking it is moving faster than the airplane. "It’s not really a propulsion system, it’s a high lift device," says Clarke. The aircraft might be taking off at 75 miles per hour, but the wind is moving past the wings nearly twice as fast.
Efficiency is also why the X-57's main engines are on the wing tips. When flying, vortices form at the tips of a plane's wings. These create drag. "Our wingtip motors are designed to recover and take advantage of those vortices," says Clarke.
Clark and his gang of lugnuts have been ground-testing the X-57's basic wing technology for about 18 months. "Edwards Air Force Base has this giant, dry lake bed that we can drive on for miles without having to take a turn," he says. They put the wing on top of a truck and drove it back and forth at 40, 60, 80 miles per hour, testing performance all along.
If you think driving around the desert with a wing on your truck sounds insane, remember that insanity is the most important ingredient to advancing avionics. Edwards Air Force Base is where Chuck Yeager broke the sound barrier, where Neil Armstrong flew a jet into space. And whenever NASA or the Air Force were testing any of their experimental propulsion systems, the lake bed provided a place for pilots to safely glide to landing after cutting the engines. "That lake is the real reason aircraft testing has been done at Edwards for 50 years," says Clarke.
That said, the X-57's wonky wing configuration is nowhere near ready for first flight. Over the next year or so, Clarke's team will systematically retrofit an Italian designed twin-prop until it becomes the aircraft they've promised. They'll start by replacing the pistons and mechanics that currently control the wing's propellers and flaps with an an all-electric system. "We need to evaluate how this compares to the original engines," says Clarke. He expects they'll be doing flight tests with that configuration by next fall. "After that, we'll take those baseline wings off the aircraft and put the experimental wing back on," he says.
The X-57 is itself just the first step in the longer term New Aviation Horizons project announced by NASA several months ago. The space agency's engineers will spend the next 10 years building experimental aircraft to solve problems in fuel efficiency, noise, and emissions. "We are in a build, learn, fly spiral of development, taking what we've learned and putting it into new models," says Clarke. NASA has plans for five successors to the X-57.
Eventually, Clarke believes their innovations will trickle into commercial aviation. First into smaller aircraft, but someday your dream vacation to Borneo could be borne upon lanky, silent wings that were born in a dry lake in California.
