When Wilbur and Orville Wright’s famous airplane, the Wright Flyer, first flew in 1903 it must have made quite a racket, with its crude gasoline engine spinning twin propellers via drive chains. Nearly 115 years later, another type of plane has taken flight as quiet as a ghost, without a single moving part. The new type of aircraft could usher in silent drones and perhaps far simpler planes—if researchers can overcome the daunting task of scaling up the technology.
Instead of relying on a propeller or a jet engine, the plane, about the size of a single-person kayak, pushes itself through the air using electroaerodynamics (EAD). This form of propulsion uses electric effects to send air backward, giving the plane an equal push forward.
Aeronautical engineers have long theorized that planes could be powered by EAD, says Steven Barrett, an aeronautical engineer at the Massachusetts Institute of Technology (MIT) in Cambridge. But no one had ever constructed an EAD plane capable of lifting its own weight. When Barrett and colleagues finally succeeded, they stood in awed silence, he says. "It had taken about 7 years of work just to get off the ground.”
In an EAD propulsion system, a strong electric field generates a wind of fast-moving charged particles called ions, which smack into neutral air molecules and push them behind the plane, giving the aircraft a push forward. The technology—also called ion drive, ion wind, or ion propulsion—has already been developed for use in outer space by NASA, and is now deployed on some satellites and spacecraft. Because space is a vacuum, these systems bring along a fluid, like xenon, to ionize, whereas Barrett’s aircraft is designed to ionize nitrogen molecules in the ambient air.
It’s far easier to deploy ion drive in space than in the atmosphere, however. Gravity guides a satellite around the planet, with ion drive applying small course corrections. In contrast, a plane must produce enough thrust to keep itself aloft and to overcome the constant drag of air resistance.
After running multiple computer simulations, Barrett’s team settled on a design for a plane with a 5-meter wingspan and a mass of 2.45 kilograms, about the weight of a chicken. To generate the needed electric field, sets of electrodes resembling Venetian blinds run under the plane’s wings, each consisting of a positively charge stainless steel wire a few centimeters in front of a highly negatively charged slice of foam covered in aluminum. The plane also carries a custom battery stack and a converter to ramp the voltage from the batteries from about 200 volts to 40 kilovolts. Although the highly charged electrodes were exposed on the plane’s frames, they could be turned on and off by remote control to avoid safety risks.
The team tested the airplane inside a gymnasium at MIT, working at odd hours to avoid running into sports teams. “There were some pretty epic crashes,” Barrett says. Eventually, the team devised a slingshotlike apparatus to help launch the aircraft. After hundreds of failed attempts, the aircraft was finally able to propel itself enough to remain airborne. Over 10 test flights, the plane flew up to 60 meters, a little farther than the Wright brothers’ first flight, in about 10 seconds, with an average altitude of half a meter, the researchers report this week in Nature.
“This is a great first step,” says Daniel Drew, an electrical engineer at the University of California, Berkeley, who is working on EAD microrobots and was not involved with the study. However, he cautions “if they try to go much bigger with the plane size, they’re going to run into a lot of issues.” The basic problem comes down to scaling, Drew says. As the size of the plane increases, its weight will grow faster than the area of its wings. So to stay aloft, a bigger plane must produce much more thrust per unit of wing area, he explains, something that “would be extremely difficult to achieve from a physics standpoint.”
Barrett isn’t ready to rule out the possibility of one day transporting humans. “We’re still a long way off obviously, and there’s a lot of things we need to improve to get there,” he says, “but I don’t think there’s anything that makes it fundamentally impossible.” Thrust could be improved by making the power converter system and the batteries more efficient, testing different strategies for creating ions, or integrating the thrusters into the plane’s frame to reduce drag, he says. Franck Plouraboué, a fluid mechanics researcher at France’s national research agency CNRS and the University of Toulouse, says one way to power EAD aircraft could be through ultralight solar panels attached to the top of the plane.
Drew thinks we’re more likely to one day see a swarm of smaller EAD aircraft. In that context, Barrett thinks the biggest advantage of EAD aircraft will be the lack of noise. “If we want to use drones all around our cities for delivering things and monitoring air quality, all that buzzing and noise pollution would get quite annoying.”