Today’s aircraft fly by pulling their wings through the air fast enough to generate the flow that produces lift. Drop below that speed, or pitch the plane at the wrong angle, and you stall out—the wings stop producing enough lift, and the plane drops like a brick.
But put the motors on or even inside the wings, and the propellers blow air directly over them, at the optimal angle. You start producing lift while the plane is standing still. It’s not enough for the plane to shoot straight up, but it allows a shorter take-off and a lower chance of stalling.
The green plane faces the same challenge as any other electric device—battery life. A dead battery cutting short a cell phone call is annoying; but cutting short a flight could be deadly.
In the US, engineer Greg Cole is developing a fully powered, two-seater plane with ultra-high efficiency. It should require a maximum of 36 horsepower when climbing and as little as 7 horsepower to stay aloft. But even with those modest requirements, it will be able to carry only enough lithium-ion batteries (230 pounds worth) for one hour in the air—not quite enough for even recreational flight. While planes commonly rent out for an hour at a time, they are required to carry enough fuel for nearly two hours, in case of emergencies that keep them in the air longer than planned.
The limited battery life mirrors the dilemma that electric cars face. And in both cases, hybrid gas-electric engines provide a solution, according to Ron Gremban. A pilot since 2001, Gremban is also technology director of the California Cars Initiative (CalCars), an organization promoting super-efficient plug-in hybrids that can run longer on electricity alone than today’s hybrids. (Gremban hacked the first plug-in Toyota Prius.) He proposes the same fix for small airplanes that would run mostly on battery power but extend their range with small gas engines that drive a generator. According to Gremban, just one gallon of gas could keep a small plane in the air for an extra half-hour.
As with cars, future super batteries could also make long-range electric-only planes possible. Professor Yi Cui of Stanford University believes he can pack more power into batteries by replacing the rigid framework in today’s cells with a flexible mat of silicon nanowires that swells to hold up to ten times as many lithium ions.
Brien Seeley believes that clean, long-range planes, together with automatic or semiautomatic navigation systems will open the skies for more short-range flights—cleaning up both traffic and smog. “It’s time to get off the pavement,” said Seeley.single page
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.