Solar-Powered Plane Would Fly by Flapping

Our neighboring planets may someday be explored by aircraft with no motors or jets or props, but with solar-powered wings that flap and soar like an eagle.

The world hasn't seen such a contraption since Icarus donned his wax wings and flew out of that Greek mythological prison, only to fly a tad too close to the sun and plummet back into the sea. But that doesn't mean it was a bad idea, according to researchers at the University of Missouri-Rolla, who are breathing new hope into the old idea of flying like a bird.

They are riding the crest of a technological wave, trying to combine today's — and tomorrow's — breakthroughs in a project that sounds almost like science fiction. The National Aeronautics and Space Administration is interested enough to provide $63,000 in seed money from its Institute for Advanced Concepts.

But nobody has ever built a flying machine powered totally by solar energy, and pushed aloft by flapping its wings, so can these folks really do it?

"I think so," says Kakkattukuzhy Isaac, professor of mechanical and aerospace engineering at the university. "Definitely."

The idea is so far out that Isaac says he and his fellow researchers aren't planning to build their aircraft in the next few months. Instead, they are projecting five or six years down the road, when some really exotic materials should become available. Then, they hope to bring about a marriage of their aerospace concepts with these new materials.

"I think that is quite realistic," Isaac says.

Light as a Feather

The machine they plan to build will be a bit larger than an eagle, and it will use solar energy to flex its wings during the day, climbing higher and higher, and then soar through the night. That substitutes altitude for a battery as an energy storage device, thus helping to solve one of the main obstacles to making this baby fly. A battery would be just too heavy.

Weight is the primary enemy of the concept, which is why Isaac doesn't think the technology will ever be used to carry humans. Instead, it will be used to carry sensors and cameras and navigational devices so that it can monitor everything from weather to battlefield maneuvers.

But why resort to a technology so radical?

"The main driving force is cost," Isaac says. "We feel that by tightly integrating all the technologies we will be able to reduce the cost of manufacturing and maintaining" the aircraft. Such a machine could explore Mars, for example, at a fraction of the cost of the gold-plated Mars rovers, and cover far more territory.

If, of course, it works.

That will depend primarily on the pace of development of what Isaac calls an "exotic" material. It's called an "ionic polymer-metal composite," and while that may be a new term to most of us, it's one we're likely to hear often in the years ahead.

The material is ideal for this purpose in that it can be made to deform when an electric current passes through it, and then return to its original shape as soon as the current is turned off. One positive and one negative electrode, placed on opposite sides of the material, cause charged particles, called ions, to move inside the material, because they are attracted to one of the electrodes. The movement of the ions, along with water molecules that also move, causes the material to deform.

"So by controlling the applied voltage of just a few volts, you will be able to control the motion itself," Isaac says. In effect, by switching the current on and off, the wings of the aircraft can be made to flap, much like a bird.

The current will come from highly flexible solar panels — another cutting-edge technology — that form the outer skin of the wings. Many mechanical parts, like motors and hydraulics, are eliminated, thus reducing the chance of failure.

Smooth Sailing Required

So theoretically, it could stay up forever, either guided to its destination by ground controllers, or using onboard sensors to operate independently. Of course, Isaacs isn't counting on it staying up forever.

"If everything works as expected, it could keep on going, but we know that things take place, like weather, and that can be a big factor," he says.

On Earth, for example, the craft would have to stay out of the jet stream, where powerful winds would rip it to pieces. And on Mars, there's those sand storms. But at least clouds won't be a problem. It should operate comfortably at 30,000 feet, or more, staying mostly above the clouds where it can capture those solar rays.

It won't be able to carry the kitchen sink, because its very nature will keep it fairly small, probably less than 15 to 20 feet in wingspan.

"The basic idea of flapping wings obviously limits the size," Isaac says. A large craft would not be able to handle the considerable stresses and strains caused by flapping wings.

"It won't be feasible to have a large structure with flapping wings," he says. "That limits the size of the payload, so it won't be for a manned vehicle."


Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.