28 April 2004

Solar-Powered Robot Bird Set For Exploration Duties

by Kate Melville

Thanks to a grant from NASA, University of Missouri-Rolla researchers are developing the world's first flapping-wing, solar-powered unmanned aircraft.

Using thin-film solar arrays and an ionic polymer-metal composite material that can deform in an electric field like an artificial muscle and return to its original shape when the field is removed, the vehicle would be able to flap its wings without using conventional mechanical parts. Designed to complement other types of exploration vehicles, the solid-state aircraft could provide high-resolution data on a larger scale than the Mars rovers.

With a flight profile similar to that of an eagle, the proposed solid state aircraft could soar for long periods, flapping its wings to regain altitude. By using a renewable power source, the vehicle would be capable of long-duration flights on Earth, Mars, or Venus.

"During the day, the solar cells will collect solar energy," says Dr. K.M. Isaac, professor of mechanical and aerospace engineering at UMR. "Instead of storing the energy in a battery, like in the UMR solar car, we plan to store it as potential energy. When it has a lot of energy, it will start flapping its wings like a big bird. By doing that, it will go up in altitude. Then in the night, for example, when there is no solar power available, it will start gliding just like a bird until the sun comes up again."

Isaac said the aircraft would be much lighter than more conventional craft because most of the surplus energy collected during the day would be used to gain altitude. "In addition, the proposed vehicle has advantage over other more conventional craft because the wing performs aerodynamics, controls and solar energy collection tasks."

Because the vehicle would fly at altitudes of 30,000 to 40,000 feet, cloudy skies wouldn't be a problem, Isaac says, but stormy weather could spell trouble. "Wind shears are pretty high at those altitudes," he added.

Exploration of planets would be limited to the three planets because the solar intensity would be too low to power the vehicle beyond Mars. The planet also needs to have an atmosphere for the vehicle to operate. Solar flight on the outer planets, such as Saturn or Jupiter, or moons just isn't feasible, Isaac says.

For missions on the Earth, the aircraft would be taken up in a hot air balloon and released. "The controls would have to be either remotely controlled or autonomous, meaning that it would by itself decide what kind of flight to take," Isaac says. "Missions could include taking photographs of remote locations or looking for minerals as well as reconnaissance."

The aircraft's flexibility and light weight would also make it ideal for space exploration. "To explore Mars and Venus, it will have to be launched from an entry module," Isaac adds.

Jessica Rolwes, a graduate student, and two other UMR students are working with Isaac to analyze the aerodynamics of the proposed vehicle. Inside UMR's Fluid Dynamics and Combustion Laboratories, they have placed a motorized wing inside a 10-gallon tank of saltwater. The wing moves back and forth and electrodes mounted on the wing react with the saltwater to produce hydrogen bubbles. Illuminated by two powerful pulse lasers, the bubbles are photographed twice within nanoseconds to capture their movement. A software program then analyzes the photos to determine the velocity.

"What we're trying to do is image the flow around a wing," Rolwes says.

The information they receive during these tests will help them in their next step of building models, Isaac says. "We plan to test some models in the wind tunnel to prove that the material will be able to flap at whatever frequency we desire," Isaac says. "We will also test to make sure it will clear sufficient lift, for example, to sustain the flight."