Solar airplanes don't have much on board. They're ridiculously flat and thin, inviting the wind to lift them instead of knocking them around. The body is strong and light, often made of carbon-fiber pipes for the frame, with a strong fabric like Kevlar stretched across it. Somewhere in the structure, you'll see an "X" or "V," which prevents the plane from rolling.
Most planes run on batteries at night, although some have used fuel cells. The batteries are light and energetic and are usually arranged in a sheet. QinetiQ's solar plane "Zephyr," which holds the current endurance record, runs on a sheet of lithium-sulfur batteries [source: Bush]. The batteries are wired to motors that turn propellers.
You can't miss the solar panels, which are the skin and heart of the plane. They are unlike the rigid, bulky solar panels on satellites or a solar house. These panels are millimeters thick, are flexible enough to roll, and are incredibly efficient and expensive [source: Bush, USO]. The solar panels are also wired to the propellers.
On board, the plane will carry light, voltage and wind sensors, and it will have a method for relaying that information to the pilot.
If you're wondering where the wheels are -- there's no need to bother. "Some solar airplanes basically drop off the landing gear in flight because you're not going to need them. The plane may land on skids or crash-land. Engineers are getting rid of every bit of weight you can possibly imagine," says John Del Frate, an engineer at NASA's Dryden Flight Research Center.
Some solar airplanes are true UAVs, or unmanned aerial vehicles. Except for takeoff and landing, an autopilot flies the plane. Pilots use onboard systems to track the plane and control its motors from the ground. Unmanned planes include NASA's deceased Helios, the Zephyr, and Aurora Flight Sciences' Odysseus and SunLight Eagle.
Other solar planes can support a pilot. Examples of piloted solar aircraft are NASA and AeroVironment's retired Gossamer Penguin and Solar Challenger, and a different group's Solar Impulse, which aims to circumnavigate the globe.