If there are enough clear summer days in a row, Bye Aerospace's Sun Flyer may just live up to its name and charge in sunlight. Bye just sold twenty of their distinctive training airplanes to the Spartan College of Aeronautics and Technology. If the Sun Flyers take off, they could signal a future of more sustainable flying.

That said, even under the best circumstances, it takes a lot of time for the sun to recharge the batteries of a Sun Flyer. Bye Aerospace CEO George Bye tells Popular Science:

Solar panel efficiency of course varies with the time of day, the weather, the time of year, so there's several factors impacting how much the solar power contribution might be to the plane's batteries. If you're flying the airplane slowly, the amount of energy required to maintain flight, combined together with the best solar combination of clear, sunny skies and summertime, the solar energy contribution can be fairly significant, up to 20 percent of the energy required for flight, even a little bit more. If it's cloud and a winter time flight, the solar energy contribution diminishes.