Weighing just 4 grams, the drone is the smallest solar-powered aircraft ever to fly, thanks to special electrostatic motors that generate extremely high voltages and tiny solar panels. Though the hummingbird-sized prototype only lasted an hour, developers say the approach could lead to insect-sized drones that can remain airborne indefinitely.

Small drones are an attractive solution to a variety of problems in communications, espionage and search and rescue, but they suffer from short battery life, while solar-powered drones struggle to generate enough power to be self-sustaining.

When solar-powered drones are made smaller, the solar panels become smaller and the amount of available energy decreases. Minjin Chee Researchers from China's Beihang University say the efficiency of electric motors also declines as more energy is lost as heat.

To avoid this decay cycle, Qi and his colleagues developed a simple circuit that boosts the voltage generated by solar panels to between 6,000 and 9,000 volts. They powered the 10-centimeter rotors using an electrostatic propulsion system, rather than using electromagnetic motors like those used in electric cars, quadcopters, and a variety of robots.

The motor works by alternately attracting and repelling charged parts arranged in a ring, generating torque to spin a single rotor blade like a helicopter. The lightweight parts are made from ultra-thin carbon fiber covered with very delicate aluminum foil. The high voltage requirement is actually an advantage, as the current is reduced, resulting in very little heat loss.

“T“The motor generates very little heat because the operating current is very low for the same power output. The motor's high efficiency and low power consumption allow the vehicle to be powered by very small solar panels,” Qi said. “For the first time, we have successfully flown a micro air vehicle using natural light; previously, this was only achievable with very large ultralight aircraft.”

The machine, which the researchers call the “CoulombFly,” weighs just 4.21 grams and could fly for an hour before it failed. Qi says these weaknesses can be eliminated by design, and future versions could fly essentially indefinitely, using solar panels during the day and powering themselves from radio signals like 4G or Wi-Fi at night.

CoulombFly has a payload capacity of 1.59 grams, allowing it to carry small sensors, computers, and cameras, but with improved designs, the researchers believe this can be increased to 4 grams, and the fixed-wing version could carry up to 30 grams. An even smaller version of CoulombFly, with rotors less than 1 centimeter in diameter, is also in development.