Sun's Out, Drones Out

Shrinking drones down to tiny proportions allows them to go where no other drone can go. Called micro aerial vehicles (MAVs), these diminutive drones are useful for a number of important applications, such as inspecting the insides of pipes and fuel tanks, or running reconnaissance operations where stealth is a key consideration. But today’s MAV technology is quite limited with respect to range and flight time. When you shrink an aerial vehicle down to the size of an insect or bird, engineering challenges abound.

Traditional MAVs rely on DC motors, which are driven by electromagnets, for propulsion. Since the efficiency of these motors rapidly decreases with shrinking sizes, and lightweight aerial vehicles can only carry very small amounts of energy to power them, they cannot stay airborne for long. Typical flight times for a MAV may be just a few minutes. This makes them completely impractical for long-distance operations.

Researchers have recently developed a new type of MAV, called CoulombFly, that can stay aloft indefinitely — well, as long as the sun is shining, anyway. As you have probably guessed, CoulombFly is powered by solar energy. However, that alone would not be enough to keep the drone in the air. Due to efficiency issues with tiny DC motors, the team also needed to develop a novel propulsion system to make the most of the energy captured by the limited surface area of the vehicle.

The vehicle weighs in at 4.21 grams and has a wingspan of 20 centimeters, making it about 10 times smaller and 600 times lighter than existing sunlight-powered aircraft. The frame of CoulombFly is made of lightweight wood that contains the control system and solar cells. Positioned above the lower airframe is a pair of rings that make up the electrostatic motor. An inner ring is composed of 64 sheets of carbon fiber coated with aluminum foil. The outer ring has eight pairs of positive and negative electrode plates, alternating in polarity. These electrode plates also have aluminum brushes that make contact with the inner ring’s sheets as they spin.

Electrical energy from the solar cells charges the electrodes of the outer ring, causing them to generate an electric field. This exerts a force on the sheets of the inner ring, causing them to spin. A rotor is attached to this inner ring, which gives lift to the vehicle. The motor was measured as producing 5.8 grams of lift, which leaves 1.59 grams of available payload capacity. That is not much, but it is sufficient for many modern sensors or cameras.

In a laboratory setting, CoulombFly was shown to be capable of sustained flight for a period of an hour with no degradation of performance. In theory, this MAV could stay aloft indefinitely as long as the sun is shining. Of course it is not always shining, however, but the team believes there is just enough payload capacity available to include a small rechargeable battery that could keep the vehicle in flight, potentially even for 24 hours.

Looking ahead, the researchers plan to experiment with using their technology in both fixed-wing vehicles and quadcopters. Doing so could enable the vehicles to carry far more weight, which would make them useful for many more applications.