The vast majority of drones have four rotors, because that’s an inherently stable configuration. In the absence of any kind of external force, the counter-rotation of adjacent propellers moving at the same speed will keep the drone stable. Small tweaks to those speeds can make the drone change direction on a dime. But that configuration is also inefficient and consumes a lot of power, which ultimately limits battery life. But a new drone design from Flybotix, a startup originating from École polytechnique fédérale de Lausanne (EPFL) in Switzerland, flies with just two propellers to double the battery life.
In the EPFL press release, they state that this drone design was inspired by the way helicopters fly. But that’s not the most accurate description of how this drone is able to remain stable with just two propellers. Most helicopters use their primary rotor for lift and the tail rotor to control spin. The axis of rotation of those rotors is perpendicular. The Flybotix drone has a completely different configuration with two matching propellers stacked on top of each other along the same axis. The propellers turn in opposite directions to keep the drone from spinning. Theoretically, if no wind is present and the drone is perfectly balanced, that setup would remain stable as it moves up and down — but it wouldn’t be able to move in any other direction.
To make the design maneuverable and stable in real-world conditions, Flybotix developed a unique control system. Like other drones, accelerometers and gyroscopes are used to monitor the drone’s roll, pitch, and yaw. But, in addition, the exact speed and tilt of the rotors is also constantly measured. If an air current or collision pushes the drone, that’s immediately detected and compensated for to keep the drone stable. To maneuver the drone, the speed and tilt of the propellers is carefully adjusted so it responds like a conventional quad-rotor drone.
It may not be immediately obvious, but this dual-rotor setup is much more efficient than a quad-rotor drone of the same size. That isn’t just because of the number of motors, but because the propellers themselves can be larger without adding to the drone’s overall size. Larger propellers are more efficient, which means the Flybotix drone can fly twice as long as a conventional quad-rotor. The drone is being marketed for professional use, and would be perfect for lengthy inspections in tight areas.