If you’ve ever been to a circus, you may have seen a performer drive a motorcycle across a tight rope. The stunt is designed to look like a feat of skill, but in reality it’s a simple demonstration of physics. A second performer rides on a platform hanging from the motorcycle, and below the rope. The weight of that platform and the performer act as a pendulum to keep the motorcycle upright and perfectly stable as it moves across the tight rope. A somewhat common theory among hobbyists is that this same principle can be used to keep a drone stable during flight, and Tom Stanton has tested that theory in his newest video.
In the first part of the video, Stanton sets up a practical test to see if a pendulum can keep a drone stable. To do that, he used a standard quadrotor drone with all of the normal gyroscope-based stability controls disabled. All the flight controller can do is provide equal thrust to all of the rotors. Stanton mounted that drone at the end of a wood rod with the batteries mounted to the other end to act as the weight of the pendulum. He then rested the rod on a post at a slight angle and gave the drone thrust to see if it would naturally pivot to an upright position as a result of the pendulum.
As the video demonstrates, it doesn’t. The rest of the video is an explanation of why the pendulum doesn’t stabilize the drone. That comes down to simple physics. Because the rotors are fixed and provide thrust in a single direction, the pendulum actually causes the drone to become less stable. To illustrate that, Stanton enabled the normal stability controls to show that they have to fight to overcome the effect from the pendulum.
In the motorcycle tight rope example, the pivot point of the pendulum is right where the wheels touch the rope. The pendulum swings perpendicular to the direction of travel, so the pivot point doesn’t change. On the other hand, the pivot point of the drone’s pendulum moves with it, which negates the correction torque. At the end of the day that means that pendulums do not make a drone more stable, and in fact do the opposite. To keep a drone as stable as possible, you want as much of the weight as possible to be very close to its center of mass.