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Drone Pendulum Fallacy

What is static stability?

Hanging a pendulum below a pivot is an example of naturally stable system. When the pendulum is moved from it's original position, there is a restoring torque that brings it back below the pivot.

The opposite to this is an inverted pendulum, which is an unstable system. When the pendulum is disturbed by an external force, it will accelerate away from it's original position

A drone is neither stable or unstable, it is neutral. When rotated to a new angle, a drone won't to move back towards or away from it's original angle without the aid of a flight controller. This is due to all forces being balanced about the natural pivot point. For example, hold a stick at it's balance point, then rotate the stick to a different angle, the stick will remain at the new angle. This is exactly how a drone behaves in the air.

A pendulum requires an offset between the pivot point and the centre of mass, however, objects in mid air naturally pivot around their centre of mass. Therefore it isn't possible to experience a pendulum effect. This is the misconception that leads to the drone pendulum fallacy.

It's often assumed that a drone rotates about it's centre of thrust and a low centre of mass will hang below it, like a pendulum. But in fact, the drone will rotate around it's centre of mass. Therefore a low centre of mass only causes it to rotate about a lower point. There is no torque produced due to the centre of mass and pivot point being in the same position, therefore no rotation.

Do aerodynamics play a role in the pendulum fallacy?

No. The rocket pendulum fallacy is a common misunderstanding that rockets are more stable if the thrust is produced above the centre of mass. But due to rockets having fins and travelling at high speeds, it's often misunderstood that this doesn't apply to drones. However, the rocket pendulum fallacy and the drone pendulum fallacy are the same concept.

The drone pendulum fallacy assumes that aerodynamic forces are negligible due to the hovering in stationary air. However, it is possible to experience a pendulum effect using aerodynamic drag.

As a drone increases in speed, other external forces can cause the drone to rotate unexpectedly. In the example shown above, the drone will experience a 'pendulum like' effect due to the imbalance of drag about the centre of mass.

FPV drones

The centre of mass on an FPV (First Person View) drone can have an affect on how it handles. The single largest mass object on most FPV drones is the battery, therefore it can have a large effect on the centre of mass position. This will shift the pivot point and also change the rotational inertia of the drone.

Shifting the pivot point either side of the drone camera can cause minor changes in how the drone feels to a pilot during certain flight maneuvers.

Depending on the mass of the motors and height of the frame, the rotational inertia will change with different battery positions. This is most likely the largest cause for the drone to handle different.

Even if the drone pendulum effect existed, it most likely wouldn't be noticeable with today's modern drones. The compensation rate of flight controllers and high torque brushless motors make it possible for drones to correct for every minor disturbance. Fortunately it's just a fallacy.