Rybinsk State Aviation Technical University named after P. A. Solovyov (RSATU) has patented a utility model of a quadcopter with a balancing ball. Information about this is contained in the patent published by the Federal Service for Intellectual Property.
A circular groove with a spherical cross-section is made on the lower surface of the quadcopter body. A balancing ball moves along it. When the device tilts, the ball moves as a counterweight in the direction opposite to the one the quadcopter is tilting.
The stated goal is to increase aerodynamic stability under wind influence. At the same time, the mass of the device increases insignificantly — compared, for example, to a quadcopter with four pairs of coaxial propellers.