These quadcopters are topping out at more than 50 kilometers per hour while flying in a circle with a radius of just 1.7 meters. The tether is exerting over 13 gs of centripetal force to keep them from flying off into oblivion. Despite the speed, there seems to be no problem getting four quadcopters to do a bunch of coordinated acrobatics, which is fun (if a bit dizzying) to watch. I had to keep reminding myself that the vid wasn’t sped up.
The reason to do this kind of thing from a research perspective is that it’s the only way (that I can think of) to fly a quadcopter really really fast, or to subject it to a bunch of gravities, in a confined, controlled space. At ETH Zurich, the ability to do this is helping them get familiar with how quadcopters handle at very high speeds, how big of an issue drag is, the efficiency of different propeller designs, and stuff like that. Also, it gives them a chance to practice controlled maneuvering at high speed, including the emergency braking that’s featured in the vid.