How does this RC helicopter keep itself upright?
The rotor is gyro stabilized. The balance bar is the gyro. If the machine rolls right, the balance bar wants to stay in a level plane and generates a correction by influencing the rotor blades to go where the balance bar wants to be.

The Bell 2 blade teetering rotor system used on the '47 and the Huey used a much smaller version of the same thing, to provide a little bit of inherent stability to the rotor disc, without inhibiting pilot control.