From the full official documentation of US Patent 8944764

The invention claimed is:

1. A main rotor arrangement for an UAV-helicopter, the main rotor arrangement comprising: a rotor mast defining a vertical axis of rotation, a hub for attachment of rotor blades, wherein the hub is mounted to the rotor mast in a tiltable way so as to reduce loads acting on the rotor blades during operation of the UAV-helicopter, a damping assembly comprising a hollow receptacle being attached to a lower side of the hub and concentrically about the rotor mast, wherein the receptacle comprises a detachable damper element adapted to act as a damping support between the hub and the rotor mast for damping said tilting of the hub, wherein the receptacle comprises a lower opening comprising internal threads and a disc comprising external threads for engagement with the internal threads, whereby the disc can be moved into or out from the receptacle by the threads so as to compress or expand the damper element contained within the receptacle to make the damping performance of the damping assembly adjustable. 

2. The main rotor arrangement according to claim 1, wherein the hub is hollow and concentrically arranged about the rotor mast so as to form a circumferential gap between the hub and the rotor mast. 

3. The main rotor arrangement according to claim 1, wherein the damper element is ring-shaped and arranged concentrically about the rotor mast. 

4. A UAV-helicopter, comprising: a main rotor arrangement according to claim 1.