Ultralight Aircraft (UL's) for reasons of noise reduction usually equipped with a modern three-blade propeller, which is operated at a relatively low speed. The drive of the propeller is frequently done by a V-ribbed belt. Figure 1 shows a part of the drive unit of a UL. The illustrated rotor with the total mass m is composed of the three-bladed propeller, the propeller shaft and a pulley. The rotor is mounted on two ball bearings (bearing A, B stock). When the angular velocity £ 1 due to unbalance the bearing forces FA and Fb appear in the bearings A and B (Fig. 2).