The armature can be on either the rotor or the stator. The distance between the rotor and stator is called the air gap. There are no contact surfaces between the rotor and stator, and no working fluid. Disc brakes consist of a rotor and caliper at each wheel. It consists of a rotating cylinder divided into multiple metal contact segments on the rotor. Mechanical unbalance of the rotor will generate vibration. Wind is another source of rotor blade loading. B.G. Lamme later developed a rotating bar winding rotor. The rotor consists of a mast, hub and rotor blades. BIM involved pressurizing the interior of the rotor blades with nitrogen. Yaw control develops through opposing cyclic pitch in each rotor. Solidity is measured by blade area divided by the rotor area. In a rotor with blades moving say towards right. The Mi-26 was the first factory-equipped helicopter with a single, eight-blade main lift rotor. This greatly eases the problem of starting the massive rotor of a large synchronous motor.