The critical speed of the rotor can be obtained by looking at the intersection of the Campbell diagram with ω = Ω curve in the fixed frame. If the diagram is generated in the corotating frame, the intersection of the backward whirl eigenfrequency curve with ω = 2Ω curve is the backward whirl mode critical speed. Similarly, the intersection of the forward whirl eigenfrequency curve with the abscissa is the forward whirl mode critical speed. Generally, in the corotating frame, the backward whirl eigenfrequency curves increase with the rotor speed, whereas the forward whirl mode curves decrease.