The paper presents the experimental unbalance response of two slightly different rigid rotors supported by two identical aerodynamic foil bearings. Impulse (Pelton) turbines manufactured directly on the outer surface entrain both rotors at rotation speeds comprised between 50 krpm and 100 krpm. The displacements in the two foil bearings are measured during coast down and are depicted as waterfall plots. They show typical non-linear behavior, i.e. subsynchronous vibrations accompanying the synchronous component. The measurements show clearly that the subsynchronous components bifurcate at typical rotation speeds (mostly rational fractions of the rotation speed). The non-linear behavior of the rigid rotor supported on foil bearings is also underlined by varying the added unbalance: with increasing unbalance the vibration spectrum becomes gradually richer as new subsynchronous vibrations appear.
The experimental results are compared with very simplified theoretical predictions based on the assumption that the air film in the two bearings is infinitely stiff compared to the foil structure. This latter is characterized by a cubic stiffness and a structural damping coefficient. The comparisons show only a rough qualitative agreement and enlighten the actual limits of foil bearings theoretical models.