Some considerations on the basic assumptions in rotordynamics

The dynamic study of rotors is usually performed under a number of assumpti ons, namely small displacements and rotations, small unbalance and constant angular velocity. The latter assumption can be substituted by a known time history of the spin speed. The present paper develops a general non-linear model which can be used to study the rotordynamic behaviour of both fixed and free rotors without resorting to the mentioned assumptions and compares the results obtained from a number of non-linear numerical simulations wit h those computed through the usual linearized approach. It is so possible t o verify that the validity of the rotordynamic models extends to situations in which fairly large unbalances and whirling motions are present and, abo ve all, it is shown that the doubts forwarded about the application of a mo del which is based on constant spin speed to the case of free rotors in whi ch the angular momentum is constant have no ground. Rotordynamic models can thus be used to study the stability in the small of spinning spacecrafts a nd the insight obtained from the study of rotors is useful to understand th eir attitude dynamics and its interactions with the vibration dynamics. (C) 1999 Academic Press.