The enforcement of zero eigenvalue for rigid body modes in the state space matrix assembled by the ASE module is now only activated if an
RBRED bulk data card is specified. This enforcement process was activated in the earlier versions of ZAERO if either the SUPORT entry in the
“ASSIGN FEM=” Executive Control Command or an
RBRED bulk data card was specified and ensures that the number of zero eigenvalues of the state space matrix is equal to the number of rigid body modes. Otherwise, a small but unstable eigenvalue may appear due to numerical truncation error resulting from the Rational Function Approximation.
RBRED affects the following modules: ASE, GLOADS, ELOADS, MLOADS, and NLFLTR. For those modules that involve rigid body modes, if an input deck contains a SUPORT entry but without
RBRED, may give different results between version 7.2 and earlier versions of ZAERO. To get the older results back, you must add an
RBRED card.
Due to this change, the bulk data card description of the RBRED has been changed.
The SUPORT entry in the “ASSIGN FEM=” Executive Control Command now only cleans-up the mode shape of the rigid body modes. Please see Remark 10 of the “ASSIGN FEM=” Executive Control Command for details on the SUPORT entry.