Dear Matthias,
To add to what Marshall said, FAST uses a 4th order Adams-Bashforth-Adams-Moulton predictor-corrector fixed-step integrator. This integrator is initialized over the first few steps with a 4th order Runga-Kutta integrator. Marshall gave a good rule-of-thumb for choosing the aerodynamic time step. When choosing the structural step, a good rule-of-thumb is that the time step (input DT) should be set less than or equal to one over ten times the highest full system natural frequency. That is, in equation form:
DT <= 1 / ( 10 * <highest full system natural frequency in Hz )
The full system natural frequencies can be found by running a FAST linearization analysis (AnalMode = 2) about the initial conditions. See the Linearization chapter of the FAST User’s Guide for more information.
There are several reaons why one could see long periods of time with no change in output:
*The solution could be in equilibrium.
*A controller could be defining output discretely.
*The response could be changing with smaller precision than the precision used for output (FAST input parameter OutFmt).
I hope that helps.
Best regards,