Dear Pascal,
Warnings regarding a “small angle approximation violation,” warnings regarding “supersonic blades,” a simulation crash, or very large deflections that occur in the time series near the start of a simulation are good signs of a numerical instability. Without knowing more about your simulation settings, it’s hard to know that what the problem is.
If you believe your structural mass/stiffness properties are all set properly, my guess is that the integrator time steps are not set properly, which is what is leading to the numerical instability. See my post dated Nov 4, 2010 in the forum topic found here for rules of thumb on how to choose proper time steps: Error in FAST, working in ADAMS.
Best regards,