Dynamic stall and twist instability


I’ve modified FAST to account for swept blades with elastic twist, which also includes an additional DOF for which I use the first torsion mode. For a particular model, I have found some unstable behavior in twist that goes away if I turn off dynamic stall, or if I use dynamic stall parameters from a different airfoil. With a “working” airfoil, the instability arises if either the stall angle is modified (along with extrapolated CN) or if the CN slope is modified. I am not well versed in dynamic stall theory, so I was wondering if you might understand what could be going on in this situation.


Well, I have not encountered this instability again but I revisited it out of curiosity. After reading the AeroDyn theory document (Moriarty and Hansen, 2005) I noticed the section in Dynamic Stall on the pitching moment modification. So I set the CM of the offending airfoil to zero and the model ran fine without the instability.