Lift Coefficient Problems
 

I'm running Fluent on a simple UAV design and I'm trying to get a lift coefficient curve. I have a set of design points that run from an angle of attack of 0 to 25 degrees. I get a max lift coefficient around 16 degrees. It goes down at 18 degrees. Then goes back up when it gets to 20 degrees! What is going on?

I've tried both steady state and transient with no real difference.

Running steady state solver is out of question at such a high angle of attack because there is lot of flow separation at these angle of attacks but the problem you are telling looks quite strange. Give it a try by changing your time step and then let me know what happens.
Regards

It turned out to be as simple as a mesh problem in that the mesh wasn't fine enough around the UAV. The wind tunnel was so large that even when the cells looked small, it wasn't enough. I have fixed it now, and I'm getting a beautiful coefficient curve even with steady state. (I'm not going too far past the stall angle) Thank you!