Thanks! The airbrake works pretty well. Slows down cruise slightly which I can adjust. Just requires some thinking about parameters to prevent each change from having unintended consequences on something else.
W.r.t. props, I tested lowering the governor minimum pitch to 0 which made the plane sink like a stone at idle albeit with the engine maintaining cruise RPM. So the negative alpha of the prop blades w.r.t incoming air in this case is properly working to slow the plane down rapidly. At least thats what i'm assuming is happening. I also tried increasing the lift coefficient of the blades thinking this would also increase negative lift in this negative alpha scenario. That also seemed to work. But, it would be useful if the lift coefficients for airfoils had optional separate values for positive and negative alpha. What I did also was influencing the lift in positive alpha cases.
My sense is part of the issue is related to what you mentioned -- the RPM is staying too high even at relatively low manifold pressure settings. I.e., the engine in real life has more of a braking effect than what is currently being modeled. I am going to see what RPM the actual plane drops to at 11 inches MP and 110 mph -- the conditions from abeam the numbers and base leg.
2. Related to my second question, I found that lowering the tailwheel spring constant to ~200 makes the ground handing fairly realistic at ~30-45mph, a little too squirrely below that, and a little too stable above that. So I thought it would be useful to have the spring constant a function of speed.