On Monday, April 18, 2016 at 7:18:31 AM UTC+12, xcnick wrote:
Bruce, I did give you garbage. When the xcsoar numbers came out as a parabola I knew I screwed up big time. I guess those are the number generated when you choose discus.

Any three constraints will always give you a parabola. That's just maths. Unless they lie on a straight line. Which you could look at as a section of a veeery flat parabola.

You can of course fit a cubic or higher to those three constraints, but there are an infinite number that will fit, and you've got no basis on which to choose between them. So parabola is the best choice.


The actual numbers from Johnson show the kink some talk about so I don't expect them to fit a parabola.

Yes. The famous Discus "laminar flow drag bucket". Also present in many later designs.

I *think* (without having analysed it) that both the portion below the kink and the portion above the kink correspond very closely to parabolas -- just two different parabolas.

Does any flight optimisation software actually model that? I suspect not. Except in very strong conditions (or final glide), it's the part slower than the kink that you care about.

A bit of a bugger that the CAI software wants the 2 m/s sink speed, as that's past the kink. I reckon the thing to do would be to solve for the parabola from min sink to the kink, and then figure out where 2 m/s sink would occur if the kink didn't happen.


Is the A,B,C of see you x y and z of a quadratic?

Very probably. I don't have seeyou handy. The names don't matter. A,B,C being z,y,x would matter :-) Do the values look similar?