We want glider flight models that:
X-Plane looks like it has quite detailed control over how the flight model behaves although this involves complex manipulation of a hidden 3d model. But with effort it seems possible to get good flying characteristics.
An example of x-plane glider flight model improvement is lordauriel's work on the ASK21 Realistic Flight Model . There is an interesting forum thread with comment as the flight model was improved here
XPlane glider airfoil design
XPlane aircraft parameters affecting the flight model
... more coming soon...