Boundary Features Rotates Incorrectly

[BHill96]

When a boundary feature is centered on a point other than (0,0), it follows a circular path around its center instead of rotating in place.

[markstock]

This is a known issue, and there is a workaround. When creating a boundary feature that "moves according to formula", there are two positional inputs: "center" pair and "x position", "y position" formula fields. This obviously confusing. Consider the formula inputs (for position and rotation) to define the movement of a virtual marker. And consider the "center" positional input to be the position of the new boundary relative to that marker.

Eventually, we will need to support these markers as first-class objects, creating them individually, drawing them, allowing them to attach to other markers, while allowing all kinds of features attach to them.

[markstock]

As an example, if you wanted to dynamically pitch an airfoil about its quarter-chord point, you would generate an airfoil, set the "Leading edge position" to (-0.25, 0.0), and set the "angular position" to sin(t).