LiftDrag plugin moment defined incorrectly

[treforevans]

In the lift_drag plugin, the moment magnitude is presently defined as:
moment = cm * q * area * spanwiseI
where spanwiseI is a unit vector to determine the direction of the moment magnitude. This computation is incorrect as the moment is in units of N instead of Nm as expected (assuming SI units).

The moment coefficient requires a reference length in addition to a reference area. Commonly, this is chosen as the chord of the airfoil so the moment would be (in pseudo-code):
moment = cm * q * area * chord * spanwiseI

[arjo129]

Thanks for spotting this. This would break the behaviour of existing simulations, in addition we would need to take in a chord parameter. We don't currrently have a way to calculate the chord from a mesh. If you have suggestions as to how to address these issues I'd be open to helping you review a PR.

[treforevans]

Perhaps a default chord parameter could be set to 1 so that the behaviour of existing simulations would not change (since the parameter is unspecified).