Corrected a typo in a variable name (claStall) of the LiftDragModel

[achille-martin]

Dear UUV Sim Devs,

I was playing around with the Lift/Drag models for the fins and I realised a couple of things:

• A typo in a variable name (cdaStall instead of claStall) in the LiftDragModel.cc file
• A non-realistic model for the Drag component (two lines approximation)

Concerning the two lines model and the Drag component, it is consistent to use the same "S" shaped graph for both the Lift coefficient CL=f(alpha) and the Drag coefficient CD=f(alpha), as described http://gazebosim.org/tutorials?tut=aerodynamics&cat=physics. When I refer to "S" shaped graph, I mean the Cl vs alpha graph like presented http://gazebosim.org/tutorials?tut=aerodynamics&cat=physics.

However, it is not physically correct to have a drag vector oriented along the inertial velocity of the sub when the alpha is negative: it should always be oriented opposite to the inertial velocity (as shown http://gazebosim.org/tutorials?tut=aerodynamics&cat=physics whatever the alpha value. Therefore I have updated the code to generate a "V" shaped two lines approximation for the drag component (as shown http://airfoiltools.com/airfoil/details?airfoil=naca0015-il, CD vs alpha).

Best regards,

Achille

[achille-martin]

Will create branches to enable discussions on both commits