lift and drag

[andrewning]

A few minor issues:

• The CD calculation should use the full CL corrected for all terms and not just CL0 + CL_alpha * alpha (this will be very wrong post-stall, and with things like elevator deflection). This would require calculating the full CL and CD before rotating to the X/Z c.s.
• The Oswald efficiency factor should be a user input. 0.9 is too optimistic, particularly for the small planes that are often used in this sim.
• The use of CD_.p is confusing in the current file. As used in line 251 it should be the parasitic drag coefficient (which is generally denoted CDp), however based on ordering on line 106 it appears to be the derivative of the drag coefficient w.r.t. roll rate (CD,p). The drag params also has CD_.0, which I'm assuming is the zero-lift drag coefficient (actually it appears to be the drag coefficient at zero alpha, which is not quite the same, it would be better to use the zero-lift drag coefficient and to allow for a non-centered drag polar which requires one additional parameter). It appears that CD_0 should be used in line 251 instead.
• not all of the stability/control derivatives for lift and drag are currently used. That's misleading if one can input them (though the important ones are currently used).

(Of course, all of these issues are drag related, which will have very little effect on the dynamics.)