Lift coefficients in BAR airfoil files

[bjonkman]

While debugging some differences in a new method of calculating default unsteady airfoil coefficients, I noticed some unusual characteristics in the lift coefficients in several of the aerodyn/BAR_Baseline files (1-6).

./modules/aerodyn/BAR_Baseline/Airfoils/BAR0_AeroDyn15_Polar_01.dat ./modules/aerodyn/BAR_Baseline/Airfoils/BAR0_AeroDyn15_Polar_02.dat ./modules/aerodyn/BAR_Baseline/Airfoils/BAR0_AeroDyn15_Polar_03.dat ./modules/aerodyn/BAR_Baseline/Airfoils/BAR0_AeroDyn15_Polar_04.dat ./modules/aerodyn/BAR_Baseline/Airfoils/BAR0_AeroDyn15_Polar_05.dat ./modules/aerodyn/BAR_Baseline/Airfoils/BAR0_AeroDyn15_Polar_06.dat

I am not sure what those local minima just below alpha=0 are:

Other aerodyn/BAR_Baseline airfoils look more normal. For example

./modules/aerodyn/BAR_Baseline/Airfoils/BAR0_AeroDyn15_Polar_07.dat ./modules/aerodyn/BAR_Baseline/Airfoils/BAR0_AeroDyn15_Polar_08.dat ./modules/aerodyn/BAR_Baseline/Airfoils/BAR0_AeroDyn15_Polar_09.dat

Here are values for 7-9:

If those local minima are expected, we will have to look into some different methods for computing values for c_lalpha and c_nalpha. Also, I'm not sure how the UA models will handle it.

Let me know what you think.