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I'm using D DATCOM to estimate cm-alpha curve for a small tail planform with NACA-0012 airfoil using the namelist ($HTPLNF).

When taking the Tail planform aerodynamic center, ac (about 25% of MAC), as the reference point, the pitching moment coefficient CM should be at almost a constant value (0, for the airfoil according to reference from Abbott, page attached) for a range of angles of attack.

But the result from DATCOM is a line with a negative slope as shown in the attached picture. why?!

When I define the same planform geometry and airfoil as a Wing Alone configuration using ($WGPLNF), the result is almost constant, which is reasonable.

I have checked the user manual, but couldn't find a clarification.

Does the program define ($HTPLNF) differently? or is there any assumptions when defining a tail?

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