Principal Axis Inclination α

\tan (2 α) = \frac{2 \cdot I_{yz}}{I_{z} - I_{y}}

α	Principal axis angle
I_yz	Biaxial second moment of area
I_z	Moment of inertia about the z-axis
I_y	Moment of inertia about the y-axis

Do you have any questions?