A single$-$cell thin$-$wall airfoil cross section that is approximated $($from left$)$ by a half ellipse for the leading edge, followed by upper and lower horizontal skins, and a linear taper region for the trailing edge, where a$=4$ inch and t$=0\mathrm{.}11$ inch. If the applied torque T $=3$ kip$-$ft$.$ The length of the airfoil section is $10$ feet. The airfoil is made of Aluminum $60-64.$ Find the angle of twist by hand calculations.