3. Question Three [20 marks total] (a) A wing box cross-section is shown in Figure 3a. This closed thin-walled tube provides the torsional stiffness of an aeroplane wing along its long axis. The front (left hand) edge of the wing is a semi-circular section of inner radius 0.2 m. The remainder has inner width 1.5 m and inner height 0.2 m at the right hand end. The trailing edge of the wing (which would be to the right) is not included as this part contains the moveable ailerons which do not contribute to the stiffness significantly. All walls are 4 mm thick, except the vertical right hand end, which is 10 mm thick. The wing is assumed not to taper along its length of 7 m. It is manufactured from aluminium with a shear modulus of 30 GPa. Determine: A1 14mm 0.2 m 10 mm 0.2 m A2 torsional stiffness = GJT 1.5 m For Closed section 4A JT $ds (b) A 0.6 m section of the lower wing box panel is removed to accommodate a retractable under- carriage, as shown in Figure 3b. (i) Determine the torsional stiffness for this open cross-section. [6 marks] For Open section JT - bt 0.6 m