Question: The geometric and aerodynamic data for a wing of a large white butterfly is as follows: Flight speed, U∞ = 1.35 m/s; average chord, c = 25 mm; average span, s = 50 mm; air density = 1.2 kg/m3; air viscosity, µ = 18 × 10-6 Pa/s; drag at zero lift = 120µN (measured on a miniature wind-tunnel balance). Estimate the boundary-layer thickness at the trailing edge. Also compare the measured drag with the estimated skin-friction drag. How do you account for any difference in value?