5.3. Describe Prandtl's classic lifting-line theory. 5.4. An aircraft wing may be represented by a single horseshoe vortex of span 20m and circulation strength 25ms-1. Calculate the downwash angle of a point 15m downstream of the bound vortex, in the plane of the vortex system on its centreline. The flight speed is 104 ms. (Answer: 0.4830).