Induced drag and wake kinetic energy

This 2007 Lanchester Lecture covered a theoretical result, thought to be new and in print in the Journal of Fluid Mechanics, and some lines of thought on the interpretation of induced drag and recurring paradoxes which interfere with teaching ‘how wings work’. Some of these were presented in the author's 1998 Annual Review. The difficulties are not confined to drag, but also concern lift; they are no less severe in two dimensions than in three. The flow-field in the far wake of an aircraft is examined with the primary purpose of identifying lift and drag in a simple and unified manner, preferably an intuitive one acceptable by novices (and even by those who mind using Bernoulli's equation!).