The interesting article by Archer, Sapuppo and Betteridge leads to the problem of determining the optimum lift distribution along the wing during flapping motion. Adopting the assumptions made in Ref. 1, the problem may be reduced to one of minimising the induced drag for a specified, periodically varying, bending moment at the wing root. The wing-root bending moment should be of primary importance to a bird or, for that matter, to a mechanical flapping-machine, since it is at this point that most of the flapping energy is introduced.

A modification sometimes used to increase the effectiveness of a control surface is thickening of the trailing-edge by attaching small spanwise strips, normal to the upper and lower surface of the control, along the trailing-edge (see Fig. 1). While the effect of such a modification is fairly well known for steady flow, little evidence is available of the effect in unsteady flow. There are no theoretical methods of predicting the unsteady aerodynamic forces acting on an oscillating control surface which has trailing-edge strips (T-strips) attached and so flutter calculations are based on classical thin-liftingsurface theories. This practice is very dubious and two instances where flutter has occurred but has not been predicted by use of the classical unsteady aerodynamic forces, are described in Refs. 2 and 3.

The drag of slant-based three-dimensional bodies such as the upswept rear fuselage of some aircraft (see for instance McCluney and Marshall and Peake) and car shapes (Morel) can be high and it is generally recognised that this high drag is associated with powerful longitudinal vortices in the flow direction. Morel has shown that the formation of these vortices, and therefore the drag, is a function of the slant angle of the base. His experiments were conducted with the body at zero incidence only and it appears that the effect of body incidence on the drag is unrecorded, although Bearman mentions that it has a significant effect.