Reductions in drag can also be obtained by careful attention to the shape of the wing tip. This is particularly true in the case of aircraft with untapered wings. Although untapered wings are not the best shape in terms of minimising drag, they are often used on light aircraft because of their relative simplicity of construction, and their docile handling characteristics. (The inboard section tends to stall first.)
Two simple approaches; the bent and the straight-cut tip are illustrated in Fig. 4.13. Both of these tip designs are said to reduce drag by producing
Fig. 4.11 Influence of lifting fuselage on lift distribution and drag
(a) Fuselages of cylindrical cross-section produce little or no lift, so there is a gap in the lift distribution at the centre (b) By using a lifting fuselage shape, the lift distribution can be brought closer to the optimum for low induced drag
separation of the spanwise flow at the tip, resulting in a beneficial modification of the tip flow-field. It should be noted, however, that unusual tip shapes are often intended primarily to inhibit tip stall, rather than reduce drag. Upward bent tips are evident on the Aerospatiale Robin shown in Fig. 4.14.