Another benefit of high aspect ratio

Another way of increasing the proportion of laminar boundary layer on a wing of given area, is to reduce the chord of the section, while increasing the wing span: in other words, by increasing the aspect ratio. Thus, high-aspect-ratio wings can be beneficial in reducing both trailing vortex and surface-friction drag.

Artificially induced laminar flow

In order to preserve a low-drag laminar boundary layer over an even larger proportion of the surface of an aircraft, the engines can be used to provide suction to remove the boundary layer through slots, as described in the previ­ous chapter, or through a porous skin. Several research aircraft have been flown with experimental porous or slotted surfaces. A good description of early postwar experiments is given by Lachmann (1961). Although very low drag values were often obtained, it was discovered that there were consider­able practical difficulties, particularly in keeping the holes free of debris and suicidal insects. A boundary layer suction system would increase the cost, complexity and weight of the aircraft. The engine performance, and the air­craft handling properties may also be adversely affected. Thus far, there has been no widespread application of suction-induced laminar flow in production aircraft.

For many years, the main concession to the idea of using engine suction in this way, has been the occasional use of a pusher propeller situated at the rear of the fuselage or engine nacelle, as in the Beech Starship shown in Fig. 4.10. The rear-mounted propellers ensure that there is a favourable

Fig. 4.10 Vertical surfaces on the wing tips of the canard-configuration Beech Starship combined the functions of drag-reducing winglet and fin (Photo courtesy of Beech Aircraft Corp.)

pressure gradient (air moving towards a lower pressure) over the nacelles, and a large area of the wing. This in turn delays the transition to a turbulent flow, and inhibits separation. Proponents of the aft-mounted pusher propeller claim considerable reductions in drag by this method, but this may be partially offset by a deterioration in propeller efficiency. A more significant advantage of this arrangement is the reduction in cabin noise.