CRESCENT SHAPED WINGS

Research into wing planforms in recent years suggests that some saving of vortex drag may be made by adopting a distinctly crescent shaped wing plan, resembling that of the swallow bird wing, or the curved fin of a shark. The planform shown in Fig 6.3e, to be seen on some modem full-scale sailplanes, represents a move part way in this direction but the full crescent wing is more extreme, with the trailing edge curving progressively more backwards towards the tip and a relatively sharp extremity. The basic distribution of chord across the span remains nearly elliptical but the ellipse is progressively sheared backwards to produce the shark fin form.

Advanced calculation methods were used in the first place, to establish a theoretical basis for this study. Wind tunnel and flight tests lend some further support to the idea. More work is being done.

A point that needs to be taken into account is that sweptback wings are more prone to tip stalling and to wing flutter (see Sections 6.9 and 13.9 here). There may well be some improvement in theoretical performance with such wings, but they are more difficult to construct and if handling difficulties appear in flight, the gain will not be realised. The author has built and flown two sailplanes with wings approximating this plan. One of them exhibited tip stalling problems and the other developed severe wing flutter at moderate airspeeds, despite corrective surgery. Whether any real saving in drag resulted remains uncertain.