TIPS FOR SLOW FLIGHT

Sailplanes and other models trimmed for flight at high angles of attack should gain something from a tip design which reduces the strength of the tip vortex or, if this can be done, compels it to form further out in the spanwise sense. Aerodynamically, the effective span of a wing is not determined merely by the geometric span. If the vortex forms somewhere inboard of the tip, which it nearly always, if not always, does, the wing loses efficiency in proportion to the inboard migration of the vortex. This is often represented by a ‘span efficiency’ figure and very few real wings are better than 95% efficient in this sense. Attempts to improve the wings of sailplanes and gliding models have tended to concentrate on devices to prevent the vortex forming inboard, thus seeking to increase the span efficiency as much as possible. It has not, however, been established that these methods succeed. Early full sized aircraft and many models have wing tip shapes which encourage premature formation of the wing tip vortex. If, for example, the tip is raked forward (Fig. 6.6a) or generously curved at the trailing edge (Fig. 6.6b), the vortex may be expected to form close to the point where the trailing edge curvature begins.

6.7 IMPROVING THE WING TIPS

Evidence from wind tunnel tests and flight tests on full sized sailplanes and some powered aircraft has been gathered over recent years to show that the airflow near the tip of a wing can be improved by adopting a generally upturned form.

The first support for this came from the German aerodynamicist S. Horner, whose book Aerodynamic Drug was published in 1951. A development of the Horner tip was widely adopted for full sized sailplanes, as illustrated in Fig. 6.6c. The tip is essentially square but the leading edge is curved back to meet the trailing edge approximately at right angles. On the underside the wing tapers in thickness upwards to a crisp edge, rather than a rounded form. The purpose of this is to allow the high pressure air below the wing to sweep easily to the tip where its energy may serve to carry the vortex to the extreme limit of the span. In practice the full effect does not seem to occur but the extension of the trailing edge to the extremity probably does carry the vortex slightly further out than with a rounded tip.

The Hdmer tip is easy to make, quite elegant in appearance, and practical in service.

More recently, sailplanes with distinctly up curved and back swept tips have appeared, as illustrated in Fig. 6.12. These are combined with the type of wing plan shown in Fig. 6.3e. The outermost panel of the wing, usually made detachable, has a slightly increased dihedral angle which blends to a Hdmer tip, and at the trailing edge of this tip a small winglet is added (see section 6.19). Because the winglets have the effect of changing the general lift distribution, the main wing is slightly less tapered than usual. For the Schempp Hirth Ventus 2 sailplane a reduction in the minimum rate of sink of 6% has been claimed. (This amounts to a matter of 3 cm per second or 7 inches per minute.) What is probably of more importance to the model flier is that the slight additional dihedral and the smoother flow over the tips, improves the handling of the aircraft at low speeds and gives better aileron control. Tip stalling is less likely.

Not all aerodynamicists are fully convinced of the benefits and it is always difficult to distinguish gains in performance made by improved wing section design, turbulators and the introduction of new structural materials, allowing wings to be thinner and stiffer, from the wing tip effects.