ASPECT RATIO

The aspect ratio of a wing or any other surface is found by dividing the span by the mean chord, or in cases where the mean chord is hard to determine, by dividing the square of the span by the total area, thus: A = b2/S, in the standard symbols. Among models, ‘ A-2’ sailplanes tend to the high aspect ratio side, aerobatic power models to the low. Full-sized

, delta winged aircraft like the Concorde have very low aspect ratio, at the other extreme ‘Open Class’ sailplanes currently average about 30 to 1, while some experimental types ; such as the British ‘Sigma’ and the Brunswick S. B. 10 exceed 36:1. With a span of 29 І metres the S. B. 10 has an average wing chord of just under 80 cm (93 ft 2 ins, 26 W ins). ; Such long, narrow and thin wings present great problems to the engineer and to the pilot, = or operator of a model. In spite of the difficulties, high aspect ratio wings are essential for |agood performances at the low speed end of the Cl scale to reduce the vortex-induced і drag.

Fig. 5.7 Aspect ratio

I,

I b= span

I £» mean chord о

A«* span-i-mean chord; – s – , or span x span area; – H-

£• C w

$(

J);

Ш the wing* Ф<нт m ofeque/ ere* but of їмШу tittering aspect ratio*

): How powerful the effects of A. R. may be can be judged from the standard formula for ^ estimation of induced drag of wing. Where CDi is the vortex-induced drag coefficient, and : A is the aspect ratio,

The factor к in this equation is a correcting figure to allow for wing planform. For a well – ; designed wing it is only a little over 1.0. It will be given more attention in the next chapter, і Otherwise, the formula shows that doubling the aspect ratio halves the vortex drag coefficient. The effect on the sinking speed of a contest glider or any gliding duration ‘ model is very large. As shown in Chapter 4, the sinking speed depends on the maximum I value of the ratio Cl’ VCd – Most of the Cd in slow flight is the vortex-induced drag

coefficient An example is worked in more detail in the appendix, where it is shown that increasing the aspect ratio of a model from 7.5 to 15 increases the power factor from about 16:1 to nearly 26:1. It is for this reason that full-sized sailplanes have such high aspect ratios. The reduction in sinking speed is vitally important for staying aloft in weak lift and also for climbing as rapidly as possible in stronger upcurrents. With a high aspect ratio sinking speeds can be very low even if the wing loading is high. High aspect ratio with high wing loading is one way of achieving a good soaring performance combined with good penetration. For absolute minimum sink, both high a. r. and low wing loading are needed, i. e., low span loading, W/b.