LOW ASPECT RATIO

A very low a. r. wing can fly safely at a wide range of angles, is easier to trim and less critical all round. The usable range of angles of attack is wider and gusts have less influence. A very low a. r. wing tends not to reach such high values of Cl, since such a wing is, in a sense, all tips and there are very strong cross flows. But the stall is postponed, possibly even to 45 degrees. This explains why low a. r. aircraft and deltas adopt nose-up attitudes on the approach to land.

5.6 TAIL UNITS AND ASPECT RATIO

The steeper slope of the lift curve of high aspect ratio wings is significant also for tail units. Fins in particular are often very insensitive and on radio controlled models the rudder attached to a low a. r. fin also may be ineffective. Because of the low aspect ratio a large change of angle of attack is needed to bring about a moderate change of Cl, so the stabilising or control force is weak unless the fin area is considerably enlarged to compensate. The habit of designing fins for their fashionable appearance rather than for efficiency is partly to blame. A fin is a small wing and should be treated as such. Sweepback adds nothing aerodynamically other than extra drag. The higher the aspect ratio, the more sensitive the surface will be to small disturbances or control movements. Perhaps the only modellers who have always had to recognise this are the magnet-steered – sailplane enthusiasts who obtain satisfactory control with very tall and light rudders. The low aspect ratio fin may have advantages on an aerobatic model for spin recovery. In a spin, the fin may be required to provide a correcting force in conditions of very marked cross flow. The high a. r. surface might be stalled at such an angle, with disastrous results, while the comparatively insensitive low a. r. fin will be capable of stopping the rotation. The dorsal fin extension sometimes added to full-sized aircraft has a similar effect, adding some area, reducing the fin a. r., without requiring major structural alterations.

Tailplanes and foreplanes on canards too are more sensitive if they have high a. r. It is vital that the stabiliser should not stall before the main surface on an orthodox layout. The tail aspect ratio must, for safety’s sake, be somewhat lower than the wing, but within that limit should still be as high as possible. If the tail is called on to carry a proportion of the total lift in normal flight (although this is not the most efficient way to design a model), it generates vortex drag which can be reduced somewhat if its a. r. is high. With high a. r., even a ‘non-lifting’ tail is more responsive to small departures from the desired trim angle than a low a. r. surface, and can be reduced in area. The drag of a small, symmetrical – sectioned tail-plane at zero lift angle of attack is very low. The high a. r. tail also has a smaller proportion of its area in danger of being blanketed by the fuselage wake or cross flow from the fin. With canards, stalling of the rear plane before the foreplane is disastrous and leads to uncontrollable nose-up pitch. The foreplane must stall first and may therefore have a high aspect ratio.