THE STRONGLY TAPERED WING

The converse of a rectangular wing is one such as that shown in Figure 6.1b, strongly tapered with tips almost pointed. This is not only very inefficient but dangerous. The strength of downwash over the various parts of such a wing is such that the local angle of attack increases towards the tips, where the area is smaller. There is an aerodynamic ‘wash-in’, the tips are over-loaded and stall first, indeed, with a wing like that sketched, they would be almost permanently stalled. The narrow outer panels are called upon to provide far more lift than their section cj max. permits, while the roots contribute little. The model would fly better if its ends were cut off altogether, squaring the tips.

6.3 WASHOUT

The strongly tapered wing does possess one advantage. Because it has a very broad and thick root, it may be very lightly built without loss of strength. For this reason some early full-sized sailplanes such as the Rhonadler of 1932 adopted this type of wing. The tip stalling problems were avoided by giving the whole wing a marked twist or wash-out to reduce die geometric angle of attack over the outer panels by approximately the amount needed to equalise the downwash across the span. This tended to distribute die load more in proportion to the area and so reduced the vortex drag. The result was an efficient wing, but at only one airspeed. At the designed speed, the whole wing was working at roughly constant aerodynamic angle of attack, but at any other speed the distribution changed. In particular, as the speed increased, the wing tips reached their local zero angle of attack quite soon as the average angle of attack of the whole wing decreased. At any higher speed than this, the outer wing panels actually operated at negative angles of attack to the local airflow, and began to ‘lift’ downwards. Although this lift force was directed down, the resolved drag component was still directed aft. Thus, not only did the tips of these highly twisted wings throw extra down loads onto the rest of the wing, but they added vortex drag. More importantly, as the speed increased, the profile drag at the tips, operating at negative angles of attack, rose rapidly. From the cockpit the wing tips could be seen to bend downwards at some quite moderate airspeed, and the penetration suffered accordingly. The same effect may be observed on many model sailplanes with too much washout Too much wing twist, introduced to cure a bad choice of planform, results in a ‘one speed’ wing. This may be exactly what is desired for an FI A (‘A2’) sailplane, but not for any type that needs to fly at varying speeds. Even for an ‘A2’, wing twist renders the model more sensitive to slight trimming errors. A small departure from the ideal airspeed causes a disproportionate rise of drag.

For reasons to do with high profile drag and premature stalling at low Reynolds numbers, calculations show that almost all the aerodynamic advantages of the tapered wing are lost on small free flight models. For this reason rectangular wings are preferred for all these. By careful use of washout, the tip vortices may be reduced in power even on such a wing, at the single trim for minimum rate of sink.

Washout often proves very useful in preventing dangerous tip stalling on all models, particularly for scale types where the wing of the prototype is strongly tapered. Washout also aids aileron control at low speeds (see Chapter 7).