APPLICATION OF RESULTS

Even in full-sized practice, wind tunnel results are not applied directly. A suitable wing profile may be chosen in the first place on the basis of its comparative success against other profiles in the tunnel, but when final calculations are made, it is assumed, rightly as a rule, that the wing in service will not be accurate enough to give the same performance. Corrections are applied to reduce the tunnel results to those expected in reality. These corrections are arrived at in much the same ways as modellers arrive at their results: wings

are built and tested in service and experience is accumulated in this way for future designs. Often so called ‘practical construction’ wing profiles, produced by normal factory methods, are tested in the wind tunnel.

There has also been extensive work on the effect of roughness and polish on wings. In full-sized work this almost invariably shows the advantage of a smooth, wave-free surface. At model sizes, a difference appears. Some wing profiles tested at Stuttgart were •made by aeromodellers using balsawood and traditional methods of construction with frameworks of ribs and spars covered with sheet balsa or tissue and doped. Several of the open framed, tissue-covered examples performed better at very low Re numbers than did smooth and polished solid wood models of the same nominal profile. The exact shape of a tissue covered aerofoil is very hard to find, since the tissue always sags to some extent between the ribs and, if spars protrude, these too change the profile. The precise shape is hardly under control. The two most important features of any aerofoil, camber of the mean line and general thickness form, are probably by far the most important factors for the free flight modeller to worry about

Radio controlled models, except for the very smallest hand-launched gliders, fly at Re numbers about 100,000 and upwards (see Chapter 3, paragraph 3.3). At about Re 100,000 a tissue or film-covered framework wing often seems to perform just as well as a perfectly smooth and wave-free wing. Some modellers find that a different covering material, such as slightly rough fabric rather than glossy smooth plastic film, can improve the behaviour of the model, suggesting that such a surface may promote transition in the boundary layer and so delay flow separation. Pressnell’s invigorator effect may also be working with these slightly roughened surfaces (see 8.9). Turbulator strips too may be of use. However, as the size and flight speed of the aircraft increases, die benefits of a perfectly accurate and smooth wing become increasingly obvious. Wind tunnel test results also show greater reliability and predictability as the Re numbers rises to 200,000 and above, so it is here that such test results will And their greatest use. Faster, larger models are in this Re region.