OTHER FORMS OF TURBULATOR

The fineness of the tape used for turbulators in all wind tunnel testing is worthy of note. Modellers have sometimes employed much thicker ones, sometimes using strips of limn or 1.5mm balsa where thicknesses one tenth of this should be sufficient Turbulators, or; invigorators, which are too thick can cause flow separation rather than improving the boundary layer conditions.

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8.3 ZIG ZAG TURBULATORS

There is much to be said for laying the tuibulator tape in zig-zag fashion. Tests at Delft і University have shown that a zig zag tape tuibulator just ahead of the separation point on the wing has a better effect than a straight strip. Hie best spacing of the zig zags is a matter i for experiment on a given wing. The best results are obtained when there is a definite { relationship between the natural tendency of the flow within the separation bubble to develop waves and small, chordwise vortices (see paragraph 3.9). The modeller is unlikely to know what this is without very costly tunnel tests and some calculation, so trial and error is the likely way of discovering the optimum arrangement Possibly pinking shears of different sizes could be used to produce tapes for trial.

8.4 PNEUMATIC TURBULATORS 1

Perforations, such as a row of pin holes through the wing skin instead of a tape strip, can | act as a tuibulator. 3

The air pressure inside a wing is usually somewhat greater than that on the upper 1 surface, so air is drawn through the perforations and injected into the boundaiy layer. Ibis ] in effect trips the flow and may be sufficient to make it tuibulent (The effect was first j noticed by M. M. Gates in wind tunnel tests carried out in the 1950s.) Many full-sized і sailplanes also use pneumatic boundary layer control, especially on the underside of the wing near the trailing edge, where a separation bubble commonly forms. High pressure air is taken in by a small intake, positioned under the wing, and this raises the pressure in the і hollow chamber inside the wing. Very fine holes are drilled, at small spacing, through the skin just ahead of the separation point of the bubble. The injected air blows the boundary layer off the wing altogether and reduces the profile drag. Keeping the many small holes open is a problem of maintenance.