BOUNDARY LAYER INVIGORATORS

Research by Martyn Presnell in a wind tunnel at Hatfield has shown that considerable improvements in the performance of free-flight model sailplanes and rubber driven aeroplanes can be achieved by the use of multiple ‘trip strips’ or, in Presnell’s terminology, ‘invigorators’.

Test wings using the Benedek 6356b aerofoil (see Appendix 3) were constructed from materials exactly like those used in a typical FI A (A2) sailplane model. Balsa wood wing ribs and spars were used, the framework being covered with tissue paper, doped, and in one case, the forward third of the wing was skinned with thin sheet balsa. Not only were lift and drag forces measured, but some flow-visualisation tests were done. These involved coating the test wing with pigmented kerosene to reveal the nature of the boundary layer. Where the b. l. was turbulent the kerosene evaporated rapidly, leaving a film of pigment. Within the laminar separation bubble, the evaporation was less rapid so the flow of the air nearest the wing skin could be seen as the liquid moved upstream (See Figure 3.6). In the fully laminar flow regions the kerosene remained liquid longer still and flowed in the normal downstream direction. The flow separation point and re-attachment downstream of the bubble could then be discovered for each angle of attack. (Modellers have sometimes noticed that, when flying in the late afternoon or early evening at dewfall, dew deposited on a wing before flight will still sometimes be present after the flight on the leading edges where the flow is laminar, but evaporates from the rear parts of the wing where turbulent boundary layers are expected.)

The addition of a single turbulator at 5% of the wing chord improved the measured lift and drag figures, as expected, at Reynolds numbers below 40,000, although the separation bubble was still present. The turbulator consisted of a thin strip of adhesive plastic tape 0.15mm thick and 0.75mm wide, running spanwise.

It was then found that the addition of further strips of the same thin tape at various positions on the chord aft of the turbulator resulted in further improvements of lift and drag figures. The best results at Re below 70,000 were found with five of these invigorators in the positions shown in Figure 8.8. The original 5% turbulator remained in place throughout

Presnell noted that placing an invigorator within the separation bubble, as revealed by the kerosene, made no detectable difference. The first invigorator must be placed just aft of the re-attachment point and the others spaced over the rear part of the wing in the turbulent boundary layer. The exact mechanism of the invigorators is not fully understood at present. It may be that they aid the already turbulent boundary layer to remain attached to the wing after the bubble has been passed. Presnell points out that several leading contest model fliers have used invigorators with success.