THE EFFECTS OF STRUCTURE AND SURFACE

Models constructed on traditional lines may in effect have turbulators built in. The sag of tissue or other thin covering behind the leading edge spar between the ribs creates a bump in the profile. This may have an entirely beneficial effect on transition, and the good performance of some models can be explained only in this way. Among his tests on the Gd 801, Kraemer included tests of a paper-covered model which showed that sub-critical flow prevailed down to Re 42,000, comparable with the same aerofoil with a turbulator wire. Wind tunnel results on a number of balsawood and tissue covered wings, carried out at Stuttgart University and reported by Dr. D. Althaus (Profilpolaren fur den ModeUflug, Vol. 2) have shown the same effect at free flight model wing sizes and speeds. This suggests that attempts by modellers to preserve very accurate profiles over the front part of low speed model wings are sometimes misguided. The simple tissue-covered leading edge may prove more efficient than one with smooth sheet balsa covering, especially if the wing profile used is on the thick side with a large leading edge radius. It should be emphasised, nevertheless, that when the model is large enough or fast enough to avoid sub-critical Re problems, turbulators and surface irregularities at the leading edge cause drag to rise and ci max. to fall. This may be confirmed by study of Appendix 2.