TURBULENT FLOW AEROFOILS

Arising directly out of Schmitz’s researches, Sigurd Isaacson and Georges Benedek designed a whole series of aerofoil sections for use on models. The Wortmann М2 is also of this type. These are all intended to fly with turbulent boundary layers, at low speeds. All are thin. They have enjoyed wide popularity, many modellers use them without knowing their principles and may defeat the designer’s purpose by rounding the leading edge too much during the final stages of sanding before covering the wing. As suggested above, this may not matter much if there is covering sag behind the leading edge, but in some cases the inaccuracy may cause a deterioration of performance.

The Seredinsky type of wing (Fig. 8.6) resembles the wing profile of some larger soaring birds. Although difficult to construct, it may prove very effective on smaller models, or models with very high aspect ratio, and hence small wing chords. The leading, edge is similar to that of a simple curved plate, but the thickening of the profile on the underside provides room for a strong main spar without much effect on the upper surface flow.

Some of the Benedek aerofoils are intended for the Jedelsky type of structure, in which the necessary strength and stiffness is obtained by building the whole wing of solid balsa

sheet, thick at the front with a thin sheet over the trailing portion, stiffened by ribs but without tissue covering. There is no doubt some penalty in higher drag on the underside of such profiles, but this may be acceptable if the aerofoil is more efficient at low Re. Unfortunately no wind tunnel tests have been published on such profiles (Fig. 8.7).