The bristle of a rotating duct cleaning brush was modelled using large deformation elastic theory. In addition to contact and centrifugal forces, especially the effect of air drag was studied in the article. The dependence of the drag coefficient on air speed was simulated for numerical computations by fitting a fourth degree polynomial in logarithmic co-ordinates on point-wise data for a cylindrical body from literature. The results show that the effect of air drag on the deflection of the single bristle (L = 1 mm, d = 1 mm, rho(b) = 1140 m(3)/kg, EI = 2 GPa) is negligible at normal rotating speeds of the brush (300-1000 rpm). However, air drag may affect considerably the deflection of a bristle, which has a large diameter and low density. As to be expected, air drag decreases the contact force between the single bristle tip and duct surface even at low rotation speed of bristle (<50 rpm). Thicker or stiffer bristle decreases the deflection of the rotating bristle and increases the magnitude of the contact normal force between the bristle tip and the duct surface. The model presented combined with the Mathcad software seems to be a useful tool in the cleaning brush design work.