Zero-lift drag estimation of symmetrical airfoils at low Reynolds numbers based on rotor experiments

Determining the zero-lift drag of an airfoil at low Reynolds numbers is challenging because the aerodynamic forces of the wing model are small and wind tunnel experiments require stringent control of airflow turbulence intensity and extensive sampling to yield accurate results. Research on this topic is constrained owing to the difficulty of reliable measurement, leading to variability in the drag coefficients reported at identical Reynolds numbers. This paper proposes a method for estimating the zero-lift drag of airfoils from rotor thrust and power measurements. The proposed approach is used to estimate the minimum drag coefficients of symmetrical National Advisory Committee for Aeronautics (NACA) airfoils at low Reynolds numbers ranging from 1.7 × 10 4 to 5.6 × 10 4 . The estimated zero-lift drag coefficient of NACA0012 shows agreement with results obtained from previous wind tunnel experiments. Further, the relationship between the Reynolds number and thickness is examined for symmetrical NACA 4-digit airfoils and flat plates. The results confirm that the proposed method is particularly effective for thin airfoils. Although the validation of our results is limited by the scarcity of reference data, the proposed method provides systematic estimates of airfoil drag corresponding to varying Reynolds numbers and airfoil thickness at low Reynolds numbers. Thus, it offers a viable alternative to wind tunnel experiments for evaluating airfoil zero-lift drag at low Reynolds numbers. Complementary application of the proposed method and wind tunnel testing is expected to enable the reliable estimation of the minimum drag of symmetrical airfoils at low Reynolds numbers.