SECONDARY CORRECTION FORMULA FOR 2-DIMENSIONAL LIFT COEFFICIENT MEASURED IN A WIND-TUNNEL

An improvement of the secondary correction formula for the lift coeffi cient of an airfoil measured in a wind tunnel is presented, taking int o account upwash and blockage effects. The formula allows the extensio n of the limits of correctability in the presence of strong variations of wall-induced velocity. An adaptive flexible-wall wind tunnel has b een used to introduce known streamwise distributions of interference v elocity components into the test chamber, to monitor their effects on the model aerodynamic performance and to find relations with the known interference-free lift coefficient. A method to impose simultaneously streamwise variations of wall-induced upwash and blockage has been de veloped. The basic aims of the work are the investigation of the limit s of validity of primary and secondary wind-tunnel wall corrections, a nd the establishment of semiempirical correction rules for nonuniform flows. Tests were performed on a NACA 0012 airfoil at Mach 0.4, at ang les of attack between 4 and 10 deg. The experimental results show that by using the secondary correction significant improvements, in terms of correctability Limits, are obtained with respect to the primary one , even if the percent error on the lift-coefficient evaluation remains unacceptable for high incidences and/or high blockage ratio tests. Th e correlation of experimental data has suggested that it is possible t o improve the secondary correction for the Lift coefficient of an airf oil section by taking into account the effects of variations of blocka ge and upwash on the model.