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12 Krzysztof Olasek, Maciej Karczewski

Fig. 2 Surface plots - lift coefficient results, a) fora=5°, b) fora=13°

Visible is the asymptotic character of obtained surfaces forthe integral linę sizes, for a=5° values of c_z approach the reference value for x/c slightly bigger than 1 and y/c«1). Similar behaviour, and morę easily identifiable, is seen for a=13°, where beyond y/c=0.5, c_z abruptly decreases for the entire rangę of x/c values. A conclusion can be madę that rectangle size (or integration linę length) should be chosen in such a way as to encompass the investigated airfoil as closely as possible. Therefore, the same width/chord (x/c) ratio, equal to 1.25, can be used for a wide rangę of angles of attack. Likewise, the height/chord ratio tends to a minimum, remembering that y/c dimension must increase as the airfoil angle of attack increases.

Another important factor is the size of the flow structures induced by the airfoil's inclination. For higher angles of attack (beyond the static stall) a separation region appears above the airfoil's top surface. As mentioned, it can be noticed that for a=13° and y/c ratio<0.5, a sudden drop of lift coefficient occurs. Thus, another recommendation forthe choice of the integral curve size, should be to the inclusion of separation region inside the integral rectangle. Otherwise, velocity data used for the circulation function gives unreliable lift results. Fig. 3 demonstrates the changes of the integration window size for 3 cases.