The evolution of the normal aerodynamic coefficient of 19 configurations of
square plates with various porosity patterns, ranging from solid plate to
homogeneous porous plate, is experimentally characterized. The variation of the
porosity pattern is obtained by partially covering the holes of a commercial
fly-swatter using adhesive tape. Evolution of the normal aerodynamic
coefficient is assessed from the measurement of the angular position of the
porous plate, placed as a freely rotating pendulum swept by a flow in a wind
tunnel. These angular measurements are also supported by PIV measurements of
the structure of the wake. We show that the porosity pattern determines whether
or not an abrupt stall occurs. In particular, the details of the porosity
pattern on the edges of the plate are decisive for the existence of abrupt
stall