bottom edge of the flap wing 661 by a second removable rivet 666 which passes through aligned holes not visible in the flap wing 661 and the piston 662 the contact of the head 356 and the ends 359 of the legs 357 of the removable rivet 665 circular cam arm 131 39 bead 664 and piston 665 is made loose enough that rotation of the piston 662 relative to the circular cam arm 131 39 is possible as shown by the sequence of depictions of fig8 a and 8b as the pulley wheel 150 rotates the bottom of the flap wing 661b repeatedly swings away from the circular cam arm 131 39 and back towards it 131 39 pivoting about the fold 661c between it 661b and the flap roof 661a in particular when the circular cam arm 131 39 is oriented so that the bead 664 is at the bottom of the circular cam arm 131 39 as shown in fig5 b the piston 662 is oriented horizontally so the bottom of the flap wing 661b is a maximum distance from the circular cam arm 131 39 as the circular cam arm 131 39 rotates and the bead 664 moves upwards the orientation of the piston 662 becomes less horizontal and the distance of the bottom of the flap wing 661b from the circular cam arm 131 39 decreases and when the circular cam arm 131 39 is oriented so that the.