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Figure 2


Fig. 2. Flapping mechanism as mounted on the soap-tunnel framework (further illustrated in Fig. 4). The flapping mechanism consists of a crank mechanism that generates a stroke and angle of attack amplitude that are 90° out of phase with respect to each other. The angle of attack amplitude is reduced with a series of pulleys. The stroke amplitude is reduced with the aid of a pantograph. The flapper is driven by a DC motor. We mounted a special dial-plate with one microswitch (four are drawn) on the motor housing. This switch is pressed by a disk with a small knob in a phase we predetermined with the dial; in this way the camera can be triggered in a specific phase of the stroke. The angle of attack and stroke amplitude can be varied independently by changing the distance between the motor and the sled, indicated by `set distance', and the arm length of the crankshaft (`set crank amplitude'), which is hidden under the motor house in its current position. Finally, the angle of the stroke plane of the foil can be set with respect to the free-stream direction by rotating the whole crank mechanism, which is indicated by `set body angle'; in this study it is zero (as drawn).





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