In moving from the realm of stationary wing flight towards the complexity of flapping wing flight as evolved in birds, we wish to introduce the simplest means of flapping as a first step in an attempt to simplify as much as possible our wind tunnel model and also to confine our attention to the dominant thrust producing mechanism. Our model half wings flap only in the plane normal to flight. The flapping motion is sinusoidal with time and symmetrical about the mid position (Fig. 6). No wing bending was allowed. The wing section shape had no camber and also was rigid in shape but free to twist in torsion about its shear centre. However, the model root support was fixed and therefore could not simulate vertical plunging of a bird associated with ideal elliptic span loading (Ref. 1).