Fig. 7. Photographs near mid-downstroke using bubble rake. Left column (A,B) at Re=120. Right column (C,D) at Re=1400. Full wing views (A,C) taken at approximately mid-downstroke when wing is parallel to camera. Close-ups of the leading edge (B,D) show the growth of flow within the core of the leading edge vortex. Note the lack of a tight helix at low Re (B).