Fig. 3. Composite wake velocity and vorticity (s^–1) plots of the near wake in the streamwise plane [x, z] for the outer wing (lz) at flight speeds of 2, 3, 5 and 7 m s^–1. At 2 m s^–1 the images show double start vortices (white arrows) and a single stop vortex. Vorticity is scaled according to the colour bar to the right of each image and vectors are scaled according to the reference vectors, representing the flight velocity, plotted above each composite plot. The size of the images is indicated by the 5 cm scale bar to the left. The upwash during the upstroke is due to the laser sheet cutting outside the tip vortex.