1930 INTRODUCTION Studies of animal flight have attained a new level of detail over the last decade, due to a tremendous progress in measurement techniques and heightened interest in flapping flight among the academic, military and industrial spheres. This interest is, for the most part, inspired by the very high maneuverability allowed by flapping flight compared with conventional propulsion systems. Initially, the focus was primarily on insect flight (Ellington et al., 1996; van den Berg and Ellington, 1997; Willmott et al., 1997; Ellington, 1999). These early studies demonstrated that flow behavior at the size, speed and flapping frequency of insects is distinctly different from well-studied aeroplane aerodynamics. For example, flapping wings possess mechanisms of lift generation beyond those of fixed wings, such as wake capture, rotational lift and delayed stall, and these unsteady effects have been found to provide a substantial portion of the lift during insect flight (Dickinson et al., 1999; Sane, 2003). Interest in bird and bat flight has increased significantly over the last few years and with it the desire to understand vertebrate flight in its full complexity. Beyond experimental investigations and calculations regarding the wake pattern and aerodynamic forces (Hedrick et al., 2002; Spedding et al., 2003; Hedenström et al., 2006; Hedenström et al., 2007; Hubel et al., 2009), there is an increasing interest in complementary issues, such as power consumption (Rayner, 1999; Tobalske et al., 2003), maneuverability (Tobalske et al., 2007; Iriarte-Diaz and Swartz, 2008; Hedrick et al., 2009), kinematics (Hedrick et al., 2004; Riskin et al., 2008) and wing structure (Swartz et al., 1996; Swartz and Middleton, 2008), as well as the correlation of the morphological and physiological conditions, kinematics and generated aerodynamic forces (Rosen et al., 2004; Swartz et al., 2007; Tobalske, 2007). Traditionally, vertebrate flight, unlike insect flight, has been assumed to exclude unsteady effects due to the much larger Reynolds number (Re) regime; however, recent work has shown that leading edge vortices (LEVs) also play a role in lift generation in the flight of small vertebrates. Well before this was revealed for in insect flight, LEVs have been known to contribute to lift generation in technical applications such as delta-winged aeroplanes operating at much higher Reynolds numbers than any animals. Contrary to conventional wings, where high angles of attack lead to stall and decreasing lift generation, these swept wings with a sharp leading edge take advantage of controlled stall conditions and flow reattachment on the upper surface (LEV), generating high lift and drag at high angles of attack (Videler, 2005). Studies on a fixed model swift wing show that swifts under gliding conditions are probably capable of developing stable LEV conditions (Videler et al., 2004). However, additional studies on real wings confirm the presence of a LEV but contradict additional lift gain (Lentink et al., 2007) and the direct analogy between LEVs on swept and flapping wings (Lentink and Dickinson, 2009). Recent work has shown that LEVs are not limited to swift wings and gliding conditions. So far they have been observed on flapping wings of small (<10 g) bats (Muijres et al., 2008) and in hummingbird flight (Altshuler et al., 2004; Warrick et al., 2005; Warrick et al., 2009). This raises the question of whether the previous quasi-steady approach of bird flight might have to be reconsidered. Comparatively little is known about the influence of unsteady mechanisms over the wide range of Reynolds numbers and reduced The Journal of Experimental Biology 213, 1930-1939 © 2010. Published by The Company of Biologists Ltd doi:10.1242/jeb.040857 The importance of leading edge vortices under simplified flapping flight conditions at the size scale of birds Tatjana Y. Hubel* and Cameron Tropea Fachgebiet Strömungslehre und Aerodynamik, Technische Universität Darmstadt, 64287, Germany *Author for correspondence at present address: Structure and Motion Laboratory, The Royal Veterinary College, North Mymms, Hatfield, Hertfordshire AL9 7TA, UK (thubel@rvc.ac.uk) Accepted 21 February 2010 SUMMARY Over the last decade, interest in animal flight has grown, in part due to the possible use of flapping propulsion for micro air vehicles. The importance of unsteady lift-enhancing mechanisms in insect flight has been recognized, but unsteady effects were generally thought to be absent for the flapping flight of larger animals. Only recently has the existence of LEVs (leading edge vortices) in small vertebrates such as swifts, small bats and hummingbirds been confirmed. To study the relevance of unsteady effects at the scale of large birds [reduced frequency k between 0.05 and 0.3, k(pfc)/U  ; f is wingbeat frequency, U  is free-stream velocity, and c is the average wing chord], and the consequences of the lack of kinematic and morphological refinements, we have designed a simplified goose-sized flapping model for wind tunnel testing. The 2-D flow patterns along the wing span were quantitatively visualized using particle image velocimetry (PIV), and a three-component balance was used to measure the forces generated by the wings. The flow visualization on the wing showed the appearance of LEVs, which is typically associated with a delayed stall effect, and the transition into flow separation. Also, the influence of the delayed stall and flow separation was clearly visible in measurements of instantaneous net force over the wingbeat cycle. Here, we show that, even at reduced frequencies as low as those of large bird flight, unsteady effects are present and non-negligible and have to be addressed by kinematic and morphological adaptations. Key words: leading edge vortex, bird flight, delayed stall.