Leading edge vortex in a slow-flying passerine

Abstract

Most hovering animals, such as insects and hummingbirds, enhance lift by producing leading edge vortices (LEVs) and by using both the downstroke and upstroke for lift production. By contrast, most hovering passerine birds primarily use the downstroke to generate lift. To compensate for the nearly inactive upstroke, weight support during the downstroke needs to be relatively higher in passerines when compared with, e.g. hummingbirds. Here we show, by capturing the airflow around the wing of a freely flying pied flycatcher, that passerines may use LEVs during the downstroke to increase lift. The LEV contributes up to 49 per cent to weight support, which is three times higher than in hummingbirds, suggesting that avian hoverers compensate for the nearly inactive upstroke by generating stronger LEVs. Contrary to other animals, the LEV strength in the flycatcher is lowest near the wing tip, instead of highest. This is correlated with a spanwise reduction of the wing's angle-of-attack, partly owing to upward bending of primary feathers. We suggest that this helps to delay bursting and shedding of the particularly strong LEV in passerines.

Bat flight: aerodynamics, kinematics and flight morphology

“…The dominating unsteady mechanism used in nature is the dynamic stall and associated LEV, which have been shown for many insects (Ellington et al, 1996;Birch and Dickinson, 2001;Sane, 2003;Johansson et al, 2013), and a few bird species (Muijres et al, 2012a, Warrick et al, 2009, Wolf et al, 2013. In this context, wind tunnel experiments of on-wing flow measurements of slow flying bats have demonstrated the presence of LEVs in the relatively small Palla's long-tongued bats ( ) the LEV contributes up to 40% of the total aerodynamic force (Table 1).…”

Flapping wing aerodynamics: from insects to vertebrates

“…2) that range in complexity from stiff-winged plant seeds ) to bird wings that can change shape (i.e. morphing wings) (Muijres et al, 2012;Videler et al, 2004). Based on studies of robotic insects, this aerodynamic mechanism may enhance lift by up to 45% in hovering fruit flies (Dickinson et al, 1999) and support up to two-thirds of the lift during the downstroke of hovering hawkmoths (van den Berg and .…”

“…Based on studies of robotic insects, this aerodynamic mechanism may enhance lift by up to 45% in hovering fruit flies (Dickinson et al, 1999) and support up to two-thirds of the lift during the downstroke of hovering hawkmoths (van den Berg and . Among vertebrates, the LEV can increase lift by up to 40% in slow-flying bats (Muijres et al, 2008), and it represents up to 26% of the total lift in hovering hummingbirds (Warrick et al, 2009) and up to ∼50% of the lift in some forwardflying birds (Muijres et al, 2012). However, these estimates of vertebrate lift enhancement may only be rough approximations of the true values, because they are based on the quasi-steady method that uses inviscid vortex theory (Box 1).…”

“…3, and provide the context for comparing insect and vertebrate aerodynamics. (Ellington et al, 1996), Pallas' long-tongued bat, Glossophaga soricina (Muijres et al, 2008), the rufous hummingbird, Selasphorus rufus (Warrick et al, 2009), and a pied flycatcher, Ficedula hypoleuca (Muijres et al, 2012). The LEV images shown for the seed and insect wing are from smoke visualizations, and those for the vertebrate wings are from particle image velocimetry (PIV) measurements.…”

“…Vertebrates can use their arm and hand muscles to actively twist and bend the wing surface within a stroke. For birds, this wing twist reduces the angle of attack along the wingspan, which may help to stabilize the LEV and prevent flow separation (Muijres et al, 2012). Birds can also use their alula, a small winglet attached to the bird's thumb, as a wing slot to prevent flow separation (Lee et al, 2015;Hedenstrom et al, 2009).…”