Avian wing geometry and kinematics of a free-flying barn owl in flapping flight

Abstract

to produce thrust and lift in an efficient way in this Reynolds number regime. The flapping motions carried out by birds follow very complex movements of the wing and body which can only be characterized by a set of parameters. Besides the four main motion components (pitch, plunge, in and out fold and sweep), which can vary in amplitude, motion form, frequency and velocity, the phase relations between these components are also important for the aerodynamic performance. Furthermore, the wings of birds are highly flexible and adaptive, which means that wing parameters as the incidence angle, camber, thickness distributions are changing during flapping motion as well. Recent studies show the influence of single parameters on the aerodynamic performance of a flapping wing (e.g. phase angle between pitch and plunge, wing flexibility, Reynolds number Send 1996; Anderson et al. 1998, wing flexibility Heathcote andGursul 2007; Reynolds number Ol et al. 2009). However, due to the amount of parameters which can be modulated, the insight remains selective. Therefore, assuming that natural flyers use highly efficient flight mechanisms, their movement parameters can be used as a base for further investigations on flapping motions for technical applications (Platzer et al. 2008).Revealing the mysteries of nature is, however, often a challenging task, since living animals restrict the application of measurement techniques. Therefore, the history of bird flight examination is close related to the development of nonintrusive measurement techniques. One of the first quantitative measurements of birds in flight was performed by Bilo (1971). He applied the measurement technique stereophotogrammetry to sparrows in a wind tunnel. However, he used manually determined correspondences of distinct points on the bird, and therefore, the resolution (≈120 points, only on the upper side) and the accuracy were limited. Nevertheless, stereophotogrammetry is still in use for Abstract This paper presents results of high-resolution three-dimensional wing shape measurements performed on free-flying barn owls in flapping flight. The applied measurement technique is introduced together with a moving camera set-up, allowing for an investigation of the free flapping flight of birds with high spatial and temporal resolution. Based on the three-dimensional surface data, a methodology for parameterizing the wing profile along with wing kinematics during flapping flight has been developed. This allowed a description of the spanwise varying kinematics and aerodynamic parameters (e.g. effective angles of attack, camber, thickness) of the wing in dependence on the flapping phase. The results are discussed in detail using the data of a single flight, whereas a comparison of some kinematic parameters obtained from different flights is given too.

From wing shape to fluid dynamics in the barn owl (Tyto alba)

“…The full capacity for wing-shape change in gliding birds is unknown due to difficulty of measurement. Wings dramatically morph during flapping flight (Wolf and Konrath, 2015), but it is unclear how many active degrees of freedom are available for controlling morphing. Bird wings are highly coupled; the internal structure of a bird’s wing acts as a linkage, where the bones and feathers generally move together in prescribed ways (Matloff et al, 2020; Harvey et al, 2019), and much of the obvious shape change is controlled by a single degree of freedom.…”

High-speed surface reconstruction of a flying bird using structured-light

“…Spatially encoded projection patterns can reconstruct a sequence of stand-alone frames and are hence called 'single-shot' (Salvi et al, 2010;Zhang, 2012), which gives the advantage of being robust to inter-frame movement. Some existing spatially encoded structuredlight methods rely on binary pseudo-random dots but either have relatively low frame rate and accuracy (Saberioon and Cisar, 2016;Sarbolandi et al, 2015) or require manual digitizing of numerous points per frame (Wolf and Konrath, 2015;Zhang et al, 2008). Other existing spatial methods use grayscale patterns which cannot be projected at high frame rates (Guan et al, 2003;Lenar et al, 2013;Sagawa et al, 2012;Su and Liu, 2006).…”

Features of owl wings that promote silent flight

“…Thus, the geometry of the barn owl wing shows an indirect adaptation towards silent flight by being optimized for low flight velocities. Flight speed was estimated to range from 2.5 to 7 m s 21 [33,34]. This also means that these birds operate at Reynolds numbers around 60 000 [34].…”

“…Flight speed was estimated to range from 2.5 to 7 m s 21 [33,34]. This also means that these birds operate at Reynolds numbers around 60 000 [34]. Johnson [32] compared wing loading and aspect ratios in 15 species of North American owls.…”

“…These new data confirm the high camber observed in fixed wings (figure 2; see also fig. 29 in [34]). More precisely, camber changed during a wing stroke, reaching high values during the downstroke.…”