Gliding swifts attain laminar flow over rough wings.

David Lentink,Roeland de Kat,Daniel Osorio

doi:10.1371/journal.pone.0099901

David Lentink, Roeland de Kat + Show 1 more

Open Access

https://doi.org/10.1371/journal.pone.0099901

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Abstract

Swifts are among the most aerodynamically refined gliding birds. However, the overlapping vanes and protruding shafts of their primary feathers make swift wings remarkably rough for their size. Wing roughness height is 1–2% of chord length on the upper surface—10,000 times rougher than sailplane wings. Sailplanes depend on extreme wing smoothness to increase the area of laminar flow on the wing surface and minimize drag for extended glides. To understand why the swift does not rely on smooth wings, we used a stethoscope to map laminar flow over preserved wings in a low-turbulence wind tunnel. By combining laminar area, lift, and drag measurements, we show that average area of laminar flow on swift wings is 69% (n = 3; std 13%) of their total area during glides that maximize flight distance and duration—similar to high-performance sailplanes. Our aerodynamic analysis indicates that swifts attain laminar flow over their rough wings because their wing size is comparable to the distance the air travels (after a roughness-induced perturbation) before it transitions from laminar to turbulent. To interpret the function of swift wing roughness, we simulated its effect on smooth model wings using physical models. This manipulation shows that laminar flow is reduced and drag increased at high speeds. At the speeds at which swifts cruise, however, swift-like roughness prolongs laminar flow and reduces drag. This feature gives small birds with rudimentary wings an edge during the evolution of glide performance.

Highlights

Feathers are a key adaptation that distinguishes bird wings [1,2,3] from wings of other flying animals such as bats [4] and insects [5]
At the macro scale the protruding feather rachis, together with the valleys created by the overlap between primary feathers [11], make the hand wing of swifts corrugated with a roughness height of 1–2% (Fig. 1A–C)
We mapped the spanwise distribution of laminar flow (Fig. 2)

Summary

Introduction

Feathers are a key adaptation that distinguishes bird wings [1,2,3] from wings of other flying animals such as bats [4] and insects [5]. We measured boundary-layer laminar-turbulent transition locations on the upper surface of 3 wing pairs, with 5u, 30u, and 50u sweep, for all combinations of wind speeds (5– 25 ms21 in 5 ms21 increments) and angles of attack (0u–18u with 4.5u increments) relevant for gliding swifts. At the speed (5 to 20 ms21), sweep (5u–50u), and angle of attack combinations for which our lift and drag measurements [7] predict maximal glide distance and flight duration, we find that transition occurs well behind the turbulators on the hand wing.

Results

Conclusion