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Gliding swifts attain laminar flow over rough wings

Gliding swifts attain laminar flow over rough wings
Gliding swifts attain laminar flow over rough wings
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.
1932-6203
Lentink, David
25389990-daa2-4511-80aa-3f60f184df33
de Kat, Roeland
d46a99a4-8653-4698-9ef4-46dd0c77ba5d
Lentink, David
25389990-daa2-4511-80aa-3f60f184df33
de Kat, Roeland
d46a99a4-8653-4698-9ef4-46dd0c77ba5d

Lentink, David and de Kat, Roeland (2014) Gliding swifts attain laminar flow over rough wings. PLoS ONE, 9 (6), [e99901]. (doi:10.1371/journal.pone.0099901).

Record type: Article

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.

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More information

e-pub ahead of print date: 25 June 2014
Published date: 25 June 2014
Organisations: Aerodynamics & Flight Mechanics Group

Identifiers

Local EPrints ID: 367091
URI: http://eprints.soton.ac.uk/id/eprint/367091
ISSN: 1932-6203
PURE UUID: 0d73592d-00e1-47a9-88f3-f79cfe1480c4
ORCID for Roeland de Kat: ORCID iD orcid.org/0000-0002-6851-4409

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Date deposited: 22 Jul 2014 10:03
Last modified: 14 Mar 2024 17:23

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Contributors

Author: David Lentink
Author: Roeland de Kat ORCID iD

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