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Gliding swifts attain laminar flow over rough wings
David Lentink1, Roeland de Kat2
1Department of Mechanical Engineering, Stanford University, Stanford, California, United States of America; Experimental Zoology Group, Wageningen University, Wageningen, The Netherlands.
Plos One
|June 26, 2014
Summary
Swift wings are rough, yet maintain smooth airflow for efficient gliding. This surprising roughness actually enhances glide performance in small birds by prolonging laminar flow and reducing drag at cruising speeds.
Area of Science:
- Aerodynamics
- Bio-inspired engineering
- Avian biomechanics
Background:
- Swifts possess highly refined gliding abilities, yet their wings exhibit significant roughness due to feather structure.
- High wing roughness is counterintuitive for aerodynamics, as smooth surfaces are typically required for laminar flow and reduced drag in gliders like sailplanes.
Purpose of the Study:
- To investigate how swifts achieve efficient gliding with rough wings.
- To understand the aerodynamic function of wing roughness in swift flight dynamics.
Main Methods:
- Utilized a stethoscope to map laminar airflow over preserved swift wings in a low-turbulence wind tunnel.
- Conducted aerodynamic analyses combining laminar flow area, lift, and drag measurements.
- Employed physical models to simulate the effect of swift-like roughness on smooth wings.
Main Results:
- Swift wings maintained a significant area of laminar flow (69%) during gliding, comparable to high-performance sailplanes.
- Aerodynamic analysis suggests swifts achieve laminar flow due to their wing size relative to the airflow's transition distance after perturbation.
- Simulations revealed that swift-like roughness prolongs laminar flow and reduces drag at typical swift cruising speeds, while increasing drag at higher speeds.
Conclusions:
- Swift wing roughness is not detrimental but functionally advantageous for glide performance at their specific flight speeds.
- This roughness mechanism provides a potential evolutionary edge for small birds with less developed wings.
- The findings challenge conventional aerodynamic principles by demonstrating the benefits of controlled roughness in specific biological contexts.
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