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Air-permeable hole-pattern and nose-droop control improve aerodynamic performance of primary feathers

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Abstract

Primary feathers of soaring land birds have evolved into highly specialized flight feathers characterized by morphological improvements affecting aerodynamic performance. The foremost feathers in the cascade have to bear high lift-loading with a strong bending during soaring flight. A challenge to the study of feather aerodynamics is to understand how the observed low drag and high lift values in the Reynolds (Re) regime from 1.0 to 2.0E4 can be achieved. Computed micro-tomography images show that the feather responds to high lift-loading with an increasing nose-droop and profile-camber. Wind-tunnel tests conducted with the foremost primary feather of a White Stork (Ciconia ciconia) at Re = 1.8E4 indicated a surprisingly high maximum lift coefficient of 1.5 and a glide ratio of nearly 10. We present evidence that this is due to morphologic characteristics formed by the cristae dorsales as well as air-permeable arrays along the rhachis. Measurements of lift and drag forces with open and closed pores confirmed the efficiency of this mechanism. Porous structures facilitate a blow out, comparable to technical blow-hole turbulators for sailplanes and low speed turbine-blades. From our findings, we conclude that the mechanism has evolved in order to affect the boundary layer and to reduce aerodynamic drag of the feather.

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Abbreviations

A L :

Lifting area (m2)

AR:

Aspect ratio (AR = b 2 /A L)

b :

Wingspan (m)

c :

Profile chord (m)

C D :

Total drag coefficient

C Lmax :

Lift coefficient at maximum lift

CL/CD:

Glide ratio (L/D)

d/c:

Profile thickness related to c

D :

Drag force (N)

f/c:

Profile camber related to c

L max :

Maximum lift-force (N)

P :

Static pressure (N/m2)

P D :

Drag power (W)

P S :

Dissipation power (W)

q :

Dynamic pressure (N/m2)

Re :

Reynolds Number (Re = V·c/ν)

V :

Volume (m3)

V :

Forward velocity (m/s)

VRs :

Specific volume-rate or permeability (m3/(s N))

V ∞ :

Free stream velocity (m/s)

\( \rho \) :

Standard air density 1.22 (kg/m3)

ν :

Dynamic viscosity (m2/s)

I :

ΔPD/PS (efficiency factor)

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Acknowledgments

We are very grateful for the support from the following institutions: Institute for Medical Physics, University of Erlangen-Nürnberg (Micro-CT-Scans), Institute for Experimental Physics, University of Augsburg (SEM), Institute for Aerodynamics, Technical University of Munich (wind-tunnel tests), Central Library of the DLR (Deutsches Forschungszentrum für Luft und Raumfahrt, Köln). We are also grateful to two reviewers who made valuable suggestions on an earlier version of the manuscript.

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Authors and Affiliations

  1. Max Planck Institute for Ornithology, Vogelwarte Radolfzell, Schlossallee 2, 78315, Radolfzell, Germany

    Heinrich Eder & Wolfgang Fiedler

  2. Institute for Aerodynamics, Technische Universität München, Boltzmannstr. 15, 85748, Garching, Germany

    Xaver Pascoe

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  1. Heinrich Eder
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  2. Wolfgang Fiedler
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  3. Xaver Pascoe
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Correspondence to Wolfgang Fiedler.

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Eder, H., Fiedler, W. & Pascoe, X. Air-permeable hole-pattern and nose-droop control improve aerodynamic performance of primary feathers. J Comp Physiol A 197, 109–117 (2011). https://doi.org/10.1007/s00359-010-0592-7

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  • DOI: https://doi.org/10.1007/s00359-010-0592-7

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