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Low Reynolds number aerodynamics of free-to-roll low aspect ratio wings

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Abstract

The aerodynamics of thin, flat-plate wings of various planforms (rectangular, elliptical and Zimmerman) have been studied in free-to-roll experiments in a wind tunnel. Non-zero trim angles at low angles of attack, self-induced roll oscillations with increasing angle of attack and even autorotation in some cases were observed. The rectangular wings with round leading-edge had non-zero trim angles at low incidences due to the asymmetric development of the three-dimensional separation bubble at these low Reynolds numbers. With increasing angle of attack, the bubble increases in length and once reattachment is lost, large amplitude roll oscillations develop. The Strouhal number of the roll oscillations is of the order of 10−2, which is in the same range as those expected for small aircraft experiencing atmospheric gusts. Velocity measurements revealed that variations in the strength of the vortices drove the rolling motion. At the mean roll angle, because of the time lag in the strength of the vortices, an asymmetric flow is generated, which results in a net rolling moment in the direction of the rolling motion.

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Abbreviations

b :

Wingspan

c :

Wing chord

I xx :

Moment of inertia about X-axis

Re :

Reynolds number, ρU ∞ c/μ

S :

Surface area

St :

Strouhal number, fc/U ∞

t :

Thickness

U ∞ :

Free-stream velocity

x :

Chordwise coordinate

α :

Angle of attack

Γ:

Circulation

μ :

Viscosity

Φ:

Roll angle

ρ :

Density

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Acknowledgments

This work is sponsored by the Air Force Office of Scientific Research, Air Force Material Command, USAF, under grant number FA8655-06-1-3058 and also by the RCUK Academic Fellowship in Unmanned Air Vehicles.

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Authors and Affiliations

  1. Department of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UK

    Nicholas T. Gresham, Zhijin Wang & Ismet Gursul

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  1. Nicholas T. Gresham
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  2. Zhijin Wang
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  3. Ismet Gursul
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Correspondence to Ismet Gursul.

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Gresham, N.T., Wang, Z. & Gursul, I. Low Reynolds number aerodynamics of free-to-roll low aspect ratio wings. Exp Fluids 49, 11–25 (2010). https://doi.org/10.1007/s00348-009-0726-2

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  • DOI: https://doi.org/10.1007/s00348-009-0726-2

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