Aspect-Ratio Effects on Aeromechanics of Membrane Wings at Moderate Reynolds Numbers

The aspect ratio of a membrane wing at moderate Reynolds number Re=67,500 affects aerodynamic performance as well as membrane deformations. Wind-tunnel experiments are conducted using three different low-aspect-ratio wings (aspect ratios of 1, 1.5, and 2). Aerodynamic performance is determined from force and moment measurements, which are performed using a six-component force transducer. Membrane deformations are obtained using photogrammetry. Mean values and unsteady effects are examined for both aerodynamic performance and membrane deformations. Mean deflection results indicate that lower-aspect-ratio membrane wings show defined U-shape deflections along the span, whereas higher aspect ratios display a progressive rise in deformation to the wing tip. Dominant chordwise vibration modes of the membrane and their spectral content show that lower aspect ratios exhibit higher mode shapes and frequencies, likely caused by increased downwash, which delays the influence of vortex shedding into higher incidences. The frequencies of the dominant modes in membrane motions are found to correlate with the frequencies of lift and drag fluctuations.