Andrew Wrist

Geometry and Prestrain Effects on the Aerodynamic Characteristics of Batten-Reinforced Membrane Wings

To lessen the deterioration of fixed-wing aerodynamic performance associated with chord Reynolds numbers below 100,000, flexible membrane wing designs have been studied and proposed as an alternative for micro air vehicle use. The beneficial effects of a flexible membrane can include higher lift, steeper lift-curve slope, delayed stall, gentle stall characteristics, and greater efficiency. These benefits have been attributed to both the time-averaged and dynamic deformation of the membrane. This work discusses the geometric and prestrain effects on a batten-reinforced, free trailing-edge membrane wing in low-Reynolds-number (50,000) flow. The global aerodynamic forces on the wings with varying wing aspect ratio, cell aspect ratio, and prestrain level were measured. The results show that the aerodynamic advantages of the flexible membrane are retained for the low-aspect-ratio wings. The optimal membrane cell aspect ratio is found to be approximately 1. The comparison of the aerodynamic forces between the low-aspect-ratio membrane wings and the corresponding three-dimensional-printed wings with the time-averaged deformation indicates the importance of membrane dynamic motion for the derived aerodynamic benefits.

Aerodynamic Comparison of Flat and Cambered Frames for Flexible MAV Wings

This paper compares the aerodynamic properties for flexible membrane MAV (micro air vehicle) wings with flat frames and cambered frames. The frames for the wings were designed in SolidWorks and constructed using an Objet30 Pro 3D printer. The membranes are composed of silicone rubber. The wings were tested in a The University of Alabama’s low speed wind tunnel at a Reynolds number of 50,000 (10 m/s) while varying angles-of-attack from -4 to 24°. A load cell was used to gather force measurements that were used to develop lift, drag, and pitching moment coefficients. The results show that cambering the frames for the flexible MAV wings increases maximum lift and aerodynamic efficiency when compared to flat frames.

Wake Characteristics of a 2D Spanwise Tensioned Membrane with Aerodynamic Loading

Abstract This paper compares the wake characteristics for spanwise tensioned membranes with free and fixed leading and trailing edges. Membrane vibration was generated by placing the membrane, attached to two rigid plates, within an air flow and constraining the leading and trailing edges as well as interior spanwise locations, independently. The flow velocity in the wake of the membrane wing was measured using a hot-wire anemometer sensor placed downstream of the trailing edge. Silicon rubber membranes of different thicknesses were tested at varying angles-of-attack, applied tension and freestream velocity. The wake mean and rms velocity profiles are compared and analyzed. The power density spectra of the wake velocity fluctuation are also presented and discussed. The results show that the existence of leading-edge vibration, no matter the placement of the interior constraint, increased the wake size and created a less organized energy spectrum.

Aerodynamic comparisons of flexible membrane micro air vehicle wings with cambered and flat frames

Flexible membrane wings at the micro air vehicle scale can experience improved lift/drag ratios, delays in stall, and decreased time-averaged flow separation when compared to rigid wings. This research examines the effect of frame camber on the aerodynamic characteristics of membrane wings. The frames for the wings were 3D printed using a polymer-based material. The membranes are silicone rubber. Tests were conducted at Re ∼50,000. Aerodynamic force and moment measurements were acquired at angles-of-attack varying from −4 to 24°. Additionally, digital image correlation data were acquired to assess time-averaged shapes of the membrane wings during wind tunnel tests. An in-house program was developed to average the deflection plots from the digital image correlation images and produce time-averaged shapes. Lifting-line theory based on the time-averaged shapes was then used to calculate theoretical lift and induced drag coefficients, showing that the time-average shape of the membrane under load contributes extensively to the aerodynamic performance. The results show that introducing camber to the frames of membrane wings increases aerodynamic efficiency.

Theoretical Investigation of the Aerodynamics of Membrane MAV Wings with Cambered Frames

This paper builds on a previous study that investigated aerodynamic properties for flexible membrane MAV (micro air vehicle) wings with cambered frames. The frames for the wings were designed in SolidWorks and fabricated using an Objet30 Pro 3D printer. The membranes are composed of silicone rubber. The wings were tested in a The University of Alabama’s low speed wind tunnel at a Reynolds number of 50,000 (10 m/s) while varying angles-of-attack from -4 to 24°. In the previous study, results showed that cambering the frames for the low aspect ratio (AR = 2) flexible MAV wings increases maximum lift and aerodynamic efficiency when compared to flat frames. In this study, lifting-line theory was used to perform a theoretical analysis on the time-averaged shapes for the membrane wings. The calculated values were compared with the experimental values to determine the effectiveness of using the theory for approximating time-averaged aerodynamic coefficients. It was determined that lifting-line theory approximates the lift coefficients reasonably well despite the assumptions of inviscid, attached flow, while also indicating higher relative induced drag for frames with less camber and at larger angles-of-attack.