Rye M. Waldman

Aerodynamic Characterization of a Wing Membrane with Variable Compliance

Membrane wings with variable compliance have a great potential to improve the maneuverability and performance of micro air vehicles. Moreover, changes in membrane wing compliance might be used by flying animals, such as bats, to control aerodynamic performance. In this work, the mechanical properties and aerodynamic performance of a low-aspect-ratio membrane wing with variable compliance was characterized. The membrane was made of dielectric material coated with compliant electrodes and supported by a rigid frame. The compliance of the wing was controlled by applying a voltage across a membrane. The wing model was tested in a wind tunnel. It was found that, when a fixed voltage is applied across the wing membrane, the camber increases, accompanied by a small increase in lift. However, lift is significantly increased when the wing is forced with an oscillating field at specific frequencies. In addition, stall is delayed, and for a range of angle of attacks, there is an increase in lift-to-drag ratio. Fluid...

The aerodynamic cost of flight in the short-tailed fruit bat (Carollia perspicillata): comparing theory with measurement

Aerodynamic theory has long been used to predict the power required for animal flight, but widely used models contain many simplifications. It has been difficult to ascertain how closely biological reality matches model predictions, largely because of the technical challenges of accurately measuring the power expended when an animal flies. We designed a study to measure flight speed-dependent aerodynamic power directly from the kinetic energy contained in the wake of bats flying in a wind tunnel. We compared these measurements with two theoretical predictions that have been used for several decades in diverse fields of vertebrate biology and to metabolic measurements from a previous study using the same individuals. A high-accuracy displaced laser sheet stereo particle image velocimetry experimental design measured the wake velocities in the Trefftz plane behind four bats flying over a range of speeds (3–7 m s−1). We computed the aerodynamic power contained in the wake using a novel interpolation method and compared these results with the power predicted by Pennycuicks and Rayners models. The measured aerodynamic power falls between the two theoretical predictions, demonstrating that the models effectively predict the appropriate range of flight power, but the models do not accurately predict minimum power or maximum range speeds. Mechanical efficiency—the ratio of aerodynamic power output to metabolic power input—varied from 5.9% to 9.8% for the same individuals, changing with flight speed.

Aerodynamic Behavior of Compliant Membranes as Related to Bat Flight

We present computations of membrane airfoil behavior subject to aerodynamic loading and compare them with in vivo measurements of membrane wings of bats during flight. The computational method assumes an inviscid potential flow (with net circulation determined by a Kutta condition), is computed using XFOIL and iteratively coupled with a finite element model describing the membrane behavior. We find that a simple model assuming uniform loading is largely confirmed, particularly for very compliant membranes in which the pressure loading is focused at the center of the airfoil. Stiffer wings transition to the more traditional pressure distribution predicted by thin airfoil theory for rigid wings. Comparisons with sail theories are also made, illustrating the effect of compliance. Additionally, the in vivo measurements of membrane deformation during bat flight are acquired from detailed kinematics recorded from Cynopterus brachyotis, flying in a wind tunnel. We demonstrate that the expansion of the wing area during the downstroke of the flight cycle exhibits area increases of up to 100% during the downstroke. In addition, comparisons with the computational theory show good qualitative agreement.

Shape, lift, and vibrations of highly compliant membrane wings

Membrane wings are common in flying animals such as bats, lemurs, flying squirrels, and pterosaurs, as well as in low Reynolds number Micro Air Vehicles. Vortices shed from the sharp leading and trailing edges and wingtips of membrane wings, and the vortex interactions with the membrane play an important role in the wing’s performance. With compliant membrane wings that are initially tension-free at rest, there are two issues to consider: (a) the static relationship between the net aerodynamic forces and the bulk wing deformation, and (b) the interaction between the membrane dynamics and unsteady flow structures. We present a simple model of the finite deformation and natural frequency of an initially tension-free membrane wing, which depends only on an aeroelastic parameter. We extend our theory with a computer model that accounts for nonuniform chordwise load. We conduct experiments on low aspect ratio membrane wings with different support structures and thicknesses, over a broad range of freestream velocities and angles of attack. We measure wing shape and dynamics, aerodynamic force, and the wake, and find good agreement between the experimental results, the computer model, and our theoretical model. Membrane deformation affects the membrane vibration modes, which in turn affects the coupling between the membrane and vortex shedding. Wings with different wingtip support, but similar stiffness show similar static behavior, but exhibit markedly different dynamic behavior.

Aerodynamic Characterization of Wing Membrane with Adaptive Compliance

Membrane wings with adaptive compliance have a great potential to improve the maneuverability and performance of micro air vehicles. Moreover, adaptive compliance might be used by flying animals with membrane wings such as bats, to control aerodynamic performance. In this work, we characterized the mechanical properties and aerodynamic performance of a low aspect ratio membrane wing with adaptive compliance. The membrane was made of dielectric material coated with compliant electrodes, and supported by a rigid frame. The compliance of the wing was controlled by applying a voltage across membrane. We tested the wing model in a wind tunnel. We found that when a fixed voltage is applied across the wing membrane the camber increases, accompanied by a small increase in lift. However, lift is significantly increased when the wing is forced with an oscillating field at specific frequencies. In addition, stall is delayed, and for a range of angle of attacks there is an increase in lift to drag ratio. Fluid dynamics measurements are needed to identify the source of lift enhancement and analyze the fluid-membrane interaction.

Measurement of streamwise vortices in low-speed flight

Low Reynolds number aerodynamic experiments with flapping animals (such as bats and small birds) are of particular interest due to their application to Micro Air Vehicles (MAVs) which operate in a similar parameter space. Previous PIV wake measurements described the structures left by bats and birds and provided insight to the time history of their aerodynamic force generation; however, these studies have faced difficulty drawing quantitative conclusions based on said measurements. The highly three-dimensional and unsteady nature of the flows associated with flapping flight are major challenges for accurate measurements. The challenge of animal flight measurements is finding small flow features in a large field of view at high speed with limited laser energy and camera resolution. Cross-stream measurement is further complicated by the predominately out-of-plane flow that requires thick laser sheets and short inter-frame times, which increase noise and measurement uncertainty. Choosing appropriate experimental parameters requires compromise between the spatial and temporal resolution and the dynamic range of the measurement. To explore these challenges, we do a case study on the wake of a fixed wing. The fixed model simplifies the experiment and allows direct measurements of the aerodynamic forces via load cell. We present a detailed analysis of the wake measurements, discuss the criteria for making accurate measurements, and present a solution for making quantitative aerodynamic load measurements behind free flyers.