Dragos Viieru

Membrane wing aerodynamics for micro air vehicles

Abstract The aerodynamic performance of a wing deteriorates considerably as the Reynolds number decreases from 106 to 104. In particular, flow separation can result in substantial change in effective airfoil shape and cause reduced aerodynamic performance. Lately, there has been growing interest in developing suitable techniques for sustained and robust flight of micro air vehicles (MAVs) with a wingspan of 15 cm or smaller, flight speed around 10 m / s , and a corresponding Reynolds number of 104–105. This paper reviews the aerodynamics of membrane and corresponding rigid wings under the MAV flight conditions. The membrane wing is observed to yield desirable characteristics in delaying stall as well as adapting to the unsteady flight environment, which is intrinsic to the designated flight speed. Flow structures associated with the low Reynolds number and low aspect ratio wing, such as pressure distribution, separation bubble and tip vortex are reviewed. Structural dynamics in response to the surrounding flow field is presented to highlight the multiple time-scale phenomena. Based on the computational capabilities for treating moving boundary problems, wing shape optimization can be conducted in automated manners. To enhance the lift, the effect of endplates is evaluated. The proper orthogonal decomposition method is also discussed as an economic tool to describe the flow structure around a wing and to facilitate flow and vehicle control.

Membrane wing-based micro air vehicles

Micro air vehicles (MAVs) with a wingspan of 15cm or shorter, and flight speed around 10m∕s have attracted substantial interest in recent years. There are several prominent features of MAV flight: (i) low Reynolds number (104–105), resulting in degraded aerodynamic performance, (ii) small physical dimensions, resulting in certain favorable scaling characteristics including structural strength, reduced stall speed, and impact tolerance, and (iii) low flight speed, resulting in order one effect of the flight environment and intrinsically unsteady flight characteristics. Flexible wings utilizing membrane materials are employed by natural flyers such as bats and insects. Compared to a rigid wing, a membrane wing can better adapt to the stall and has the potential for morphing to achieve enhanced agility and storage consideration. We will discuss the aerodynamics of both rigid and membrane wings under the MAV flight condition. To understand membrane wing performance, the fluid and structure interaction is of critical importance. Flow structures associated with the low Reynolds number and low aspect ratio wing, such as pressure distribution, separation bubble, and tip vortex, as well as structural dynamics in response to the surrounding flow field are discussed. Based on the computational capabilities for treating moving boundary problems, an automated wing shape optimization technique is also developed. Salient features of the flexible-wing-based MAV, including the vehicle concept, flexible wing design, novel fabrication methods, aerodynamic assessment, and flight data analysis are highlighted.

Static Aeroelastic Model Validation of Membrane Micro Air Vehicle Wings

A low-aspect-ratio, low-Reynolds-number membrane wing has been identified as a viable platform for micro air vehicle applications. Desirable flying qualities include high lift and larger stability margins. Several challenges are associated with the numerical modeling of such a wing, including highly three-dimensional flows, separation bubbles, and nonlinear membrane behavior. A thorough model validation and system identification effort is therefore required. A novel experimental setup integrates a wind tunnel with a visual image correlation system for simultaneous measurement of wing displacements, strains, and aerodynamic loads. These three metrics are used for a direct comparison of numerical and experimental data for both pre- and poststall angles of attack. Suitable correspondence is demonstrated for moderate angles of attack; methods for increasing the model fidelity can be made for angles with poor predictive capability. Computed flow structures reveal further information concerning the aeroelastic behavior of membrane wings.

Flapping and flexible wing aerodynamics of low reynolds number flight vehicles

For flight vehicles operated at the low Reynolds number regime, such as birds, bats, insects, as well as small man-made vehicles, flapping and fixed wings are employed in various ways to generate aerodynamic forces. For flapping wings, the unsteady fluid physics, interacting with wing kinematics and shapes determine the lift generation. For fixed wings, laminar-turbulent transition, three dimensional flows around low aspect ratio vehicles, and coupling between flexible wing structures and surrounding fluid flows are of major interest. In the present paper we discuss recent progress in understanding the low Reynolds number unsteady fluid dynamics associated with flapping wings, including leading-edge vortices, pitching-up rotation and wake-capturing mechanisms. For fixed wings, recent efforts in fluid-structure interaction and laminar-turbulent transition are highlighted.

A Study of Aerodynamics of Low Reynolds Number Flexible Airfoils

The interaction between aerodynamics and structural flexibility in a low Reynolds number environment is of considerable interest to biological and micro air vehicles. In this study, coupled fluid-structure computations of the Navier-Stokes fluid flow and a flexible airfoil in low Reynolds number environments are conducted to probe the aerodynamic implications. While a flexible airfoil deforms in response to the aerodynamic loading, it exhibits an equivalent pitching motion, which modifies the effective angle of attack, causing noticeable differences in lift and thrust generation. Within the range of the flexibility considered, the flow fields are similar in all cases. Even at Re=100, in the plunging motion, the force acting on airfoil is dominated by pressure and the viscous force is of little impact on the overall lift and thrust generation. Detailed airfoil shape is secondary compared to the equivalent angle of attack.

Effect of Tip Vortex on Wing Aerodynamics of Micro Air Vehicles

Tip vortex induces downwash movement, which reduces the effective angle of attack of a wing. For a low-aspect. ratio, low-Reynolds-number wing, such as that employed by the micro air vehicle (MAV), the induced drag by the tip vortex substantially affects its aerodynamic performance. In this paper we use the endplate concept to help probe the tip-vortex effects on the MAV aeredynamic characteristics. The investigation is facilitated by solving the Navier-Stokes equations around a rigid wing with a root-chord Reynolds number of 9 x 10 4

Investigation of Tip Vortex on Aerodynamic Performance of a Micro Air Vehicle

Tip vortex induces downwash movement that reduces the effective angle of attack. When a wing has a relatively low aspect ratio, such as that employed by the micro air vehicle (MAV), the induced drag by the tip vortex is relatively large and therefore the aerodynamic performances of the vehicle are deteriorated. In this paper we study the MAV wing aerodynamics using the endplate to help probe the tip vortex effects. The investigation is facilitated by solving the Navier-Stokes equations around a rigid wing with a root chord Reynolds number of 9x10. It is confirmed that with modest angle of attack, the endplate can reduce the downwash, and therefore increase the effective angle of attack and the lift. However, as the angle of attack becomes higher than 15, the wing tip vortex is stronger and the endplate can no longer affect the vortex structure to improve lift. Furthermore, drag also increases along with the endplate.

A Numerical and Experimental Investigation of Flexible Micro Air Vehicle Wing Deformation

A class of micro air vehicles (MAVs), developed at the University of Florida, implements a flexible-wing approach as a viable method for dealing with environmental disturbances such as wind gusts. This paper investigates the steady-state deflection field across a membrane wing. Experimental results are found using a low-speed wind tunnel in conjunction with a visual image correlation system. Numerical models are also formulated, consisting of the first iteration of a fluid-structure interaction problem. The pressure redistribution corresponding to the wing’s change in shape can be ignored for small deformations. The computational results from this single iteration are compared to the experimental deformation field, with good correlation between the two through a range of flight conditions.

Aerodynamics of Low Reynolds Number Flyers: Flapping-Wing Aerodynamics

Flying animals flap wings to create lift and thrust as well as to perform remarkable maneuvers with rapid accelerations and decelerations. Insects, bats, and birds provide illuminating examples of utilizing unsteady aerodynamics to design future MAVs. Pioneering work on flapping-wing aerodynamics was done by Lighthill (1969) and Weis-Fogh (1973). Recent works, both in experiments and simulations, were documented by Katz (1979), Ellington (1984a), DeLaurier (1993), Smith (1996), Vest and Katz (1996), Liu and Kawachi (1998), Dickinson et al. (1999), Jones and Platzer (1999, 2003), Wang (2000), and Chasman and Chakravarthy (2001). A review of the characteristics of both flapping wings and fixed wings was given by Shyy et al. (1999a). The spectrum of animal flight with flapping wing was presented by Templin (2000). Ho et al. (2003) further reviewed the recent effort in developing flapping-wing-based MAVs. Computational and experimental studies regarding rotating-wing MAVs were made by Bohorquez et al. (2003). Aerodynamic phenomena associated with biological flight prominently features unsteady motions, characterized by large-scale vortex structures, complex flapping kinematics, and flexible-wing structures. Furthermore, knowledge gained from studying biological flight shows that the steady-state aerodynamic theory can be seriously challenged to explain the lift needed for biological flyers (Brodsky, 1994; Ellington, 1984a; Ellington et al., 1996). The quasi-steady theory is constructed based on the instantaneous velocity, wing geometry, and AoA when the steady-state aerodynamic model is used.

Aerodynamics of Low Reynolds Number Flyers: Flexible-Wing Aerodynamics

General Background of Flexible-Wing Flyers In the development of MAVs, there are three main approaches, which are based on flapping-wings, rotating wings, and fixed wings for generating lift. We focus on the fixed, flexible-wing aerodynamics in this chapter. It is well known that flying animals typically have flexible wings to adapt to the flow environment. Birds have different layers of feathers, all flexible and often connected to each other. Hence, they can adjust the wing planform for a particular flight mode. The flapping modes of bats are more complicated than those of birds. Bats have more than two dozen independently controlled joints in the wing (Swartz, 1997) and highly deforming bones (Swartz et al., 1992) that enable them to fly at either a positive or a negative AoA, to dynamically change wing camber, and to create a complex 3D wing topology to achieve extraordinary flight performance. Bats have compliant thin-membrane surfaces, and their flight is characterized by highly unsteady and 3D wing motions (Figure 3.1). Measurements by Tian et al. (2006) have shown that bats exhibit highly articulated motion, in complete contrast to the relatively simple flapping motion of birds and insects. They have shown that bats can execute a 180° turn in a compact and fast manner: flying in and turning back in the space of less than one half of its wingspan and accomplishing the turn within three wing beats with turn rates exceeding 200°/s.