Flexible Membrane Wing Research Articles

Investigation of a robust tendon-sheath mechanism for flexible membrane wing application in mini-UAV

Two inherent issues manifest themselves in flying mini-unmanned aerial vehicles (mini-UAV) in the dense area at tropical climate regions, namely disturbances from gusty winds and limited space for deployment tasks. Flexible membrane wing (FMW) UAVs are seen to be potentials to mitigate these problems. FMWs are adaptable to gusty airflow as the wings are able to flex according to the gust load to reduce the effective angle-of-attack, thus, reducing the aerodynamic loads on the wing. On the other hand, the flexible structure is allowing the UAV to fold in a compact package, and later on, the mini-UAV can be deployed instantly from the storage tube, e.g. through a catapult mechanism. This paper discusses the development of an FMW UAV actuated by a tendon-sheath mechanism (TSM). This approach allows the wing to morph to generate a rolling moment, while still allowing the wing to fold. Dynamic characteristics of the mechanism that exhibits the strong nonlinear phenomenon of friction on TSM are modeled and compensated for. A feed-forward controller was implemented based on the identified nonlinear behavior to control the warping position of the wing. The proposed strategy is validated experimentally in a wind tunnel facility by creating a gusty environment that is imitating a realistic gusty condition based upon the results of computational fluid dynamics (CFD) simulation. The results demonstrate a stable and robust wing-warping actuation, even in gusty conditions. Accurate wing-warping can be achieved via the TSM, while also allowing the wings to fold.

Unsteady flow over flexible wings at different low Reynolds numbers

In this study, unsteady flow around flexible membrane wing which had aspect ratio of 1 (AR=1) was investigated experimentally at various Reynolds numbers (Re = 25000 and Re = 50000). Smoke-wire technique for flow visualization over the flexible membrane wing was utilized in the experiments. Digital Image Correlation system (DIC) was used for measuring deformation of AR = 1 flexible membrane wing. Instantaneous deformation measurements of membrane wing were combined with the flow field measurements. In low aspect ratio flexible membrane wings, unsteadiness includes tip vortices and vortex shedding, and the combination of tip vortices. In these types of wings, complex unsteady deformations occurred due to vortex shedding. The results showed that the increasing angle of attack results in increase of membrane deformation. Moreover, it was concluded that analysis of the instantaneous deformation revealed chordwise and spanwise, modes which were due to the shedding of leading-edge vortices as well as tip vortices. Consequently, vibrational mode decreased and maximum standard deviation location approached to the trailing edge by reason of increasing angle of attack.

Effect of tip vortices on membrane vibration of flexible wings with different aspect ratios

In this study, the effect of the aspect ratio on the aerodynamics characteristic of flexible membrane wings with different aspect ratios (AR = 1 and AR = 3) is experimentally investigated at Reynolds number of 25000. Time accurate measurements of membrane deformation using Digital Image Correlation system (DIC) is carried out while normal forces of the wing will be measured by helping a load-cell system and flow on the wing was visualized by means of smoke wire technic. The characteristics of high aspect ratio wings are shown to be affected by leading edge separation bubbles at low Reynolds number. It is concluded that the camber of membrane wing excites the separated shear layer and this situation increases the lift coefficient relatively more as compared to rigid wings. In membrane wings with low aspect ratio, unsteadiness included tip vortices and vortex shedding, and the combination of tip vortices and vortex shedding causes complex unsteady deformations of these membrane wings. The characteristic of high aspect ratio wings was shown to be affected by leading edge separation bubbles at low Reynolds numbers whereas the deformations of flexible wing with low aspect ratio affected by tip vortices and leading edge separation bubbles.

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.

Experimental Study on the Effect of Skin Flexibility on Aerodynamic Performance of Flapping Wings for Micro Air Vehicles

In the present study, the aerodynamic characteristics such as time-averaged lift and drag generation of two flexible membrane (latex thin and thick) wings with different skin flexibilities are compared with those of a conventional rigid (wood) wing to assess the effects of skin flexibility (rigidity) on the aerodynamic performance for flapping flight applications. The experiments are performed in an open circuit wind tunnel of non-return airflow with a test section of (0.3m x 0.3m) and is capable of speeds from 0.5 to 30 m/s. The time-averaged lift and drag as functions of flapping frequency, forward flight velocity and the orientation angle of the flapping motions with respect to the incoming flows are measured by using a strain gauge balance and KYOWA PCD-300A sensor interface data acquisition system. It has been found that flapping motion would bring significant aerodynamic benefits when the flapping flight is in unsteady state regime, with advance ratio less than 1.0. The aerodynamic benefits are found to decay exponentially with the increasing advance ratio. Flapping motion is found to become detrimental for high speed flight applications. It is also observed that the skin flexibility has considerable effect on the aerodynamic performance. The flexible latex thick wing is found to have better overall aerodynamic performance over the rigid wing, especially for low speed applications. The wood (rigid) wing exhibited better lift production performance in quasi steady regime.

Autonomous formation flight of multiple flapping-wing flying vehicles using motion capture system

This study demonstrates an autonomous formation flight of multiple flapping-wing flying vehicles for the first time. In order to extract the position and attitude information of each flying vehicle, an external motion capture system is applied. A path-following controller is designed so that each flapping-wing flying vehicle has a time-independent circular path with the predetermined radius, and the constant forward flight speed and altitude. Due to the variations in flight characteristics among four different flapping-wing flying vehicles, the gain of the directional control, which generates nonlinear rolling moment using the changes of the tension in flexible membrane wings, is individually tuned for each vehicle. The rotational speed of the circular path is kept constant by controlling the desired radius rather than the forward flight speed so that the formation angles between two closest flapping-wing flying vehicles are maintained at 90 degrees. The performances of the individual path-following controllers and the circular formation controller are statistically evaluated in terms of the mean and standard deviation of the flight state variables.

Impact of flexibility on the aerodynamics of an aspect ratio two membrane wing

Development of an aeroelastic solver with application to flexible membrane wings for micro air vehicles is presented. A high-order (up to sixth order) Navier–Stokes solver is coupled with a geometrically nonlinear p-version Reissner–Mindlin finite element plate model to simulate the highly flexible elastic membrane. An implicit LES approach is employed to compute the mixed laminar/transitional/turbulent flowfields present for the low Reynolds number flows associated with micro air vehicles. Computations are performed for an aspect ratio two membrane wing at angles of attack α=10°, 16° and 23° for a Reynolds number, Re=24300. Comparisons of the computational results with experimental PIV and surface deflection measurements demonstrated reasonable agreement. Reduced separation and enhanced lift are obtained due to favorable interactions between the flexible membrane wing and the unsteady flow over the wing. The impact of flexibility on the aerodynamic performance comes primarily from the development of mean camber with some further effects arising from the interaction between the dynamic motion of the membrane and the unsteady flowfield above. At lower angles of attack this lift enhancement comes at the cost of reduced L/D. The nose-down pitching moment increases with flexibility at the lowest angle of attack but is reduced for the higher two angles of attack. These results suggest that membrane flexibility might provide a means to reduce the impact of a strong gust encounter by maintaining lift and reducing the effect of the gust on pitching moment.

Modal Analysis of a Flexible Membrane Wing of Micro Air Vehicles

Modal Analysis of a Flexible Membrane Wing of Micro Air Vehicles

Flapping Wing Structural Deformation and Thrust Correlation Study with Flexible Membrane Wings

This experimental study investigates the relationship between flapping wing structure and the production of aerodynamic forces for micro air vehicle hovering flight by measuring full-field structural deformation and thrust generation. Results from four flexible micromembrane wings with different skeletal reinforcement demonstrate that wing compliance is crucial in thrust production: only certain modes of passive aeroelastic deformation allow the wing to effectively produce thrust. The experimental setup consists of a flapping mechanism with a single-degree-of- freedom rotary actuation up to 45 Hz at 70 deg stoke amplitude and with power measurement, a force and torque sensor that measures the lift and thrust, and a digital image correlation system that consists of four cameras capable of capturing the complete stroke kinematics and structural deformation. Several technical challenges related to the experimental testing of microflapping wings are resolved in this study: primarily, flapping wings less than 3 in. in length produce loads and deformations that are difficult to measure in an accurate and nonintrusive manner. Furthermore, the synchronization of the load measurement system, the vision-based wing deformation measurement system, and the flapping mechanism is demonstrated. Intensive data analyses are performed to extract useful information from the measurements in both air and vacuum.

An experimental investigation on the aerodynamic performances of flexible membrane wings in flapping flight

An experimental study was conducted to assess the aerodynamic benefits of flapping flight compared with fixed-wing soaring flight for the development of flapping-wing Micro-Air-Vehicles (MAVs). The time-averaged aerodynamic performances (i.e. mean lift and thrust/drag) of two flexible membrane wings with different skin flexibility (i.e., a flexible nylon wing and a very flexible latex wing) were compared with that of a conventional rigid wing to evaluate the effects of skin flexibility of the tested wings on their aerodynamic performances for flapping flight applications. The measurement results revealed clearly that, for all the tested wings, flapping motion would bring significant aerodynamic benefits when the flapping flight is in unsteady state regime with advance ratio (i.e., the ratio of forward flight speed to wingtip velocity) of the flapping flight being smaller than 1.0. The aerodynamic benefits of flapping flight were found to decay rapidly as the advance ratio increases. The skin flexibility of the tested wings was found to have considerable effects on their aerodynamic performances for both soaring and flapping flights: The flexible membrane wings were found to have better overall aerodynamic performance (i.e., lift-to-drag ratio) over the rigid wing for soaring flight, especially for high speed soaring flight or at relatively high angle of attack. The rigid wing was found to have better lift production performance for flapping flight in general. The latex wing, which is the most flexible among the three tested wings, was found to have the best thrust generation performance for flapping flight. The less flexible nylon wing, which has the best overall aerodynamic performance for soaring flight, was found to be the worst for flapping flight applications.

High fidelity computational simulation of a membrane wing airfoil

Computations and analysis for a two-dimensional flexible membrane wing airfoil are presented. A well-validated, robust, sixth-order Navier–Stokes solver is employed coupled with a membrane structural model suitable for the highly nonlinear structural response of the membrane. A low Reynolds number, Re=2500, consistent with MAV flight is chosen for the majority of the calculations. The most notable effect of the membrane flexibility is the introduction of a mean camber to the membrane airfoil. A close coupling between unsteady vortex shedding and the dynamic structural response is demonstrated. The dynamic motion of the membrane surface is also shown to significantly alter the unsteady flow over the membrane airfoil at high angles of attack. The coupling of this dynamic effect and the mean camber results in a delay in stall with enhanced lift for higher angles of attack. Exploratory computations investigating the effects of angle of attack, membrane rigidity, membrane pretension and Reynolds number on the membrane airfoil response are also presented.

Investigation of Membrane Actuation for Roll Control of a Micro Air Vehicle

A class of micro air vehicles uses a flexible membrane wing for weight savings and passive shape adaptation. Such a wing is not amenable to conventional aileron mechanisms for roll control, due to a lack of internal wing structure. Therefore, morphing (in the form of asymmetric twisting) is implemented through the use of a torque-actuated wing structure with thousands of discrete design permutations. A static aeroelastic model of the micro air vehicle is developed and validated to optimize the performance of the torque-actuated wing structure. Objective functions include the steady-state roll rate and the lift-to-drag ratio incurred during such a maneuver. An optimized design is obtained through the use of a genetic algorithm presenting significant improvements in both performance metrics compared with the baseline design.

Shape Optimization of a Membrane Wing for Micro Air Vehicles

§† Micro air vehicles with a maximal dimension of 15cm require original design concept due to their low Reynolds number flight regime. It has been empirically observed that a flexible membrane wing can improve the aerodynamic performance of the vehicles. To help advance our knowledge in this area, we investigate shape optimization of a membrane wing. A direct membrane wing optimization employing a coupled Navier-Stokes and flexible structure analysis is computationally expensive. Therefore, we use a rigid wing as a surrogate model. We employ a moving grid technique to facilitate automatic grid generation. Our objective is to maximize the lift-to-drag ratio under aerodynamic and geometry constraints. The optimized design exhibits reduced camber from root to tip. Overall, the aerodynamic improvement is primarily derived from the inner 70% of the wing. The optimized platform is checked for performance of the membrane wings and found to improve the performance by about the same margin. Furthermore, both optimized membrane and rigid wings improve the liftto-drag ratios mainly by reducing the form drag. However, the membrane wing demonstrates less variation in lift-to-drag ratio as the angles of attack vary.

Computation of aerodynamic coefficients for a flexible membrane airfoil in turbulent flow: A comparison with classical theory

In the present paper an aeroelastic model of flexible membrane wing aerodynamics which incorporates the Reynolds-averaged Navier–Stokes equations is presented. The Reynolds stresses are prescribed by the k–ω shear-stress transport eddy-viscosity model recently proposed by Menter. The computed coefficients are compared with classical inviscid membrane airfoil theory and with a portion of the available experimental data for membrane wings. The results indicate that classical potential-based membrane airfoil theory can provide a meaningful description of membrane wing aerodynamics only for a small range of incidence angles near ideal and then only for membrane airfoils with small excess length ratios. For larger excess lengths and incidence angles viscous effects dominate the aerodynamics. The agreement of the computed results with the experimental data is mixed. The current status of the available experimental data for membrane airfoils is also reviewed.

Computational model of flexible membrane wings in steady laminar flow

A computational procedure is presented that models the interaction of a two-dimensional flexible membrane wing and laminar, high-Reynolds-number fluid flow. The membrane wing model is derived by combining a spatial-coordinate-based finite difference formulation of the equilibrium statement for an elastic membrane with a pressure-based control volume formulation of the incompressible Navier-Stokes equations written in general curvilinear body-fitted coordinates. The model is applied to initially flat membrane wings of both vanishing and finite material stiffness as well as to flexible inextensible wings with excess length. Computational results are presented for Reynolds numbers between 2 x 10 3 and 10 4 . The results from the viscous-flow-based membrane wing model are compared with predictions using a potential-flow-based model as well as with experimental data for membrane wings in turbulent flow. Although the assumption of laminar flow precludes a quantitative comparison with the available experimental data, the solutions obtained capture many of the significant features of the aeroelastic interaction that are unaccounted for with a potential flow description of the fluid dynamics.

Aerodynamic characteristics of a flexible membrane wing

The results of experimental studies of the aerodynamic characteristics of single-surfac e membrane wings constructed of inextensible flexible material are described and compared with theory. The experiments included measurement of the lift and drag characteristics of semispan wing models of three different planforms (triangular, parabolic, and elliptical). The wing surfaces were constructed of 2- and 4-mil stainless steel shim stock attached to a streamline leading-edge spar. This very thin metal membrane is flexible and adjusts its shape in response to the airflow over the wing surface. The experimental apparatus permitted variation of the camber and twist distribution of the wing. A unique method of describing the geometry of the wing is presented and used to correlate the experimental results and to compare experiment with theory. The experimental results agree well with lifting line theory below stall using the method presented for determining the geometric shape of the wing. The wing with the parabolic planform exhibited the best aerodynamic characteristics compared to the triangular and elliptical planforms. The maximum lift coefficients and lift-to-drag ratios measured are comparable to conventional rigid wings. Various dynamic phenomena encountered during the experiments are described.

3-D program predicting the flexible membrane wings aerodynamic properties

This paper presents a technique to determine the aerodynamic forces coefficients for a flexible membrane wing (sail). The method is based on a numerical technique using the vortex lattice method. The sections of the wings are described as cubic polynomial shapes. The method is based on the matricial solving of a set of simultaneous equations with the equivalent vortices strengths as variables. An example of calculations are provided for a parrellipedic sail planform. The solution is expressed in terms of a range of excess length of fabric. This technique can be used in coordination with a constrained numerical optimizer program to design optimal sails shapes.