Wing Twist Research Articles

Tailless control of a four-winged flapping-wing micro air vehicle with wing twist modulation.

This paper describes the tailless control system design of a flapping-wing micro air vehicle in a four-winged configuration, which can provide high control authority to be stable and agile in flight conditions from hovering to maneuvering flights. The tailless control system consists of variable flapping frequency and wing twist modulation. The variable flapping frequency creates rolling moments through differential vertical force from flapping mechanisms that can be independently driven on the left and right sides. The wing twist modulation changes wing tension, resulting in vertical and horizontal force variations during one flap cycle and generating pitching and yaw moments. We presume that the wing geometry and implementation method of wing-root actuation are related to the control authority of wing twist modulation. Then, the control system's performance is analyzed for various wing geometries and implementation methods, including wing length, leading-edge thickness, camber angle, and vein configuration. Furthermore, the cross-coupling effect is examined for the wing twist modulation, and a control surface interconnect is designed to compensate for the decrease of pitch control authority and adverse rolling moment. The refined wing and control mechanism demonstrated its high control authority without significant loss of vertical force and power efficiency. The flight experiments validated that the control system based on wing twist modulation is suitable for four-winged flapping-wing micro air vehicles, providing sufficient control moment and minimizing the cross-coupling effect.

Hovering flapping wings with dynamic twist

The role of (active) dynamic wing twist on aerodynamic performance of three-dimensional hovering flapping flight is explored using numerical simulations. A variety of cases with different pitch angles and with (flexible wings) or without (rigid wings) dynamic twist are compared. The results show that changes in aerodynamic performance due to dynamic twist are comparable to those obtained without twist (rigid wing cases) by pitching the whole wing and that lift and lift-to-power ratio generally collapse onto a single curve when plotted as a function of the mid-stroke pitch angle at 2/3 wing radius. However, in some cases dynamic twist yields enhanced time-averaged efficiency. Using the force and power partitioning method, it is shown that this enhancement results from the absence of vortical structures near the wing root lower surface and to the presence of an extended leading edge vortex on the wing upper surface, when compared to the most efficient rigid wing case. These differences in flow topology lead to enhanced lift during the early phase of the strokes without changes in power consumption.

АНАЛІЗ ВПЛИВУ КУТА СТРІЛОПОДІБНОСТІ НА ХАРАКТЕРИСТИКИ СТАТИЧНОЇ АЕРОПРУЖНОСТІ КРИЛА НАВЧАЛЬНО-ТРЕНУВАЛЬНОГО ЛІТАКА

The article is dedicated to the analysis of static aeroelastic phenomena in the wing of a trainer aircraft and the general influence of wing sweep on critical speeds. Aeroelasticity studies the interaction between aerodynamic and elastic forces and the impact of this interaction on the aircraft's structure. There are two types of aeroelasticity: static and dynamic. The article reviews and analyzes publications related to this issue. The analysis of the research shows that when designing an aircraft, it is necessary to determine the aeroelastic characteristics of its aerodynamic surfaces. In addition to structural rigidity, the sweep angle also influences these phenomena. The scientific novelty of the study lies in the fact that, unlike the analyzed research, the designed wing structure of a trainer aircraft is considered.For the designed trainer aircraft, the critical speeds of static aeroelastic phenomena were determined. The calculation of the influence of the wing sweep angle on the divergence speeds of the trainer aircraft confirms previously obtained results regarding this influence. When the sweep is increased or decreased for wings with the same span, the torsional rigidity of the wing decreases due to the increase in the length of the line of centers of rigidity. However, the most significant impact on the aeroelastic characteristics is made by the shift of the bending center when the sweep angle changes.With forward sweep, the arm of the aerodynamic forces relative to the bending center increases, thereby reducing the critical divergence speeds. For straight sweep, as the sweep increases, the bending center moves forward, and at a certain point, the bending center is located in front of the aerodynamic center. This means that the wing twist angle decreases as the aerodynamic force increases, which, in the considered planar case, eliminates the occurrence of divergence.The preliminarily determined maximum flight speed does not exceed the critical speeds of divergence and aileron reversal. This result represents the practical value of the study. To achieve higher flight speeds, wings with straight sweep should be used. More precise computational or experimental methods for determining aeroelastic characteristics should be used in the subsequent design stages to prevent aeroelastic phenomena across the entire operational range of speeds and altitudes of the trainer aircraft.

Wing deformation improves aerodynamic performance of forward flight of bluebottle flies flying in a flight mill.

Insect wings are flexible structures that exhibit deformations of complex spatiotemporal patterns. Existing studies on wing deformation underscore the indispensable role of wing deformation in enhancing aerodynamic performance. Here, we investigated forward flight in bluebottle flies, flying semi-freely in a magnetic flight mill; we quantified wing surface deformation using high-speed videography and marker-less surface reconstruction and studied the effects on aerodynamic forces, power and efficiency using computational fluid dynamics. The results showed that flies' wings exhibited substantial camber near the wing root and twisted along the wingspan, as they were coupled effects of deflection primarily about the claval flexion line. Such deflection was more substantial for supination during the upstroke when most thrust was produced. Compared with deformed wings, the undeformed wings generated 59-98% of thrust and 54-87% of thrust efficiency (i.e. ratio of thrust and power). Wing twist moved the aerodynamic centre of pressure proximally and posteriorly, likely improving aerodynamic efficiency.

The spatiotemporal richness of hummingbird wing deformations.

Animals exhibit an abundant diversity of forms, and this diversity is even more evident when considering animals that can change shape on demand. The evolution of flexibility contributes to aspects of performance from propulsive efficiency to environmental navigation. It is, however, challenging to quantify and compare body parts that, by their nature, dynamically vary in shape over many time scales. Commonly, body configurations are tracked by labelled markers and quantified parametrically through conventional measures of size and shape (descriptor approach) or non-parametrically through data-driven analyses that broadly capture spatiotemporal deformation patterns (shape variable approach). We developed a weightless marker tracking technique and combined these analytic approaches to study wing morphological flexibility in hoverfeeding Anna's hummingbirds (Calypte anna). Four shape variables explained >95% of typical stroke cycle wing shape variation and were broadly correlated with specific conventional descriptors such as wing twist and area. Moreover, shape variables decomposed wing deformations into pairs of in-plane and out-of-plane components at integer multiples of the stroke frequency. This property allowed us to identify spatiotemporal deformation profiles characteristic of hoverfeeding with experimentally imposed kinematic constraints, including through shape variables explaining <10% of typical shape variation. Hoverfeeding in front of a visual barrier restricted stroke amplitude and elicited increased stroke frequencies together with in-plane and out-of-plane deformations throughout the stroke cycle. Lifting submaximal loads increased stroke amplitudes at similar stroke frequencies together with prominent in-plane deformations during the upstroke and pronation. Our study highlights how spatially and temporally distinct changes in wing shape can contribute to agile fluidic locomotion.

Effects on Aircraft Performance Due to Geometrical Twist of Wing

This paper provides an overview of the effects of Geometric Twist on Aircraft performance by introducing a washout condition in an aircraft wing. This condition results in an effective Angle of Attack at the wingtip that is lower than the Angle of Attack at the wing root. Using CFD analysis, the variation of aircraft performance factors such as lift, drag coefficients, and aerodynamic efficiency is calculated for different Twist angles. A plot of lift and drag coefficients at varying Angle of Attack angles has been generated based on these analyses. These results illustrate the advantages of wing twist variation, particularly at higher angles of attack. One significant advantage of Geometric Twist is that it causes stall conditions to first occur at the wing root, providing a signal to the pilot to control the aircraft before the stall reaches the wingtip. This ensures the effectiveness of control surfaces, such as Ailerons and Flaps, located at the wing trailing edge. A comparison is made between the lift, drag coefficients, and aerodynamic efficiency of twisted wings and untwisted wings with identical parameters. While twisting the wingtip yields favourable results at higher Angles of Attack (AOA) compared to an untwisted wing, the aerodynamic efficiency of the wings decreases at lower AOA. However, applying the twist angle at high angles of attack, such as 10, 12.5, 15, and 20 degrees, leads to an increase in aerodynamic efficiency.

Controlling roll-yaw coupling with aileron and twist design

AbstractMost simple ailerons produce adverse yaw. However, with proper aileron placement and wing twist, an aileron can produce proverse or neutral yaw, eliminating the need for aileron-rudder mixing, differential aileron deflection or Frise ailerons. The relationship between wing planform, aileron placement and lift distribution is studied here for a special class of optimal lift distributions that minimise induced drag for a variety of design constraints. It is shown that a wing employing the elliptic lift distribution will always produce adverse yaw, independent of aileron design or operating condition. However, for wings employing other optimal lift distributions, the ailerons can be placed to produce proverse or neutral yaw. A numerical lifting-line algorithm is used to explore the impact of aileron design on a wide range of wing planforms and lift distributions. Results can be used in the early stages of design to correctly place ailerons with respect to desired roll-yaw coupling.

Fluid-structure analysis of flapping-wing rotorcraft considering stiffness influence

As a novel aerial vehicle, flapping-wing rotorcrafts (FWRs) leverage both aerodynamic advantages of flapping and rotary wings, which can enhance the lift by using the passive rotational effect generated by the flexible deformation of their center symmetrical flapping wings. Due to FWRs’ unique wing kinematics, considering the aeroelastic deformation of wings is essential for the aerodynamic optimization of FWRs. In this paper, we propose a computational method of fluid-structure interaction for FWRs based on a simplified structural model. Such a method guarantees accurate results and works with high computational efficiency. To experimentally validate the proposed computational method, motion capture testing is constructed to simultaneously measure FWR wing deformation and aerodynamic forces during flapping. By comparing the experimental and computational results, the proposed method is capable of predicting the flapping lift effectively. Moreover, it can interpret the wing rotation and twist of the tested FWR. The computational efficiency in this paper is extremely close to that of the CFD calculation without considering the fluid-structure interaction. Based on this proposed computational method, the lift force of the test FWR platform is increased by 36.2 % by changing wing stiffness.

Twisting morphing wings with tight geometric constraints for biomimetic swimming or flying robotic vehicles

Active twisting morphing (ATM) wings have significant implications for biomimetic swimming or flying robotic vehicles. This article considers an ATM wing that has one shaft with many ribs, each rib is driven to swing in its individual plane that is perpendicular to the shaft, showing increasingly swing angles from the wing base to the wing-tip and thus form active wing twisting. For the overall twisting angle of the wing that is less than 90 degrees (e.g., during cruising operations of the robotic vehicles), we show that the geometry of mechanical implementations of the twisting wing can be further tightened using segment gears for transmission, instead of normal whole gears, which constraints have been largely unexplored. Here we provide the geometric constraints on the transmission with segment gears for such wing and give tighter feasible solutions of the constraints as well as provide the mechanical implementations using segment gears that are compact for ATM wings for biomimetic robotic vehicles.

Wing Shape Optimization Using FFD and Twist Parameterization

Optimization of wing shapes for aerodynamic performance is presented using a combination of particle swarm method and surrogate models. The wing shape deformations are parameterized using free form deformation together with wing twist. The developed strategy is applied to the lift-constrained drag minimization of Onera M6 wing.

Lift enhancement of a butterfly-like flapping wing vehicle by reinforcement learning algorithm

In order to enhance the take-off lift of a butterfly-like flapping wing vehicle (FWV), we implemented an integrated experimental platform and applied a reinforcement learning algorithm. The vehicle, which has a wingspan of 81 cm and is mounted on a stand with a force sensor, is driven by two servos that are powered and controlled wirelessly. To achieve the goal of enhancing take-off lift, we used a model-free, on-policy actor-critic proximal policy optimization algorithm. After 300 learning steps, the average aerodynamic lift force increased significantly from 0.044 N to 0.861 N. This enhanced lift force was sufficient to meet the take-off requirements of the vehicle without the need for any additional aids or airflow. Additionally, we observed a strong lift peak in the upstroke after analyzing the learning results. Further experiments showed that this lift peak is directly related to the elastic release of the wing twist and the opening and closing of the gap between the forewing and hindwing in the early stage of the upstroke. These findings were not easily predicted or discovered using traditional aerodynamic methods. This work provides valuable reinforcement learning experience for the future development of FWVs.

Wing twist angle predictions using finite element model unit load cases

To develop a wing twist angle predictive model, fiber Bragg gratings were installed on a wing and were simulated with rod elements in the wing's finite element model. The finite element model was preliminary used to determine the optimal orientations in which the fibers should be installed to measure the principal strain induced by twist. Simple load cases, created using finite element unit load cases linear combinations, were used to fit parameters of a linear model. Model predictions were first validated on bending and twist simulated load cases and then on experimental twist tests. The predicted wing twist angle relative errors were two times inferior to the targeted error range. This new method seems more accurate than methods presented in previous studies.

Development of a morphing UAV for optimal multi-segment mission performance

AbstractThe need for innovative solutions to enable aerial platforms to fly faster, higher, and longer continues to remain a primary focus for airframe designers. This paper outlines work undertaken to apply a morphing wing warping technology onto a generic Unmanned Aerial Vehicle to deliver enhanced flight performance, efficiency and control capabilities. The prototype employs wings of novel construction which provide both near resistance-free compliance in twist as well as adequate structural stiffness to resist applied loads; all while preserving an aerodynamically smooth surface. Used in combination with developed and integrated closed-loop feedback control architecture, a real-time, non-linear, span-wise wing twist adjustment capability required for optimised flight under differing operating conditions and flight requirements, is demonstrated. Experimental results obtained from a wind tunnel test program show up to a 72% increase in lift to drag ratio under certain conditions compared to a fixed baseline providing some confidence that the combination could be used to realise a step change in flight performance.

The spatial-temporal effects of wing flexibility on aerodynamic performance of the flapping wing

In this paper, three-dimensional fluid–structure interaction simulation of flapping of a flexible wing is carried out. The aerodynamic effect of the flexible wing can be explained by analyzing the spatial and temporal effects of wing flexibility on aerodynamic performance. It is concluded that the flexible wing can increase the average lift and the aerodynamic efficiency. The spatial influence of flexible deformation mainly comes from the contribution of camber. In the mid-downstroke, wing flexibility results in significant camber near the wingtip, which is conducive to the attachment of the leading-edge vortex to the wing surface, thus enhancing the ability of the wingtip to generate lift. The temporal influence of flexible deformation mainly comes from the contribution of twist and bend. The fast pitching-down rotation due to the wing twist in the early downstroke is conducive to the accumulation of vorticity. The spanwise bend of the flexible wing due to the aerodynamic force and inertia can increase the flapping amplitude, which accounts for the lift increase. The above spatial-temporal effects make the flexible wing have better performance in generating lift and aerodynamic efficiency. The results are beneficial to systematically understand the aerodynamic effects of insect wing deformation and can provide guidance for the wing design of micro aerial vehicles.

High-Rate, High-Precision Wing Twist Actuation for Drone, Missile, and Munition Flight Control

This paper covers a new actuation and deflection controller configuration for high-aspect-ratio wings used on subsonic drones, missiles, and munitions. Current approaches to the flight control of these aircraft have unearthed challenges with friction, stiction, slop, bandwidth, and thick boundary layer nonlinearities, which degrade flight control accuracy—especially in terminal flight phases. The approach described in this paper uses directionally attached piezoelectric (DAP) actuators to actively twist a high-aspect-ratio wing for flight control. The DAP actuators were modeled analytically and computationally using linear finite element modeling. A 3″ (7.62 cm) chord × 15″ (38.1 cm) semispan rectangular wing with an NACA 0012 profile was built and structurally tested, demonstrating excellent agreement between theory and experiment. New actuation methods were used to overdrive the PZT-5H piezoelectric elements deep into the repoling range. This overdrive actuation rejuvenated the actuator elements and allowed for dramatically improved deflections with respect to configurations in previous years. Static testing demonstrated deflections in excess of ±1.6° in root-to-tip twist. Dynamic testing showed corner frequencies greater than 310 Hz. A series of wind tunnel tests at up to 180 ft/s (55 m/s, 123 mph, 107 kts, 198 kph) demonstrated excellent roll control authority, rapid manipulation of Clδ, and lift manipulation using quasi-static deflections. The paper concludes with a summary of implications for terminal guidance for drone, missile, and munition flight control in real atmospheres.

Fuel burn evaluation of a transonic strut-braced-wing regional aircraft through multipoint aerodynamic optimisation

AbstractThis paper presents a relative fuel burn evaluation of the transonic strut-braced-wing configuration for the regional aircraft class in comparison to an equivalent conventional tube-and-wing aircraft. This is accomplished through multipoint aerodynamic shape optimisation based on the Reynolds-averaged Navier-Stokes equations. Aircraft concepts are first developed through low-order multidisciplinary design optimisation based on the design missions and top-level aircraft requirements of the Embraer E190-E2. High-fidelity aerodynamic shape shape optimisation is then applied to wing–body–tail models of each aircraft, with the objective of minimising the weighted-average cruise drag over a five-point operating envelope that includes the nominal design point, design points at $\pm 10\%$ nominal $C_L$ at Mach 0.78, and two high-speed cruise points at Mach 0.81. Design variables include angle-of-attack, wing (and strut) twist and section shape degrees of freedom, and horizontal tail incidence, while nonlinear constraints include constant lift, zero pitching moment, minimum wing and strut volume, and minimum maximum thickness-to-chord ratios. Results show that the multipoint optimised strut-braced wing maintains similar features to those of the single-point optimum, and compromises on-design performance by only two drag counts to achieve up to 11.6% reductions in drag at the off-design conditions. Introducing low-order estimates for approximating full aircraft performance, results indicate that the multipoint optimised strut-braced-wing regional jet offers a 13.1% improvement in cruise lift-to-drag ratio and a 7.8% reduction in block fuel over a 500nmi nominal mission when compared to the similarly optimised Embraer E190-E2-like conventional tube-and-wing aircraft.

Exploration of the effect of wing component post-buckling on bending-twist coupling for nonlinear wing twist

A new investigated concept for passive load alleviation is to exploit the nonlinear behavior of wing design components to trigger a deformation which reduce loads once a critical load level is reached. The necessary deformation is a torsional rotation which is supposed to reduce the angle of attack. For this target, wingbox sections are investigated regarding their nonlinear behavior with finite element analysis. Parameter studies feature anisotropic carbon fiber reinforced polymer (CFRP) layups for the skins, layups and thicknesses for spars and the presence of stringers. Results show a desired nonlinear progressive bending-torsion coupling for an unstiffened wingbox section, when the upper skin and the rear spar are modified. After modification they are allowed to buckle within the load envelope. The skin has an anisotropic layup. The rear spar needs to be thinner than the front spar. Both modifications result in progressively increasing torsional rotation of the wingbox with increasing load. Stringers are not applied because they limit the nonlinearity which is not desired for the envisioned load alleviation technique.

A passive load alleviation aircraft wing: topology optimization for maximizing nonlinear bending–torsion coupling

Aircraft wings with passive load alleviation morph their shape to a configuration where the aerodynamic forces are reduced without the use of an actuator. In our research, we exploit geometric nonlinearities of the inner wing structure to maximize load alleviation. In order to find designs with the desired properties, we propose a topology optimization approach. Passive load alleviation is achieved through bending–torsion coupling. The wing twist will reduce the angle of attack, thus lowering the aerodynamic forces. Consequently, the objective function is to maximize the torsion angle. Since shape morphing should only affect loads that exceed normal maneuvering loads, a displacement constraint is enforced, preventing torsion at lower force levels. Maximizing the displacement will lead to topologies for which the finite element solver cannot find a solution. To circumvent this, we propose adding a compliance value to the objective function. This term has a weighting function, which controls how much influence the compliance value has: after a set number of iterations, the initially high level of influence will drop. We used a geometric nonlinear finite element formulation with a linear elastic material model. The addition of an energy interpolation scheme reduces mesh distortion. We successfully applied the proposed methodology to two different test cases resembling an aircraft wing box section. These test cases illustrate the methodology’s potential for designing new geometries with the desired nonlinear behavior. We discuss what design features can be deduced and how they achieve the nonlinear structural response.

Aerodynamic characteristics of flexible flapping wings depending on aspect ratio and slack angle

Experimental investigations are made for the combined effects of aspect ratio (AR), slack (βS), and pitch angles on the aerodynamic characteristics of flexible flapping wings in hover. βS is introduced as a way to indirectly alter the flexibility of the wing. An optimum AR range of 3–5 based on the lift coefficient is observed depending on the flexibility. For a constant AR, the intensity of the leading-edge vortex (LEV) with corresponding circulatory-based lift mitigates as βS increases beyond 2.5°. The variation of βS affects the magnitude of the shed trailing-edge vortices (TEVs) but the vorticity core is maintained. We found the shed TEVs to be the key vortical feature of twistable flexible wings in comparison with the rigid (untwisted) cases. More intriguingly, the negative wing twist played a significant role in sustaining the circulatory lift at the outboard section for even high AR cases. The primary LEV trace is found to be an indicator for the effective spanwise limit of the LEV. Although an increase in AR reduces the effective spanwise limit, it is found that wing flexibility further decreases the radial distance. Again, the study reveals that lift enhancement in the rigid wing requires a wider effective downwash area induced by the outward movement of the LEV traces to merge with the tip vortex. Contrarily, the flexible wing requires an elongated downwash area induced by the wing twist to enhance the aerodynamic performance.

Aerodynamic Analysis and Optimization of Wings for the Jain University Sailplane Using XFLR5

The wing of a sailplane must be designed to conform to specified mission and design requirements. High aerodynamic efficiency and good stall characteristics are some of the design requirements, while achieving level flight at a particular altitude could be a mission requirement. The aerospace engineering students of Jain University set out the task of designing a sailplane capable of flying at an altitude of 7,000 feet. The design task was undertaken using the computational code XFLR5. In the first step, the aerodynamic characteristics of five different airfoils are analyzed considering the effect of altitude. Secondly, the effect of chord length and span on rectangular wing aerodynamics is investigated. The design convergence is obtained for a wing of chord length 1 m and span 20 m. The effect of wing twist on the initially converged design to obtain a more elliptical lift distribution is then performed to obtain optimized designs.