Trailing Edge Deflection Research Articles

Aero-structural optimization and analysis of a camber-morphing flying wing: Structural and wind tunnel testing

This article presents the design, optimization and performance assessment of a novel structure-actuation morphing concept for a flying wing, enabling the flight control for straight flight and around the pitch and roll axes. The applied camber-morphing concept utilizes an optimized selectively compliant internal structure, combined with electromechanical actuators to achieve a trailing edge deflection. These deflections lead to variations of the local and global lift, permitting to control the flight of the aircraft. The aero-structural behaviour of the wing is analysed using a coupled three-dimensional aerodynamic and structural simulation tool. An optimization of the planform, aerodynamic shape, internal structure and actuation parameters is performed to attain a longitudinally stable and aerodynamically efficient flying wing. The drag increment caused by morphing is minimized through the numerical optimization, resulting in high aerodynamic efficiency across a range of flight speeds. The stiffness and morphing capabilities of the manufactured wing are characterized experimentally and are compared with the numerical predictions, and the aerodynamic and aeroelastic behaviour of the wing is investigated through wind tunnel tests. The test results indicate the ability of the flying wing to achieve sufficient variations in lift, roll and pitch to control the flight completely through camber morphing.

Modeling and control of parafoils based on computational fluid dynamics

• Leading-edge incision and trailing-edge deflection are incorporated into the calculation of aerodynamic coefficients . • A flexible dynamic model with eight degrees of freedom is established. • A characteristic modeling method is used to reduce the order of the dynamic model. • Generalized predictive control based on a characteristic model is applied to the parafoil system. The determination of aerodynamic parameters of parafoil canopies has been a crucial issue because it affects the model precision. To calculate the aerodynamic coefficients of a canopy, the lifting-line theory has been used in the traditional method. However, because of the existence of leading-edge incisions, there are some restrictive assumptions in lifting-line theory when one is calculating the aerodynamic coefficients of a canopy. Therefore in this article we calculate the aerodynamic coefficients on the basis of computational fluid dynamics . As an improvement, the effects of a leading-edge incision and trailing-edge deflection are considered. Firstly, lift and drag coefficients are obtained by use of computational fluid dynamics. Then the least-squares method is used to identify incision and deflection factors. Furthermore, an eight-degrees-of-freedom mathematical model of a parafoil system is established on the basis of the parameters obtained. Finally, a novel control algorithm, generalized predictive control based on a characteristic model, is applied to the system. The precision of the model established and the effectiveness of the proposed control method are validated by simulation and airdrop testing.

Clarifying the relationship between efficiency and resonance for flexible inertial swimmers

We study a linear inviscid model of a passively flexible swimmer, calculating its propulsive performance, eigenvalues and eigenfunctions with an eye towards clarifying the relationship between efficiency and resonance. The frequencies of actuation and stiffness ratios we consider span a large range, while the mass ratio is mostly fixed to a low value representative of swimmers. We present results showing how the trailing edge deflection, thrust coefficient, power coefficient and efficiency vary in the stiffness–frequency plane. The trailing edge deflection, thrust coefficient and power coefficient show sharp ridges of resonant behaviour for mid-to-high frequencies and stiffnesses, whereas the efficiency does not show resonant behaviour anywhere. For low frequencies and stiffnesses, the resonant peaks smear together and the efficiency is high. In this region, flutter modes emerge, inducing travelling wave kinematics which make the swimmer more efficient. We also consider the effects of a finite Reynolds number in the form of streamwise drag. The drag adds an offset to the net thrust produced by the swimmer, causing resonant peaks to appear in the efficiency (as observed in experiments in the literature).

Stall Recovery of a Morphing Wing via Extended Nonlinear Lifting-Line Theory

Aircraft morphing provides advantages to traditional flight including drag reduction and maneuverability. Previous research indicates that smooth spanwise transitions in trailing-edge camber, representative of a biological analog, provide aerodynamic benefits at small angles of attack by eliminating vortices at geometric discontinuities but lack nonlinear aerodynamic investigations. This work aims to analyze the adaptability of a spanwise morphing wing concept with respect to nonlinear aerodynamics using an optimized nonlinear extended lifting-line model. In this novel approach, it is shown that adaptation, including stall recovery, can be achieved solely through geometric tailoring as opposed to attitude correction for a range of flight conditions while reducing the drag penalty associated with operating at the unadapted condition. The range of conditions for which the wing can recover are restricted by the limited trailing-edge deflections and the inability of the actuators to substantially shift the stall angle of the section lift curve. These results provide insight into improving morphing wing designs, indicating that, by adding another degree of freedom to the chordwise deformation such as a morphing hinge capable of larger actuation and reflex camber, stall recovery via geometric tailoring may be feasible for an even larger range of conditions.

Morphing Airfoil with Thermally Activated SMA Actuators

In this paper we study a type of design of Shape Memory Alloy (SMA) wire actuated morphing airfoil having discontinuous skin by targeting trailing edge deflection and changing camber. The design aims at airfoils to have optimal directional stiffness and relaxed shear constraint to allow the required change to create a conformal target shape. The trailing edge deflection of the morphing airfoil structure is produced by the electrical actuation via Joule heating of the actuators placed in the airfoil in various different configurations. Actuation response is tested under quasi-static loading and partial loading–unloading cycles applied to the actuators. Deformation characteristics are analyzed for these actuation schemes. Response time for various actuation rates and also the functional fatigue of SMA actuators due to variable stiffness conditions are investigated. Aerodynamic characteristics of this design are analyzed under a simulated free-stream flow velocity of up to 20 m/s and up to aerodynamic stall. For a maximum trailing edge rotation of 9°, a 150% increase in the coefficient of lift is observed. These results are further compared with wind tunnel test results which show promising outcome. Various aspects of aerodynamic performance enhancement and possibilities to realize it with distributed dynamic control via morphing structures are discussed.

Prediction of Aerodynamic Characteristics of Ram-Air Parachutes

The focus of this work is on the computational methodology for aerodynamic modeling of ram-air parachutes and increasing confidence and understanding in their concept designs including new parachute control methods. The complex geometries of ram-air parachutes are modeled by two-dimensional rigid airfoil geometries with or without trailing-edge deflections and bleed air spoilers. The aerodynamic forces are then calculated from steady or unsteady Reynolds-averaged Navier–Stokes simulations using Cobalt and Kestrel flow solvers. The effects of the grid size and type, the time step, and the choice of solver parameters are investigated. The flow solvers are then used to study the flow around three-dimensional wings with open/closed ram-air inlets by comparing lift and drag coefficients with available experimental data. The results show that computational fluid dynamics simulations are a valuable aid in understanding the flow structure of ram-air parachutes, which resemble a rectangular wing with open inlets. However, the computational solutions of these geometries have initial oscillations of large amplitude and converge slowly compared to closed wings/airfoils. The simulation of open geometry should run unsteady and for a large number of time steps. It is also shown that an open-inlet geometry has smaller lift and larger drag, and it stalls earlier than a closed-inlet geometry. Although the air reaches stagnation conditions inside the cavity present in an open airfoil, the air pressure inside the wing cells is less than the stagnation pressure. The flow investigations show that eddies are formed on the lower surface of the open airfoil and wings; however, the wing eddy size varies in the spanwise direction. Finally, the grid sensitivity results show that the solutions of open geometries are very sensitive to the grid quality.

Combined Lateral and Longitudinal Control of Parafoils Using Upper-Surface Canopy Spoilers

Precision placement of guided airdrop systems necessarily requires some mechanism enabling effective directional control of the vehicle. Often this mechanism is realized through asymmetric deflection of the parafoil canopy trailing-edge brakes. In contrast to conventional trailing-edge deflection used primarily for lateral steering, upper-surface bleed air spoilers have been shown to be extremely effective for both lateral and longitudinal (i.e., glide slope) control of parafoil and payload systems. Bleed air spoilers operate by opening and closing several spanwise slits in the upper surface of the parafoil canopy, thus creating a virtual spoiler from the stream of expelled ram air. The work reported here considers the autonomous landing performance of a small-scale parafoil and payload system using upper-surface bleed air spoilers exclusively for both lateral steering and glide slope control. Landing accuracy statistics computed from a series of Monte Carlo simulations in a variety of atmospheric conditions and experimental flight tests were found to be in good agreement. Median miss distances for the combined lateral and longitudinal control logic are on the order of 13 m, indicating an improvement in landing accuracy of nearly 50% over similar systems employing only lateral steering control.

Particle-image velocimetry investigation of the fluid-structure interaction mechanisms of a natural owl wing

The increasing interest in the development of small flying air vehicles has given rise to a strong need to thoroughly understand low-speed aerodynamics. The barn owl is a well-known example of a biological system that possesses a high level of adaptation to its habitat and as such can inspire future small-scale air vehicle design. The combination of the owl-specific wing geometry and plumage adaptations with the flexibility of the wing structure yields a highly complex flow field, still enabling the owl to perform stable and at the same time silent low-speed gliding flight. To investigate the effects leading to such a characteristic flight, time-resolved stereoscopic particle-image velocimetry (TR-SPIV) measurements are performed on a prepared natural owl wing in a range of angles of attack and Reynolds numbers 40 000 based on the chord length at a position located at 30% of the halfspan from the owl’s body. The flow field does not show any flow separation on the suction side, whereas flow separation is found on the pressure side for all investigated cases. The flow field on the pressure side is characterized by large-scale vortices which interact with the flexible wing structure. The good agreement of the shedding frequency of the pressure side vortices with the frequency of the trailing-edge deflection indicates that the structural deformation is induced by the flow field on the pressure side. Additionally, the reduction of the time-averaged mean wing curvature at high Reynolds numbers indicates a passive lift-control mechanism that provides constant lift in the entire flight envelope of the owl.

Development of a Wind Turbine Test Rig and Rotor for Trailing Edge Flap Investigation: Static Flap Angles Case

One of the strategies used to improve performance and increase the life-span of wind turbines is active flow control. It involves the modification of the aerodynamic characteristics of a wind turbine blade by means of moveable aerodynamic control surfaces. Trailing edge flaps are relatively small moveable control surfaces placed at the trailing edge of a blade's airfoil that modify the lift of a blade or airfoil section. An instrumented wind turbine test rig and rotor were specifically developed to enable a wide-range of experiments to investigate the potential of trailing edge flaps as an active control technique. A modular blade based on the S833 airfoil was designed to allow accurate instrumentation and customizable settings. The blade is 1.7 meters long, had a constant 178mm chord and a 6° pitch. The modular aerodynamic parts were 3D printed using plastic PC-ABS material. The blade design point was within the range of wind velocities in the available large test facility. The wind facility is a large open jet wind tunnel with a maximum velocity of 11m/s in the test area. The capability of the developed system was demonstrated through an initial study of the effect of stationary trailing edge flaps on blade load and performance. The investigation focused on measuring the changes in flapwise bending moment and power production for different trailing edge flap spanwise locations and deflection angles. The relationship between the load reduction and deflection angle was linear as expected from theory and the highest reduction was caused by the flap furthest from the rotor center. Overall, the experimental setup proved to be effective in measuring small changes in flapwise bending moment within the wind turbine blade and will provide insight when (active) flap control is targeted.

Aerodynamic behavior of an airfoil with morphing trailing edge for wind turbine applications

The length of wind turbine rotor blades has been increased during the last decades. Higher stresses arise especially at the blade root because of the longer lever arm. One way to reduce unsteady blade-root stresses caused by turbulence, gusts, or wind shear is to actively control the lift in the blade tip region. One promising method involves airfoils with morphing trailing edges to control the lift and consequently the loads acting on the blade. In the present study, the steady and unsteady behavior of an airfoil with a morphing trailing edge is investigated. Two-dimensional Reynolds-Averaged Navier-Stokes (RANS) simulations are performed for a typical thin wind turbine airfoil with a morphing trailing edge. Steady-state simulations are used to design optimal geometry, size, and deflection angles of the morphing trailing edge. The resulting steady aerodynamic coefficients are then analyzed at different angles of attack in order to determine the effectiveness of the morphing trailing edge. In order to investigate the unsteady aerodynamic behavior of the optimal morphing trailing edge, time- resolved RANS-simulations are performed using a deformable grid. In order to analyze the phase shift between the variable trailing edge deflection and the dynamic lift coefficient, the trailing edge is deflected at four different reduced frequencies for each different angle of attack. As expected, a phase shift between the deflection and the lift occurs. While deflecting the trailing edge at angles of attack near stall, additionally an overshoot above and beyond the steady lift coefficient is observed and evaluated.

Fluid structure interaction of a morphed wind turbine blade

SUMMARY One serious challenge of energy systems design, wind turbines in particular, is the need to match the system operation to the variable load. This is so because system efficiency drops at off-design load. One strategy to address this challenge for wind turbine blades and obtain a more consistent efficiency over a wide load range, is varying the blade geometry. Predictable morphing of wind turbine blade in reaction to wind load conditions has been introduced recently. The concept, derived from fish locomotion, also has similarities to spoilers and ailerons, known to reduce flow separation and improve performance using passive changes in blade geometry. In this work, we employ a fully coupled technique on CFD and FEM models to introduce continuous morphing to desired and predetermined blade design geometry, the NACA 4412 profile, which is commonly used in wind turbine applications. Then, we assess the aerodynamic behavior of a morphing wind turbine airfoil using a two-dimensional computation. The work is focused on assessing aerodynamic forces based on trailing edge deflection, wind speed, and material elasticity, that is, Young's modulus. The computational results suggest that the morphing blade has superior part-load efficiency over the rigid NACA blade. Copyright © 2013 John Wiley & Sons, Ltd.

Parafoil Control Authority with Upper-Surface Canopy Spoilers

Current autonomous parafoil and payload aircraft are controlled by deflection of the canopy trailing edge. Asymmetric trailing edge deflection results in a turn, symmetric deflection results in a change in speed with little change in glide slope. The inability to control glide slopewith these traditional canopy controlmechanisms is severely detrimental to the landing accuracy of parafoil and payload aircraft, especially when turbulent atmospheric conditions or difficult terrain are present near the landing area. An alternative canopy control mechanism is investigated where internal air is vented through the upper surface of the canopy to create an aerodynamic spoiler. Flight-test results for various configurations of the spoiler concept are presented. It is shown that a dramatic change in glide slope can be generatedwith relatively small spoiler actuations. Furthermore, it is shown that the same lateral control authority availablewith trailing edge brakes can be achievedwith asymmetric actuation of the upper-surface spoilers.

Parametric dependence of a morphing wind turbine blade on material elasticity

A few recent works have suggested a morphing blade for wind turbine energy conversion. The concept is derived from fin and wing motions that better adapt to varying load conditions. Previous research has provided the fluid mechanic justification of this new concept. This paper establishes a parametric relationship between an asymmetric wind turbine blade and constituent material modulus to predict the geometric response of the morphing blade for a given material characteristic. The airfoil’s trailing edge deflection is associated to a prescribed fluid exit angle via the Moment Area (MA) method. Subsequently, a mathematical model is derived to predict material deformation with respect to imparted aerodynamic forces. Results show that an airfoil, much like a tapered beam, can be modeled as a non-prismatic cantilevered beam using this well established method. ►A mathematical model relating morphing airfoil thickness and elastic modulus was established. ►For non-prismatic beam under a uniform distributive load, the slope and deflection of the airfoil’s trailing edge were related to the fluid exit angle. ►The main driver of blade deformation was the angular drag force. The Moment Area method was used, verified by Finite Element method. ►Displacement to the exit angle is predicated upon the elastic modulus value given that other parameters are constant. ►Optimum power output is obtained in part load conditions when the blade deforms to the applicable exit angle.

Examination of Rotor Loads due to On-Blade Active Controls for Performance Enhancement

On-blade active controls with trailing-edge deflection, leading-edge deflection, and active-twist are studied for improvements in rotor aerodynamic efficiency and their influence on structural loads. A full-scale UH-60A Blackhawk rotor at two key flight conditions (high-speed forward flight and high-thrust forward flight) is studied using coupled computational fluid dynamics and computational structural dynamics simulations. A simulationbased trade study is carried out comprising parametric variations of geometric sizing and deployment schedules of the blade morphing. The study shows that active controls improve rotor performance and reduce rotor loads at the sametimewithcarefulselectionofdeploymentscheduleanddesign.Inhigh-speedforward flight,usingtrailing-edge deflection, an improvement of 7.3% in performance and a reduction in the hub vibratory loads of up to 54% is achieved,and using active-twist animprovement of 7.0% in performance and upto 22% reduction in hub vibratory loads is achieved. In high-thrust forward flight, a 15.0% improvement in performance and up to 40% reduction in hub vibratory loads is achieved using leading-edge deflection.

Shape Memory Alloy Actuator Integrated Morphing Aerofoil Structure

In this paper, we report new design and analysis of aerofoil morphing using Shape Memory Alloy (SMA) wire actuator. A finite element based computational model of the coupled SMA-aerofoil is developed incorporating the non-linear hysteretic deformation of the SMA wire actuator and low-speed aerodynamic pressure. This model is used for analysis of the preliminary aerofoil design. Simulation results from this model are reported. Based on these results, the aerofoil is redesigned and detailed analysis is carried out. Trailing edge deflection is optimized, and at the same time, it is ensured that the aerofoil is stiff enough for sustaining aerodynamic loads at velocities up to 20m/s. Experimental results obtained from a laboratory scale setup of SMA wire integrated aerofoil are reported and they are found to be in close agreement with the simulated results.

Simulating the dynamics of flexible bodies and vortex sheets

We present a numerical method for the dynamics of a flexible body in an inviscid flow with a free vortex sheet. The formulation is implicit with respect to body variables and explicit with respect to the free vortex sheet. We apply the method to a flexible foil driven periodically in a steady stream. We give numerical evidence that the method is stable and accurate for a relatively small computational cost. A continuous form of the vortex sheet regularization permits continuity of the flow across the body’s trailing edge. Nonlinear behavior arises gradually with respect to driving amplitude, and is attributed to the rolling-up of the vortex sheet. Flow quantities move across the body in traveling waves, and show large gradients at the body edges. We find that in the small-amplitude regime, the phase difference between heaving and pitching which maximizes trailing edge deflection also maximizes power output; the phase difference which minimizes trailing edge deflection maximizes efficiency.

Flexible-Membrane Airfoils at Low Reynolds Numbers

An experimental study was conducted to assess the benefits of using flexible-membrane airfoils/wings at low Reynolds numbers for micro air vehicle applications compared with using a conventional rigid airfoil/wing. In addition to measuring aerodynamic forces acting on flexible-membrane airfoils/wings, a high-resolution particle image velocimetry system was used to conduct flowfield measurements to quantify the transient behavior of vortex and turbulent flow structures around the flexible-membrane airfoils/wings to elucidate the associated underlying fundamental physics. The aerodynamic force measurements revealed that flexible-membrane airfoils could provide better aerodynamic performance compared with their rigid counterpart at low Reynolds numbers. The flexibility (or rigidity) of the membrane skins of the airfoils was found to greatly affect their aerodynamic performance. Particle image velocimetry measurements elucidated that flexible-membrane airfoils could change their camber (i.e., crosssectional shape) automatically to adapt incoming flows to balance the pressure differences on the upper and lower surfaces of the airfoils, therefore suppressing flow separation on the airfoil upper surfaces. Meanwhile, deformation of the flexible-membrane skins was found to cause significant airfoil trailing-edge deflection (i.e., lift the airfoil trailing edge up from its original designed position), which resulted in a reduction of the effective angles of attack of the flexible-membrane airfoils, thereby delaying airfoil stall at high angles of attack. The nonuniform spanwise deformation of the flexible-membrane skins of the flexible-membrane airfoils was found to significantly affect the characteristics of vortex and turbulent flow structures around the flexible-membrane airfoils.

Design Optimization of a Controllable Camber Rotor Airfoil

A conformable airfoil is proposed as an alternative to trailing-edge flaps used for active helicopter vibration reduction through high-frequency changes in camber. The design consists of several compliant mechanisms of predetermined topology that are placed serially within the airfoil along the chord, aft of the leading-edge spar. A shape optimization approach is used to design the compliant mechanisms, in which the objective is to maximize trailing-edge deflection while minimizing airfoil deflections due to aerodynamic loads. Solutions were obtained using a sequential linear programming method coupled with a finite element analysis. An optimized shape is predicted to achieve a trailing-edge deflection of ±6.0 mm or a ±4.6- deg equivalent flap deflection angle using the tip deflection objective. Results indicate that the deflection is dependent on the amount of passive material allowed and the objective function used. The aerodynamic loads are found to cause only small deformations in comparison with those caused by the actuation. Prototype fabrication and bench-top tests demonstrated that rotor airfoil camber is controllable using the proposed concept.

Morphing Wing Flight Control Via Postbuckled Precompressed Piezoelectric Actuators

The design, modeling, and testing of a morphing wing for flight control of an uninhabited aerial vehicle is detailed. The design employed a new type of piezoelectric flight control mechanism which relied on axial precompression to magnify control deflections and forces simultaneously. This postbuckled precompressed bending actuator was oriented in the plane of the 12% thick wing and mounted between the end of a tapered D-spar at the 40% chord and a trailing-edge stiffener at the 98% chord. Axial precompression was generated in the piezoelectric elements by an elastic skin which covered the outside of the wing and served as the aerodynamic surface over the aft 70 % of the wing chord. A two-dimensional semi-analytical model based on the Rayleigh-Ritz method of assumed modes was used to predict the static and dynamic trailing-edge deflections as a function of the applied voltage and aerodynamic loading. It was shown that static trailing-edge deflections of ±3.1 deg could be attained statically and dynamically through 34 Hz, with excellent correlation between theory and experiment. Wind tunnel and flight tests showed that the postbuckled precompressed morphing wing increased roll control authority on a 1.4 meter span uninhabited aerial vehicle while reducing weight, slop, part-count, and power consumption.

Design of a Comfortable Rotor Airfoil Using Distributed Piezoelectric Actuators

In the present study, several design methodologies are developed for determining the optimal distribution of a limited amount of piezoelectric material aft of the spar in a conformable rotor airfoil section. The design objectives are to maximize the trailing-edge deflection under actuation loads and simultaneously minimize the airfoil deflection under aerodynamic loads. Energy-like functions, mutual potential energy (MPE) and strain energy (SE), are used as measures of the deflections created by the actuation and aerodynamic loads, respectively. The design objectives are achieved by maximizing a multicriteria objective function that represents a ratio of the MPE to SE. Several design optimization techniques are evaluated including topology, geometry, sequential topology-geometry, and concurrent topology-geometry optimizations. The results of the study indicate that the optimized conformable airfoil section obtained using the concurrent topology-geometry optimization can produce a downward trailing-edge deflection equivalent to 4.24 deg of effective flap angle from the actuation loads, with the peak-to-peak deflection being nearly twice the downward deflection. The airfoil deformation caused by the aerodynamic loads alone is extremely small (less than 0.24 deg). Key features of the optimized airfoil are arrangement of actuators near the spar that act to stretch and shrink the skin and a bimorph like mechanism from midchord to the trailing edge. Additional results include a strain analysis, aerodynamic lift-and-drag increment study, and an examination of the effects of skin thickness and volume constraint of the active material.