Airborne wind energy systems (AWESs) are tethered flying devices used for electricity generation. During the power-generation phase, the aerial component usually flies in a circular or figure-of-eight pattern. This paper examines the control surface movements required for circular flights in rigid-wing AWESs. In the absence of gravity, steady trim with equilibrium solutions can be achieved if the orbit plane is normal to the wind. The radius depends on how much the aircraft leans into the turn: leaning in reduces the radius and is statically stable, while leaning out achieves a larger radius but is unstable. For the latter case, artificial stabilisation can be done by cross-feeding the pitch and roll responses to the aileron. For circular trajectories that are not normal to the wind (i.e. experiencing out-of-plane wind), energy needs to be added to the system through the periodic forcing of a control surface. The correct timing of the forcing will excite the orbit's natural frequency, enabling full control of the circle centre and orientation for navigation in the 3D space. This can be done even in the presence of gravity, which is discussed in the second half of this paper. The aileron is the most effective control effector for forcing. Although the trimming method presented in this paper is only suitable for theoretical studies, it provides insights into the flight dynamics of rigid-wing AWESs and lays the groundwork for future flight control developments.

We propose a feedback architecture that enables effective flight control of rigid-wing airborne wind energy systems during circular-pattern reel out. The controller performs well with only proportional-integral regulators, thereby presenting a significantly simpler solution compared to the existing literature. The key idea is in tracking the roll angle on a non-static reference frame, effectively reducing the control problem to one degree of freedom. This method of navigation does not require users to define an exact path for the kite to follow, which contributes to stability and robustness. In its minimum viable form, the controller can function with only ailerons while requiring no pitot-tube measurement, although the addition of elevators and rudder enables angle-of-attack and zero-sideslip tracking for more efficient power generation. Simulation-based verification is conducted on an industrial six-degree-of-freedom model with a flexible tether, nonlinear aerodynamics, and realistic wind conditions, showing satisfactory performance. Three expansions to the control law are then presented. The first one reduces angle of attack fluctuation during reel out by adding a proportional pitch angle feedback term to the elevator, resulting in more power. In the second expansion, the reel-out radius is automatically adjusted to enable phase synchronisation of multiple kites in a farm configuration, where minimum separation rules may apply. The third expansion implements a proportional feedback rule that enables figure-of-eight flight. By using proportional-integral architecture, the controller is easy to implement, making it a suitable baseline system for benchmarking more advanced control laws.

This paper explores different reel-out strategies in a simulated environment to improve the power generation capability of Kitemill's KM1 prototype. The KM1 is a rigid-wing groundgen airborne wind energy system that flies in circular loops during its power production phase. Firstly, we investigate the impact of flight and ground winch control. A major improvement of 47% more power over an existing reel-out method can be achieved by combining winch control to maintain a constant reel-out speed, along with using flight control to track a constant angle of attack and zero sideslip. Subsequently, a series of parameter sweeps were conducted to determine the best reel-out trajectories. The parameter space considered includes wind speed, circle radius and reel-out speed. Whilst there are many parameter combinations that provide similar power outputs, their effects on other operation metrics are different. For instance, excessively large circles cause a small drop in power generated (relative to the optimal value) and impose significant penalties on structural load, power quality (how much it fluctuates), airspace and ground space usage and flight control. Regarding the reel-out speed, faster reel-out provides more power and alleviates structural load at the cost of lower airspeed relative to the wind, which increases the risk of stalling the airframe. The proposed reel-out strategies are compared with a published optimisation-based study, which uncovered the importance of pitch control in power production. All analyses were conducted on a high-fidelity simulator with tether dynamics. The simulator's accuracy was verified against past flight test data.

Aviation is widely recognised to have global-scale climate impacts through the formation of ozone (O3) in the upper troposphere and lower stratosphere (UTLS), driven by emissions of nitrogen oxides (NOX). Ozone is known to be one of the most potent greenhouse gases formed from the interaction of aircraft emission plumes with atmospheric species. This paper follows up on previous research, where a Photochemical Trajectory Model was shown to be a robust measure of ozone formation along flight trajectories post-flight. We use a combination of a global Lagrangian chemistry-transport model and a box model to quantify the impacts of aircraft NOX on UTLS ozone over a five-day timescale. This work expands on the spatial and temporal range, as well as the chemical accuracy reported previously, with a greater range of NOX chemistry relevant chemical species. Based on these models, route optimisation has been investigated, through the use of network theory and algorithms. This is to show the potential inclusion of an understanding of climate-sensitive regions of the atmosphere on route planning can have on aviation’s impact on Earth’s Thermal Radiation balance with existing resources and technology. Optimised flight trajectories indicated reductions in O3 formation per unit NOX are in the range 1–40% depending on the spatial aspect of the flight. Temporally, local winter times and equatorial regions are generally found to have the most significant O3 formation per unit NOX; moreover, hotspots were found over the Pacific and Indian Ocean.

This paper develops a geometrically nonlinear mathematical model of a flexible cantilevered beam hosting a guided axially elastic tendon. The model is intended as an enabling tool to explore the guided tendon-based dynamic enhancement methods in application to highly flexible and lightweight structures. To both validate the model and to generate further insights, an experiment-guided study is performed on a beam-tendon configuration. Uniquely, this is done with moderately large beam deformations used to exercise nonlinearity. The model-based study of the problem identifies and reflects on the individual stiffness-modulating terms corresponding to various geometrical and axial elasticity-driven mechanisms. The susceptibility to these individual modulating mechanisms is shown to vary between different groups of modes. These are systematically identified using contrasting frequency variations created by the different mechanisms. The second novel aspect of this research is the treatment of the resistive effects posed by the guides on the tendon's axial movement. It is shown that the progressive locking of the tendon's motion with increased guide impedance results in higher extents of stiffening due to the segmented activation of the tendon's elasticity. It is further demonstrated that the effective tailoring of the impedance posed by the guides can be used to generate dynamically optimal conditions. This is illustrated for the specific case of modal damping maximisation, assuming guide-impedance originating in the form of viscous damping. Given these insights, this research recognises potential extensions of the axially activated tendon for vibration suppression applications.

Highly flexible wings can experience complex aerodynamic phenomena, such as dynamic stall, which induce nonlinear forces and lead to instabilities or limit-cycle oscillations that are challenging to predict and mitigate. A low-order accurate and efficient numerical model is in demand to give near real-time on-site nonlinear aeroelastic prediction during the wind tunnel testing, to help ensure that suitable tests are safely conducted. For this purpose, reducing the states of either the aerodynamic or structural model is extremely important in lowering the computation cost for numerical continuation. This paper explores a low-order modeling approach to analyze dynamic stall and bifurcation behavior in highly flexible wings, with applications in the design and control of next-generation highly efficient air transport. In this paper, the low-order unsteady aerodynamic model is derived by reducing the order of the attached flow portion of the original Beddoes-Leishman model (BLM). The reduced set of governing equations is tailored to retain significant aerodynamic and structural couplings while minimizing computational overhead; the nonlinear beam shape (NBS) formulation is used to model the flexible wing structure. Numerical comparisons for a NACA 0012 airfoil show that the modified BLM with six states is close to matching the original BLM with twelve states when prescribed periodic motion is given. The caveat is that the model is useful in a relatively low-frequency range; however, this is considered to be enough to cover the frequency range of aerodynamic phenomena associated with limit cycle oscillations (LCOs) related to highly flexible wings. The numerical model is expected to quickly identify and predict parameter regimes where stable, unstable, and oscillatory behaviors emerge, enabling a deeper understanding of aerodynamic load variations and structural deformation patterns. To further validate the low-order aerodynamic model, the final version of the paper will incorporate open-sourced experimental data to assess the model's accuracy and evaluate the capability of the developed numerical tool to predict the nonlinear aeroelastic behavior of the high-aspect-ratio wing. This includes key features such as dynamic stall characteristics and post-stall behavior.

The ongoing quest for more efficient aircraft operations has led to several high aspect ratio wing concepts that may take flight in the next decade. A wing tends to become more flexible as the aspect ratio is increased, which results in the interaction between the wing structural response and aircraft flight dynamics, with an increased likelihood of nonlinear response of the wing structure and aerodynamics as well as an increase in the complexity of the aircraft systems that must be designed in the nonlinear domain. In this context, developing nonlinear numerical tools is crucial to predict the behaviour of this new generation of aircraft. This paper discusses the numerical and experimental work on a 2.4 m very flexible high aspect ratio wing, where the results of these tests can be used to validate nonlinear aeroelastic software. The wing was designed to exhibit specific nonlinear dynamic behaviour and does not represent any wing that would be used on an aircraft. Following a brief description of the experimental test model, three different numerical models are presented: two low-order aeroelastic models, one based on the multibody toolbox Simscape, one based on a low-order geometrical exact beam formulation and one model based on the industrial standard MSC Nastran software. The low-order models were used to carry out numerical continuation and bifurcation analysis to predict the unstable nonlinear behaviour of the wing and the onset of limit cycle oscillations. Ground Vibration Tests and static tests were carried out to update the numerical models. The wing model was tested in the Airbus' low-speed wind tunnel in Filton (UK). Experimental data were processed with the eigensystem realisation algorithm, and the Zimmerman flutter-margin criterion was applied to predict the flutter onset boundary. Results showed that the driving mode for the flutter onset is the first in-plane mode. The comparison between experimental and numerical data shows the ability of low-order models to capture the nonlinear effects and predict the reduction in the flutter onset speed as the angle of attack of the model is increased.

There has been a growing interest in utilizing flared folding wingtips as an in-flight load alleviation device to enable increased wing spans that meet airport gate limits but with little increase in wing weight. The semi-aeroelastic hinge (SAH) concept is implemented in high-aspect-ratio wings to enable wingtips to be released during severe load cases such as maneuvers and gusts to alleviate the bending moments while maintaining optimum aerodynamic shape for the rest of the flight. In this paper, scaling methods for wings incorporating the SAH are explored, allowing for the development of equivalent scaled unmanned aerial vehicles or wind tunnel models with similar aeroelastic behavior as full-size aircraft. Three scaling approaches are considered in this study, namely, Iso-Froude, Iso-Frequency, and Iso-Strain, where a set of governing nondimensional quantities and scaling factors are determined. Despite the significant nonlinearities resulting from large wingtip fold angles, it is shown that a linear scaling approach can be appropriate for such a wing configuration. Furthermore, the aeroelastic properties of each scaled model are compared to those of the full-scale model, where the best match was obtained from the Iso-Strain model, although it is challenging to meet the required operational conditions.

High aspect ratio wings promise greater aerodynamic efficiency with subsequent savings for airlines. However, these types of designs could be prone to unstable aeroelastic responses within the standard flight envelope, which may result from the reduction in the natural frequencies. This paper proposes a fully nonlinear low order aeroelastic framework able to predict the onset of any potential subcritical and supercritical limit cycle oscillations in very flexible wings, a critical tool required for the design of modern wings. Numerical continuation is used to detect unstable aeroelastic oscillations triggered by nonlinear geometrical effects, nonlinear aerodynamics or both. For this work, a 2-state aerodynamic model is used to predict the unsteady aerodynamic response of the model. Numerical results are verified and validated against wind tunnel data. The comparison shows a close match of the wing static response and the prediction of the flutter speed. The subject of this work is a very flexible wind tunnel model of 2.4 m span tested in 2018 in the 12 x 10 ft subsonic Airbus wind tunnel in Filton, which was part of the Agile Wing Integration (AWI) project. The model was specifically designed for the purposes of numerical code validation with certain dynamic characteristics and does not resemble any wing that would be used on an aircraft. It was equipped with accelerometers, strain gauges, pressure sensors and markers for video tracking to provide a complete picture of the onset of aeroelastic instabilities. The bifurcation analysis presents the equilibrium branches for the wing model in different configurations for different angles of attack and structural damping. Results show that the Hopf bifurcation velocity (flutter) strongly depends on the structural damping for the clean wing configuration, which also presents pitchfork-like behaviour at zero incidence due to the asymmetry in the inertia of the model.

This paper describes the development and performance verification of a unique wind tunnel model and traversing rig for testing the use of active flow control technology as a novel control effector. The rig allows the model to be set within the University of Bristol’s 7′ × 5′ low speed tunnel and moved through a wide �� and �� range (−10◦ to 35◦ and ±25◦ respectively) whilst supplying blowing air to the model. The development and verification of taring procedures and methods for converting ‘in-tunnel’ measured data to ‘free air’ conditions are described. A set of tests featuring a generic combat aircraft with a 65◦ leading edge is used in order to demonstrate the capabilities of the system. This work serves as a precursor to future publications where further test results and comparisons with CFD simulations will be presented.

This paper investigates the possibility of developing a novel vibration absorber aimed towards slender continuous structures such as wings or blades. The absorber essentially consists an axially elastic tendon guided along a series of points along the span of cantilevered laboratory beam demonstrator, such that an aggregate of its induced extensional activity is observed at a specific location. The central idea is to introduce a discrete absorber at this strategically selected point, with the tendon providing the stiffness between the absorber and the primary structure. To explore the concept, the presented experiment is aimed at providing a comparison against a reduced 2 degree of freedom representation of the combined and mutually coupled absorber system. The specific focus of this arrangement is to support the interpretation of the underlying modal characteristics of the proposed concept.

This paper presents the atmospheric flight test results of a remotely piloted airliner model under normal and upset flight conditions. The aircraft, which has been designed and built in- house, is a 3.7% scale model of NASA’s Generic T-tail Transport airplane and resembles a typical T-tailed, twin engine regional jet. Weight and moments of inertia are dynamically scaled to cruising altitude of the full-scale aircraft, allowing realistic simulation of its dynamic behavior. A robust structural design and a ballistic recovery system allow high risk tests to be carried out which would normally cause a loss of the vehicle. The model is equipped with extensive instrumentation, including a custom 5-hole air data probe for accurate measurement of the airflow under upset conditions. A custom flight computer handles data acquisition, real- time state estimation, control augmentation, automated flight test execution, and data logging. Flight tests have been carried out to validate and enhance previously obtained wind tunnel data across a wide flight envelope, including flight under stalled conditions. System identification has been performed on a six-degrees-of-freedom aerodynamic model, which also includes a novel nonlinear stall model for the longitudinal dynamics. The estimated aerodynamic derivatives are directly compared to wind tunnel data. Time traces and frequency responses are shown as well, comparing flight test data to nonlinear simulations based on the wind tunnel data and the identified aerodynamic model.

This paper presents low Reynolds number experimental wind tunnel investigations of flexible high aspect ratio aircraft models with body pitch degree-of-freedom. The aim is to assess nonlinear dynamic interactions of highly flexible wings with aircraft dynamics. Two sets of wings, one designed to be highly flexible and the other to be near-rigid, were considered for comparison. The first test category involves the aircraft being trimmed for three lift cases of 20, 25, and 30N within a wind speed range of 18 to 30 m/s. The second category involves a trimmed pitch sweep ranging from -3.5 to 10.7 degrees at a single wind speed of 25 m/s. The aircraft mode frequencies and damping were identified experimentally using the Eigensystem Realization Algorithm on autocorrelated responses to random excitation of the model using gust vanes. Highly nonlinear trends in the short-period mode frequency and damping with respect to wind speed and pitch angle were observed, as well as differences between the wing design cases. A low-order geometrically-exact structural model was employed for numerical comparisons. Three aerodynamic models were considered: steady Vortex Lattice Method (VLM), steady 2-state Leishman, and unsteady 2-state Leishman aerodynamics. It was found that wing-to-tail influence captured by VLM aerodynamics is vital to accurately simulate the short-period mode and for its frequency magnitude to match with experiments. However, the local nonlinearity observed experimentally is not captured by any of the aerodynamic models considered. Introducing aerodynamic unsteadiness significantly reduces the damping of the short-period mode and has very minor impact on the wing-bending dominant mode. The trends in wing-bending mode were captured well by all aerodynamic models, with minor differences between them. In general, the short-period mode frequency increases with wind speed and decreases with pitch angle (for wings-up deflections). An increase in the magnitude of wing deflection results in an increase in wing-bending mode frequency and a decrease in damping. A reduction in wing stiffness or increase in flexibility reduces the frequency of the short-period mode. Increasing the magnitude of wing deflection also reduces the short-period mode frequency, which was observed to be more significant in experiments than in numerical results.

This paper presents the development and testing of a new aeroelastic demonstrator realized as a cantilevered highly flexible wing with built in aeroelastic behavior of interest. The wing was configured and sized to exhibit the bending-torsion coupling under aeroelastic conditions and to undergo flutter within an airspeed range of 20-30 m/s. The tests focused on both static and dynamic aeroelastic aspects under stable airspeeds. In particular, operational modal analysis was performed across a range of airspeeds to experimentally map the evolution of the modal properties leading to flutter. In addition to this, the Limit Cycle Oscillation (LCO) responses, following the experimentally identified flutter onset airspeeds, are studied. This systematic research aims to experimentally explore aeroelastic modal coupling behavior and its role in shaping the LCO responses. In addition to experimentally replicating such aspects which are generally limited to numerical studies, the results presented herewith are also intended to inform further studies related to developing highly targeted instability suppression methods for aeroelastic structures.

Aircraft like the Boeing 777-X use on-ground folding wingtips to meet the airport gate size restriction while increasing the aspect ratio during flight to reduce induced drag. A recent concept of aircraft design is to utilise in-flight floating wingtips as a means of load alleviation, which is known as semi-aeroelastic hinge (SAH). This device allows wingtips to be released during manoeuvre and severe gusts to alleviate wing loads, while locking the wingtips during cruise to maintain an optimum aerodynamic shape. This paper develops a flight mechanics model incorporating flexible wings to study the influence of the SAH device on multiple aspects of aircraft flight dynamics. The dynamic responses of the aircraft to gusts and control surface inputs are computed and compared for various hinge and wingtip configurations and release times. It was found that the gust load measured from the wing with free hinge configuration was approximately 40% lower compared to that of the fixed hinge case. It is also shown that when the SAH is released, it can significantly reduce an aircraft's roll damping and the frequency of the short-period mode, leading to higher roll and pitch rates. Furthermore, it shows that the transient responses following the wingtip release will exacerbate the responses induced by gusts, and greater load alleviation can be achieved by manipulating the hinge release time for each gust length. Finally, a step input of the elevator during the hinge release was found to be beneficial for enhancing the gust load alleviation.