Inflatable Wing Research Articles

Wind Tunnel Testing of Tethered Inflatable Wings

A wind tunnel testing approach for tethered inflatable wings is presented. Use cases for such wings range from airborne wind energy systems to high-altitude communication platforms. The tests were conducted for two tethered inflatable wings, one made out of nylon fabric and the other an ultra-high-molecular-weight polyethylene fabric. They were tested within a speed range of 15–32.5 m/s for three tether attachment configurations. Stereo photogrammetry data, force and moment measurements, and wake pressure measurements were recorded for each speed and test configuration. These measurements provide an experimental database for aeroelastic model validation and comparison with high-fidelity computational fluid dynamics studies. The effects of wing fabric material and tether attachment configurations on aerodynamic performance were explored and found to have a profound impact. These tests also highlight the possibility of passive aeroelastic tailoring of the wing configuration to achieve desired aerodynamic performance in the form of high lift and load alleviation. Some testing challenges and possible sources of measurement uncertainty are also discussed.

Wrinkling and Buckling of a New Swept Baffled Inflatable Wing Structure

Wrinkling and Buckling of a New Swept Baffled Inflatable Wing Structure

Methodology for shape prediction and conversion of a conventional aerofoil to an inflatable baffled aerofoil

AbstractInflatable wings for UAVs are useful where storage space is a severe constraint. Literature in the field of inflatable wings often assumes an inflated aerofoil shape for various analyses. However, the flexible inflatable aerofoil fabric might deform to another equilibrium shape upon inflation. Hence accurate shape prediction of the inflated aerofoil is vital. Further, no standardised nomenclature or a process to convert a smooth aerofoil into its corresponding inflatable aerofoil counterpart is available. This paper analytically predicts the equilibrium shape of any inflatable aerofoil and validates the analytical prediction using non-linear finite element methods. Further, a scheme for the generation of two types of inflatable aerofoils is presented. Parameters such as the number and position of compartments and aerofoil length ratio (ALR) are identified as necessary to define the aerofoil’s shape fully. A process to minimise the deviation of the inflatable aerofoil from its original smooth aerofoil using particle swarm optimisation (PSO) is discussed. Research presented in this paper can help in performing various analyses on the actual equilibrium shape of the aerofoil.

Feasibility Investigation of Attitude Control with Shape Memory Alloy Actuator on a Tethered Wing

This study is aimed at assessing the feasibility of employing an innovative, smart-material-based control effector for an inflatable wing. A shape memory alloy (SMA) actuator is primarily investigated as a control effector in this work for its advantages of a simple actuation mechanism and a high force-to-weight ratio. This paper presents the design, control strategy and simulation results of the SMA actuator used as a stability augmentation system for a small-scale prototype kite. Stable flight of the kite is achieved during open wind tunnel tests using the SMA actuator. Based on experimental and simulation analyses, it is evident that the current SMA actuator is better for low-frequency actuations rather than stability augmentation purposes, as its performance is sensitive to practical conditions. The study also discusses potential improvements and applications of the SMA actuator.

Conceptual Design of Hybrid Aerial Vehicle for Venus Exploration

The conceptual design of a hybrid aerial vehicle for the exploration of the upper Venus atmosphere is presented. The vehicle will float like a balloon and harvest solar energy which is stored in batteries. The neutral buoyancy reduces the energy consumption and makes the vehicle robust and durable. Energy stored in the batteries can be used for powered flight with good horizontal and vertical mobility to explore aspects of the atmosphere. The vehicle is intended to operate near 55.3 km altitude and to explore the cloud layer of the planet. The vehicle takes its inspiration from the Stingray inflatable wing by Prospective Concepts. Based on a trade study, the wing span was set to 25 m. Equations are developed for the altitude, gas and skin temperature, and skin stress during neutrally buoyant flight. To keep the equations in a simplified analytical form, the complex compartmentalized gas pockets of the vehicle are lumped into a single gas sphere. The equations take into account the volumetric expansion of the structure and the requirement that the differential pressure needs to be large enough to allow for brief periods of powered flight without significant structural deformation. An aerodynamic analysis provides the lift and drag coefficient curves and indicates that the vehicle is pitch-stable. A powered flight analysis shows that an airspeed of 30 m/s can be maintained for 31 min at 55 km and 69 min at 69 km altitude.

Structural design and dynamic analysis of an inflatable delta wing

Structural stiffness and strength, as well as the maintenance of wing shape are very important for the usage of inflatable wings. Firstly, inspired by the deployable dorsal fin of fish, structural design on an bionic inflatable delta wing is proposed. The outer flexible surface of delta wing can be stretched through the interval inflatable airbeams to unfold the spanwise-rotatable deployable ribs to achieve the maintenance of wing shape. Then, a dynamic theoretical model of equivalent multi-beam-springs array is established to describe the interval inflatable airbeams and the ribs by the Lagrange equations. The multi-point excitation modal test method is also used to identify the modal of the inflatable delta wing. Moreover, the transient response is analyzed by the wavelet spectrum for shock excitation. The equivalent multi-beam-springs array model is verified by experimental testing for inflatable structural framework of delta wing. Finally, the effects of airbeam spacing and internal pressure on the structural frequency of inflated wing are predicted by numerical simulation. These results can provide a new way for the design of the inflatable delta wing.

Conceptual Design of a Robotic Ground-Aerial Vehicle with an Aeroelastic Wing Model for Mars Planetary Exploration

This paper presents the technical barriers and an analysis to advance the conceptual development of novel robotic ground-aerial vehicles (RGAVs) for exploration missions to Mars prior to human arrival and the establishment of a base. The concept for RGAVs for Mars planetary exploration is novel, and will require innovations that are at various stages of development or use by the aerospace community. The RGAV concept will utilize inflatable wing technology, which increases the flexibility of the wing, and thus the possibility of structural dynamic instabilities that must be studied in the context of the Martian atmosphere. An aeroelastic model for wing bending is proposed, which considers wind gusts where the change in wind direction is up to ±6° from the mean, and a turbulence intensity of up to 20%. Their effect on the bending displacement of a semi-elastic wing is quantified, resulting in a maximum wing tip displacement of 16.2 cm. Low-fidelity computational aerodynamic analysis is performed using OpenVSP (3.31.1, NASA, Washington, DC, USA) to compute mean aerodynamic loads during cruise conditions at a cruise Mach number of 0.70. Finally, a non-linear adaptive control system is proposed for the longitudinal aerial dynamics and a proportional integral derivative (PID) controller is outlined for the ground roving lateral dynamics.

Conformable Inflatable Wings Woven Using a Jacquard Technique

Inflatable wings are of interest for applications where low weight, compact transport volume, and easy set-up are important. Examples are unmanned aerial vehicles with inflatable wings, paragliders and softkites for sport or airborne wind-energy applications. In this paper, a new method of designing and fabricating conformable inflatable wings by Jacquard three-dimensional weaving is presented. Depending on the weaving pattern, plane-parallel, tapered, or even curved structures can be produced. An analytical framework was developed to determine the shapes of pressurized structures produced by Jacquard weaving. Based on this theory, several design patterns suitable for inflatable wings are proposed. It is shown that the structural efficiency of the woven structure is identical to the structural efficiency of a cylinder. To validate the concept, different wing prototypes were built with the methods used for the mass production of airbags. The new method allows for the cost-efficient fabrication of inflatable structures, pressure vessels, and liquid containers with applications in the automotive, aerospace, and leisure industries.

Structural design and modal behaviors analysis of a new swept baffled inflatable wing

Inflatable wing has significant application value in the design of loitering munitions because of its advantages such as lightweight and foldability. However, due to the flexible characteristics, aeroelastic behaviors of inflatable wings such as flutter are nonnegligible in flight. By designing a certain angle between the inflatable beam and the wing span, the structural dynamic and even the aeroelastic performance of the inflatable wing can be effectively improved. Based on the analysis of the mechanical and geometric characteristics of the inflatable structure, a new inflatable wing with sweep arranged inflatable beams is proposed, and the main design variables and methods are analyzed. For purpose of investigating the aeroelastic performance of the swept baffled inflatable wing, the modal behaviors by considering the wet mode are studied. In consideration of the deficiencies of the traditional wet modal analysis method, by introducing the influence on the additional stiffness of flow field, an added mass-stiffness method is proposed in this paper, and the advantages are verified by ground vibration experiments. On this basis, the effects of baffles sweep angle, pressure, and boundary conditions on the modal parameters and aeroelastic performance of inflatable wing are analyzed. The results show that the aeroelastic performance of the inflatable wing can be designed by changing the baffles sweep angle, which is enlightened for the aeroelastic tailoring design on inflatable wings.

Aerodynamic Exploration of Corrugated Airfoil Based on NACA0030 for Inflatable Wing Structure

The flow structures and surface pressure distributions on corrugated airfoils significantly differed from those on a conventional, smooth airfoil. An unsteady, two-dimensional computational simulation was carried out to investigate the flow behavior and associated aerodynamic performance of a group of corrugated airfoils with different levels of waviness at angles of attack from 0° to 20° with an interval of 2° at a low Reynolds number regime (Re = 1.2 × 105) and were quantitatively compared with those of its smooth counterpart. Time-averaged aerodynamic coefficients demonstrated that the corrugated airfoils have a lower lift and higher drag because of trapped vortices in the corrugations. The pressure drag of the corrugated airfoils was greater than that of the smooth airfoil. In contrast, the viscous drag of the corrugated airfoils was smaller than that of the smooth airfoil because the recirculation generated in the corrugation could reduce the viscous drag. The averaged velocity gradient in the boundary layer showed that the thickness of the boundary layer increased significantly for the corrugated airfoils because of recirculating flow caused by the small-standing vortices trapped in the valley of corrugations. The smoother the corrugated surface, the closer the aerodynamic characteristics are to those of the smooth airfoil.

Minimisation of Shape Deviation Between an Inflatable Aerofoil and a Target Areofoil Profile

The use of inflatable wings for unmanned aerial vehicles over their fixed-winged counterparts has many advantages. However, there are difficulties associated with predicting the profile of the inflatable wing, which makes achieving a desirable aerofoil profile non-trivial. This research aimed to formulate a design methodology which would determine the uninflated geometry for an inflatable aerofoil profile, accurately fitting that of a target or prescribed profile. The aspects omitted from the scope of this work includes the aerodynamic performance and stiffness of the inflated aerofoils. The resulting uninflated geometry is suitable to construct a physical model. The methodology proposed incorporates numerical shape optimisation on finite element models. A series of structured numerical experiments determine robusticity of the established methodology by altering the thickness of the target aerofoil profile and increasing its complexity. For each case, the methodology successfully satisfied its aim, producing accurate fits between the inflated numerical model and the target aerofoil profile. When validated, the inflated shape of the numerical model proved to predict that of its corresponding physical model accurately for both simple and more complex geometries.

Effect of Chordwise Struts and Misaligned Flow on the Aerodynamic Performance of a Leading-Edge Inflatable Wing

Leading-edge inflatable (LEI) kites use a pressurized tubular frame to structurally support a single skin membrane canopy. The presence of the tubes on the pressure side of the wing leads to characteristic flow phenomena for this type of kite. In this paper, we present steady-state Reynolds-Averaged Navier-Stokes (RANS) simulations for a LEI wing for airborne wind energy applications. Expanding on previous work where only the leading-edge tube was considered, eight additional strut tubes that support the wing canopy are now included. The shape of the wing is considered to be constant. The influence of the strut tubes on the aerodynamic performance of the wing and the local flow field is assessed, considering flow configurations with and without side-slip. The simulations show that the aerodynamic performance of the wing decreases with increasing side-slip component of the inflow. On the other hand, the chordwise struts have little influence on the integral lift and drag of the wing, irrespective of the side-slip component. The overall flow characteristics are in good agreement with previous studies. In particular, it is confirmed that at a low Reynolds number of Re=105, a laminar separation bubble exists on the suction side of this hypothetical rigid wing shape with perfectly smooth surface. The destruction of this bubble at low angles of attack impacts negatively on the aerodynamic performance.

Research on Biaxial Damage Evolution Characteristics of Flexible Woven Composite Film

The flexible braided composite film structures have been widely used in many fields such as near space aerostats, inflatable wings, flexible spacecraft due to their unique advantages such as small folding volume, light weight, high specific strength, excellent comprehensive performance, etc. However, the space environment is so harsh, and the flexible film structure composite material could be easily affected by various environmental factors such as long-term sunlight, ultraviolet radiation and space particle impact. So the weathering damage evolution characteristics directly determine the use life and working efficiency. The typical biaxial fiber reinforced laminated flexible composite material was studied with the damage mode, and the composite meso-mechanical theory was also used to construct the braided composite mechanics model during the damage process. The damage model of the warp and weft fiber bundles with the base film layer was introduced into the independent flexible film composite component damage degree, and the damage evolution model of the damage-containing composite material characterized by the mesoscopic component parameters was also established. Moreover, the load-bearing stress characteristics of each component under different damage conditions were calculated and analysed. The research results could provide an effective theoretical reference for the weathering optimization design and the improvement of the service life of flexible film woven composite structures.

Wind-Tunnel Measurement of Differential Pressure on the Surface of a Dynamically Inflatable Wing Cell

An instrumentation system for in-situ measurement of the inner-outer pressure differential at the upper and lower surfaces of dynamically inflatable wings is designed and tested, revealing important insights into the aerodynamic characteristics of inflatable airfoils. Wind tunnel tests demonstrated full capability of low-pressure differential readings in the range of 1.0–120 Pa, covering speeds from 3 to 10 m/s at angles of attack from −20 to +25°. Readings were stable, presenting coefficients of variation from 2% to 7% over the operational flight envelope. The experimental data confirmed the occurrence of a bottom leading-edge recirculation bubble, linked to the low Reynolds regime and the presence of an air intake. It supported the proposition of a novel approach to aerodynamic characterization based on local pressure differentials, which takes in account the confined airflow structure and provides lift forces estimations compatible with practical observation. The results were also compatible with data previously obtained following different strategies and were shown to be effective for parameterizing the inflation and stall phenomena. Overall, the instrumentation may be applied straightforwardly as a flight-test equipment, and it can be further converted into collapse alert and prevention systems.

Reynolds-averaged Navier-Stokes simulations of the flow past a leading edge inflatable wing for airborne wind energy applications

In this work we present Reynolds-averaged Navier-Stokes (RANS) simulations of the flow past the constant design shape of a leading-edge inflatable (LEI) wing. The simulations are performed with a steady-state solver using a k − ω SST turbulence model, covering a range of Reynolds numbers between 105 ≤ and ≤ 15 × 106 and angles of attack varying between −5° and 24°, which are representative for operating conditions in airborne wind energy applications. The resulting force distributions are used to characterize the aerodynamic performance of the wing. We found that a γ − transition model is required to accurately predict the occurrence of stall up to at least Re= 3 × 106. The work highlights similarities with the flow past a two-dimensional LEI airfoil, in particular, with respect to flow transition and its influence on the aerodynamic properties. The computed values of the lift and drag coefficients agree well with in-flight measurements acquired during the traction phase of the LEI wing operation. The simulations show that the three-dimensional flow field exhibits a significant cross flow along the span of the wing.

Textile wing fabric for emergency response UAS

The fabrics used to manufacture parachutes and paragliders must have several specific characteristics: the mass of fabric per unit of surface must be low while the other physical-mechanical characteristics (the axial breaking strength load, the relative and absolute elongation, the tear resistance of the fabric and the assemblies, air permeability) must have high values. The paper deals with the analysis of qualitative aspects of several parachute fabrics that are used as a baseline in the development of a novel fabric. The results of experiments have materialized in statistical data, diagrams and graphs and their interpretation leads to the determination of the fabric variant that best meets the requirements of the destination. The destination is a patent pending inflatable wing design that utilizes a single skin construction and solid reinforcements in the sewing for shape stability. It is worth noting that the experimental results were compared with values indicated in specific international testing norms.

Towing Test Data Set of the Kyushu University Kite System

Kites can be used to harvest wind energy with substantially lower material and environmental footprints and a higher capacity factor than conventional wind turbines. In this paper, we present measurement data from seven individual tow tests with the kite system developed by Kyushu University. This system was designed for 7 kW traction power and comprises an inflatable wing of 6 m2 surface area with a suspended kite control unit that is towed on a relatively short tether of 0.4 m by a truck driving at constant speed along a straight runway. To produce a controlled relative flow environment, the experiment was conducted only when the background wind speed was negligible. We recorded the time-series of 11 different sensor values acquired on the kite, the control unit and the truck. The measured data can be used to assess the effects of the towing speed, the flight mode and the lengths of the control lines on the tether force.

Study on Inflatable Deployment Characteristics of Flexible Film Wing

The inflation deployment process directly affects the forming and bearing deformation characteristics of the flexible wing. In order to optimize the structural design of the flexible inflatable wing, the inflation deployment process was studied with the finite element simulation calculation for the distribution characteristics of pressure and displacement when the wing is not inflated and after inflation molding. The results show that the inflation process is: for the first stage, when the inflation pressure is lower than the critical forming pressure, due to the effect of gravity, the flexible film wing downward bending, and the skin surface pressure is quite small, while the wing is inflated, the overall deformation and displacement is quite large. for the second stage of inflation molding, the flexible wing has a bearing stiffness, but due to aerodynamic loading upward warping occurred, the pressure of the wing root is large, but the displacement of the wing tip is large, which could provide a valid reference method to study the precision and stability of the flexible wing inflation process.

Power Prediction of Airborne Wind Energy Systems Using Multivariate Machine Learning

Kites can be used to harvest wind energy at higher altitudes while using only a fraction of the material required for conventional wind turbines. In this work, we present the kite system of Kyushu University and demonstrate how experimental data can be used to train machine learning regression models. The system is designed for 7 kW traction power and comprises an inflatable wing with suspended kite control unit that is either tethered to a fixed ground anchor or to a towing vehicle to produce a controlled relative flow environment. A measurement unit was attached to the kite for data acquisition. To predict the generated tether force, we collected input–output samples from a set of well-designed experimental runs to act as our labeled training data in a supervised machine learning setting. We then identified a set of key input parameters which were found to be consistent with our sensitivity analysis using Pearson input–output correlation metrics. Finally, we designed and tested the accuracy of a neural network, among other multivariate regression models. The quality metrics of our models show great promise in accurately predicting the tether force for new input/feature combinations and potentially guide new designs for optimal power generation.

Study on influence of ambient temperature on biaxial stress and strength of flexible inflatable wing film

Abstract With advantages of small folding volume, light weight, simple structure, low cost and so on, the flexible inflatable wing has attracted many researchers' attention. However, the change of ambient temperature has a significant influence on the bearing stress of the flexible inflatable wing. Here the typical flat inflatable wing was taken as the object, and the biaxial stress analysis and the Mises stress of the influence of the ambient temperature of the inflatable wing were constructed adopting the mechanical theory and balance principle. And the finite element simulation calculation under different ambient temperature was also performed with the theoretical model. It is found that the biaxial stress and strength of the flexible wing film are closely related to the ambient temperature, and the temperature changings will cause the wing stress to change the Mises stress distribution law of the upper and lower airfoil surfaces, and the wing skin is a two-phase plane stress state. Moreover, with the increase of the ambient temperature, the changes of the Mises stress of the wing root, middle and wingtips of the whole airfoil are inconsistent, but the overall warping deformation is improved, which provides a theoretical reference for the structural optimization design and strength analysis of the flexible inflatable wing.