The Constitutive Relation of a Fabric Membrane Composite for a Stratospheric Airship Envelope Based on Invariant Theory

The study of stratospheric airships has become the focus in many countries in recent years,because of its potential applications in many fields.Lightweight and high strength envelopes are the keys to the design of stratospheric airships,as it directly determines the endurance flight performance and loading deformation characteristics of the airship.A typical envelope of any stratospheric airship is a coated-fabric material which is composed of a fiber layer and several functional membrane layers.According to composite structure,nonlinearity and viscoelasticity are the two main characteristics of such envelope.Based on the analysis on the interaction between the different components in the micro-mechanical model of the coated-fabric,several invariant values reflecting the characteristics of the envelope material are obtained according to invariant theory.Furthermore,the constitutive equation that describes the viscoelasticity of the envelope material is derived.The constitutive equation can represent both the individual roles of the warp and weft fibers,and their further coupled interactions.The theoretical computation results were verified by off-axial tension tests.The results can help gain a deeper understanding of the mechanical mechanism and provide a reference for structural design of envelope material.

The Mechanical Characteristics and Experimental Study of the Stratospheric Airship

Stratospheric airship is a special near-space air vehicle which has lots of advantages than other traditional flying aircrafts, such as long endurance, strong survival ability, low cost, excellent resolution detector etc. In addition, the stratospheric airship can be an ideal stratospheric bearing platform. This paper firstly gave an overview describing some technical differences between the stratospheric airship and the traditional airship, including the working environment, design specifications, structure characteristics, energy system, flying modes, and so on. Some technical difficulties including the materials, power system which apply to the stratospheric airship and deformation of the huge hull, super-heating effect, and station-keeping were discussed. Furthermore, technical target, technical specifications, design concept, and overview of flying tested about two stratospheric demonstration airships which were representative achievements of the research on the stratospheric airship in China were introduced. Finally, the predictions about the progress and direction of development were discussed.

Mismatch loss analysis of solar array output power on stratospheric airship during flight

Stratospheric airships are long-endurance aerostats and have broad applications.All of the energy required for their operation is obtained from solar radiation,which makes accurate calculation of the energy output from the solar array crucial to the design and flight planning of the airships.However,the status of each photovoltaic module in the solar array may differ due to the airship curvature,resulting in mismatch losses and lowered output power,which has not been widely studied.In this paper,an irradiation model and a thermal model are established based on the actual arrangement of the modules.The output power model is established considering the non-uniform radiation in the array.The mismatch losses of the array are analyzed under different flight conditions.The output power of the solar array is decreased by up to 31.6%compared to the ideal state.Moreover,the proportion of mismatch losses increases with latitude,but the maximum mismatch loss power occurs at mid-latitudes.Then,an array reconfiguration method is proposed based on the irradiance dispersion index and position dispersion index.The reconfigured array increases output power by 11.5%and can maintain energy balance in continuous flight.The results can be used to correct the overestimation of the output power during the airship design or to guide the configuration of the solar array.

Change rules of a stratospheric airship's envelope shape during ascent process

Stratospheric airship is a special near-space air vehicle,and has more advantages than other air vehicles,such as long endurance,strong survival ability,excellent resolution,low cost,and so on,which make it an ideal stratospheric platform.It is of great significance to choose a reasonable and effective way to launch a stratospheric airship to the space for both academic research and engineering applications.In this paper,the non-forming launch way is studied and the method of differential pressure gradient is used to study the change rules of the airship＇s envelope shape during the ascent process.Numerical simulation results show that the head of the envelope will maintain the inflatable shape and the envelope under the zero-pressure level will be compressed into a wide range of wrinkles during the ascent process.The airship＇s envelope will expand with the ascent of the airship and the position of the zero-pressure level will move downward constantly.At the same time,the envelope will gradually form a certain degree of stiffness under the action of the inner and external differential pressure.The experimental results agree well with the analytical results,which shows that the non-forming launch way is effective and reliable,and the analytical method has exactness and feasibility.

Yaw controller design of stratospheric airship based on phase plane method

Recently, stratospheric airships prefer to employ a vectored tail rotor or differential main propellers for the yaw control, rather than the control surfaces like common low-altitude airship. The load capacity of vectored mechanism and propellers are always limited by the weight and strength, which bring challenges for the attitude controller. In this paper, the yaw channel of airship dynamics is firstly rewritten as a simplified two-order dynamics equation and the dynamic charac- teristics is analyzed with a phase plane method. Analysis shows that when ignoring damping, the yaw control channel is available to the minimum principle of Pontryagin for optimal control, which can obtain a Bang-Bang controller. But under this controller, the control output could he bouncing around the theoretical switch curve due to the presence of disturbance and damping, which makes adverse effects for the servo structure. Considering the structure requirements of actuators, a phase plane method controller is employed, with a dead zone surrounded by several phase switch curve. Thus, the controller outputs are limited to finite values. Finally, through the numerical simulation and actual flight experiment, the method is proved to be effective.

Light weight optimization of stratospheric airship envelope based on reliability analysis

A stratospheric airship is an essential flight vehicle in the aviation field.In this paper,optimal design approach of stratospheric airships is developed to optimize envelope shape considering three failure modes and multidisciplinary analysis models,and could also reduce the mass of a stratospheric airship to be deployed at a specific location.Based on a theoretical analysis,three failure modes of airships including bending wrinkling failure,hoop tearing failure and bending kink failure,are given to describe and illustrate the failure mechanism of stratospheric airships.The results show that the location,length and size of the local uniform load and the large fineness ratio are easier to lead to bending wrinkling failure and bending kink failure.The small fineness ratio and the increasing differential pressure are more prone to cause hoop tearing failure for an airship hull.The failure probability is sensitive to the wind field.From an optimization design,the reliability analysis is essential to be carried out based on the safety of the airship.The solution in this study can provide economical design recommendations.

Turning mechanism and composite control of stratospheric airships

The parametric model of stratospheric airships is established in the body axes coordinate system. In this paper we study the turning mechanism of stratospheric airships including the generated forces and the key parameters for steady turning. We compare and analyze the different driven-characteristics between aerodynamic control surfaces and vectored thrust in turning. We design a composite control combining aerodynamic control surfaces and vectored thrust according to different dynamic pressure conditions, to achieve coordinated turning under high or low airspeed situations.

Meso-Scale Tearing Mechanism Analysis of Flexible Fabric Composite for Stratospheric Airship via Experiment and Numerical Simulation

Stratospheric airships are controllable lighter-than-air aircraft and have great potential application in surveillance and communication.The envelopes,one of the main structures of a stratospheric airship,are generally made of flexible fabric composites to be lightweight,high strength,capable of containing lifting gas,and resistant to the harsh stratospheric environment.The composites,however,are prone to tearing.Hence,their tearing behavior has attracted great attention.This paper explores the meso-scale tearing mechanism of an envelope and the temperature influence on its tear strength via experiment and numerical simulation.Biaxial tear tests were conducted on cruciform specimens,which were contacted with liquids(cold alcohol or hot water)at different temperatures including-25,20,50,80℃.The specimens’tear stresses were measured and the meso-scale tearing behavior was captured with a microscope.Besides,a novel finite element analysis model based on truss and spring elements was established to simulate the tearing behavior.It was found that the simulation result has a relative agreement with the tests.The simulation results show that the maximum tear stress of the envelope drops by 39.62%as the temperature rises from-60℃ to 80℃ and the tensile properties of yarns and matrix account for stress concentration around a crack tip.This work deeply reveals the meso-scale tearing mechanism of the envelope and provides a valuable reference for exploring tearing properties of flexible fabric composites.

Kinetic Analysis of Vectored Electric Propulsion System for Stratosphere Airship

To enhance the controllability of stratosphere airship,a vectored electric propulsion system is used.By using the Lagrangian method,a kinetic model of the vectored electric propulsion system is established and validated through ground tests.The fake gyroscopic torque is first proposed,which the vector mechanism should overcome besides the inertial torque and the gravitational torque.The fake gyroscopic torque is caused by the difference between inertial moments about two principal inertial axes of the propeller in the rotating plane,appears only when the propeller is rotating and is proportional with the rotation speed.It is a sinusoidal pulse,with a frequency that is twice of the rotation speed.Considering the fake gyroscope torque pulse and aerodynamic efficiency,three blade propeller is recommended for the vectored propulsion system used for stratosphere airship.

A novel inflatable rings supported design and buoyancy-weight balance deformation analysis of stratosphere airships

Owing to the unique advantages in flight altitude,dwelling time and wide coverage area,stratospheric airships provide permanent monitoring and surveillance for both civil and military applications.Here we propose a semi-rigid stratosphere airship design with circumferential high-pressure inflatable rings and a longitudinal carbon fiber skeleton supported inside.We perform numerical simulations to analyze the deformation characteristics during the whole ascending and descending process.An equivalent internal gradient pressure model of helium is established based on the capsule shape and buoyancy-weight equilibrium conditions.The implicit dynamic method is used to deal with the large deformation of the airship capsule under a low negative pressure condition.Deformation and load-bearing performance of the airship capsule,inflatable ring,skeleton,and suspension line are obtained under different working conditions.The results show that the airship,supported with the inflatable rings and the suspension lines,effectively maintains the shape and ensures the stiffness during the ascending,dwelling,and descending stages,especially suffering from negative pressure.

Aerodynamic Characteristics of Airship Zhiyuan-1

An airship named "Zhiyuan-1" was designed/fabricated/flied as a technical demonstration for strato- spheric airship during 2007—2009 by Shanghai Jiaotong University. The calculation method and procedure of aerodynamic parameters were introduced, and the optimized configuration of the hull and the aerodynamic layout were given in this paper. Wind tunnel tests with diferent configurations, diferent pitch angles and diferent yaw angles were performed to study the wind load characteristics of the rigid model of the airship "Zhiyuan-1" in the 3.2 m wind tunnel at China Aerodynamics Research & Development Center. Also the numerical calculation about the test model was carried out to investigate the aerodynamic behavior. According to the results of wind test and numerical calculation, the excellent hull configuration of the airship "Zhiyuan-1" with lower drag charac- teristic was confirmed, which is based on optimism of the Michel transition law. And the phenomena of pressure coefcient distribution were discussed according to the results of wind tunnel test and numerical calculation at diferent flight attitudes.

Station-keeping control for a stratosphere airship via wind speed prediction approach

Purpose–The purpose of this paper is to improve the control precision of the station-keeping control for a stratosphere airship through the feedforward-feedback PID controller which is designed by the wind speed prediction based on the incremental extreme learning machine(I-ELM).Design/methodology/approach–First of all,the online prediction of wind speed is implemented by the I-ELM with rolling time.Second,the feedforward-feedback PID controller is designed through the position information of the airship and the predicted wind speed.In the end,the one-dimensional dynamic model of the stratosphere airship is built,and the controller is applied in the numerical simulation.Findings–Based on the conducted numerical simulations,some valuable conclusions are obtained.First,through the comparison between the predicted value and true value of the wind speed,the wind speed prediction based on I-ELM is very accurate.Second,the feedforward-feedback PID controller designed in this paper is very effective.Originality/value–This paper is very valuable to the research of a high-accuracy station-keeping control of stratosphere airship.

Height Variation in the Summer Quasi-Zero Wind Layer over Dunhuang, Northwest China: A Diagnostic Study

This study investigates the variation in the stratospheric quasi-zero wind layer(QZWL)over Dunhuang,Gansu Province,China,on 9 August 2020 using sounding observations from the Dunhuang national reference station and the fifth generation of ECMWF atmospheric reanalysis data(ERA5).The QZWL over Dunhuang was located between 18.6 and 20.4 km on 9 August 2020.The South Asian high(SAH)and subtropical westerly jet jointly affected the QZWL.As the SAH retreated westward,the upper-level westerly jet over Dunhuang strengthened,and the jet axis height increased.As a result,the zonal westerly wind was lifted to a higher altitude,and the wind speed of 100–70 hPa increased,raising the QZWL.In addition,the east–west oscillation of the SAH occurred earlier than the adjustment of the QZWL altitude,which can be used as a forecasting indicator for the QZWL.To further explore the mechanism responsible for the QZWL adjustment,the forcing terms in the equations for zonal wind,kinetic energy,and vertical wind shear were analyzed.The results showed that the upper-level geopotential gradient was the basic physical factor forcing the local change in zonal wind and kinetic energy.The change in zonal wind and kinetic energy led to the uplift of the QZWL.The results revealed that the vertical shear of horizontal wind could adequately indicate the stratospheric QZWL location.