Exploration on the Frontiers of Flexible and Deformable Cross-Domain Intelligent Flight

BAO Weimin ,Lü Xiaozhou *,BAI Guanghui ,LI Xiaoping *,LIU Yanmin ,BAI BowenSHI Yaoguang

1 School of Aerospace Science &Technology,Xidian University,Xi’an 710071

2 China Aerospace Science and Technology Corporation,Beijing 100048

3 Science and Technology on Space Physics Laboratory,Beijing 100076

Abstract:As human aeronautic and aerospace technology continues to prosper and the aerial flight space domain further expands,traditional fixed-shape air vehicles have been confronted with difficulties in satisfying complex missions in cross-domain scenarios.Owing to their flexible and deformable appearance,morphing air vehicles are expected to realize cross-domain intelligent flight,thus emerging as the most subversive strategic development trend and research focus in aeronautic and aerospace fields.This paper primarily reviews the research background and challenges of flexible and deformable cross-domain intelligent flight,proposing a corresponding research framework and mode as well as exploring the scientific issues and state-of-the-art solutions,where key research progress is introduced.The explorations covered in this paper also provide ideas and directions for the study of deformable cross-domain intelligent flight,which has critical scientific significance in promoting the study itself.

Key words:cross-domain flight,flexible,intelligent,deformable configuration

1 INTRODUCTION

With the advancing development of human aerospace technology and the continuous expansion of space flights,cross-domain flight is gradually becoming one of the most urgent strategic development areas in the aeronautic and aerospace fields,which will enable free access to and full use of space.It has become more difficult for traditional fixed-shape air vehicles to accomplish complex missions for cross-domain scenarios.On the contrary,by continuously changing the external shape of the vehicles in part or in whole,flexible and deformable air vehicles can adapt to diverse missions,satisfying practical requirements in real time while keeping optimal efficiency and performance in various flight environments,thus this has become one of the most subversive and revolutionary research interests in aeronautic and aerospace fields.

A large amount of research has been conducted on deformable air vehicles worldwide.With the flight speed ranging from 185 to 220 km/h,the morphing aircraft designed by Defense Advanced Research Projects Agency (DARPA) realized diverse functions of flight,involving integrated reconnaissance,patrol and strike.As shown in Figure 1,the morphing aircraft of BAE Systems plc is expected to realize morphing flight with a speed from subsonic to transonic.The U.S.Air Force’s smart morphing wing achieved morphing flight at an altitude of 12 km at 0.75 Ma speed.The smart wings from the European H2020 Project achieved low-speed morphing flight.Unfortunately,most of the research on these deformable air vehicles focused on the sub/transonic aviation field.In the aerospace field,few studies to date have focused on the greater challenge of high-speed-generated aerodynamic heat effecting the variable wings of high-speed morphing air vehicles.As the speed of cross-domain flight continues to increase,the importance of studying high-speed deformable cross-domain air vehicles has become self-evident.

In response to the practical demands required by highspeed and cross-domain morphing flight,this paper proposes the concept of flexible and deformable cross-domain intelligent flight.Literally,the deformation mentioned in this paper refers to the continuous changing of different spatial scales (in part,in distribution and/or in whole) during the flight of the air vehicle.The scope of cross domain involves both cross-airspace and cross-speed domains.Cross-airspace mainly refers to the continuous domain,including slip domain and the thin domain of near space.Cross-speed domain refers to cross-domain from high speed to ultra-high speed (Mach 3 -25).The notion of intelligent is that the air vehicle can adjust its aerodynamic,electromagnetic (EM) and maneuvering characteristics through continuously changing its flexible and deformable shape to realize intelligent flight based upon autonomous decision-making and real time learning.

The flexible and deformable cross-domain intelligent air vehicle is a major change to the traditional fixed-shape highspeed air vehicles,where the characteristics include a variable lift-to-drag ratio,on-demand orbital changes and intelligent flight enabled by its continuously changeable form.Therefore,the flexible and deformable cross-domain intelligent air vehicle is expected to realize continuous changing of its external form to complete its task,as well as conducting an environment-oriented intelligent flight,i.e.,on-demand change and intelligent flight,which is a disruptive innovation for the flight mode based on historical fixed-shaped air vehicles.

Nevertheless,the flexible and deformable cross-domain intelligent air vehicle is still confronted with certain great challenges.The first challenge is that the thermal-resistance limit of existing flexible materials is lower than 400 ℃,whereas the aerodynamic heat generated by air friction during high-speed flight exceeds a thousand degrees celsius.Therefore,the prerequisite for morphing flight lies in achieving the long-term survival of the flexible skin of the high-speed air vehicle in extreme heat flow.

The second challenge to achieve morphing is the vehicle’s deformation mechanism.When the vehicle conducts large-scale deformation,miniaturizing of the driving mechanism is required from the beginning,capable of high load-bearing performance.

Figure 1 Deformable air vehicles in the world

Furthermore,due to the nature of the deformable highspeed air vehicle featuring nonstationary and multi-degree-offreedom (MDOF) characteristics,there are no mature control models available for reference,which poses a potential encumbrance to its flight control.The key to realize intelligent morphing flight is to invariably maintain stable optimal aerodynamic characteristics of the deformable air vehicle.

In addition,the deformable air vehicle concept is still facing great challenges in the area of making decisions and learning strategies throughout the process of realizing intelligent flight.Therefore,implementation of autonomous intelligent flight control which matches the environment and mission is a key difficulty in achieving intelligent morphing flight.These challenges have become the main issues of study on deformable cross-domain intelligent flight worldwide,demanding higher requirements on research models and ideas.

2 RESEARCH FRAMEWORK AND MODE

The traditional fixed-shape high-speed air vehicle was mainly designed to serve established missions.The overall objectives firstly conceived,according to the established tasks,then various subsystems of the air vehicle are devised,after which each subsystem performs the operation of flight control in accordance with a specific set of indices and parameters.Flight control mainly includes trajectory planning,guidance and attitude control,which are based on dynamic and kinematic models.Hence,the existing research modes for air vehicles can be summarized as a study of multi-interdisciplinary,decoupling sequence,characterized by fixed shape,control model with hierarchical control.Specifically,the control model is first established according to the fixed shape of the air vehicle,then the hierarchical control is established based on the control model,and finally the vehicle’s shape is determined in accordance with the hierarchical control,thereby realizing overall flight control.

The design of the flexible and deformable high-speed air vehicle presented in this project mainly focuses on variable multiple missions,in which a novel research mode for intelligent flight is proposed.This mode can be concluded as an in-depth multi-interdisciplinary coupled research featured with the characteristics of deformable configuration,multimodality and smart control.Based on continuous flexible and deformable configurations,the construction of a smart-learning mode is introduced.Through adaptive high-dimensional smart control,based on the mode of space,an integrated control of aerodynamic,electromagnetic and trajectory characteristics during flight process is conducted,thereby realizing on-demand high-speed cross-domain intelligent flight.

The key to realize a flexible and continuous deformable configuration mode lies in innovating a variable configuration,multi-modality and smart control.

Figure 2 Different research modes

The deformable configurations enable over-limit heat protection on the external form of the air vehicle by studying the over-limit thermal-resistance of flexible materials.Meanwhile,the intelligent perception for the vehicle’s deformation state is realized through flexible sensing.Finally,the deformable mechanical structure is studied,which is combined subsequently with the flexible skin to form the vehicle's deformation unit,thereby realizing continuously controllable and deformable configurations with flexible external form,i.e.,on-demand deformation.

Through mechanism modeling and offline learning,the multi-modality can realize the cognition of task requirements,aerodynamic characteristics,motion characteristics,and electromagnetic characteristics to achieve online perception and autonomous learning,thus establishing a new mechanism to conduct recurrent learning.

Finally,the multi-modal autonomous decision-making and smart control for high-speed cross-domain flight is realized through real-time intelligent perception.

3 SCIENTIFIC PROBLEMS AND EXPLORATIONS

In realizing flexible and deformable cross-domain intelligent flight,a series of issues need to be overcome,involving aerodynamic shape,material fabrication,structure design,driving power,flight control,etc.Among these issues,some of the major scientific problems are described as follows,which include but are not limited to flexible materials and deformation mechanisms in a high-speed cross-domain flight environment;interaction mechanism and description of deformation configuration and flight characteristics for a high-speed air vehicle;integrated smart control for deformation and flight for a high-speed cross-domain air vehicle.

In view of the aforementioned problems and resultant research directions required for flexible and deformable cross-domain intelligent flight,our research group has accumulated some research findings in flexible skin and flexible sensing,as well as in smart control of electromagnetic characteristics.

Exploration 1:Flexible and continuously deformable materials and deformation mechanisms.

Currently,the existing deformable-configured flexible materials and deformation mechanisms encounter three major challenges,i.e.,the over-limit thermal protection of flexible materials,the decoupling measurement of multiple physical parameters,and how to achieve deformation mechanisms with high load-bearing performance.

It has been introduced above that the existing flexible materials cannot survive during the period of high-speed or ultra-high speed flight due to the thermal-resistance limit being lower than 400℃.

In order to realize the long-term survival of the flexible skin of the high-speed air vehicle under extreme heat flow,in our preliminary research,our research group imitated the structure of the human skin sweat glands and proposed a method for creating a flexible skin for high-speed aircraft based on the Lost-Wax Casting (LWC) method,and prepared a prototype which has passed its ground FD test assessment.

In our preliminary work,our research group proposed a fabrication method based on LWC for the flexible skin of the high-speed air vehicle to realize its long-term survival under extreme heat flow,which imitated the sudoriferous gland structure in the human skin,and the prototype has been verified through ground FD test.

Figure 3 Bionic flexible skin

Exploration 2:Theory and method of autonomous-learning control for high-speed cross-domain intelligent flight

The autonomous-learning control for high-speed cross-domain intelligent flight is that control after completely utilizing the information acquired by itself and/or from the surroundings,so the smart morphing air vehicle can respond appropriately to external changes and influences through learning.So the smart control technology is used to perform online identification,evaluation and planning,as well as online control reconfiguration of the vehicle itself adapting to its surroundings during the intelligent flight process.

The smart control can be realized by conducting autonomous decision-making,through information acquisition,and the transmission of system information between element modals through multimodalities.The control method for the autonomous learning needed for high-speed cross-domain intelligent flight should concentrate on not only the multiscale decomposing and matching strategies of multimodalities,but also on the rapid autonomous decision-making appropriate to deal with the required diverse environmental tasks.

Our research group has proposed a variety of flexible sensors and made prototypes to realize the detection of some physical parameters such as three-dimensional stress,flexible large strain.Our prepared prototype of flexible three-dimensional graphene foam exhibits excellent characteristics for electrical conductance (approximate 0.4 S·m) and pressure-sensitivity,which can be utilized in flexible tactile sensors,flexible three-dimensional interface stress sensors and large strain sensors,thereby realizing decoupling measurement of multiple physical parameters.

Exploration 3:Smart control of electromagnetic characteristics of deformable air vehicle.

This approach uses the electromagnetic scattering echo of the object to obtain certain properties of the scatter,such as profile,size,position,electromagnetic parameters.

In view of cross-domain flight,our future research interest will be focused on certain problems involving the spatiotemporal electromagnetic response mechanism of flexible,fluid and solid polymorphic coupling,the electromagnetic effect,as well as spatiotemporal variation of deformable configured air vehicles.

Our research group has made relevant explorations on the multi-field coupling mechanism and the control technology of electromagnetic characteristics of deformable configured air vehicles.

As shown in Figure 5,multi-field coupling can be used to solve the issue of control technology of the flexible morphing air vehicle’s electromagnetic characteristics,and to proactively adjust not only the vehicle’s deformation for flowing and mixing with the flow field of the plasma sheath,but also the parameters and distribution properties of the plasma,thus realizing the effect of electromagnetic characteristics control with respect to the vehicle.Consequently,the intelligent flexible morphing air vehicle is capable of analyzing and simulating the multi-field coupling mechanism of deformation configuration and high-speed flow field.

Figure 4 Flexible 3D graphene foam

Figure 5 Multi-field coupling mechanism

Moreover,we have established an electromagnetic research platform for low-pressure high-enthalpy high-speed plasma,which is used to conduct plasma force-thermal-electromagnetic multi-physical field effect experiments in plasma.The established platform is also used to verify the active control technology of high-speed plasma flow field,and to measure the high-precision multi-dimensional electromagnetic characteristics of a target enveloped with plasma.

Regarding the technical regulation of the electromagnetic characteristics of the flexible morphing air vehicle,the experiments were conducted by using the shear mixing flow characteristics of a double-nozzle flow rate difference to form a high-dynamic and non-steady flow mixing process.

Through using the equivalent experimental method,the flow mixing process between the deformation of the vehicle and the high-speed flow field was preliminarily simulated,during which a discernible and stable distribution of brightness and dark can be observed,where the vortex scale can reach millimeter to centimeter orders of magnitude.

In addition,the local surface of the air vehicle and the flow field were flow-mixed at different inclination angles,and the simulation measuring the electromagnetic scattering characteristics under the local deformation state of the vehicle was conducted.Our preliminary experiments have verified that the electromagnetic scattering characteristics and the target characteristics of a one-dimensional range profile can be adjusted and regulated under the morphing state of the vehicle.

The above explorations and approaches lay theoretical foundations for the research of flexible and deformable cross-domain air vehicles in terms of over-limit thermal protection,a deformable flexible skin,electromagnetic control,supporting flexible and deformable cross-domain intelligent flight.

4 PERSPECTIVES

With respect to the current state of flight control and smart control technologies,the cross-domain deformable intelligent air vehicle still has great potential for development,for which some most detrimental encumbrances and barriers must be overcome.

The first task is to achieve adaptive matching of the aerodynamic characteristics of the air vehicle under its flexible morphing state,which can be solved by adopting diverse morphing configurations to realize a variable lift-to-drag ratio at different flight stages,and by enabling the optimal aerodynamic layout of the full mission profile.

The second task lies in realizing the electromagnetic control and the stealth of the flexible morphing air vehicle.

Last but not least,it is of critical importance to realize autonomous trajectory change of the flexible morphing air vehicle,and to change the external shape of the vehicle to attain multi-function capability.In addition,the morphing vehicle can facilitate its change of trajectory,thus achieving super maneuverability for on-demand autonomous trajectory changes.

The research and development of the deformable air vehicle still has a long way to go.Both the deformable driving mechanism,together with the modeling and the control law design of the morphing vehicle need to be further studied.The relevant studies will produce far-reaching significance for the sustainable development of various multidisciplinary fields,including but are not limited to smart materials,structural design,sensing technology,information processing,as well as control law design.