Instill the bull's nose: intelligence? Deformable aircraft is efficient and flexible.
How is the research progress of intelligent morphing aircraft? What are the recent breakthroughs in its key technologies?
Inlaid ox characters:
The North Atlantic Treaty Organization defines intelligent variant aircraft as: by changing the shape of the aircraft locally or wholly, the aircraft can adapt to various mission requirements in real time and maintain the best efficiency and performance in various flight environments. It can be seen that the intelligent variant aircraft is a new concept aircraft with flight adaptability, and its research involves unsteady aerodynamics, time-varying structural mechanics, pneumatic servo elasticity, intelligent materials and structural mechanics, nonlinear system dynamics, intelligent sensing and control science and other frontiers and hotspots. It represents a development direction of advanced aircraft in the future. Intelligent morphing aircraft has great application prospects. Taking the future intelligent variant aircraft envisioned by NASA as an example, through the comprehensive application of new intelligent materials, actuators, sensors and control systems, the aircraft can constantly change its shape with the change of external environment, not only maintaining the best performance during the whole flight, but also improving comfort and reducing costs.
Because of its great advantages and application potential, many intelligent morphing design concepts and attempts have emerged at home and abroad, such as adaptive wing, active flexible wing, active aeroelastic wing, intelligent wing, intelligent rotor, variant aircraft and so on. In this paper, the latest progress of intelligent morphing aircraft is classified and summarized according to the wing surface deformation mode, and various types of intelligent morphing of wings, such as variable span, variable chord length, variable thickness, variable sweep angle and variable camber, are introduced in detail, and several key technologies to realize intelligent morphing of wings are refined. Through the introduction of this paper, we can have a richer understanding and understanding of the design ideas and key technologies of intelligent variant aircraft.
Classification and progress of deformed wings
Changing the plane shape of the wing reasonably can improve the aerodynamic performance of the aircraft. The following table lists the effects of wing parameters on aerodynamic performance. It can be seen that by reasonably changing the wing shape parameters, the aerodynamic characteristics and maneuverability of the aircraft can be improved, and the benefits of increasing lift, reducing drag, increasing range and flight time can be produced, so that the aircraft can complete a variety of flight tasks efficiently. Due to the influence of wing shape parameters, the design methods of wing deformation are also varied. In this paper, the deformation forms such as variable spread length, variable chord length, variable thickness, variable sweep angle and variable camber are introduced respectively.
1. Variable expansion length
Stretching is the simplest and most direct way of wing deformation. The change of wingspan has the following advantages: increasing the wingspan of variant aircraft is equivalent to increasing its wing area and aspect ratio, which can lead to the increase of lift-drag ratio and the increase of range and endurance; When parking, the wing shrinks, which can significantly reduce the occupied area of the aircraft; When the wingspans of the two sides are different, the rolling moment caused by the asymmetry of left and right lift can facilitate the lateral control of the aircraft.
As early as 1929, American designer Vinent first put forward the design idea of wing with variable span, and successfully manufactured GX-3 verification machine for flight test. 193 1 year, Russian scientist Makhonine designed and manufactured MAK- 10 aircraft, and its span can be increased from 13 m to 2 1 m, with a change of more than 60%. MAK- 123 aircraft appeared in 1947, and FS-29 aircraft appeared in 1972, both of which belong to variable extension aircraft, but the early deformation mechanism was too bulky to be popularized.
"Telescopic wing" is a new design concept with variable span that has appeared in recent years. In the deformable aircraft structure project (MAS) implemented by the National Defense Research Projects Agency in 2003, the retractable wing is one of the three main deformation schemes (the rest are folding wings and sliding skin wings, which will be introduced in detail later). This design is aimed at Tomahawk cruise missiles. The wings are deployed to obtain maximum lift during cruise flight, and the wings are contracted during high-speed dive to improve maneuverability. However, due to wing loading's heavy weight and thin wing, it is impossible to install telescopic mechanism, so this scheme cannot be popularized. Wang Jianghua of Northwestern Polytechnical University and others optimized the aerodynamic shape of the telescopic wing cruise missile. The research shows that the telescopic wing design can reduce the fuel consumption of the missile by 65,438+02%, and obviously improve the missile performance. In 2007, Julie and others of the University of Maryland took the inflatable telescopic beam as the driving mechanism, and changed the lift and controlled the roll through wing expansion and contraction, and conducted wind tunnel tests. After testing, the aspect ratio can be changed by up to 230%, and the lift-drag ratio can be as high as 16. However, the parasitic resistance caused by soft skin affects the aerodynamic performance to some extent.
Generally speaking, the variable-span wing still needs to solve a series of problems such as structural weight reduction design of telescopic mechanism, wing thinning design of high-speed flight and continuous sealing design of elastic skin, which is far from engineering application.
2. Variable chord length
Similar to the control effect of the variable-length wing, the variable-length wing also changes the aspect ratio and wing area reasonably through wing deformation, thus optimizing the lift-drag ratio, flight speed and maneuverability of the aircraft.
The most typical application of the concept of variable chord length is the flaperon design of traditional aircraft. The take-off and landing performance and rolling maneuverability of the aircraft can be significantly improved by driving the aileron to deform chordally through the screw mechanism. As for the airfoil itself, it is very difficult to design the variable chord length because of the interference of the beam frame, oil tank and other equipment, or the airfoil is small and lacks space, so there are relatively few related studies at home and abroad. As early as 1937, Russian scientist Bakasayev designed and manufactured the first variable chord aircraft RK- 1. The chord length of the aircraft was changed from the six-string telescopic overlapping fuselage, and the wing area of the original aircraft changed by 44%, and the improved aircraft changed by as much as 135%, which verified the feasibility of changing the chord length through the telescopic mechanism.
In recent years, the technology companies represented by CRG Company in the United States have re-developed the research of variable chord wing by using composite materials and intelligent materials. In 2004, Perkins of CRG Company and others used shape memory alloy materials with compression ratio as high as 400% for variable chord length design. The experimental results show that the material can achieve the expected deformation after heating, but the shape memory alloy can not return to its original shape after cooling because of its instability. In 2005, Reid of CRG and others designed a variable chord wing with ribs interspersed with each other. Driven by DC motor and guide rod, the wing area can be increased by nearly 80%, but the design also has the problems of complex mechanism and low restoring force of skin material, which makes it difficult to return to the initial state of deformation. 20 1 1 Barbarino of Pennsylvania State University and others applied the compressible honeycomb structure to the chord deformation design of helicopter blades. The deformed honeycomb structure can withstand cyclic driving, and its chord deformation can be increased by about 30%. In addition, it is worth mentioning that the designer ensures the continuous smoothness of the wing surface by pre-stretching the flexible skin.
Driven by new materials and technologies such as shape memory alloy and composite honeycomb structure, many concepts of variable chord wing have appeared in recent years, but the performance stability of these new materials needs to be improved to facilitate engineering application.
3. Variable thickness
Variable thickness design refers to adjusting the contour line of the wing without causing obvious changes in the shape of the wing, which belongs to slight deformation design. The change of wing thickness can improve the aerodynamic performance of airfoil at high and low speeds, and has the advantages of avoiding or delaying boundary layer separation, controlling transition position and shock wave to reduce wave resistance and suppress buffeting.
As early as 1992, Austin and others in the United States designed a variable thickness wing based on truss structure. The designer arranged a linear displacement driver on the truss. By exciting the driver, the length of each strut on the truss can be adjusted, thus adjusting the airfoil thickness and optimizing the aerodynamic efficiency. In recent years, the National Research Center of Canada has carried out a series of theoretical research and experimental verification of variable thickness wings. In 2007, Coutu and others of the center designed an adaptive variable thickness wing. The wing consists of a rigid fuselage, a flexible skin and a driver installed inside the wing. The wing skin is made of carbon fiber composite material, which has good flexibility and sufficient support stiffness. Under the driver's excitation, the thickness of the wing changes and the laminar flow effect of the wing is effectively improved. In 2008, Andrei et al. of the center designed an excitation device in the thickness direction of the upper surface of the wing. Through the numerical simulation of different airfoils of 17, the conclusion that the transition position is delayed is obtained, which proves that periodic driving excitation can be applied to transition control. In 2009, based on Andrei's research, Grigoli designed an adaptive neuro-fuzzy controller for deformation control. The controller calculates the pressure change between the reference airfoil and the optimized airfoil according to the airfoil surface pressure collected by the pressure sensor, and realizes the direct correlation between the pressure change and the transition position for the first time. In addition, in 2009, Steven and others of the University of Bristol in the United Kingdom used piezoelectric materials as actuators and installed them on the upper surface of the wing skin. When energized, the exciter generates vibration with a fixed frequency, thus changing the boundary layer flow on the skin surface. The wind tunnel test shows that this driving mode can reduce the wing drag and improve the lift.
Variable thickness wing design can adjust the flow field and improve aerodynamic performance by changing the airfoil slightly. With the development of new intelligent materials such as piezoelectric ceramics, more application attempts and greater economic value will be produced in future engineering applications.
Step 4 change the sweep angle
Small sweep angle is helpful to improve the efficiency of the wing at low speed, and large sweep angle is helpful to reduce the wave-making resistance at high speed. Sweep angle changes independently in different flight conditions, which becomes the most effective means to take into account different aerodynamic performances at high and low speeds. Because of this, the variable sweep technology has also become the first mature technology to change the shape of the wing.
From the 1940s to the 1970s, the variable sweep technology has been successfully applied to many fighters and bombers, such as MiG -23, F- 14, Gael, B- 1B bomber and so on. However, due to its complicated mechanism and operation, high failure rate and difficult maintenance, the early variable sweep technology was gradually replaced by double triangle design, canard design, large side strip design and wing body fusion technology, which limited the improvement of aircraft load, shape and stealth performance.
2 1 century, with the development and application of new materials and technologies, the performance of variable sweep aircraft has also been developed and improved. In 2004, Neil of Virginia Tech and others designed a UAV model with adaptive deformation. Except that the wingspan can be changed by 65,438+07%, the tail of the fuselage can be compressed by 65,438+02% and the wing can be twisted by 20, the sweep angle of the UAV can be changed from 0 to 40. Wind tunnel experiments verify the effectiveness of UAV model in various deformation forms. In 2006, Grant of the University of Florida and others designed a multi-node variable sweepback micro air vehicle by studying the flying attitude of seagulls. The inner and outer wings of the aircraft wing have independent variable sweep angle mechanism, and the simulation shows that it has good steering ability and crosswind resistance. 20 13 Chen Qian et al. of China Aerospace Aerodynamics Research Institute designed and analyzed the large-scale shear variable swept mode of aircraft outer wing, and verified that the variable swept wing can meet the needs of aerodynamic characteristics research such as skin, structure, drive and control through wind tunnel tests. The quasi-steady aerodynamic characteristic curve shows that the variable swept wing has great aerodynamic benefits.
Most notably, the sliding skin variable swept wing aircraft MFX- 1 designed by American NextGen Company for MAS project is different from the traditional variable swept wing mode with wings embedded in the fuselage, and the chord length of the aircraft can be changed independently of the swept angle. In 2006, the first flight of MFX- 1 was successful. At the speed of 185~220km/h, the wingspan changes by 30%, the wing area changes by 40%, and the sweepback angle changes from15 to 35, and the whole journey does not exceed 15s. The test results successfully confirmed that the plane was flying.
5. Variable curvature
The camber is the most basic factor for the wing to generate lift. Changing camber can effectively control the airflow separation on the wing surface, and can significantly improve the flight maneuverability of aircraft, especially for low-speed aircraft, which is usually in a low Reynolds number flight state and its performance mainly depends on laminar boundary layer flow.
There are many researches on the variable camber wing at home and abroad, such as the mechanically hinged variable camber wing in the MAW project of 198 1, the mechanical variable camber wing installed on the F-1/fighter plane by Powers of 1992, and the University of Maryland in 2004, due to mechanical reasons.
In recent years, the development of intelligent materials and advanced manufacturing technology has provided a good material and technical foundation for variable camber wings. In 2003, Elzey of the University of Virginia and others designed a connecting-rod variable camber wing driven by shape memory alloy, which caused great bending deformation of the wing section. In 2009, Pierre of Texas A&M University and others made their own mechanism to drive the deformation of the leading edge and trailing edge of the wing by pressurizing the airbag in the central wing box. After testing, the maximum deformation of the airfoil head is 14, and the maximum deformation of the airfoil tail is 13. After deformation, the skin can still remain smooth and continuous. 20 1 1 Hasse of the Swiss Center for Structural Science and Technology put forward the concept of "rib structure" and applied it to the design of morphing wings. The designer replaced the traditional hinge structure with distributed flexible rib structure, which has the advantages of large geometric deformation, high bearing capacity and light weight. The ground test shows that the wing rib structure design can realize the airfoil from NACA00 12 to NACA 242. In 20 15, James et al. of the US Air Force Laboratory designed a conformal airfoil based on "compliance mechanism". The compliant mechanism can amplify the actuation displacement of smart materials and transfer it to the leading edge and the trailing edge, so that the energy required for airfoil control is lower. Removing the control surface also reduces the weight and cost of the wing. The experimental model is1.8m, and the camber changes more than 6% under aerodynamic load. 20 15 Alessandro and others in Italy designed a conformal wing based on "asymmetric structure". The design idea is similar to "compliant mechanism", and it is also a cleverly designed force transmission mechanism, which can amplify the dynamic displacement and transmit it to the front and rear edges. This design can effectively avoid local stress caused by deformation. The designer has proved the advancement of independent deformation of asymmetric honeycomb structure through ground tests, and analyzed the typical failure forms of the structure and the strong nonlinearity caused by large deformation. 20 15 Benjimin and others of Swansea university in England put forward the concept of "active bending deformation of fishbone" under the inspiration of biology. The fishbone structure is adopted to reduce the chord stiffness of the airfoil and realize the camber control of the airfoil. The wind tunnel test shows that the lift-drag ratio of the deformed wing can be increased by 20%~25% compared with the traditional wing under the same test conditions. This concept can be applied to fixed wings, helicopters, wind turbines and tides. In 20 16, Francesco and others of the Swiss Laboratory for Composite Materials and Adaptive Structures designed a "wrinkled skin-reinforced" wing that could replace ailerons. Under the action of water flow, the folded skin of the trailing edge can stretch and deform, pushing the tail to bend up and down. The wind tunnel test shows that this design can provide high-frequency rolling control force and effectively replace the aileron function. In addition, due to the continuous shape of the wing, this design can significantly reduce the zero-lift resistance.
At present, foreign countries attach great importance to the research of variable camber wings. With the development of intelligent materials, various design concepts have emerged. The conformal airfoil design based on variable camber can not only control the separation of airflow by changing the camber of the airfoil and improve the aerodynamic performance of the aircraft, but also realize the warpage of the airfoil and control the rolling maneuver of the aircraft by setting different camber for different chord sections, which can effectively replace the control surfaces such as flaperons, and has high application value and engineering feasibility.
Key Technology of Deformed Wing
According to the above introduction, although there are various ways of wing deformation, all deformed wings are inseparable from large-area smooth and continuous flexible skin structure, light and efficient deformation drive system and fast and sensitive sensing control system. Therefore, the key technologies to realize wing deformation can be divided into the following categories:
1. Smooth and continuous flexible skin technology
Compared with the conventional wing, the deformable wing puts forward new requirements for the skin structure, that is, the skin should not only maintain the characteristics of light weight, high in-plane normal stiffness, and be able to bear and transmit aerodynamic loads, but also have sufficient smooth continuity and large-scale deformation characteristics. Therefore, it is an important challenge for the design of intelligent deformable aircraft in the future to combine traditional materials with new materials, innovate structural design, and design a flexible skin structure with weight, deformability and bearing capacity meeting the deformation scheme.
2. Light and efficient deformation drive control technology.
The driving and control of deformable wings is also one of the key technologies in the design of intelligent deformable aircraft. The driving device of intelligent variant aircraft should have the characteristics of light weight, distribution, high efficiency, quick response, low energy consumption and easy control. The traditional motor and hydraulic driving methods are too complicated and complicated to meet the design requirements. New driving devices based on smart materials should be the focus of the follow-up development, such as magnetostrictive actuators, piezoelectric ceramic actuators and shape memory material actuators.
3. Distributed sensor network technology adapted to large deformation.
Intelligent deformation of structures requires real-time detection and perception of changes in the surrounding environment and self-state, which requires that the wings are covered with sensing elements capable of sensing all kinds of information to form a distributed multi-sensor network system. Sensor elements should not only ensure sufficient accuracy and fast response characteristics, but also adapt to the motion characteristics of intelligent deformable aircraft with large displacement and strain, which puts forward new requirements for sensor elements and sensor networks and is also one of the challenges in the future.
Intelligent variant aircraft design is a new technology with broad application prospects in civil and military aircraft fields, which can promote the development of new intelligent materials, bionic design, structural optimization design, advanced sensing technology and multi-information fusion technology, and has far-reaching significance for the pre-research and technical reserve of new concept aircraft in the future. The summary of intelligent variant technology in this paper can provide corresponding reference for the design and development of intelligent variant aircraft.