A bird-inspired wing surface retraction and extension trans-medium deformable aircraft

Abstract

This invention discloses a bird-inspired wing-shaped deformable aircraft that can move across media. Using birds such as pelicans, which possess transmedia capabilities, as biomimetic prototypes, and based on research into their water-entry and exiting behaviors and morphological characteristics, an innovative structural design scheme for a biomimetic deformable wing aircraft is proposed. Based on the biomechanical mechanisms of multi-joint linkage in bird skeletons and the hydrophobic properties of feathers, the aircraft employs a deformable wing and tail design; combined with lightweight materials and a multi-degree-of-freedom actuation strategy, it achieves efficient coordination between dynamic morphological switching and transmedia motion.

Description

Technical Field

[0001] This invention belongs to the field of aircraft technology, specifically relating to a bird-inspired wing surface that can deform across media. Background Technology

[0002] Hybrid Aquatic-Aerial Vehicles (HAAVs), with their unique cross-sea and air operational capabilities, have become a research hotspot in the field of special-purpose unmanned aerial vehicles (UAVs). Currently, my country is deeply implementing its maritime power strategy, accelerating the pace of marine science and technology innovation, and the low-altitude economy has risen to become a national strategic emerging industry, becoming an important direction for cultivating new productive forces. Against this backdrop, HAAVs, capable of freely switching between sea and air domains, align with multiple national strategic needs such as three-dimensional marine resource development, emergency rescue in complex environments, and high-end defense applications. As an unmanned system with both air and underwater capabilities, its core advantage lies in expanding the operational range and mission flexibility through medium switching. In the civilian sector, HAAVs can perform deep-sea three-dimensional exploration, disaster relief, and efficient material delivery; in marine scientific research, they can support seabed topographic mapping, three-dimensional hydrological parameter acquisition, and cross-medium environmental monitoring; in the military sector, they can utilize underwater navigation to evade radar detection or achieve emergency deployment through high-speed aerial maneuverability, completing tasks such as communication relay, amphibious reconnaissance, penetration missions, and even electronic jamming relay. Compared to traditional single-media platforms, HAAV's cross-domain collaboration capabilities can significantly reduce energy consumption and improve mission success rates.

[0003] Air and water differ in density by approximately 800 times and viscosity by 49 times. This significant difference in physical properties presents a dual challenge to the structural design and dynamic performance of aircraft. The different media result in vastly different shapes for airborne and underwater vehicles. Traditional fixed-wing aircraft rely on large-area wings to generate lift; however, underwater navigation is hampered by the high-density fluid creating significant drag, leading to low underwater efficiency. Furthermore, the streamlined hull of underwater vehicles struggles to meet lift requirements in air. Research indicates that, with the same structural shape and speed, lift in water can be up to 14 times greater than in air, highlighting the limitations of a single-configuration design. Achieving efficient cross-medium switching through structural optimization has become a major challenge in the development of HAAVs (Habitat-Assisted Aerial Vehicles).

[0004] Bionics offers a new approach to solving these problems. In nature, some creatures, such as cormorants and flying fish, can actively adapt to different media environments by changing their limb morphology. Cormorants quickly fold their wings to reduce drag the moment they enter the water, and then unfold their wings to generate lift after emerging from the water. Research based on bio-inspired principles shows that deformable wings can effectively adapt to differences in cross-medium flow fields. However, most existing research focuses on single media or simplified deformation mechanisms. Bionic modeling of the dynamic changes in the wing surface during the entire process of a bird entering the water still lacks a systematic approach, and the impact of wing and tail deformation on the stability of cross-medium aircraft has not yet been fully explored.

[0005] Existing technology research on cross-medium aircraft: “Aqua MAV”: In 2017, Siddall et al. designed a transmedium aircraft called “Aqua MAV”, breaking through the limitations of traditional aircraft through a deformable wing design. This design employs a biomimetic folding wing structure, with a total mass of 201.3g, a wingspan of 592mm, and an endurance of 14 minutes. The aircraft simulates the diving behavior of the northern gannet, adjusting the wing sweep angle within a range of 0°-90° to optimize aerodynamic performance. The aircraft maintains a high lift-to-drag ratio during cruise, while in the folded state, drag is reduced by 62% and lift by 92%. The research also included wind tunnel and underwater experiments to verify the aerodynamic characteristics of the wing at different sweep angles, and combined with a steady-state trajectory model to predict the aircraft's trajectory from the air to the water surface. The aircraft achieves underwater propulsion by compressing CO2 and jetting water, combined with a hollow tail fin to regulate buoyancy and maintain the aircraft's balance, resolving the contradiction between structural strength and fluid resistance during transmedium water exit. This research provides a technical approach for miniaturized biomimetic environmental monitoring devices that combines high efficiency and engineering robustness, demonstrating broad application potential in the fields of wetland ecological sampling and disaster emergency response.

[0006] "Dipper": In 2021, Rockenbauer et al. proposed the "Dipper" transmedium aircraft. It weighs 3100g, has a wingspan of 2100mm, and a maximum dive speed of 130km / h. Inspired by the dive behavior of cormorants, it employs a four-bar linkage to achieve a wing folding angle of up to 87°, effectively reducing water entry impact and optimizing underwater drag, thus improving attitude stability during transmedium flight. The maximum test depth reached 5m. It uses a single motor to drive a bidirectional clutch, separately controlling the air propeller (maximum thrust 42.6 N) and the underwater propeller (peak thrust 60 N). It achieves efficient aerial flight with a maximum level flight speed of 110 km / h and rapid underwater maneuverability with a maximum speed of 3 m / s. It leaps out of the water by switching motor directions in 0.3 s, reducing reliance on additional propulsion or jet systems.

[0007] "Nezha": In 2021, a team from Shanghai Jiao Tong University proposed a multi-modal hybrid innovative design aircraft. Its core breakthrough is the integration of the functions of a fixed-wing UAV, a rotary-wing UAV, and an underwater glider, achieving multi-modal motion such as horizontal / vertical flight, hovering, water surface drifting, and underwater gliding. It uses 30 MPa ultra-high-pressure gas cylinders and lightweight valves, has a 50-meter deep-diving capability, and utilizes a 360° rotatable mass block to adjust pitch attitude, achieving rudderless steering. The six-rotor arms fold synchronously to reduce underwater drag and improve hydrodynamic performance. The vertical takeoff and landing mechanism completes the water entry / exit transition in 5 seconds under wave interference, and the power consumption during level flight is 79.9% lower than that during vertical flight.

[0008] "Qianxiang II": In 2023, a team from Peking University and others proposed a gliding trans-domain vehicle. Based on a hydrofoil-foldable wing collaborative design, the vehicle is 2.0 m long, has a wingspan of 2.4 m, a maximum takeoff weight of 7.5 kg, and possesses water-surface takeoff capability. The research, through comparisons with and without hydrofoils, points out the crucial role of hydrofoils in reducing impact loads and improving fluid-structure interaction stability, resulting in a 21% reduction in vertical overload during landing.

[0009] Research on existing biomimetic deformable wing structures: In 2007, NextGen developed and validated the MFX-1 transformable aircraft, its key feature being the dynamic adjustment of its wings. The aircraft employs a flexible skin and lightweight kinematic substructure design, using a single motor to achieve continuous adjustment of wingspan, area, and sweep angle, with a configuration change time of less than 15 seconds. The aircraft weighs 100 pounds, has a wingspan of 9.3 feet, is powered by a jet engine, equipped with a V-tail and a three-way camera system, and has an endurance of 20 minutes. The research validated its performance through three phases of flight testing: the first test used a fixed-wing configuration to verify basic performance; the second test resulted in instability and a crash due to wing asymmetry and airspeed errors; after structural corrections and sensor calibration, the final test successfully achieved real-time configuration changes, and with laser velocimetry assistance, the flight trajectory was relatively stable.

[0010] In 2017, DiLuca's team proposed a biomimetic morphing wing inspired by the structure of bird feathers. This wing, designed with artificial feathers, can fold rapidly, reducing wing area by 41%. In its deployed state, its maximum lift coefficient increases by 32%, and its minimum turning radius decreases to 3.9 meters; in its folded state, the minimum drag coefficient decreases by 45.1%, making it suitable for high-speed flight. Roll control is achieved through asymmetric folding, achieving a maximum roll moment coefficient of 0.139. Under high lift conditions, the roll rate is 20% higher than traditional ailerons. Flight tests show that the wing can fold within 146ms, and combined with pitch control, it can effectively compensate for control lag in the low CL region. This design, through its biomimetic structure, provides a new solution for the multi-mission adaptability of unmanned aerial vehicles (UAVs).

[0011] In 2020, the Ajanica team proposed a biomimetic cooperative deformable unmanned aerial vehicle (UAV) based on the flight characteristics of the Northern Goshawk, enhancing flight performance through synchronized morphological adjustments of the wings and tail. The research employed a feather-like structure design with variable sweep angle and area, allowing the wing area to expand by 41% and the tail area by a maximum increase of 214.3%. Wind tunnel experiments showed that when the wings were deployed, lift increased by 70.8%, drag by 63.8%, and the pitch moment coefficient increased by a maximum of 253.9%. Tail deployment improved longitudinal stability. In the folded state, thrust decreased by 59% during high-speed cruise, while the minimum flight speed dropped to 4.0 m / s when fully deployed, flight tests verified the practical benefits of cooperative deformability. With both wings and tail fully extended, pitch acceleration increased by 445%, linear acceleration by 383%, while roll moment remained stable at high angles of attack. The research confirms that biomimetic cooperative deformability, through dynamic adjustment of aerodynamic characteristics, expands the agility and maneuverability of the UAV.

[0012] In 2024, Jeger's team proposed a bird-inspired adaptive deformable wing cooperative control strategy to improve the stability, anti-interference capability, and energy efficiency of unmanned aerial vehicles (UAVs) in complex airflow conditions. The deployed UAV has a wingspan of 1.52m and a mass of 752g, employing an 8-DOF dynamic adjustment mechanism. Experiments show that the controller can maintain flight stability even under physical disturbances, turbulence, and single-sided actuator failure. By using Bayesian optimization to adjust the sweep angle, twist angle, and tail wing shape online, energy consumption is reduced, and the optimal shape matches the behavior of birds retracting their wings at high speeds and spreading their wings at low speeds. Wind tunnel data and flight tests confirm the effectiveness of the method, demonstrating its applicability to long-endurance missions. Summary of the Invention

[0013] To overcome the shortcomings of existing technologies, this invention provides a bird-inspired wing-shaped deformable aircraft capable of folding and unfolding across media. Using pelicans and other birds with trans-media capabilities as biomimetic prototypes, and based on research into their water-entry and exiting behaviors and morphological characteristics, an innovative structural design scheme for a biomimetic deformable wing aircraft is proposed. Based on the biomechanical mechanisms of multi-joint linkage in bird skeletons and the hydrophobic properties of feathers, the aircraft employs a deformable wing and tail design; combined with lightweight materials and a multi-degree-of-freedom actuation strategy, it achieves efficient coordination between dynamic morphological switching and trans-media motion.

[0014] The technical solution adopted by this invention to solve its technical problem is as follows: A bird-inspired wing-shaped trans-medium deformable aircraft includes a rotor, a brushless motor, a fuselage center tube, fuselage skin, wings, a flexible connector, a power supply and control unit, and a tail fin. The rotor is mounted at the front end of the central tube of the fuselage; The brushless motor is connected to the central tube of the machine body via a motor mount; The tail end of the central tube of the fuselage is connected to a flexible connector; The power supply and control unit are located below the fuselage center tube and can slide back and forth along the fuselage center tube to adjust the position of the aircraft's center of gravity. The wings include a left wing and a right wing, which are symmetrically mounted on both sides of the fuselage center tube via wing fixing plates; the wing folding is achieved using a three-section hinged frame; The wing comprises a frame, feathers, and wing surfaces. The frame is the integral fixed part of the wing and includes a wing fixing plate, a second truss, a third truss, a fourth truss, a fifth truss, and a sixth truss. The wing fixing plate has two hinge points, A and B. Hinge point A is hinged to the second truss, allowing the second truss to rotate freely around hinge point A. A servo motor is mounted at hinge point B, with its fuselage fixed to the wing fixing plate and its servo disc fixed to the third truss. The second truss is hinged to the fourth and fifth trusses via two hinge points. The third truss is hinged to the fourth truss. The sixth truss is hinged to the fourth and fifth trusses via two hinge points. The feather includes a feather root, a feather shaft, and a feather surface; one end of the feather root has a hinge point hole, which connects to the fifth and sixth trusses and can rotate freely, and the other end is inserted into a carbon fiber rod as a feather shaft; the feather surface is attached to the feather shaft; Wing surfaces are arranged on the feather surfaces of the fifth and sixth trusses. The wing surface of the sixth truss is hollow with an open trailing edge. When the wing is folded, the wing surface of the fifth truss is housed inside to avoid structural interference. The tail section includes a tail support frame, a vertical tail section, and a horizontal tail section. The horizontal tail section consists of nine feathers that retract and deploy synchronously via a carbon fiber-rubber composite transmission. The tail support frame is hinged to an elastic connector. The tail support frame is fan-shaped and consists of two layers of carbon fiber plates, with the nine feathers fixed between the two layers via hinges. The vertical tail section is fixedly connected to the tail support frame, and the directional control surface is changed by altering the angle between the tail support frame and the elastic connector, thus controlling the aircraft's steering in conjunction with changes in the wing. The fuselage skin covers the central tube of the fuselage.

[0015] Preferably, the wing and control surfaces of the aircraft are controlled by five servo motors, two of which control the deployment and folding of the left and right wings respectively, one is fixed to the tail plate and is responsible for controlling the deployment and retraction of the tail, and two are placed at the rear of the fuselage to control the deflection of the horizontal and vertical tail respectively.

[0016] Preferably, the brushless motor is a Chengfeng V2306 V2 KV2400 brushless DC motor with a maximum power of 494W, a maximum current of 34.1A, and a maximum tensile force of 1242g.

[0017] Preferably, the central tube of the fuselage is made of 10 mm*10 mm carbon fiber square tube, which is responsible for supporting the fuselage.

[0018] Preferably, the skeleton is made of carbon fiber material.

[0019] Preferably, the second, third, and fourth trusses are designed with a single layer thickness of 3mm, while the fifth and sixth trusses are designed with a double layer thickness of 1.5mm.

[0020] Preferably, the feathers are made using 3D printing.

[0021] Preferably, the carbon fiber rod has a diameter of 2 mm.

[0022] Preferably, the fifth truss and the sixth truss have 6 and 8 holes respectively for installing feathers.

[0023] Preferably, the carbon fiber plate has a thickness of 1.5 mm.

[0024] The beneficial effects of this invention are as follows: The deformable design of the aircraft's wings and tail is controlled collaboratively by independent servos, supporting asymmetric wing surface adjustment and multi-degree-of-freedom deflection, thus improving the aircraft's attitude stability during cross-medium transitions. The tail design incorporates an elastic connector to simulate the flexible tail cone structure of birds, combined with the synchronous retraction mechanism of the horizontal tail, enabling precise control of pitch and yaw attitudes. The entire aircraft utilizes a modular design to optimize weight distribution, employing waterproof motors and servos to ensure underwater operational reliability and avoiding the additional load from complex sealing devices. The feasibility of the structural design was verified using SolidWorks motion simulation. Simulation results show that the velocity and acceleration curves during wing deployment meet dynamic stability requirements, and the feedforward control strategy effectively suppresses terminal impacts and extends mechanism life. The elastic connector and composite transmission design of the tail ensure the synchronicity and controllability of the retraction and deployment actions. The overall design meets the functional requirements of rapid form switching and high maneuverability for cross-medium aircraft. Attached Figure Description

[0025] Figure 1 This is a diagram illustrating the process of a pelican entering and exiting the water. Figure 2 This is a simplified diagram of the wing frame structure; Figure 3 It is an assembly drawing of the wing frame; Figure 4 This is a schematic diagram of the wing deployment process; Figure 5 This is a schematic diagram of the wingtip deployment trajectory; Figure 6 This is a schematic diagram of the velocity change at the final point; Figure 7 This is a schematic diagram of the change in terminal acceleration; Figure 8 This is a schematic diagram illustrating the unfolding and retraction of feathers using fluorocarbon and rubber threads. Figure 9 This is the assembly drawing of the right wing; Figure 10 This is a schematic diagram of the rubber cable pulling the tail fin deployment; Figure 11 This is a schematic diagram of a flexible connector; Figure 12 This is the tail fin assembly drawing; Figure 13 This is a schematic diagram showing the installation position of the tail fin servo. Figure 14 This is the assembly drawing of the horizontal tail fin; Figure 15 This is the assembly drawing of the vertical tail fin; Figure 16 This is a diagram of the overall structure of the aircraft; Figure 17 It is an assembly drawing of the actual aircraft skeleton; Figure 18 It is a design architecture diagram.

[0026] Reference numerals: 1-Wing fixing plate, 2-Second truss, 3-Third truss, 4-Fourth truss, 5-Fifth truss, 6-Sixth truss, 7-Inner wing skin, 8-Outer wing skin, 9-Wing fixing plate, 10-Tail wing, 11-Rotor, 12-Fuselage center tube, 13-Fuselage skin, 14-Flexible connector. Detailed Implementation

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0028] This invention focuses on the structural design of cross-medium aircraft, starting with biological prototypes and biomimetic mapping to construct an engineering implementation approach for biomimetic deformable wings. Based on biomimetic results and the scope of mission operations, design requirements and directions are clarified. A three-dimensional model is constructed, primarily focusing on the design of the deformable mechanisms of the wings and tail. Through a progressively in-depth research process, from mechanism motion and shape construction to material selection and physical prototyping, an engineering-feasible system prototype is provided, such as... Figure 18 As shown.

[0029] The process of a pelican diving into the water to forage is divided into the following steps: Figure 1 The five stages are illustrated. When searching for prey, pelicans fly low, close to the water's surface, with their wings fully extended at a high angle of attack, maintaining a stable hovering position with a high aspect ratio. Upon spotting prey in the water, the pelican adjusts its wing tilt, changing its altitude and posture, and uses its tail to adjust its entry angle. During the dive, they fold their wings completely against their bodies, extend their necks forward, and stretch their legs backward, assuming an arrowhead shape that aligns with the direction of entry into the water, reducing the impact and drag. To minimize the impact of high-speed water entry, pelicans have air-sac-like structures throughout their skeletons that act as protective cushions upon impact with the water.

[0030] When prey is in deep water and the initial velocity upon entry is insufficient for further descent, the pelican flaps its wings for a second acceleration, using gliding and the action of its wing surfaces to dive to a depth of 24 meters. After completing its hunt, the pelican, aided by the buoyancy provided by its hydrophobic and hollow feathers, combined with rhythmic body flapping and the action of its webbed feet, quickly surfaces. Upon re-taking off, its wings spread and continuously flap the water's surface, gliding until it reaches takeoff speed, at which point it leaves the water, completing its foraging process.

[0031] 1. Design requirements and parameters; The design of a cross-medium aircraft requires several key characteristics. First, the wings and tail must be able to fold and unfold rapidly within 2 seconds, freely switching between media to ensure stability when entering water from the air or taking off from underwater. Second, during air flight, the wings must be in an unfolded state to meet the lift requirements of a fixed-wing UAV, providing low drag and a stable flight attitude to ensure long-endurance cruising capability. Third, during underwater navigation, the wings and tail are folded to effectively reduce water resistance, achieve neutral buoyancy adjustment, and possess pressure resistance, ensuring the aircraft's maneuverability and stability underwater.

[0032] Based on the above design requirements and incorporating biomimetic principles, the design integrates the shape of pelican wings and the method of water entry into the aircraft design. It adopts a conventional fixed-wing layout, equipped with horizontal and vertical tail fins. The fuselage serves as the main structure, carrying key components such as mission equipment and the power system. The wings and horizontal tail fins utilize deformable structures. Considering the limitations of the actual shape, after scaling down, the proposed wingspan when fully extended is 1000mm, the fuselage length is 750mm, the aspect ratio can vary within the range of 2 to 7, and the fuselage weight is 800g. When fully folded, the wingspan folding ratio is over 60%, and the area folding ratio is over 40%. The maximum cruising speed in the air is 30m / s, the maximum diving depth is 10m, and the maximum underwater speed is 10m / s.

[0033] 2. Variable wing structure design; 2.1 Perform dynamic modeling of the wing frame; Extracting skeletal and muscular features of bird wings to establish... Figure 2 The diagram shows a simplified representation of the mechanism. In this mechanism, the vertical distance between the frame hinge points AB (link 1) is 50mm; the angle between the line connecting AB and the perpendicular is 7°; the distance between hinge points AE (link 2) is 100mm; the distance between hinge points BC (link 3) is 60mm; the distance between hinge points CF (link 4) is 180mm; the distance between hinge points EG (link 5) is 150mm; the distance between hinge point F and endpoint H (link 6) is 180mm. The distance between hinge points CD is 30mm, the distance between hinge points DE is 20mm, and the distance between hinge points FG is 20mm.

[0034] Degrees of freedom calculation for a mechanism: In a mechanism, the number of degrees of freedom represents the number of independent motions that the mechanism can accept from external inputs, that is, the number of independent position parameters that the mechanism is allowed to be given externally. The calculation formula is shown in the following formula:

[0035] In the formula n Indicates the number of active components. Describe the number of higher sub-sub ... Indicates the number of lower subclasses.

[0036] In the simplified diagram of the wing structure, link 1 represents the frame, and the remaining 5 moving parts form a total of 7 revolute joints (A~G), all of which are lower pairs, thus defining the degrees of freedom of the mechanism. Link 3 is the driving link, and the number of driving links equals the number of degrees of freedom; this means that under the drive of link 3, the mechanism will have definite motion.

[0037] 2.2 Perform kinematic simulation on the wing frame; Motion simulation analysis of the wing structure was performed using SolidWorks' motion module. In the simulation, the third truss was rotated counter-clockwise at 1 rpm with pivot point B as the center of rotation. Initial conditions were set with the fourth truss in a vertical position, at which point the angle between the wing fixing plate 1 and the vertical plane was 36.8507°. The rotation time was set to 15 seconds, at which point the wing fixing plate 1 had just rotated 90°, and the wing was fully deployed. (All angles mentioned below are relative angles; 0° is taken when the wing is fully folded and the driving component, the third truss, rotates around pivot point B). The counter-clockwise direction is considered positive. Figure 4 The rotation process of the wing structure during the 15-second motion was demonstrated.

[0038] Motion simulation analysis of the wing structure was performed using SolidWorks' motion module. In the simulation, the third truss was rotated counter-clockwise at 1 rpm with pivot point B as the center of rotation. Initial conditions were set with the fourth truss in a vertical position, at which point the angle between the wing fixing plate and the vertical plane was 36.8507°. The rotation time was set to 15 seconds, at which point the wing fixing plate had just rotated 90°, and the wing was fully deployed. (All angles mentioned below are relative angles; 0° is taken when the third truss, the driving component, rotates around pivot point B when the wing is fully folded). The counter-clockwise direction is considered positive. Figure 4 The rotation process of the wing structure during the 15-second motion was demonstrated.

[0039] During the motion, the trajectory, velocity, and acceleration information of the final point were monitored and the relevant data were exported. The following results were obtained by fitting the trajectory, X-axis velocity, Y-axis velocity, X-axis acceleration, and Y-axis acceleration of the final point using Origin software. Figure 5 The trajectory represents the final point of the mechanism's motion, with the final point position at 0s as the origin. In the X-direction, the trajectory tends to move away from the origin, reaching its maximum value at coordinates (252.15, 151.46), at which point the wings are fully deployed and the skeletons are in their limiting positions. In the Y-direction, the trajectory first moves away from the origin and then closer, reaching its furthest point along the Y-direction at coordinates (219.51, 179.33). At this point, the leading edge of the aircraft experiences the greatest change along the fuselage centerline, representing the extreme position of the center of gravity change during wing deployment.

[0040] Figure 6This is the time-velocity curve during the mechanism's motion. Due to the use of multi-link mechanisms, when the prime mover is input at a constant speed, drastic velocity changes may occur at certain positions, which is detrimental to the precise control of the aircraft. Analyzing the velocity information during wing deployment and adding feedforward control to the prime mover input can significantly increase the stability and reliability of the fuselage during wing surface changes.

[0041] Along the X-direction, the velocity is consistently positive, decreasing first and then increasing, reaching a minimum of 12.4119 m / s at a deployment radius of 1.1435 (65.52°). Along the Y-direction, the velocity is negative before a deployment radius of 1.3546 (77.62°), with its absolute value increasing first and then decreasing, reaching an extreme value of -17.104 m / s at a deployment radius of 0.6241 (35.76°). This extreme value is reached along the Y-axis trajectory at a deployment radius of 77.62°. To maintain smooth wingspan deployment, the velocity value in the X-direction needs to be compensated. The input of the prime mover during wing deployment needs to be increased first and then decreased, reaching its highest rotational speed at a deployment radius of 65.52°. Since the Y-direction velocity changes drastically after reaching a positive value, the velocity of the prime mover needs to be relatively reduced to decrease the impact on the fuselage at the end of the deployment.

[0042] Figure 7 This is a time-acceleration curve of the mechanism's motion. Acceleration information can reflect the changes in force during wing deployment, allowing for appropriate feedforward to maintain load stability. Acceleration along both the X and Y directions starts from negative values ​​and increases, reaching zero at deployment angles of 66.52° and 35.76°, corresponding to extreme velocity values. After a deployment arc of 1.4 (approximately 80°), acceleration increases rapidly, corresponding to the period of drastic velocity changes at the end of deployment. This surge in acceleration generates considerable force at the wingtip. In the control of the prime mover, significant deceleration should occur after 80°, reducing the velocity to zero at the end of full 90° deployment to minimize impact loads between limiting devices and extend aircraft life.

[0043] 2.3 Design of passive wing retraction and extension structure; The feathers of the fifth and sixth trusses are each connected by a set of fluorocarbon wires, with the connection point located at the base of each feather. The fluorocarbon wires possess excellent flexibility and are used to guide the feathers and adjust their orientation and position during wing deployment and retraction. On the sixth truss, the base of the outermost feather extends backward, with the twist point located in the center. One side of the feather is connected to the fluorocarbon wire running through each feather, and the other end is connected to the fuselage by a highly elastic rubber wire, providing lateral tension. The other end of the rubber wire connects to the fifth truss. Figure 8The installation positions and deployment states of the fluorocarbon wires and rubber wires are demonstrated. When the wing retracts, the fluorocarbon wires relax, and the elastic rubber wires provide tension, pulling the connected feathers to retract and reducing the wing area. When the wing extends, the fluorocarbon wires tighten, and the tension is greater than that of the elastic rubber wires, pulling the feathers to unfold and increasing the wing area.

[0044] 2.4 Overall wing structure design; The overall structure of the right wing is as follows Figure 9 As shown, it mainly consists of three parts: the skeleton, feathers, and wing surfaces. Figure 3 In the wing truss structure, the frame is designed as a wing fixing plate 1, serving as the overall fixed part of the wing. It has two hinge points, A and B. Hinge point A is hinged to the second truss, allowing the second truss to rotate freely around hinge point A. A servo motor, model PTK 7462W MG, is mounted at hinge point B. The servo motor fuselage is fixed to plate 1, and the servo disc is fixed to rod 3, serving as the power input for the extension device. Due to interference requirements between mechanisms and the limitations of wing extension and retraction, rod 3... horn( The angle between rod 3 and the vertical line (downward direction) passing through the twisting point B is limited to 36.8507°~126.8507°.

[0045] The feather consists of a root, shaft, and surface. One end of the root has a hinge point for connecting to the truss and allowing it to rotate freely; it is 3D printed. The other end is inserted into a 2mm diameter carbon fiber rod as the shaft. The surface simulates the shape of a real feather but is simplified, and is glued to the shaft. Six and eight holes are provided on the fifth and sixth trusses respectively for installing the feather. The trusses are two-layered, with the root sandwiched between them for better torsional resistance.

[0046] The two wings are symmetrically arranged, each controlled independently by a servo motor. Both the feathering of the wings and the extension and retraction of the frame are controlled by this servo. Independent control of the servos allows for adjustment of asymmetrical wing deformation, enabling verification of the impact of asymmetrical wing surface changes on flight attitude. Figure 9 As shown, wing surfaces are arranged on the feather planes of the fifth and sixth trusses. The wing surface of the sixth truss is hollow with an open trailing edge, so that the wing surface of the fifth truss can be housed in it when the wing is folded, thus avoiding structural interference.

[0047] 3. Variable tail fin structure design; 3.1 Design an active tail fin retraction / extension structure; The central feather is fixed to the tail fin support, and four feathers are symmetrically arranged on each side, hinged to the tail fin support and connected by elastic rubber lines. The installation positions of the lines are as follows: Figure 10As shown, the outermost feather roots on both sides are connected by fluorocarbon wire and wound around the servo rotor. When the fluorocarbon wire is loosened, the rubber wire pulls the feather to the retracted position. By controlling the tension and loosening of the fluorocarbon wire, the change in the tail fin surface is controlled.

[0048] 3.2 A flexible connection structure for the tail fin was designed; Figure 11 The design showcases the shape and structure of a flexible connector, which functions similarly to a gimbal, connecting the tail assembly to the fuselage and allowing it to deflect in both horizontal and vertical directions to control surface variations. Made of thermoplastic polyurethane rubber (TPU), it possesses a degree of flexibility and is fabricated using 3D printing. The connector's mounting angle allows for flexible deformation in the vertical direction, controlling the aircraft's pitch attitude via the horizontal stabilizer.

[0049] 3.3 Overall structural design of the tail fin; The tail fin consists of a flexible connector, a tail fin bracket, a vertical tail fin, and a horizontal tail fin, and its assembly is as follows: Figure 12 As shown. The horizontal tail fin consists of nine feathers that can be deployed and retracted. The tail fin frame is fan-shaped and consists of two layers of 1.5mm thick carbon fiber plates, with the feathers hinged between them. Figure 13 The mounting position of the servo motor responsible for controlling the deployment and retraction of the horizontal tail fin is shown, which is fixed to the lower tail fin mount.

[0050] The vertical tail is fixedly connected to the tail fin mount. The directional control surfaces can be changed by altering the angle between the tail fin mount and the flexible connector, thus controlling the aircraft's steering in conjunction with changes in the wing. Figure 14 and Figure 15 The deflection control process of the horizontal and vertical tail fins was demonstrated, controlled by two servos fixed to the fuselage center tube.

[0051] 4. Overall aircraft design; 4.1 Distribution and weight calculation of aircraft components; The aircraft is assembled from rotors, brushless motors, a fuselage center tube, fuselage skin, wings, flexible connectors, and a tail. It is powered by a Chengfeng V2306 V2 KV2400 brushless DC motor, with a maximum power of 494W, a maximum current of 34.1A, and a maximum pull of 1242g. The fuselage center tube, made of 10mm x 10mm carbon fiber square tubing, supports the fuselage and connects to the brushless motor via motor mounts. Wing mounting plates are fixedly connected to the carbon fiber square tubing, and the tail section is connected to a flexible connector. All these components are connected to the square tubing with screws.

[0052] A three-dimensional model of the aircraft was constructed, and the wing folding was achieved using a three-section articulated frame; the horizontal tail fin achieved synchronous retraction and deployment through a composite transmission of fluorocarbon wire and rubber wire. The power supply and control unit were located in the fuselage belly and could slide back and forth along carbon fiber square tubes to adjust the fuselage's center of gravity. Figure 16 This demonstrates the states of the wings and tail when the transforming aircraft is fully deployed and fully folded. Figure 17 It shows the unfolded and folded states of the actual aircraft skeleton.

[0053] The weight of the aircraft is estimated as shown in Table 1 below.

[0054] Table 1. Aircraft Weight Estimation

[0055] The wing and control surfaces are controlled by five servos. Two servos control the deployment and folding of the left and right wings respectively, one is fixed to the tail plate and controls the deployment and retraction of the tail fin, and two are located at the rear of the fuselage to control the deflection of the horizontal and vertical tail fins respectively. For waterproofing, waterproof servos and motors are used, and no additional sealing devices are used except for the control modules. When submerged, liquid can enter the wing interior through gaps, reducing the volume of water discharged and thus reducing buoyancy.

[0056] 4.2 Aircraft control strategy; (a) Control system hardware architecture; The aircraft control system adopts a distributed control architecture, using a high-performance 32-bit microcontroller as the main control chip, integrating a floating-point arithmetic unit to meet the requirements of real-time control algorithm operation. The sensor system includes gyroscopes, accelerometers, barometric altimeters, depth sensors, a GPS module, and an airspeed indicator, providing the aircraft with comprehensive attitude, position, and environmental awareness information. The actuators consist of five waterproof servos and one brushless DC motor, controlled by an electronic speed controller.

[0057] (ii) Wing deformation control strategy; Symmetrical Deformation Control: The left and right wing servos employ a master-slave synchronous control mode, achieving precise synchronization of angle commands via bus communication. Based on the terminal velocity and acceleration characteristics obtained from kinematic simulation, the deformation process is divided into three intervals: acceleration, constant velocity, and deceleration. Different servo speeds are set for each interval, and a curved deceleration is employed at the final stage of deployment to minimize the terminal velocity and reduce the impact load on the limiting devices. During deformation, the flight control system monitors the wing deployment angle and fuselage attitude angle. When abnormal attitude changes are detected, deformation is automatically paused and attitude compensation is initiated to ensure the smoothness of the deformation process.

[0058] Asymmetric Deformation Control: Asymmetric deformation is used for roll control, generating roll torque through the difference in the deployment angles of the left and right wings. It can replace or assist the function of traditional ailerons. The control law adopts a classic PID algorithm. The input is the deviation between the desired roll angle and the actual roll angle, and the output is the difference in the deployment angles of the left and right wings. Asymmetric deformation responds rapidly and can generate sufficient roll control torque in a short time. At the same time, a maximum allowable angle difference is set to prevent structural overload. Under high-speed flight conditions, the maximum angle difference is appropriately limited to ensure structural safety and flight stability.

[0059] (iii) Tail control strategy; Tail feather retraction and deployment control: The retraction and deployment of the tail feathers is driven by a single servo motor, achieving synchronized movement of multiple feathers through a composite transmission of fluorocarbon wire and rubber wire. The control strategy employs closed-loop position control, with a set range for the retraction and deployment angles. The retraction and deployment process consists of two directions: unfolding and retracting. The return elasticity of the rubber wire assists the retraction action, reducing the load on the servo motor. Feather position is monitored by sensors detecting the rotation angle of the feather root, providing feedback control to ensure retraction and deployment accuracy. The retraction and deployment actions are linked to wing deformation control, with retraction initiated before water entry and timely unfolding after water exit. The timing control is uniformly scheduled by a state machine.

[0060] Tail deflection control: The horizontal tail deflection is used for pitch control, employing a PID algorithm to output the servo deflection angle based on the deviation between the desired and actual pitch angles. The vertical tail deflection is used for yaw control, with the deflection range set according to structural limits, and the control cycle matched to the attitude control loop. During wing deformation, the tail deflection gain is appropriately reduced to prevent oscillations caused by control coupling, ensuring aircraft attitude stability.

[0061] (iv) Cross-medium state machine control logic; The state machine adopts a finite state machine model, defining five operating states: aerial cruise, dive into water, underwater submersion, surfacing, and surface takeoff. State transitions must meet preset conditions and last for a certain duration to avoid false triggering caused by noise interference. Within each state, a predefined sequence of deformation and dynamic control actions is executed to ensure timing coordination during cross-medium switching. During state transitions, the flight control system records the current attitude angle as a feedforward compensation reference to eliminate attitude disturbances that may arise during transition transients, achieving a smooth state transition.

[0062] (v) Attitude control algorithm; Airborne attitude control: Attitude control employs a cascaded PID structure, with an inner loop of angular velocity and an outer loop of angle, and the control cycles of the inner and outer loops are matched. The angular velocity loop enables rapid disturbance suppression, while the angle loop ensures attitude tracking accuracy. The control output limit is set according to the physical limitations of the actuators to ensure that the servo motors and motors operate within safe ranges.

[0063] Underwater attitude control: Due to the increased fluid density and altered dynamic characteristics, the PID parameters are adaptively adjusted for underwater control. The angular velocity loop gain is appropriately reduced to prevent overshoot, the angle loop gain is adjusted accordingly, and the integral term is enhanced to eliminate steady-state error. Simultaneously, depth loop control is introduced, adjusting the pitch angle setpoint based on depth deviation to achieve constant-depth submersion and improve underwater maneuverability.

[0064] Deformation Compensation Control: During wing deformation, the flight control system establishes a feedforward compensation table based on acceleration data obtained from kinematic simulation. It then looks up the expected disturbance torque in the table according to the current deployment angle and superimposes it onto the servo output command to achieve active compensation for deformation disturbances. When the actual disturbance deviates from the expected value beyond a set threshold, an adaptive compensation algorithm is activated to correct the compensation coefficients online, improving the robustness of the control system.

[0065] (vi) Buoyancy and center of gravity adjustment and control; Buoyancy adjustment employs a combination of passive water ingress and active water drainage. During underwater navigation, water naturally enters the internal cavities of the wings, reducing the drainage volume and lowering the buoyancy to near neutral, thus minimizing power consumption. When surfacing is required, active surfacing control is achieved through motor thrust assistance combined with tail fin deflection to generate an upward pitching torque.

[0066] Center of gravity adjustment is achieved by sliding the power module along the fuselage center tube, employing closed-loop position control. The center of gravity position is automatically adjusted according to flight conditions: during in-flight cruising, the center of gravity is located near the aerodynamic center to ensure stability; during underwater navigation, it is appropriately shifted forward to increase pitch stability; and during takeoff, it is appropriately shifted backward to assist in raising the nose. Center of gravity adjustment is decoupled from pitch control, and the adjustment rate is reasonably limited to avoid introducing additional attitude disturbances.

[0067] (vii) Fault protection and redundancy control; The system incorporates a multi-level fault protection mechanism: automatic switching to backup sensor data in case of sensor malfunction; and activation of asymmetric deformation to replace aileron function in case of single servo failure, maintaining basic controllability. Servo control employs redundant output channels, allowing switching in case of any channel failure, thus improving system reliability. All control parameters can be adjusted online via a ground station, enhancing system maintainability and adaptability.

Claims

1. A bird-inspired wing-surface morphing aircraft, characterized in that, Includes rotor, brushless motor, fuselage center tube, fuselage skin, wings, flexible connectors, power supply and control unit, and tail fin; The rotor is mounted at the front end of the central tube of the fuselage; The brushless motor is connected to the central tube of the machine body via a motor mount; The tail end of the central tube of the fuselage is connected to a flexible connector; The power supply and control unit are located below the fuselage center tube and can slide back and forth along the fuselage center tube to adjust the position of the aircraft's center of gravity. The wings include a left wing and a right wing, which are symmetrically mounted on both sides of the fuselage center tube via wing fixing plates; the wing folding is achieved using a three-section hinged frame; The wing comprises a frame, feathers, and wing surfaces. The frame is the integral fixed part of the wing and includes a wing fixing plate, a second truss, a third truss, a fourth truss, a fifth truss, and a sixth truss. The wing fixing plate has two hinge points, A and B. Hinge point A is hinged to the second truss, allowing the second truss to rotate freely around hinge point A. A servo motor is mounted at hinge point B, with its fuselage fixed to the wing fixing plate and its servo disc fixed to the third truss. The second truss is hinged to the fourth and fifth trusses via two hinge points. The third truss is hinged to the fourth truss. The sixth truss is hinged to the fourth and fifth trusses via two hinge points. The feather includes a feather root, a feather shaft, and a feather surface; one end of the feather root has a hinge point hole, which connects to the fifth and sixth trusses and can rotate freely, and the other end is inserted into a carbon fiber rod as a feather shaft; the feather surface is attached to the feather shaft; Wing surfaces are arranged on the feather surfaces of the fifth and sixth trusses. The wing surface of the sixth truss is hollow with an open trailing edge. When the wing is folded, the wing surface of the fifth truss is housed inside to avoid structural interference. The tail section includes a tail support frame, a vertical tail section, and a horizontal tail section. The horizontal tail section consists of nine feathers that retract and deploy synchronously via a carbon fiber-rubber composite transmission. The tail support frame is hinged to an elastic connector. The tail support frame is fan-shaped and consists of two layers of carbon fiber plates, with the nine feathers fixed between the two layers via hinges. The vertical tail section is fixedly connected to the tail support frame, and the directional control surface is changed by altering the angle between the tail support frame and the elastic connector, thus controlling the aircraft's steering in conjunction with changes in the wing. The fuselage skin covers the central tube of the fuselage.

2. The bird-inspired wing surface retraction and expansion trans-medium deformable aircraft according to claim 1, characterized in that, The aircraft's wing and control surfaces are controlled by five servos: two servos control the deployment and folding of the left and right wings respectively, one servo is fixed to the tail plate and controls the deployment and retraction of the tail, and two servos are located at the rear of the fuselage and control the deflection of the horizontal and vertical tail respectively.

3. The bird-inspired wing surface retraction and expansion trans-medium deformable aircraft according to claim 1, characterized in that, The brushless motor is a Chengfeng V2306 V2 KV2400 brushless DC motor with a maximum power of 494W, a maximum current of 34.1A, and a maximum tensile force of 1242g.

4. The bird-inspired wing surface retraction and expansion trans-medium deformable aircraft according to claim 1, characterized in that, The central tube of the fuselage is made of 10 mm*10 mm carbon fiber square tube, which is responsible for supporting the fuselage.

5. A bird-inspired wing-shaped trans-medium deformable aircraft according to claim 1, characterized in that, The skeleton is made of carbon fiber.

6. A bird-inspired wing-shaped trans-medium deformable aircraft according to claim 1, characterized in that, The second, third, and fourth trusses are designed with a single layer thickness of 3mm, while the fifth and sixth trusses are designed with a double layer thickness of 1.5mm.

7. A bird-inspired wing-shaped trans-medium deformable aircraft according to claim 1, characterized in that, The feathers were made using 3D printing.

8. A bird-inspired wing-shaped trans-medium deformable aircraft according to claim 1, characterized in that, The carbon fiber rod has a diameter of 2mm.

9. A bird-inspired wing-shaped trans-medium deformable aircraft according to claim 1, characterized in that, The fifth and sixth trusses have 6 and 8 holes respectively for installing feathers.

10. A bird-inspired wing-shaped trans-medium deformable aircraft according to claim 1, characterized in that, The carbon fiber plate has a thickness of 1.5 mm.

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