An Aerial–Aquatic Robot with Tunable Tilting Motors Capable of Multimode Motion

Aerial–aquatic robots have promising applications due to their ability to operate in air and water with different motion modes. However, challenges such as water/air transition and multimedia movement still limit their development. Inspired by the aerial–aquatic behavior of wild ducks, an aerial–aquatic robot with tunable tilting motors and a delta wing is introduced. The robot can perform multimode motion, including flying in air, swimming on the water surface, transitioning between air and water, and diving underwater. Besides, the buoyancy design enables the robot to float at the air–water interface without consuming energy. Herein, the performance of the robot, including vertical and horizontal cross‐media performance, power consumption, and endurance in various motion modes, is evaluated. The field experiments show that the proposed robot possesses multimode motion and high maneuverability. This study provides a new idea for the structural design of next‐generation practical aerial–aquatic robots.

weight of the Loon Copter to 2.7 kg.The aerial-aquatic hitchhike robot proposed by Li et al. [29] can fly, swim, and attach to surfaces in both air and water.It weighs only 950 g without the buoyancy adjustment device.Utilizing vertical takeoff and landing (VTOL), it can quickly transition at the air-water interface in just 0.35 s.However, owing to weak buoyancy adjustment, the robot moves downward passively due to gravity, without active downward movement capability.Tan et al. [30][31][32] developed a vector propulsion system that allows a multirotor to move underwater in multiple directions, enhancing its underwater flexibility.However, the motors are connected through a tilting mechanism, and their rotations cannot be individually controlled.Moreover, the low endurance of multirotor robots due to their high-energy consumption is a significant issue, making them unlikely to operate in larger areas of water.
In addition to fixed-wing and multirotor robots, there are other aerial-aquatic robots. [33,34]A 175-mg microrobot proposed by Chen et al. [4] can perform multimode locomotion by flapping wings and achieve the transition between water and air by igniting oxyhydrogen from water electrolysis.However, considering its size limitations, the robot has limited load capacity, and its applications must be explored.The squid-like aquatic-aerial vehicle proposed by Hou et al. [35] has pneumatically driven soft fins and arms.It uses compressed gas in the gas cylinder to provide propulsion to swim and jump out of water.Similarly, the aquatic jump-gliding robot proposed by Zufferey et al. [3] also exits the water in an impulsive manner.Nevertheless, taking off from water in this manner is not repeatable over a short period because the water jet is not reusable.
Robots face difficulties with aerial-aquatic capability, including limited water area for takeoff, insufficient speed after vertical takeoff, and nonrepeatable water takeoff.However, aerialaquatic animals do not have these issues. [36,37]Wild ducks can float on the surface, dive into the water when fishing, and fly away when predators approach.Figure 1A shows the different states of wild ducks.Unlike most aerial-aquatic robots, wild ducks float on the water surface and dive by adjusting their body posture and applying force through their webbed feet.To reach the water surface, ducks can float using their buoyancy or propel themselves with their webbed feet.They can exit the water by running and flapping their wings simultaneously.
Inspired by the aerial-aquatic behavior of wild ducks, we proposed an aerial-aquatic robot with tunable tilting motors and a delta wing.The robot can realize multimode motion, including flying in air, swimming on the water surface, transitioning between air and water, and diving underwater.Like a wild duck on water, the robot has positive net buoyancy and floats at the airwater interface without energy consumption.The robot's tunable tilting motors enable it to submerge and take off like a duck.The robot combines the vertical takeoff and landing of multirotor robots, rapid forward movement of fixed-wing robots, and diving of the UUVs with the help of the tunable tilting motors and delta wing.This combination endows the robot with powerful multimode motion, thus greatly enhancing its maneuverability.Based on the robot platform, we analyzed the air-water transition characteristics and takeoff/diving performance at the air-water interface.Additionally, we characterized the power consumption and endurance of various motion modes.Finally, to demonstrate the versatility of the aerial-aquatic robot, we tested the robot's multimode motion capability and conducted continuous experiments in the field.

Design and System
The proposed robot mimics the aerial-aquatic behavior of wild ducks.As shown in Figure 1A, the wild duck floats at the air-water interface depending on its buoyancy.Similarly, the aerial-aquatic robot has a buoyancy design that allows half of its body to stay in water and half in air without using energy.Wild ducks perform fishing from the air-water interface to underwater using their webbed feet to provide power.As the robot dives, the tilting mechanisms tilt downward to generate a downward thrust in the vertical direction.Wild ducks run quickly and flap their wings to exit the water.The robot's tilting mechanisms raise the propeller to create vertical thrust, allowing it to transition from the air-water interface to the air.The aerial-aquatic robot (Figure 1B,C) can achieve various modes of motion (Figure 1D), including vertical takeoff, horizontal hover, water landing, vertical dive, egress, horizontal dive, surface swim, and horizontal takeoff.

Configuration
The aerial-aquatic robot is configured as a delta wing (Figure S1, Supporting Information) aircraft with a wingspan, length, and width of 56.2, 35, and 1.55 kg (Figure 1B and Section S1, Supporting Information), respectively.Weight and buoyancy management are essential for an aerial-aquatic robot that can be operated in different media.Considering the different buoyancy requirements of multirotor and fixed-wing robots, the buoyancy design is often contradictory and compromising.Most multirotor robots cannot maintain depth in water without energy consumption due to their negative buoyancy, causing them to sink to the bottom.More seriously, in the event of malfunction, multirotor robots that sink into the water in a steady state may cause problems for salvage and repair.Therefore, aerial-aquatic robots should maintain positive buoyancy to remain suspended in water or on the surface without expending energy.The first design principle for fixed-wing UAVs should be lightweight, as they are not designed to stay underwater.The proposed robot is designed to be 1.05 N buoyant, requiring minimal energy for maneuvering underwater and avoiding issues faced by ordinary fixed-wing and multirotor robots.
The robot's modular design allows for easy replacement and repair through predefined interfaces (see Figure S2 and Section S2, Supporting Information).The robot consists of a carbon fiber board for the wing, an acrylic waterproof electronic capsule, and polylactic acid (PLA) tube clamps to connect the wing and waterproof cabin.
The robot's wing should avoid excessive deformation in air and water to maintain stability, particularly when crossing the air-water interface (Figure S3, Supporting Information).Additionally, the robot shouldn't exit the water covered by water droplets.Therefore, a 2 mm carbon fiber composite panel is adopted.To operate in more complex environments and reduce water droplets on the wing, [38,39] the wing is covered with a layer of superhydrophobic material (Figure 1B and Section S3, Supporting Information).
The electronic capsule is an acrylic cube with an inner diameter and thickness of 76 and 2 mm, respectively.Sealed rubber rings are placed at both ends of the electronic capsule to prevent water from entering.Two 5-mm aluminum rods are placed through the acrylic tube to fix the electronic components.To prevent the objects in the tube from turning, two aluminum rods are fastened to the acrylic covers at both ends.

Actuation
Maneuvering underwater is challenging for aerial-aquatic robots with fixed-wing or multirotor configurations due to the need for changing attitudes angle.These changes can cause issues with airframe stability and onboard sensors, particularly visual equipment.To solve this problem, the robot features three tunable tilting motors.Each tunable tilting motor consists of a motor and a tilting mechanism.The tilting mechanism includes a servo motor, a carbon fiber tube, and several aluminum bending parts.It has a maximum tilting angle of 202°, which allows the robot to perform agile underwater maneuvers and switch states without changing its attitude (Movie S1, Supporting Information).robot states.In this study, an angle of 0°is defined as the initial tilting mechanism angle when the thrust direction is forward.The tilting angle is positive and negative when the tilting mechanism rotates upward and downward, respectively.As shown in Figure 1B, four classic angles (90°, 45°, 0°, À90°) are frequently used in this robot; the other angles will be discussed in subsequent chapters.To simplify the model, a tilting angle ranging from À90°to 90°can be mapped from À1 to 1.Because this robot is equipped with three tilting mechanisms, the state of the robot's tilting mechanisms can be represented by a vector, for example, [0, 0, 1] indicates that the angles of the left, right, and rear tilting mechanisms are 0°, 0°, and 90°, respectively.Under this definition, four typical tilting angle states ([1, 1, 1], [0, 0, 1], [0.5, 0.5, 1], and [À1, À1, À1]) and their corresponding computer aided design (CAD) renderings and prototypes are shown in Figure S5, Supporting Information.The analysis of the kinematics and dynamics is presented in Section S4, Supporting Information.

Electronics
Aerial-aquatic robots require autonomous attitude control and the ability to receive commands from ground stations.Therefore, the control system (Figure 1C) consists of onboard and offboard components.The offboard components include a computer running the Mission Planner software, a 433 MHz data transmission module, a 915 MHz remote control, and a video transmission receiver.Compared with the offboard side, the layout of the onboard side is much more complicated.
The core of the onboard control system is a Mateksys flight controller running Ardupilot firmware.The flight controller has an ARM Cortex-M7 running at 480 MHz It provides eight pulse width modulation (PWM) signals for controlling five actuators and three rotors.It also utilizes an accelerometer and gyroscope to maintain its stability and orientation.
Wild ducks perceive the world using their eyes, mouth, nose, and skin.Similarly, robots perceive external environments using sensors.GPS allows the robot to determine its location, and an external compass tests the strength and direction of the magnetic field to determine its orientation.For aerial-aquatic robots, traditional barometric pressure sensors cannot be used in water because of their limited range.A pressure sensor (TE MS5837-30BA) is used to determine the water depth.However, owing to the limitation of the pressure sensor resolution, the accuracy of pressure data in air is not high.
It is common to use 2.4 GHz as the communication band for UAVs, but it is difficult to communicate with radio at GHz frequencies in water.Micro aerial-aquatics robots are unable to use the heavy buoys and sonars that are typically employed for underwater communications.Considering that the aerial-aquatic robot works mainly near the air-water interface, 915 and 433 MHz are adopted for remote control and telemetry communication, respectively.To enhance signal strength, we use a high-gain (30 dB) antenna on the ground side of the communication module.In the tests, the robot successfully received commands sent from the ground at a depth of %2 m using both 915 and 433 MHz A lightweight flight control board integrates a power distribution board (PDB) and battery elimination circuit (BEC), reducing weight by tens of grams compared with traditional Pixhawk models.A 4000-mAh 4S LiPo (14.8 V) battery is connected to the PDB to power the entire control system.The electronic speed control (ESC) uses battery voltage directly to drive the three motors.The BEC module converts the battery voltage to 9 V to power the onboard camera and video transmitter.Other electrical appliances, such as sensors, telemetry, RC receivers, and actuators, are powered by a 5 V voltage converted by the BEC module.

Vertical Cross-Media
Wild ducks maintain a steady state while floating at the air-water interface.They fish downward from the air-water interface to the water and fly away from the interface to the air.Similarly, the robot can stay at the air-water interface without energy consumption.When it transitions from the air-water interface to the air (Figure 2A and Movie S2, Supporting Information), all three tilting mechanisms tilt up to 90°([1, 1, 1]) to place the propeller above the interface.This method enhances the lift.When it transitions from the air-water interface to the water (Figure 2B and Movie S3, Supporting Information), all three tilting mechanisms tilt down to À90°([À1, À1, À1]).With the tilting mechanism, the robot can operate stably in water and air.
Evidently, propeller rotation is restricted underwater and does not provide enough thrust.According to Figure 2C,D, a single motor with an 8-inch propeller can produce a thrust of 8 N in air but less than 5 N in water.Additionally, once the throttle reaches 12%, there is no further increase in underwater thrust.Correspondingly, the motor consumes only 1/12 of the maximum power in water compared with air.Thrust is not the sole factor to consider, as resistance must also be taken into account.A multirotor can fly because its thrust is greater than its gravity, which is the resistance in the flying mode.With the tilting mechanisms, the robot converts the resistance in diving mode to the net buoyancy of the robot (buoyancy minus gravity), which is only 1.05 N. In Figure .2C,D, the black dashed line represents the resistance that needs to be overcome.The result shows that 6% and 70% of the throttle allow the robot to sink and take off, respectively.
To address the limitations of propellers in water, we explored the limits of the robot's ability to break through the air-water interface.There are two types of transition processes: top-down and bottom-up.The top-down transition ends once the robot is submerged in water, and the bottom-up ends once the robot is completely out of the water.We tested the transition time required for the robot to break through the air-water interface at different throttles.For the bottom-up transition (Figure 2E), the robot can break through the air-water interface and take off stably at 70% of the throttle, achieving the shortest transition time of 0.44 s at 100% throttle (Figure 2G).For the top-down transition (Figure 2F), the robot can break through the air-water interface, dive steadily at 6% throttle, and achieve the shortest transition time of 0.42 s at 12% throttle (Figure 2H).Compared with the proposed robot, the Loon Copter, [28] aquatic jump-gliding robot, [3] and Eagleray [26] require 13, 865, and 1.89, respectively.
Moreover, the attitude stability of the robot during interface transitions is a concern.Figure 2I,J show the fluctuations of the roll and pitch angles during the top-down and bottom-up transitions, respectively.The yellow, blue, and green blocks represent the air-water interface, air, and water, respectively.The dotted line represents the time required to completely break through the air-water interface.During the bottom-up transition, the robot's maximum roll and pitch angles are 5.98°and 3.22°, respectively, which do not affect the attitude after takeoff.The top-down transition is smoother than the bottom-up transition, with maximum roll and pitch angles of 2.51°and 1.13°, respectively.We also conducted three consecutive top-down and bottom-up cross-media operations, which is a strong demonstration of cross-media repeatability (Movie S4, Supporting Information).

Horizontal Cross-Media
Typical aerial-aquatic multirotor robots adopt the aforementioned cross-media method, which is ideally in the vertical direction, whether going up or down.When preparing to dive, a wild duck adjusts its posture diagonally downward and uses its feet to constantly stir in water for thrust.During takeoff, it sprints and constantly flaps its wings to gain lift.Wild ducks take off and land with a horizontal speed.Many fixed-wing aerial-aquatic robots use this mode to break through the air-water interface because they cannot take off and land vertically.Researchers wish to compare the two approaches numerically; however, it is often difficult to control variables.Owing to the tunable tilting motors, the proposed robot simultaneously possesses two cross-media modes.As mentioned above, the robot breaks through the air-water Figure 2. Cross-media performance of the aerial-aquatic robot.A,B) Sequence diagrams of the cross-media from the air-water interface to the air and from the air-water interface to the water, where the dotted line represents the air-water interface.C,D) Single-propeller thrust force at different throttles in air and underwater, where the dotted line represents the resistance that a single motor needs to overcome when crossing the media.E,F) The time of cross-media at different throttles from the air-water interface to the air and from the air-water interface to the water.G,H) Altitude when crossing the media, where the yellow block represents the interface, the blue block represents the air, and the green block represents the underwater.I,J) Attitude when crossing the media.
interface when the state of the tilting mechanisms is [1, 1, 1] or [À1, À1, À1].The two states correspond to the bottom-up and top-down transitions, respectively.For bio-like cross-media operations, the angle of the tilting mechanism is restricted to (À90°, 90°), which ensures the robot to generate horizontal acceleration spontaneously.
As shown in Figure 3A and Movie S5, Supporting Information, we tested and compared the vertical and horizontal takeoffs in the field (Qizhen Lake, Hangzhou).For safety reasons, the manipulator did not set the throttle to the maximum as before but dynamically adjusted it to stabilize the robot's attitude.Points V1-V4 in the line chart correspond to pictures V1-V4, which show the different phases of the vertical takeoff of the robot.The dashed line indicates that it takes 3 s for the robot to take off vertically and achieve stable flight.Compared with the vertical takeoff, more time (6.8 s) is required for the robot to achieve the horizontal takeoff.Pictures H1-H4 illustrate the horizontal takeoff, corresponding to points H1-H4.In this mode, the front tilting mechanisms tilt upward by an angle that can be dynamically adjusted, and the rear motor offers an upward thrust.When the robot is completely out of the water, the energy consumed for horizontal takeoff is almost the same as that for Similarly, there are two methods of top-down transition: 1) vertical diving and 2) horizontal diving.Horizontal diving can be classified into two forms, aileron only and tilting mechanism only.Aileron only means that the state of the tilting mechanisms is [0, 0, 1], which is a typical method for submarine diving.Here, we analyzed the horizontal diving with only tilting mechanisms.The front tilting mechanisms tilt downward by a certain angle to provide both forward and downward thrust, and the rear motor is disabled.We tested the bottoming time, energy consumption, and maximum pitch angle variation under different tilting mechanism angles in the square water tank with a depth and length of 0.8 and 2 m, respectively.Sensor data was recorded and can be seen in Figure 3B and S6, Supporting Information.All the horizontal diving experiments were performed at maximum throttle.When the tilting mechanism angle increases from 10°to 30°, the time it takes for the robot to reach the bottom of the fish tank gradually shortens to 2 s.Additionally, energy consumption exhibits a decreasing trend.The energy consumption required to reach the bottom at an angle of 30°is only 56.2% of that at an angle of 10°.However, as the time and energy consumption decrease, the pitch angle of the robot becomes larger.The maximum pitch angle changes from 32.54°to 83.98°as the tilting mechanism angle increases from 10°to 45°.This indicates that the robot can perform a somersault underwater by increasing the tilting angle.As shown in Figure 3C, when the tilting angle is 60°, the robot turns a somersault underwater at a depth of %0.6 m underwater.The entire somersault process takes 5.22 s and consumes 98.48 J.The somersaulting behavior provides the robot with greater maneuverability and can be used to make short-radius turns underwater, which is difficult for many underwater vehicles.

Energy Consumption
The aerial-aquatic robot can not only perform multimode cross-media at the air-water interface; however, it can also perform stable operations in different media owing to the tilting mechanisms.As shown in Figure 4A, the robot possesses five stable modes: vertical diving, horizontal diving, surface swimming, vertical flight, and horizontal flight.Each of these modes corresponds to the following states of the tilt mechanisms: [À1, À1, À1], [0, 0, 1], [0, 0, 1], [1, 1, 1], and [0.5, 0.5, 1].Using the onboard ammeter, we obtained data on the current, voltage, and power of the robot during operations and calculated the endurance time of the robot.For each of the outdoor motion patterns, we conducted five sets of experiments and obtained their average values, as shown in Figure 4B.
Horizontal flight is a more energy-efficient mode in air.It is capable of flying for 36.8 min at a power output of 104 W, similar to a fixed-wing flight depending on the wing.The robot is more agile in the vertical flight mode and can hover in air to achieve a power output of 465 W. The front tilting mechanisms tilt in opposite directions to counteract the counter torque generated by the rear motor.
The power required for flight is often extremely high.Compared with flight, underwater movement does not consume much energy; therefore, the robot can remain hidden underwater for a long time.Vertical diving is similar to multirotor flight, but the propellers turn straight down, requiring a current output of 3.7 A and featuring endurance of 64.9 min at the maximum throttle.When diving horizontally, the front tilting mechanisms do not have any tilting angle, and the power comes mainly from the front two motors.The robot relies on the ailerons for diving and ascent.The horizontal diving mode is the most energy efficient, with a maximum endurance of 100 min at the maximum throttle.In addition to operating in air and underwater, the robot can swim at the air-water interface with the endurance of 92.3 min at the maximum throttle.The stable motion between multiple media expands the utility and operability of the aerial-aquatic robots.

Multimode Motion and Field Experiments
We recorded the multimode motion performance of the robot in various indoor and outdoor environments.Figure 5 shows the timing diagrams for different motion modes, including vertical takeoff and horizontal hovering (Figure 5A), surface swimming (Figure 5B), vertical diving (Figure 5C), and horizontal diving (Figure 5D).
The robot started from the football field and performed vertical takeoff and landing with a tilting mechanism state of [1, 1, 1] (Figure 5A).After reaching the predetermined altitude, the robot received commands through the data transmission module to adjust its tilting mechanism state to [0.5, 0.5, 1] and entered horizontal flight mode.In horizontal flight mode, the angles of the front tilting mechanisms were not fixed.When the horizontal speed did not reach the cruising speed, the angles of the front tilting mechanisms were larger, and more lift in the vertical direction was generated.When the cruising speed was reached, the front tilting mechanisms remained approximately horizontal like a two-engine plane, and the rear motor no longer assisted the flight.As shown in Figure 5A, at 10.94 s, the state of the robot changed from vertical to horizontal flight mode.In addition, the onboard camera could transmit live videos from the sky to assist the manipulator in decision-making.
Owing to the preset buoyancy of the robot, it could float at the air-water interface (Figure 5B).The state of the tilt mechanisms was always [0, 0, 1] in this mode, and the rear motor was disabled.With two forward-tilting motors rotating at different speeds, the robot swam flexibly on the water surface and spun around a point to control its orientation.The movement of the robot on the water surface could reach 0.4 m s À1 , which is larger than its body length.
When diving vertically, the robot rotated all three tilting mechanisms downward, perpendicular to the body ([À1, À1, À1]), as shown in Figure 5C.Due to the tilting mechanisms, the body experienced less pitch change than fixed-wing robot during vertical descent.The fastest descent speed was 0.22 m s À1 .When the robot hit the bottom, it stuck to the bottom by relying on the rotation of the propeller.Thereafter, the robot relied primarily on buoyancy to ascend, a process that consumed little energy.When the robot needs to ascend quickly, it can transform into The robot's states while swimming and diving horizontally are almost the same.In Figure 5D, the robot swam at the air-water interface until T = 9.3 s.At T = 11.08 s, the robot received the command to deflect the ailerons downward and began to dive until T = 14.00 s.Subsequently, the robot was completely immersed in water and swamped underwater at a speed of 0.7 m s À1 for 1.5 s.The underwater speed is equal to 2bl s À1 , better than 0.64 bl s À1 of the Eagleray [26] and 0.98 bl s À1 of the improved Eagleray. [27]o demonstrate the feasibility of the concept shown in Figure 1D, we conducted a continuous experiment in the wild.The aerial-aquatic robot was tested in a field at Qizhen Lake, Hangzhou, Zhejiang Province, China.As shown in Figure 6 and S7, Supporting Information, the robot took off vertically and landed at the air-water interface.The robot then entered the vertical diving mode and completed three consecutive dives.Subsequently, the robot egressed out of the water.Next, the robot landed and made three horizontal dives.After completing the above tasks, the robot achieved horizontal takeoff and hovering.Images of the lake, including the aerial, water surface, and underwater environments, were recorded by the robot's onboard camera. [40]Field experiments demonstrate that the robot can achieve multimode motion with the help of tunable tilting motors and can record movies simultaneously (Movie S7, Supporting Information).

Discussion
In this study, we developed an aerial-aquatic robot with tunable tilting motors and a delta wing that imitates the aerial-aquatic modes of wild ducks.The robot can achieve repeatable and full mission profile capabilities and multimode motion.Owing to the tilting mechanisms, the aerial-aquatic robot can achieve fast and stable cross-media performance and has different cross-media modes.Multimode motion tests and field experiments demonstrate that the robot can conduct multimode movement in three media: underwater, air-water interface, and air.It has five stable working modes: vertical diving, horizontal diving, surface swimming, vertical flight, and horizontal flight.The aerial-aquatic robot combines the capabilities of multirotor and fixed-wing robots, underwater robots, and ducks, allowing for multi-mode motion and maneuverability.
Compared with the existing aerial-aquatic robots/vehicles' cross-media performance, [3,26,28] the proposed robot only takes 0.44 s to cross the media from the air-water interface to the air and 0.42 s to cross the media from the air-water interface to underwater.The Loon Copter, [28] aquatic jump-gliding robot, [3] and Eagleray [26] require 13, 865, and 1.89 s, respectively.While crossing the media, the robot's attitude is stable.During the bottom-up transition, the robot's maximum roll and pitch angles are 5.98°and 3.22°, respectively.In the top-down cross-media process, the robot's maximum roll and pitch angles are 2.51°a nd 1.13°, respectively.In addition, with the help of tunable tilting motors, robot possesses different cross-media modes with different angles, whereas the existing aerial-aquatic robots and vehicles often have only one or two modes.Hence, based on the robot platform, we studied the difference between taking off from water and diving into the water from different angles.The robot changes the angles of the tilting mechanisms and crosses the media with horizontal velocity.When diving horizontally, increasing the tilting angle results in a greater maximum pitch angle, shorter descent time, and increased energy consumption.When the tilting angle comes to 60°, the robot can perform a somersault in water.The tilting mechanisms significantly improve the flexibility of the robot.When exiting the water, the energy consumed during horizontal and vertical takeoffs is nearly equal.However, horizontal takeoff saves more energy when reaching stable flight.
Existing aerial-aquatic robots/vehicles often exhibit poor underwater performance; [41] however, the proposed robot can achieve good performance in water or air owing to the combination of the tunable tilting motors and the delta wing.When diving underwater, the speed of descent can reach up to 0.22 m s À1 .The maximum horizontal diving speed of 0.7 m s À1 is better than the speeds of the Eagleray [26] and the improved Eagleray, [27] which are 0.64 and 0.98 bl s À1 , respectively.Horizontal diving saves more energy than vertical diving and can achieve 100 min endurance with a 39 W power output.At the air-water interface, the surface swimming speed can reach 0.4 m s À1 , which is slightly greater than 1 bl s À1 .In addition, the endurance can reach 92.3 min.In air, vertical and horizontal flights can be seamlessly switched.Horizontal flight is more energy-efficient and allows for a longer flight time of 36.8 min at 104 W power, compared with only 7.27 min for vertical flight.However, the vertical power output is as high as 465 W, which allows for more maneuverable movement.
Our study is a preliminary exploration of robots with multiple cross-media modalities, and much work remains to be conducted.We explained that this method works at the principal level and put it into practice; however, there are still many areas worth improving in the future, for example, finer control of underwater movement, attitude fluctuations when breaking through the air-water interface, and size and weight optimization.In particular, optimization of the size will be an obstacle in the development of the aerial-aquatic robots, considering that the waterproofing of the robot will occupy a large amount of weight.Our parallel study suggests that micro aerial-aquatic robots with smaller sizes and lower weights can be achieved.
The robot described in this work fills the gap in aerial-aquatic robots with multiple cross-media modalities to a certain extent.It may provide some ideas for research across the air-water interface.As technology advances, faster and more stable methods of crossing the air-water interface will be discovered.This will lead to the creation of various aerial-aquatic robots for underwater observation, rescue, and research.Perhaps one day, aerial-aquatic robots could transport passengers across water and air.

Figure 1 .
Figure 1.Bioinspiration, design, system, and multimode motion of aerial-aquatic robots.A) Wild duck's behaviors and aerial-aquatic robot's multimode motion.The first row shows the wild duck, and the second row shows the robot, which corresponds up and down one by one.B) Overview of the design.The tilting motor can reach a tilting angle of 202°.C) Overview of the electronic layout, including offboard components and onboard components.D) Concept of operations for the vehicle.The robot takes off vertically i) from the ground, ii) switches to horizontal flight mode, and iii) lands on the water.Then, iv) the robot completes three vertical dives and v) takes off vertically through the air-water interface.vi) The robot lands again and completes three horizontal dives.Finally, vii) the robot accelerates at the air-water interface and viii) takes off horizontally to reach a steady state.

Figure 3 .
Figure 3. Cross-media with horizontal velocity.A) Power and energy consumption during vertical takeoff (V) and horizontal takeoff (H) from the air-water interface, where the solid line corresponds to vertical takeoff, and the dashed line corresponds to horizontal takeoff.B) Bottom contact time, energy consumption, and maximum pitch angle at different tilting angles, where the green blocks indicate that the bottom has been reached, and the gray block indicates that the bottom has not been reached.C) Altitude and the sequence diagram during horizontal diving with the tilting angle of 60°.

Figure 4 .
Figure 4. Images of multimode motion and energy consumption.A) Images of multimode motion.B) Endurance, current, and power in the five modes.

Figure 5 .
Figure 5. Outdoor experiment.A) Sequence diagram of vertical takeoff and horizontal hover, where the robot state and the first-view angle are also illustrated in the picture.B) Sequence diagram of swimming at the air-water interface.C) Sequence diagram of vertical diving, where a robot's view can be seen when touching the bottom.D) Sequence diagram of horizontal diving, where the robot was completely immersed in water at T = 5 s.

Figure 6 .
Figure 6.Field experiment.The first-view videos of the robot were recorded and the views underwater, at the air-water interface, and in air are displayed.