A flap-fan-deformed coupled variable camber flapping wing ground experiment device

Abstract

The application belongs to the technical field of fluid mechanics experiments and relates to a ground experiment device for a flap-fan-deformation coupled variable-camber flapping wing, which comprises a water tunnel frame and a deformation flapping wing movement mechanism, wherein the deformation flapping wing movement mechanism comprises linear sliding rails, a first motor, a long shaft, a second motor, a short shaft and a deformation flapping wing; the deformation flapping wing comprises a front wing section and a rear wing section, the front wing section is rotationally connected to the rear wing section through a rotating shaft, the long shaft drives the front wing section, the short shaft drives the rear wing section, and the front wing section is rotationally connected to the rear wing section; the deformation flapping wing movement mechanism is provided with a force measuring mechanism, computer assistance and other control devices are used, the torsional force and the moment acting on the deformation wing during movement are collected, the visualized image of flow field vortex of the deformation wing during pitch, heave and combined movement in water flow is obtained, data processing and analysis are facilitated, and a data foundation is laid for improvement of the endurance, propulsion performance and stability of the flapping wing aircraft.

Description

Technical Field

[0001] The invention belongs to the technical field of fluid mechanics experiments and relates to a flapping-deformation coupled variable-camber flapping wing ground experiment device. Background Art

[0002] In nature, aquatic and flying organisms have evolved over tens of thousands of years to develop remarkable locomotion and performance. For example, birds and insects fly by flapping their wings, while fish swim rapidly by swishing their fins. The locomotion mechanisms of these organisms involve complex flows at low Reynolds numbers, including laminar and turbulent unsteady flows, aero / hydrodynamics, vortex dynamics, and flow control—all important research areas in fluid mechanics. Flapping wings encompass the fins of fish, birds, and insects.

[0003] Flapping-wing aircraft still lag far behind natural flying creatures in terms of endurance, propulsion performance, and stability. Dynamic deformation can alter the vortex structure around the wing, significantly improving its propulsion performance and adjusting its shape to suit different forward motion conditions. Therefore, research on morphing flapping wings in low Reynolds number environments is particularly critical.

[0004] However, existing water tunnel experiments mainly focus on the flow field characteristics of non-deformed wings and flexible flapping wings at low Reynolds numbers. The traditional method of measuring and summarizing the deformation of different flexible flapping wings requires a large number of measurements and samples, and consumes a lot of manpower, material resources and time for statistics and comparison, resulting in low work efficiency and high time cost, and cannot effectively combine the propulsion performance and flow field changes of the complex motion of deformation and flapping wings.

[0005] Therefore, there is an urgent need for a method or device that can effectively combine the propulsion performance of the complex motion of the deformation and flapping wings and the flow field changes to solve the above technical problems. Summary of the Invention

[0006] This invention utilizes a mechanical structure to demonstrate propulsion performance during complex wing deformation and flapping motion. Combining PIV technology with computer-aided design, this method quickly generates reliable flow field visualization results. For the flexible deformation and flapping motion of the flapping wing, multi-motor coordinated control is employed. PLC control is used to achieve buoyancy, pitching, and deformation pitching. Pulse modulation between these three motions enables combined deformation at different phases. This fully simulates the motion of fish tail fins, bird and insect wings, and enhances the simulation of the coupled flapping and deformation motion of variable-curvature flapping wings.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a flapping-flapping-deformation coupled variable-camber flapping wing ground experimental device, comprising: a water tunnel frame and a deformable flapping wing motion mechanism, wherein the water tunnel frame is used to simulate fluids at different speeds and conditions, and the deformable flapping wing motion mechanism is used to simulate the flapping motion process of fish tail fins, birds and insect wings;

[0008] The water tunnel frame includes a sliding platform and a sliding rail. The sliding direction of the sliding rail is set along the flow direction of the liquid flow in the water tunnel frame. The sliding platform is slidably mounted on the sliding rail and can slide back and forth along the sliding direction of the sliding rail. The sliding platform and the sliding rail are used to drive the deformable flapping wing motion mechanism to translate along the fluid flow direction, thereby simulating the forward and backward position translation of the flapping motion in the fluid.

[0009] The deformable flapping wing motion mechanism includes: a linear slide rail, a first motor, a long shaft, a second motor, a short shaft, and a deformable flapping wing. The deformable flapping wing includes: a front wing section and a rear wing section. The front wing section and the rear wing section are rotatably connected via a rotating shaft.

[0010] The linear slide is fixedly connected to the sliding platform, and the sliding direction of the linear slide is parallel to the longitudinal section of the liquid flow direction. The fixed portion of the first motor is slidably connected to the linear slide via a slider. The first motor can slide back and forth along the sliding direction of the linear slide. The output shaft torque of the first motor is connected to the long axis, and the long axis torque is connected to the front wing segment. The linear slide is used to drive the deformable flapping wing to translate up and down and left and right in the longitudinal section of the fluid flow direction, thereby simulating the vertical and left and right position translation of the flapping motion in the fluid.

[0011] The long shaft is also fixedly connected to a fixed support plate, which rotates synchronously with the long shaft. The fixed support plate is fixedly connected to the fixing portion of the second motor. The output shaft torque of the second motor is connected to the short shaft, and the short shaft torque is connected to the rear wing section. The second motor rotates synchronously with the long shaft while driving the rear wing section to swing, so that the rear wing section can swing passively while being driven by the front wing section and also has the ability to swing on its own.

[0012] The axis of the rotating shaft connecting the front and rear wing sections is parallel to the axis of the major axis and the axis of the minor axis. The three axes are parallel so that the active swing of the front wing section, the passive swing of the rear wing section, and the active swing of the wing section itself will not interfere with each other and cause hysteresis or jamming due to the interference between the shaft connection and the motor drive.

[0013] The deformed flapping wing is fully extended into and immersed in the fluid channel of the liquid flow; the fluid in the fluid channel flows through the main swinging front wing section, the passive swinging and the self-active swinging rear wing section, thereby simulating the flapping motion process of the tail fin of fish, the wings of birds and insects.

[0014] Preferably, the experimental device further comprises a light beam and a camera, wherein the irradiation direction of the light beam is toward the wing surface of the deformed flapping wing, and the shooting direction of the camera is toward the deformed flapping wing; the light beam and the camera are used to monitor the flapping motion of the wing surface of the deformed flapping wing in real time;

[0015] A force measuring device is provided at the torque connection between the output shaft of the first motor and the long shaft. The force measuring device is used to measure the output torque and output torque change of the first motor; the force measuring device is used to monitor in real time the torque and flapping force of the wing surface of the deformed flapping wing during flapping.

[0016] More preferably, the light beam comprises a laser beam, and the camera comprises a high-speed camera.

[0017] More preferably, the output shaft of the first motor is connected with a shaft flange, the long shaft is connected with a flange, the shaft flange torque is connected with the flange, and the force measuring device is located between the shaft flange and the flange.

[0018] Preferably, the track shape of the slide rail includes: a straight track and a curved track along the length direction; tracks of different shapes can be designed or selected according to experimental needs.

[0019] Preferably, the long-axis torque connection at the front wing section and the short-axis torque connection at the rear wing section are both connected by shaft holes, and the shaft hole connection methods include: connecting three to six prisms with prism holes of matching size and shape, and connecting cylinders with round holes and then fixing them by bolts; the prism prism holes are reliable in torque transmission and the connection method is simple, and the swing balance of the deformed flapping wing is better; the cylindrical round holes are connected by bolts, which has lower processing costs but a more complicated connection method and poor balance during swinging. Different connection methods can be selected according to specific needs.

[0020] More preferably, the front wing section and the rear wing section are composed of two symmetrical halves that are split apart on both sides along the flow direction of the liquid flow; the axis-hole connection is located in the symmetrical plane of the front wing section and the rear wing section; the snapping method makes the axis-hole connection located in the central plane of the deformed flapping wing, reducing the influence of the connection on the torque, flapping force and movement during flapping.

[0021] More preferably, the shaft hole connection mode at the front wing section where the long-axis torque is connected and the shaft hole connection mode at the rear wing section where the short-axis torque is connected is a square hole and square groove connection.

[0022] Preferably, the long axis is vertically or obliquely arranged in a plane perpendicular to the flow direction of the liquid; the long axis is arranged along different positions, the long axis is inserted from above, and the uninserted surface of the water tunnel frame is sealed with a transparent material. At the same time, the position of the laser beam and the camera needs to be adjusted according to the placement of the flapping wings.

[0023] Preferably, the slide rail is horizontally or obliquely arranged in a vertical plane; when the slide rail is horizontally or obliquely arranged, the deformable flapping wing is respectively horizontally translated or obliquely translated in a longitudinal section in the fluid flow direction.

[0024] The beneficial effects of the present invention are:

[0025] 1. The present invention uses a deformable flapping wing motion mechanism to enable the deformable wing segment to simulate the motion process of fish tail fins, bird and insect wings. Furthermore, through the force measuring mechanism in the deformable flapping wing motion mechanism, computer-assisted and other control devices are used to efficiently and accurately collect the torsional forces and moments experienced by the deformable wing during motion. Simultaneously, using PIV technology, precise and rapid visualization of the flow field vortices during the deformable wing's pitching, heaving, and combined motions in water flow is obtained for data processing and analysis, thereby laying a data foundation for improving the endurance, propulsion performance, and stability of flapping-wing aircraft.

[0026] 2. The present invention can simulate the three-degree-of-freedom motion of a deformable flapping wing (sinking and buoyancy motion, pitching motion, and deforming pitching motion) and the flapping state of the flapping wing at different phases between the three motions, and adopts multi-motor collaborative control to realize active flow control in three degrees of freedom; therefore, the present invention effectively combines the propulsion performance and flow field changes of the complex motion of the deformation and flapping wings, improves the simulation degree of the flapping-fan deformation coupling motion of the variable-curvature flapping wing, so as to facilitate the measurement of the deformation of flapping wings with different flexibility, improve work efficiency, and reduce time costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is an overall structural diagram of a water tunnel test device for a flapping-flapping-deformation coupled variable-camber flapping wing ground experimental device of the present invention;

[0028] Figure 2 is a structural diagram of the deformable flapping wing motion mechanism of the present invention;

[0029] Figure 3 This is a diagram of the installation structure of the motion mechanism and the sliding guide rail of the present invention;

[0030] Figure 4 This is a diagram of the installation structure of the deformable flapping wing of the present invention;

[0031] Figure 5 is an exploded structural diagram of the deformable flapping wing of the present invention;

[0032] Figure 6 is a cross-sectional view of a deformed flapping wing of the present invention;

[0033] Figure 7 It is the measured displacement and designed displacement diagram of the present invention.

[0034] Among them, 1. Water tunnel frame; 2. Deformable flapping wing motion mechanism; 3. Sliding platform; 4. Slide rail; 5. Liquid flow; 6. Laser beam; 7. High-speed camera; 8. First motor; 9. Rigid coupling; 10. Shaft flange; 11. Force measuring device; 12. Flange; 13. Long axis; 14. Fixed support plate; 15. Deformable flapping wing; 16. Short axis; 17. Second motor; 18. Linear slide rail; 19. Mounting plate; 20. Profile; 21. Connecting plate; 22. Rear wing section; 23. Plug screw; 24. Front wing section; 25. Nut; 26. Front wing section half; 27. Rear wing section half. DETAILED DESCRIPTION

[0035] The following will provide a clear and complete description of the relevant technologies in the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0036] refer to Figures 1 to 7 This embodiment discloses a ground-based experimental device for a flapping-deformation coupled variable-camber flapping wing. Based on active flow control technology and multi-motor coordinated control, this experimental device employs a multi-stage deformation device to achieve controllable multi-degree-of-freedom motion of the deformable flapping wing. It can simulate the flapping and deformation processes of fish tail fins, bird and insect wings, and quantify the deformation of the flapping wing. Furthermore, PIV technology is used to visualize the flow field and analyze the vortex distribution during the flapping process, thereby exploring the flapping wing deformation mechanism and providing experimental support for biomimetic design.

[0037] The specific technical solution of this embodiment is:

[0038] The experimental device for ground test of variable-camber flapping wing flapping-flap deformation coupled motion includes a water tunnel frame 1, a deformable flapping wing motion structure 2, a sliding platform 3, a slide rail 4, a liquid flow 5, a laser beam 6, and a high-speed camera 7;

[0039] The water tunnel frame 1 and the deformable flapping wing motion mechanism 2 are connected to the sliding platform 3 and the slide rail 4 through bolts and the mounting plate 19. The sliding platform 3 can move linearly along the slide rail 4 in the same direction of the water flow; the water tunnel frame 1 is made of metal, and its open surface is made of transparent material, such as plexiglass, acrylic and other materials.

[0040] The deformable flapping wing motion mechanism 2 is the main experimental component;

[0041] The morphing flapping wing mechanism 2 comprises a linear slide rail 18 (RXSN40 linear module) and a first motor 8 (a stepper motor with model 60BYG250C), the linear slide rail 18 is bolted to a connecting plate 21 through a mounting plate 19 and a profile 20 (European standard 4040), and the connecting plate 21 is bolted to the sliding platform 3;

[0042] The first motor 8 is bolted to the slider of the linear slide rail 18 through a screw and a right-angle connector, which ensures that the motor can move on the linear slide rail 18, the motor shaft is connected to a belt shaft flange 10 through a set screw and a rigid coupling 9, and the belt shaft flange 10 and a flange plate 12 are coaxially positioned with a force measuring device 11 (a sensor 674) and connected through a set screw;

[0043] A long shaft 13 is inserted into the flange plate 12 and fastened by a screw. The fixed part of the first motor 8 is rotationally connected to the slider of the linear slide rail 18, so that the first motor 8 can rotate in the sliding vertical plane of the linear slide rail 18, and the long shaft 13 can also rotate in the sliding vertical plane of the linear slide rail 18. Therefore, the initial rotational position of the long shaft 13 can be adjusted, such as horizontal rotation of 0°, oblique rotation of 40°, and vertical rotation of 90°, to meet the needs of different angle flapping fan simulation.

[0044] A second motor 17 (with model 42EM150A) is bolted to a fixed support plate 14, the motor shaft is connected to a short shaft 16 through a rigid coupling and a set screw, and the position of the fixed support plate 14 is determined by the water level of the water tunnel device and the placement position of the morphing flapping wing in the experiment. Figure 1 The morphing flapping wing 15 is located in the middle section of the water level, which determines that the morphing flapping wing 15 is in the middle position of the water tunnel, and ensures that the water flow stably flows through the wing.

[0045] The morphing flapping wing 15 comprises a rear wing segment 22, a plug screw 23, a front wing segment 24 and a nut 25. The morphing flapping wing 15 is a face-symmetrical structure, the rear wing segment 22 is composed of two rear wing segment halves 27, and the front wing segment is composed of two front wing segment halves 26, and the wing segment halves are connected by set screws.

[0046] The rear wing segment 22 and the short shaft 16 in the morphing flapping wing 15 and the front wing segment 24 and the long shaft 13 are connected by inserting and bolting.

[0047] The laser beam 6 is generated by a laser generator, and the laser irradiation position is perpendicular to the wing symmetry plane.

[0048] The high-speed camera 7 is located 40-50 cm away from the bottom surface of the water tunnel frame and in the middle position of the wing segment movement path.

[0049] The morphing flapping wing 15 adopts NACA0012 airfoil (chord length 80 mm, span length 175 mm) and the material is light-cured resin.

[0050] After all the mechanisms in the device are installed and the relevant experimental circuits are connected. Adjust the sliding platform movement 3 to the middle of the three-dimensional space of the water tunnel, and place the high-speed camera 7 and laser generator. Before the formal experiment begins, it is necessary to adjust the x and y directions of the deformable wing section 15 and the force measuring device 11 to ensure that the front wing section 24 passes through the liquid flow 5 first and specifies the positive direction of the force measurement; secondly, twist the long axis 13 and observe whether the change of the computer force measurement interface curve is normal; finally, adjust the position of the laser generator irradiating the laser beam 6 to the middle of the deformable flapping wing section 15, and test whether the linear guide 18 can drive the deformable flapping wing motion mechanism 2 to move, and whether the second motor 17 can drive the short axis 16 and the rear wing section 22 to rotate normally.

[0051] The experimental instruments used in this embodiment are as follows: a dual-pulse PIV-specific laser with an output energy of 200 mJ, a wavelength of 532 nm, and a frequency of 0 to 15 Hz; a laser modulated by a light sheet can produce a 200 x 200 mm planar beam, which is used to illuminate the PSPs within the experimental area, ensuring their effective tracking and analysis; a FlowSense E05M image recorder with a resolution of 2456 x 2058, a pixel size of 3.5 μm, a maximum dynamic range of 14 bits, a maximum full-frame shooting frequency of 16 fps, and a minimum cross-frame time of 150 ns; a Nikon 50 mm / f18 lens with a high-performance narrow-band filter installed in front to avoid stray light interference; and DynamicStudio software for image analysis.

[0052] Example

[0053] Device Motion and Specific Actions: The linear slide 18 drives the pitch and buoyancy mechanism, comprised of the first motor 8, in reciprocating motion to achieve flapping wing buoyancy. The specific buoyancy speed is 0.06 m / s, and the buoyancy range is 0.06 m. The first motor 8 rotates to simulate the wing pitch motion, while the second motor 17 rotates the short shaft 16 to achieve deformation of the wing trailing edge. All three motors are controlled by a Siemens PLC (Programmable Logic Controller) to achieve buoyancy, pitch, and deformation pitch motions. Adjustments are made between the three motions to achieve deformation at different phases.

[0054] During the formal experiment, all mechanisms met the required positions, the data from the external control device was correct, the flapping wing motion mechanism was controlled to begin movement, the force measuring device collected the forces acting on the long axis during movement and output data points, the laser beam was activated, and computer software was used to capture PIV camera images. By continuously adjusting the position of the laser generator, the required experimental results and images were obtained, and finally, the captured flow field images were processed and analyzed using computer processing software.

[0055] Before the experiment, the flapping motion of the experimental model was measured. A Keyence laser displacement sensor (LK-G5000) was used to measure the three points A, B, and C on the flapping wing (e.g. Figure 6 Here we give the measured displacement and design displacement of point B and point C (as shown in Figure 7 ), ψ is the deformation phase.

[0056] Motion Process: The sliding platform 3 moves horizontally along the water flow, simulating both static and dynamic forward motion. The deformable flapping wing motion mechanism 2 moves on a linear slide 18, reciprocating perpendicular to the water flow to simulate sinking and floating motion. A second motor drives the short shaft 16 to simulate pitching motion. The coupling of these separate motion processes allows for multi-angle simulation of actual motion processes, including flapping wing deformation, pitching, sinking, and floating. For example, a fish swimming rapidly forward with its tail fin oscillating while its body remains stationary, or a bird or insect's wing with its leading edge stationary and its trailing edge actively or passively flapping.

[0057] In summary, the present invention utilizes a deformable flapping wing motion mechanism to enable the deformable wing segments to simulate the motion of fish tail fins, bird and insect wings. Furthermore, through the force-measuring mechanism within the deformable flapping wing motion mechanism, computer-assisted and other control devices are utilized to efficiently and accurately capture the torsional forces and moments experienced by the deformable wing during motion. Simultaneously, utilizing PIV technology, precise and rapid visualization of the flow field vortices during the deformable wing's pitching, heaving, and combined motions in the water flow is obtained for data processing and analysis, thereby laying a data foundation for improving the endurance, propulsion performance, and stability of flapping-wing aircraft. Therefore, the present invention can accurately simulate the complex motion of flapping wings and effectively combine deformation with the propulsion performance and flow field changes of the complex motion of flapping wings.

[0058] It should be emphasized that the above are only preferred embodiments of the present invention and do not limit the present invention in any form. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A flapping-deformation coupled variable camber flapping wing ground experimental device, characterized in that: include: A water tunnel frame (1) and a deformable flapping wing motion mechanism (2), wherein the water tunnel frame (1) is used to simulate fluids at different speeds and conditions, and the deformable flapping wing motion mechanism (2) is used to simulate the flapping motion process of fish tail fins, birds, and insect wings; The water tunnel frame (1) comprises: a sliding platform (3) and a sliding rail (4); the sliding direction of the sliding rail (4) is arranged along the flow direction of the liquid flow (5) in the water tunnel frame (1); the sliding platform (3) is slidably mounted on the sliding rail (4); and the sliding platform (3) can slide back and forth along the sliding direction of the sliding rail (4); The deformable flapping wing motion mechanism (2) comprises: a linear slide rail (18), a first motor (8), a long shaft (13), a second motor (17), a short shaft (16), and a deformable flapping wing (15); the deformable flapping wing (15) comprises: a front wing section (24) and a rear wing section (22); the front wing section (24) and the rear wing section (22) are rotatably connected via a rotating shaft; The linear slide rail (18) is fixedly connected to the sliding platform (3), the sliding direction of the linear slide rail (18) is parallel to the longitudinal section of the flow direction of the liquid flow (5), the fixed portion of the first motor (8) is slidably connected to the linear slide rail (18) through a slider, the first motor (8) can slide back and forth along the sliding direction of the linear slide rail (18), the output shaft torque of the first motor (8) is connected to the long shaft (13), and the long shaft (13) is torque-connected to the front wing section (24); The long shaft (13) is also fixedly connected to a fixed support plate (14), the fixed support plate (14) rotates synchronously with the long shaft (13), the fixed support plate (14) is fixedly connected to a fixed portion of the second motor (17), the output shaft torque of the second motor (17) is connected to the short shaft (16), and the short shaft (16) is connected to the rear wing section (22) through torque; The axis of the rotating shaft connecting the front wing section (24) and the rear wing section (22) is parallel to the axis of the major axis (13) and the axis of the minor axis (16); The deformable flapping wings (15) are completely extended into and immersed in the fluid channel of the liquid flow (5).

2. The flapping-deformation coupled variable camber flapping wing ground experiment device according to claim 1, characterized in that: The experimental device further comprises a light beam and a camera, wherein the irradiation direction of the light beam is toward the wing surface of the deformed flapping wing (15), and the shooting direction of the camera is toward the deformed flapping wing (15); A force measuring device (11) is provided at the torque connection between the output shaft of the first motor (8) and the long shaft (13), and the force measuring device (11) is used to measure the output torque and output torque change of the first motor (8).

3. The flapping-flapping-deformation coupled variable camber flapping wing ground experimental device according to claim 2, characterized in that: The light beam comprises a laser beam (6), and the camera comprises a high-speed camera (7).

4. The flapping-flapping-deformation coupled variable camber flapping wing ground experimental device according to claim 2, characterized in that: The output shaft of the first motor (8) is connected to a shaft flange (10), the long shaft (13) is connected to a flange (12), the shaft flange (10) is torque-connected to the flange (12), and the force measuring device (11) is located between the shaft flange (10) and the flange (12).

5. The flapping-flapping-deformation coupled variable camber flapping wing ground experimental device according to claim 1, characterized in that: The track shape of the slide rail (4) includes: a straight track and a curved track along the length direction.

6. The flapping-flapping-deformation coupled variable camber flapping wing ground experiment device according to claim 1, characterized in that: The torque connection of the long axis (13) at the front wing section (24) and the torque connection of the short axis (16) at the rear wing section (22) are both shaft-hole connections, and the shaft-hole connection methods include: connecting three to six prisms with prism holes of matching size and shape, and connecting cylinders with round holes and then fixing them with bolts.

7. The flapping-deformation coupled variable camber flapping wing ground experiment device according to claim 6, characterized in that: The front wing section (24) and the rear wing section (22) are both composed of two symmetrical halves that are split apart on both sides along the flow direction of the liquid flow (5); the axial hole connection is located in the symmetrical plane of the front wing section (24) and the rear wing section (22).

8. The flapping-deformation coupled variable camber flapping wing ground experiment device according to claim 6, characterized in that: The shaft hole connection mode of the torque connection of the long shaft (13) at the front wing section (24) and the torque connection of the short shaft (16) at the rear wing section (22) is a square hole and square groove connection.

9. The flapping-flapping-deformation coupled variable camber flapping wing ground experiment device according to claim 1, characterized in that: The long axis (13) is arranged vertically or obliquely in a plane perpendicular to the flow direction of the liquid flow (5).

10. The flapping-flapping-deformation coupled variable camber flapping wing ground experiment device according to claim 1, characterized in that: The slide rail (4) is arranged horizontally or obliquely in a vertical plane.

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