2.1 Establishment of folding flapping wing and gun body model
The folding flapping wing is a bionic instrument based on the flexibility of the glass fiber bar to realize the wing folding function. Its wing extension morphology is shown in Fig. 1(a) and 1(b), and its folding morphology is shown in Fig. 2(a) and Fig. 2(b).
Figure 2 Folding wing Folding state
The glass fiber has been deformed in the initial position and posture of the assembly, which is used to maintain the forward and backward extension of the wing surface. When the wings are folded into the gun muzzle, the glass fiber will undergo elastic deformation, and the elastic reaction generated by it will be used to wrap the clamping body, and then the clamping body and the flapping wing body will be loaded into the gun muzzle coated with grease to complete the preliminary gun firing preparation. The shape of the bird body folded into the gun barrel is shown in Fig. 3.
The power of the barrel firing is derived from the cylinder placed at its end, and the pressure in the cylinder is increased by external gas delivery [8]. When the input reaches a fixed air pressure, adjust the elevation Angle of the gun body, put the matched package and folding flapping aircraft into the muzzle, and then when the external conditions are stable, control the electromagnetic valve on the gun body through the remote control for instant gas release. Due to the elastic force of the glass fiber of the bird body, its own gravity and air resistance, the wrapping clamp will disperse left and right when the structure is all ejected from the muzzle, and push the bird body into the air. When the flapping wing bird and the mounting body separate and move to the highest point, the flapping wings of the flapping wing aircraft are raised at the same time, and the process of gun body ejection is finally completed. The physical state of the mounting body and the bird body from ejection to stripping is shown in Fig. 4(a), 4(b), 4(c).
2.2 Aerodynamic analysis of gun launch process
Import the established model into fluent software and place the model in a cuboid basin of 300mm100mm60mm. The body is located in the center of the flow field. One end plane of 300mm100mm is used as the inlet of the flow field wind speed, the other end plane is used as the outlet of the flow field wind speed, and the other four planes are used as fixed walls. The inlet wind speed is horizontal and perpendicular to the inlet plane of the flow field. On this basis, different special firing angles of the gun barrel are set. Under the same inlet speed and the air pressure in the cylinder is 0.6MPa, the whole aerodynamic analysis experiment of the clamping body and the ornfelling aircraft is carried out. The flight performance experimental data of flapping birds at different launch angles are shown in Table 1.
Table 1
Experimental performance data of flapping wing
Angle of fire/(°)
|
Flap exit speed /(m/s)
|
Speed of
Incoming flow /(m/s)
|
Flapping wing ejection maximum plumb height /(m)
|
Body lift /(N)
|
Airframe condition
|
30
|
19.4
|
10
|
2.3
|
6
|
sinking
|
35
|
19.4
|
10
|
2.7
|
7.4
|
sinking
|
40
|
19.4
|
10
|
3
|
8.3
|
sinking
|
45
|
19.4
|
10
|
3.5
|
10
|
normal
|
50
|
19.4
|
10
|
4.1
|
13.6
|
overturn
|
55
|
19.4
|
10
|
4.8
|
15.7
|
overturn
|
60
|
19.4
|
10
|
5.2
|
19
|
overturn
|
As can be seen from the data in Table 1, the gun firing Angle is also one of the factors affecting the steady state of flight of the bird, and the launching Angle should not be too small or too large, because too small will cause the lift force generated by the air stream to be lower than the gravity of the bird, resulting in the failure of the take-off of the bird. Excessive assembly will cause the area of the lifting force to increase, and at a certain speed of wind speed, the fuselage will eventually overturn backwards, resulting in the failure of take-off. The pressure distribution of the initial position and pose of the bird body at 45° shot Angle to the highest point is shown in Fig. 5(a), 5(b), 5(c), 5(d).
According to the pressure distribution in Fig. 5(a), 5(b), 5(c) and 5(d), the red area is mainly concentrated in the front abdomen of the bird body, while the yellow and green areas are distributed in the tail root of the body and the middle and back edge of the wingspan. The blue areas are on the back of the bird. The intensity of the pressure distribution is distinguished by color, and the intensity of the pressure distribution in the red region is the largest, followed by the yellow region, then the green region, and finally the blue region. The external pressure in Fig. 5 is mainly concentrated on the neck and belly of the bird and the root of the wing, which will exert a turning moment on the symmetrical plane of the fuselage, causing the fuselage to overturn. In order to maintain the balance of the fuselage, the relative Angle between the tail and the fuselage is adjusted when the flouncing wing body is shot to the highest point, so that the body can obtain the opposite reverse torque, and then maintain the balance of the body within a certain range. Since the force of the body under aerodynamic flight conditions is extremely complex, in order to make it easier for readers to understand the control mechanism of the tail wing, this paper simplifies the complex force state of the body, and only considers the ideal core force that has the strongest effect on the body. The ideal simplified force of the body is shown in Fig. 6.
In Fig. 6, B is the center of mass of the body, and the gravitational force is G. A is the ideal application point for the strongest pressure distribution in the front half of the fuselage, and the force exerted is F1. C is the ideal application point with the strongest tail pressure distribution, and the force exerted by it is F2. According to the stress state of the body, the flipping moment M1 generated by F1 and the flipping moment M2 generated by F2 on the body are as follows:
Where LAB and LBC respectively represent the distance between two points A and B and the distance between two points B and C. If the moment M1 and M2 calculated by formula 1 are equal, the body is stable; if they are not equal, the body is unstable. Therefore, in actual operation, the balance of the body should be maintained by changing the controllable variable Angleto indirectly change the air velocity difference between the upper and lower fuselage. According to the airflow trend and body pressure distribution in Fig. 5, the pressure application point and pressure distribution are basically in line with the controllable adjustment attitude to maintain the smooth flight of the body. The lift force generated by the body is greater than the body's own gravity in the direction of the plumb weight, and the horizontal resistance will indirectly change the body's pitching posture. If the resistance is too small, the associated lift will also decrease, and eventually lead to the bird's body sinking. If the drag is too large, although the associated lift will also increase, its acting moment will be higher than the control moment of the body tail, and eventually cause the body to overturn backwards. The analysis of the process of gun body ejection to flight steady state shows that the rapid separation of the wrapped clamp also affects the initial flight posture of the bionic bird. In order to avoid interference between the jacket body and the bionic bird when the gun reaches the highest point, it is necessary to analyze the aerodynamic characteristics of the wrap body, and explore whether the wrap body with the opening of the horn shape can not interfere with the bionic bird under the action of the elasticity of the glass fiber of the bionic bird's wings and the air flow separation. Under the external conditions of shooting Angle of 45° and flow velocity of 10m/s, the aerodynamic pressure distribution of the wrapped clamp body at the highest point of shooting is shown in Fig. 7.
As can be seen from the first half of the wrapped clamp body in Fig. 7, its pressure is mainly concentrated on the inside of the outer edge of the horn, which will cause the shell to peel outwards. From the point of view of the latter half, the lower side pressure of the end of the clamping body is relatively small, which will make the end of the shell converge toward the center of the gun trajectory, and speed up the process of the left and right stripping of the shell to a certain extent. In addition, in the process of stripping, the package clamp body will also be affected by the elastic force of the bionic bird's wing, which will also accelerate the shell into the stripping state. The stripping force of the shell is shown in Fig. 8.
At the moment of shell separation, in addition to the air stripping force Q1 and Q2, the shell will also be subjected to the force T1 and T2 of the body's wings on the shell. Under the combined action of the two forces, the shell will gain kinetic energy and move away from the body.
Therefore, the separation form of the shell machine will not cause interference between the bionic bird and the package holder, which further proves the rationality of the combination structure of the gun machine ejection.
2.3 Load characteristics of folding
ornithopter
In order to further verify the flight performance of bionic folding ornithopter, the lift and drag curves of its aerodynamic model need to be qualitatively analyzed. The total weight of the experimental prototype is 685g, that is, 6.713N, the wingspan is 1.5m, the chord length is 0.5m, and the mesh unit size is set to 0.05m. The corresponding lift curve and resistance curve of the experimental prototype are shown in Fig. 9 and Fig. 10 under the external conditions that the incoming flow velocity is 10m/s and the gun is fired to the highest point at 45° firing Angle.
As can be seen from the lift resistance curves in Fig. 9 and Fig. 10, the lift curve eventually converges to 26.7N and the drag converges to 0.014N. The lift is greater than the gravity of the bird itself, which meets the flight performance standard of the initial posture of the body and the load characteristics of the folding ornithopter. The horizontal resistance will make the body slow down in the horizontal direction before the body flapping. After the body flapping, the thrust generated by the body will overcome the horizontal resistance and do work to push the body moving forward. Generally, the resistance generated by the wingspan ratio greater than 2.5 will be greater [9–11], so the flexibility requirements of the wings are relatively high. In the flapping wing movement, the flapping frequency of the wing is also an important factor affecting the lift and thrust performance. A larger frequency of movement not only changes the velocity of the flow field near the flapping wing, but also changes the air pressure on the surface of the flapping wing, thus changing the distribution of air flow across the entire wing span [12].
For the actual environment, the ideal uniform head-on wind speed can only be adjusted and set in the wind tunnel experiment, while for the experimental premise setting of external unstable wind speed, some changes caused by external factors under actual conditions should be taken into account as far as possible to conduct a reasonable qualitative analysis of its flight state. Therefore, after the external gust dissipates, the velocity of the incoming flow exerted by the body will be transformed from the forward setting value to the relative reverse variable value. The forward setting value is the 10m/s incoming flow exerted by the outside world, and the relative reverse variable value is the air reaction incoming flow caused by the unstable thrust of the fuselage. The speed of the reaction flow depends on the flexibility and flutter frequency of the flapping wing, the greater the flexibility and flutter frequency within a certain range, the faster the air reaction flow speed obtained by the body. Under the support of the flapping lift, when the speed reaches 9 ~ 10m/s, the appropriate adjustment of the tail can maintain the original ideal attitude of the body for flight. If the wing flexibility and flutter frequency is small, the lift provided by the body wing is far from enough to maintain the body climb, the body will make the nose down due to gravity, and eventually lead to the failure of the ejection flight process.
Based on the change of actual incoming flow wind speed, the dynamic grid of the airframe is divided to simulate the flight performance and load characteristics under the relative change of actual incoming flow. Under the premise that the flapping frequency of the wings is 3Hz, the periodic dynamic flapping lift changes of the body when the incoming wind speed is stable at 10m/s are shown in Fig. 11. When the body is in a non-incoming wind speed and its own flight speed is 9m/s, the periodic dynamic flutter lift changes are shown in Fig. 12.
According to the analysis of lift variation trend in Fig. 11 and Fig. 12, the variation trend is consistent with flutter variation law. Under the condition of incoming flow, the minimum lift force of the flapping wing in steady state flight is 28.4N, which satisfies the required critical lift condition and can realize steady state free flight. Under the condition of no incoming flow, the minimum lift of the body is 20.8N when the gun is launched into the air for steady flight, which also satisfies the critical lift condition and can realize steady free flight. Therefore, the gun firing attitude can provide stable initial flight conditions for bionic folding ornithopter.
2.4 The role of grease in the shooting process
As a non-Newtonian fluid, the shape of grease is affected by many factors. During the movement of the same lubricating pair, the state of the grease will also change with time, and its viscosity value will gradually decrease with time. Similarly, in the friction and wear experiment, the shape of the grease changes from solid to liquid with the change of time to form a lubricating film, which plays a better lubricating role between the friction pairs and reduces the friction coefficient [13]. In the gun firing equipment, there is a mating space of 0.2mm between the wrapping clamp body and the inner wall of the gun barrel, and a plastic film of 0.1mm thickness is wrapped to the outer wall of the clamping body to prevent the adhesion of the lubricating material and the nylon wall. The remaining 0.1mm gap is lubrized and filled with grease. Due to the lubrication characteristics of large relative viscosity of grease at low shear speed and small relative viscosity at high shear speed [14–15], in order to avoid the relative rotation of the shell during gun firing, the grease filled between the package chuck and the inner wall of the gun muzzle is used as a mimicry directional guide rail between the gun barrel and the package chuck. To a certain extent, the standard initial posture of the bird body when the bird body and the wrapping clamp body are stripped is guaranteed. Under the conditions of the same temperature but different pressure in the cylinder, multiple sets of shooting experiments were carried out. The test parameters of the torsion and separation of the clamping body are shown in Table 2.
Table 2
Experimental parameters of torsion and separation of clamped body
Angle of fire/(°)
|
Air pressure in cylinder/(MPa)
|
Body outlet torsion Angle/(°)
|
Height of fire/(m)
|
Time of separation /(s)
|
45
|
0.4
|
0
|
2.1
|
0.8
|
45
|
0.5
|
0.4
|
2.7
|
1.1
|
45
|
0.6
|
0.6
|
3.5
|
1.4
|
45
|
0.7
|
1.2
|
4.2
|
1.6
|
45
|
0.8
|
1.8
|
4.9
|
1.7
|
Considering the coordination error between the bird body and the mounting body, the deviation range of the torsion Angle of the gun body should not exceed ± 1°. The data in Table 2 show that when the pressure in the cylinder is 0.4MPa, the torsion Angle of the wrapping body is the smallest, but the shooting height does not meet the lower limit of the flight height of the bird body, that is, the shooting height generally needs to reach 3m. When the pressure in the cylinder reaches 0.7MPa, the torsion Angle of the clamping body is greater than 1°, so it does not meet the separation standard of the shell machine. Multiple firing experiments show that under the premise of 0.6MPa in the cylinder, the torsion Angle of the wrapped sandwich body is the best under the condition that the initial flight of the bird body is satisfied, and the conditions of firing take-off are satisfied within the allowable error range. Therefore, the application of grease in the process of gun shell shell separation achieves the purpose of making the body stable flight.