What's the difference between an airplane and a dragonfly?

The shape of the upper and lower sides of an airplane wing is different. The top is convex and the bottom is flat. When the plane taxies, the wing is in air movement, which is equivalent to air flowing along the wing from the perspective of relative motion. Because of the different shapes of the upper and lower sides of the wing, the air on the upper side of the wing flows much more than the air on the lower side at the same time (the curve is longer than the straight line), that is, the air on the upper side of the wing flows faster than the air on the lower side. According to the principle of fluid mechanics, when the plane taxies, the air pressure on the upper side of the wing is less than that on the lower side, which makes the plane generate upward lift. When the plane taxies to a certain speed, the lift is enough to make the plane fly. So, the plane went to heaven. The structures related to insect flight mainly focus on the chest wing: one or two pairs of wings are attached to the chest wing, which is driven by the backboard to flutter up and down, and the rest muscle groups control the wings to twist around the torsion axis (the straight line radiating from the wing root to the wing tip), thus generating sufficient lift and thrust. Insect wings are membranous, and tubular veins are hard and elastic, supporting and strengthening the wings. Insect motor receptors are various. In order to control the speed and direction of flight, as well as the attitude of flight, insects need to change the way their wings move, and some insects can control their flight by changing the relative positions of their parts. For example, insects like dragonflies, because of their long abdomen, can bend or even curl, so the abdominal segment also plays a certain role in controlling flight. 3. Early research methods and main achievements The early research methods were mainly high-speed photography or high-speed photography. Take high-speed photos (images) of flying insects (insect bodies are tied with thin wires or placed on flying devices), shoot the process of flapping wings of insects, and then carry out image processing and image analysis. Biological observation on the flight of a large number of insects shows that several specific wing movement patterns may be beneficial to the flight of insects, but we still don't fully understand the mechanical mechanism behind them. The motion modes of these specific wings include: a. The simplest one is that the wings flutter up and down and twist along the torsion axis at the same time, so that the angle of attack changes rapidly. When the wings beat to the lowest point, they twist outward quickly, and when they are lifted to the highest point, they twist inward quickly. Dragonflies, wasps and other insects all have this flapping pattern. On the surface, this flapping mode is the simplest, but so far, we don't know the function of various control parameters (such as flapping frequency and amplitude, etc.). ) In this process. B "Flapping" and "Flapping" are to make the wing flap or partially flap, and then quickly separate from the leading edge of the wing, so as to obtain greater lift in the early stage of the next flap. Entomologists have proved that insects of Neuroptera, Lepidoptera, Hemiptera and Orthoptera can use one of them. This flapping-wing mode is the focus of early research, but the mechanical mechanism behind this flapping-wing mode is still very limited. C in this category, the transverse buckling curve (the membrane line from the leading edge to the trailing edge of the wing) is used, which appears at the end of the downshot. With the deceleration of the wing, the tip outside the lateral buckling curve folds inward. When the downbeat turns to upward lift, the wing straightens sharply and the outer section accelerates sharply, thus obtaining greater lift. However, this phenomenon has not attracted much attention, and it seems to be in a blank state because there is no application prospect. In addition, a series of meaningful flight parameters can be obtained by analyzing the images, such as flapping frequency and amplitude. The commonly used method to measure the flapping frequency is usually based on the following principle: when the frequency of high-speed photography is the same as that of insect flapping, the phase of the wings in the photo is the same, and it seems that the wings are stagnant. At this time, the frequency of high-speed photography is roughly equal to the frequency of insect flapping, so the flapping frequency of insects can be measured. Insect physiologists have done a lot of work in this field and obtained flight parameters of various insects. The statistical analysis of these parameters shows that there is a certain relationship between these parameters. For example, the wing area is proportional to the 2/3 power of the insect mass, and the flapping frequency is proportional to the-1/4 power of the insect mass. Although these results are often rough, it can still prove the following fact: although the shapes, sizes and habits of various insects are quite different due to environmental differences after tens of millions of years of long evolution, due to the limitation of flapping-wing flight, the flight parameters of various insects, such as body structure, wing shape and wing movement mode, must conform to certain mechanical laws. Therefore, mastering the mechanical principle of flapping-wing flight of insects will help to understand the evolution of insects. It is worth noting that the data obtained by image analysis often have large errors and can only be used as the basis for further accurate experiments. The wings of insects are obviously different from those of birds. The bones of birds' wings have muscles, which can control the relative movement of various parts of the wings. Insect wings are membranous and have no muscles, so the control of wing movement can only be achieved by the muscles at the root of the wings and the forces acting on the wings. Because the wings of insects are not well streamlined, the time for insects to fly by gliding is very short. In order to float in the air, insects must constantly flap their wings to get lift. But simply flapping up and down obviously can't produce effective lift, and the wings must be twisted when flapping. Insect wings are rarely rigid, but they often have certain flexibility and elasticity, so they will deform under the action of force when vibrating. Generally speaking, the action point of the resultant force of air acting on the wing is roughly behind the torsion axis of the wing, which forms the torque of the torsion axis in pairs, while the muscle tension at the root of the wing makes the wing rotate inward or outward, so the force of air acting on the surface of the wing and the muscles at the root of the wing makes the wing spiral in flight. However, due to the limitation of research methods, it is still unknown whether the wing deformation plays an important role in obtaining lift and controlling flight. In recent years, both experiments and calculations assume that the wings of insects are rigid and undeformable. In order to obtain effective lift, the wing must have a certain stiffness, and the longitudinal vein plays a major supporting role. In recent years, due to the application of electron microscope, the microstructure of the wing has been deeply understood, and people gradually realize that the film in the closed box surrounded by longitudinal veins and transverse veins actually plays a reinforcing role, just as painting cloth can increase the rigidity of the picture frame. In addition, the wing membrane also forms an "umbrella effect": after the wings are unfolded and become strong, the unsupported rear part is unfolded, just like an umbrella is opened. It has also been found that some insects have dexterous micro air vehicles on their wings. These structures can ensure that the wing is deformed into an appropriate shape in flight to obtain greater lift. For example, there is a triangular area on the dragonfly's wing, which can act on the back of the wing through the "lever effect" to make the back of the wing bend downward, and the cross section of the whole wing is winglike. V research status: traditional aerodynamics can't explain why insects can get enough lift to float in the air by flapping their wings: the lift calculated by traditional aerodynamics is far less than the gravity of insects, and the results generally verify the theoretical analytical solution (Figure 2 shows the flow field at a certain time in this process). The new phenomena observed in the experiment can be summarized as follows: ① The lift that insects can obtain through the flapping mechanism depends on the angular velocity and angular acceleration of wing opening, but has nothing to do with the initial angle α0 of wing opening. The initial opening angle significantly affects the time required for the wings to open and get out of contact with each other, especially when α 0 ≈ 0. With the decrease of α0, the time required for wing opening increases significantly. ② In theoretical analysis, it is generally assumed that the vortex core is circular, but in the experiment, it is observed that the vortex core near the wing tip is obviously elliptical, and its long axis is parallel to the chord direction of the wing. With the increase of the angle between the two wings, the radius of vortex core increases, that is, vorticity diffusion. In 1984, Ellington preliminarily discussed the three-dimensional effect of flapping mechanism and the influence of wing elasticity. So far, the research in this field is not in-depth, especially the need to introduce numerical calculation methods into it. In addition, it should be pointed out that not all insects can use this mechanism, and insects with larger wings may use other flapping methods more. Using fluid experimental technology, people observed the flow field around flying insects for the first time, and found that when the wings of insects shoot down, leading edge vortex will be generated above the wing surface. Because the vortex above the wing will produce a low pressure area, which is beneficial to produce a large lift. At this time, the mechanism and specific function of leading edge vortex are not fully understood. Later, the experiments completed by C.P. Ellington and others showed that, for example, the stress at the root of the wing could not be determined, and the influence of different vibration modes of the wing on the lift force and so on. People began to make micro flapping wings instead of living insects as experimental objects. Another advantage of this method is that it can simulate the flapping mode of free-flying insects in the air, thus overcoming the inevitable error in measuring flying insects. M.H.Di ckinson and others completed the experiment according to this idea [29]. They convert the image of insect flapping motion obtained by the camera into control signals, drive the model wings placed in the oil cylinder to move, simulate the movement of free-flying insect wings, and measure the lift and thrust acting on the wings through force sensors placed at the root of the wings (Figure 3 shows the measurement results). From this, they found that the translational force generated only by the angle of attack is not enough, but the rotating mechanism will generate greater lift, usually 2~3 times the weight (this is similar to "Carmen Vortex Street"). Surprisingly, they found that there was a lift peak in the process of alternating flapping up and down, which they interpreted as wake capture, that is, the leading edge vortex generated by the wing motion before alternating flapping up and down. Therefore, we can now be sure that for insects with long wings, the dynamic stall (or delayed stall) mechanism will play a major role, while for insects with relatively short wings, such as bees, the rotation mechanism and wake capture mechanism will play a major role. Another important problem encountered in studying how the lift force of insect flight is generated is whether the unsteady flow field caused by wing vibration is mainly a two-dimensional effect or a three-dimensional effect on the surface. Different parts of the wing have different speeds due to different distances from the wing root, wing torsion and wing deformation, so the three-dimensional effect should be the main one. In the experiment, it is also found that the leading edge vortex falls off at about the outer quarter along the longitudinal axis of the wing and flows into the tip vortex [30]. Because tip vortex is a typical three-dimensional effect, anthropologists believe that the three-dimensional effect plays a major role in the generation of lift. However, some scholars believe that since the lift of aircraft can be explained by two-dimensional effect, the lift of insects flapping their wings should also be explained by two-dimensional effect instead of three-dimensional effect. Recently, Z.JangWang simulated the two-dimensional flow field of insect flapping-wing flight by eddy current method, and found that the lift generated by two-dimensional effect was enough to make insects float in the air. Whether the two-dimensional effect or the three-dimensional effect is dominant in the lift generation of insect flapping-wing flight needs further study. Because how insects get lift by flapping their wings has not been fully explained, the research on insect flight control is still in its infancy. The existing results show that the initial time of wing twisting around the torsion axis is very sensitive to the lift and thrust in the final stage of wing flapping or flapping [27,365,438+0]. This makes it possible to explain how insects can complete highly flexible flight actions, that is, by adjusting the left and right vibration modes (delaying or advancing the start time of wing twisting in up-and-down shooting), the thrust and lift generated on the left and right wings are asymmetric, so as to control the flight direction. Insect is a kind of life, and its control problem is very different from the traditional control problem, so it should be paid attention to in the study of insect flight control. Conclusion Although the understanding of the principle of flapping-wing flight of insects has gradually become scientific and rigorous in recent decades, our research is still in the primary stage, and we need to develop a set of theories about flapping-wing flight. It is worth noting that at present, our understanding of insect flight control is very lacking. The existing research methods are still based on experiments, that is, to study the flow field around suspended flying insects or flapping-wing flight models. There are two fundamental problems: ① The flapping mode of hanging insects is very different from that of free-flying insects. In the suspended flight state, the trajectory of the wing tip relative to the insect body basically forms a closed "8" shape, while in the free flight state, the trajectory of the wing tip relative to the insect body is a flat closed trajectory (approximate ellipse). This difference will lead to the difference of lift, however, the research in this field is still insufficient. ② The flapping-wing model often adopts rigid flat wings. Although insect wings have longitudinal veins, transverse veins and wing membranes, they can still be deformed during vibration. The deformation of wings affects the flow field around the insect body, and then affects the generation of lift, but the size of this influence is unknown now. In the future, experiments will be carried out with flexible wings to determine the influence of wing deformation on the flow field and lift around the worm. The perceptual knowledge and necessary data obtained from experiments are the essential basis for establishing flapping-wing flight theory, but it is impossible to form a perfect understanding only by relying on experiments. In order to improve the principle of flapping-wing flight, theoretical analysis and numerical calculation are necessary, but it is very difficult. For example, when dealing with the flow field of flapping-wing flight numerical simulation, the moving boundary problem will be encountered. This problem is still a very difficult problem in computational fluid. Therefore, the numerical simulation of insect flapping-wing flight needs better algorithms to ensure the feasibility and calculation accuracy [32] Based on the research progress at home and abroad, I put forward some problems that need to be solved in the near future and in the future for reference. The flight mechanism of 1. wings. At present, most of the research focuses on the mechanism of single pair of wings, and the understanding of the flow field interaction between the front and rear wings during the flight of two pairs of wings is still limited. The relationship between the front and rear wings of insects flying with two pairs of wings varies with different insect species: some rear wings hang on the back of the front wings and swing up and down under the drive of the front wings; Some of them are independent of each other, and the vibrations keep the same frequency and different phases. The function of the front wing of this insect may be the same as that of the insect flying with one wing, but the function of the rear wing in flight is obviously different from that of the front wing due to the interference of the flow field generated by the front wing. 2. Influence of microstructure on wing deformation In previous studies, the wing was regarded as a rigid flat plate, but wind tunnel experiments confirmed that the lift generated by a curved flat plate was much greater than that of a rigid flat plate. Although the research focus has shifted to the influence of wing microstructure on wing deformation, the coupling problem between wing deformation and flow field is basically not involved.