CN108121855B - Flight dynamics optimization method of small unmanned aerial vehicle based on bionic flexible wing - Google Patents
Flight dynamics optimization method of small unmanned aerial vehicle based on bionic flexible wing Download PDFInfo
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Abstract
The invention discloses a flight dynamics optimization method of a small unmanned aerial vehicle based on bionic flexible wings, and belongs to the field of overall design and flight dynamics of small fixed-wing unmanned aerial vehicles. The bionic flexible wing is designed by simulating the geometrical and structural characteristics of the wing of the large gliding bird; on the basis of the bionic flexible wing, the first orders of modes with quite obvious torsion parts are obtained by analyzing the wing modes; combining the obtained first several orders of modal equations with a flight dynamics equation, and adding a aerodynamic coupling term generated by elastic deformation; the influence of the aeroelasticity effect of the bionic wing on the coupling effect of the flight dynamics is obtained, the key parameters are extracted, the sign and the size of the key parameters are adjusted by adjusting the structural design of the wing under the condition that the structural arrangement and the overall aerodynamic layout of a fuselage are not changed, the coupling form of the aeroelasticity and the flight dynamics of the wing is further adjusted, and the purposes of reducing the short-period frequency, increasing the short-period damping ratio and slowing down the gust disturbance of the small unmanned aerial vehicle are achieved.
Description
Technical Field
The invention relates to a flight dynamics optimization method based on bionic flexible wings, and belongs to the field of overall design and flight dynamics of small-sized fixed wing unmanned aerial vehicles.
Background
For a small fixed-wing aircraft, the problem of gust disturbance generated during flight at a low Reynolds number always restricts the development of related technologies. This makes its flight dynamics optimization more challenging. The flying reynolds number of the large birds flying mainly in a gliding manner is similar to that of the small unmanned aerial vehicle, and therefore the large birds can be greatly influenced by the sudden wind disturbance. This has a certain relationship with the aeroelastic effect of the bird's flexible wing.
The bionic flexible wing has the advantages that the structural characteristics of the bird wing wings are used for reference in design, so that the aeroelastic effect of the bionic flexible wing influences the flight dynamics characteristics, and the dynamics disturbance under the condition of sudden wind is relieved. The bionic flexible wing specifically uses three characteristics of the bird wing for design reference: the first is to mimic the airfoil of a bird wing with a thin turbulence airfoil; secondly, a shaft-vane structure simulating bird feathers is arranged in the chord direction by reinforcing ribs; and thirdly, the rigidity and the mass of the part close to the front edge are increased and the rigidity and the mass of the part close to the rear edge are reduced by locally modifying the thickness and the material of the film, so that the characteristic that the mass and the elastic axis of the bird wing are close to the front is simulated.
The main characteristic dimensions of the small fixed-wing aircraft are strictly controlled in the design process, and each subsystem part in the aircraft body needs to be highly integrated and densely arranged, so that the traditional overall design optimization method, such as increasing the area of a tail wing, increasing the aerodynamic moment of a tail wing, modifying the internal inertial characteristic arrangement of the aircraft body and the like, is strictly limited in practical application.
Disclosure of Invention
The invention discloses a flight dynamics optimization method of a small unmanned aerial vehicle based on a bionic flexible wing, which aims to solve the technical problem of providing a method for improving the flight stability of the small unmanned aerial vehicle based on the aeroelasticity effect of the bionic flexible wing, and achieves the purposes of enhancing the flight stability and reducing gust disturbance under the condition of not changing the structural arrangement and the overall aerodynamic arrangement of a machine body.
The purpose of the invention is realized by the following technical scheme.
The invention discloses a flight dynamics optimization method of a small unmanned aerial vehicle based on a bionic flexible wing. The low torsional rigidity generated by the thin airfoil profile can cause large torsional elastic deformation generated by aerodynamic change under gust disturbance, so that the local attack angle and lift force change of the airfoil are influenced, and the short-period characteristic of flight dynamics is directly influenced; the chord-direction wing ribs are configured to ensure that the thin film wing profile is not obviously distorted in the elastic deformation process, so that the linearization of the relationship between aerodynamic force and elastic deformation is facilitated; the forward elastic and mass axes contribute to the elastic deformation of the wing which is beneficial to alleviating gust disturbances. Based on the bionic flexible wing, the first orders of modes with quite remarkable torsion parts are obtained by performing mode analysis on the wing. And (3) connecting the obtained equation set of the first several orders of modes with the equation set of the flight dynamics, and adding an aerodynamic coupling term generated by elastic deformation to obtain a state space form of the longitudinal flight dynamics model which is quasi-analytic and coupled with the aeroelasticity of the wing. The method comprises the steps of obtaining a concrete mathematical expression of the coupling effect influence of the aeroelasticity effect of the bionic wing on flight dynamics from the quasi-analytic model, extracting key parameters, cleaning the causal relationship between the geometrical characteristics of the bionic flexible wing and the coupling effect of the aeroelasticity effect of the wing on the flight dynamics through the key parameters, guiding the structural design and modification of the bionic wing, and finally generating the expected coupling effect on the flight dynamics of the small-sized unmanned aerial vehicle, namely achieving the purposes of enhancing the flight stability and reducing the gust disturbance under the condition of not changing the structural arrangement and the overall aerodynamic layout of a machine body.
The invention discloses a flight dynamics optimization method based on a bionic flexible wing, which comprises the following steps:
the method comprises the following steps: the aerodynamic shape of the bionic flexible wing with the thin film wing section is designed according to the design requirements of the small unmanned aerial vehicle.
The design requirements include small unmanned aircraft base load requirements, cruise speed, and size limitations.
The film airfoil means that the maximum thickness of the airfoil does not exceed two percent of the chord length.
The small unmanned aerial vehicle preferably refers to a fixed wing unmanned aerial vehicle with a wing span of less than three meters and a weight of less than 10 kilograms.
Step two: and on the basis of designing the aerodynamic shape of the film wing in the step one, the overall design of the small unmanned aerial vehicle is completed, wherein the overall design of the small unmanned aerial vehicle comprises the design of the internal structure of the vehicle body and the aerodynamic shape of the vehicle body and the empennage, namely the overall design of the full-aerodynamic shape of the small unmanned aerial vehicle is completed.
Step three: and on the basis of the complete aerodynamic shape of the overall design of the small unmanned aerial vehicle, completing complete aerodynamic modeling and calculation, wherein the complete aerodynamic modeling and calculation comprise the distribution of aerodynamic force in the wing span direction.
Step four: and (3) obtaining a mass characteristic according to the overall design of the small unmanned aerial vehicle in the step two and a pneumatic characteristic obtained by the full-aerodynamic modeling calculation in the step three, establishing a small-disturbance linearization model of longitudinal flight dynamics, and converting and forming a state space form as shown in the formula (1).
Wherein: the subscript R indicates the rigidity. x is the number ofR＝[u α q θ]TIs a state quantity of four orders of rigid motion. The four state quantities are respectively forward speed, attack angle, pitch angle speed and pitch angle. w is agIs the vertical airspeed caused by wind gusts. u. of0Is the cruising speed. g is the acceleration of gravity. Matrix ARRAnd GRThe specific expressions of the respective elements therein are found in tables 1 and 2.
TABLE 1 definition of the respective derivatives in the matrices A and G
TABLE 2 Definitions of the generalized aerodynamic derivatives with respect to structural modalities
Wherein: s is the wing area, b is the wingspan, and c is the local chord length，
Is the average aerodynamic chord length, Q is the dynamic pressure, m is the total mass of the aircraft, IyRepresenting the pitch moment of inertia, x, of the small unmanned aerial vehicleEDenotes the distance between the elastic axis and the center of mass in the x direction, e denotes the distance between the elastic axis and the center of pneumatics, mi、
Respectively, the generalized mass of the ith order mode, the torsional portion in the mode shape, and the bending portion in the mode shape. Cl0(y) and Clα(y) are each CL0And CLαIn the spanwise direction.
Step five: and (4) designing a bionic flexible wing structure according to the design requirements of the small unmanned aerial vehicle and the aerodynamic shape of the bionic flexible wing obtained in the first step.
The material used for the bionic flexible wing structure is preferably carbon fiber.
Step six: the wing ribs are arranged in the wing chord direction, and the change of the thin film airfoil caused by elastic deformation is reduced, so that the nonlinear change of the quasi-constant aerodynamic force of the wing is reduced.
Step seven: finite element modeling based on a shell unit is carried out on the wing, and modal treatment for fixing the wing root is carried out to obtain the natural frequency, the modal shape and the generalized mass of the first order modes, the main torsion characteristic of the first order modal shape is analyzed, the torsion part in the first n order modal shape has larger proportion, and the first order modal shape is the first n order modal shape. The dynamics of the wing structure after the modeling process is represented by equation (2).
Wherein:
indicating the amount of elastic deformation of the wing. Etai，
ζiAnd ωiRespectively representing the generalized coordinate, the mode shape, the mode damping and the mode frequency of the ith order mode.
And seventhly, obtaining the former n-order mode according to the system accuracy, wherein the highest natural frequency of the mode obtained in the step seven is preferably not more than ten times of the longitudinal short-period mode frequency of the flight dynamics model in the step seven in order to ensure the system accuracy under the condition of not increasing the total order of the system.
Step eight: and checking the mode shapes of the first orders of modes obtained in the seventh step to ensure that obvious wing section deformation distortion cannot be generated in the mode shapes. And if the obvious wing profile deformation and distortion phenomenon is generated, modifying and reinforcing the distribution and design of the wing ribs, and repeating the fifth step to the seventh step until the main torsional mode vibration mode is ensured not to generate obvious wing profile deformation and distortion.
Step nine: integrating the flight dynamics model of the small unmanned aerial vehicle obtained in the fourth step, the natural frequency, the mode shape and the generalized mass of the wing mode obtained in the seventh step and the wing spanwise aerodynamic force distribution obtained in the third step, and simultaneously combining the flight dynamics equation set shown in the formula (1) and the structural dynamics equation set shown in the formula (2), adding a coupling term to obtain the mutually coupled structural dynamics equation set shown in the formula (3) and the flight dynamics equation set shown in the formula (4); and (3) establishing a wing aeroelasticity equation set as a formula (3) and a flight dynamics equation set as a formula (4) in parallel to obtain a state space form of a longitudinal flight dynamics model coupled with wing aeroelasticity as a formula (5).
Wherein: qηjRepresenting a generalized aerodynamic force with respect to the j-th order mode. The subscript E indicates the elastic case. Quantity of state xE＝[η1η2 ... ηn]TIs a state vector characterizing elastic deformation. The specific composition of each submatrix is shown in equation (6).
GE＝[H1α ... Hnα]T/u0
Step nine: solving for the critical generalized aerodynamic derivative in the coupling State space as shown in equation (5)
And the characteristic root of the stability matrix A, according to the generalized aerodynamic derivative
And determining the influence of the aeroelasticity of the wings on the flight dynamics of the small unmanned aerial vehicle.
The nine concrete implementation methods of the step are as follows:
solving for the critical generalized aerodynamic derivative in the coupling State space as shown in equation (5)
And the characteristic root of the stability matrix a. According to the generalized aerodynamic derivative
Determining the influence of the aeroelasticity of the wings shown in the formula (3) on the flight dynamics of the small unmanned aerial vehicle shown in the formula (4), wherein the influence of the flight dynamics of the small unmanned aerial vehicle is as follows: more negative
The value can obviously reduce the frequency of the short-period mode of flight dynamics and increase the damping of the short-period mode, so that the gust response of the small unmanned aerial vehicle is slowed down; and is more positive
The value will significantly increase the frequency of its short-period mode of flight dynamics and reduce its damping. Wherein the main torsional mode of the wing is favorable for generating larger
Value for the coupling effect of wing aeroelasticity to unmanned aerial vehicle flight dynamics is showing more.
Step ten: on the basis of determining the influence of the wing aeroelasticity on the flight dynamics of the small-sized unmanned aerial vehicle through the step nine, under the condition of not changing the structural arrangement and the overall aerodynamic layout of a fuselage, the distance e between a wing elastic shaft and an aerodynamic center in the formula (6) and the bending-torsion coupling form of the wing are changed by adjusting the wing film thickness of different chord-direction positions of the wing, and further the bending-torsion coupling form of the wing is adjusted
The sign and size of (c). And repeating the fifth step to the ninth step until the purposes of enhancing the flight stability and reducing the gust disturbance under the condition of not changing the structural arrangement of the airplane body and the overall pneumatic layout are achieved.
The bending-twisting coupling form is that
And
relative size and sign of.
The detailed implementation method of the steps is as follows: the bionic flexible wing is designed by taking the geometric and structural characteristics of the wings of the large gliding birds as a reference, the rigidity and the mass of the part close to the front edge are increased by locally modifying the thickness and the material of the film, and the rigidity and the mass of the part at the rear edge are reduced. The local adjustment of stiffness and mass changes the value of e in equation (6) and
relative size and sign of (2), so that the final
Changing towards a more negative direction.
Has the advantages that:
1. the invention discloses a flight dynamics optimization method of a small unmanned aerial vehicle based on a bionic flexible wing, which is characterized in that structural dynamics analysis and aeroelasticity analysis are carried out on the flexible film wing with a thin wing section to obtain the natural frequency, modal vibration mode and generalized mass of the first few orders of modes, and the mode vibration modes of the first few orders are analyzed to mainly twist, so that the coupling effect of the aeroelasticity of the wing on the flight dynamics of the small unmanned aerial vehicle is more obvious, and support is provided for obviously changing and optimizing the flight dynamics characteristics of the small unmanned aerial vehicle through the aeroelasticity effect of the wing.
2. The invention discloses a flight dynamics optimization method of a small unmanned aerial vehicle based on a bionic flexible wing, which adopts a method of establishing a quasi-analytic model with higher transparency to represent the coupling relation between the aeroelasticity and the flight dynamics of the bionic flexible wing, analyzes and refines key parameters from the coupling relation, adjusts the sign and the size of the key parameters by adjusting the wing structural design under the condition of not changing the structural arrangement and the overall aerodynamic layout of a fuselage, further adjusts the coupling form of the aeroelasticity and the flight dynamics of the wing, and achieves the purposes of reducing the short-cycle frequency, increasing the short-cycle damping ratio, enhancing the flight stability and slowing down the gust disturbance of the small unmanned aerial vehicle.
Drawings
FIG. 1 is a flow chart of a method for optimizing flight dynamics based on a bionic flexible wing;
in fig. 2, (a) is a bird wing airfoil profile, and (b) is a bionic elastic wing airfoil profile;
FIG. 3 is a small unmanned aerial vehicle according to an embodiment;
FIG. 4 is a schematic diagram of a vortex lattice method based pneumatic modeling;
FIG. 5 is a distribution of the major aerodynamic derivatives along the span of the wing;
FIG. 6 is a comparison of the original design airfoil (a) and the optimized design airfoil (b) of the example embodiment;
FIG. 7 shows the main torsional mode and the distribution of the torsional portion of the mode along the span direction of the bionic flexible wing;
FIGS. 8-10 are torsion and bending portions, respectively, of the mode shapes of the first to third order structural modes of the bionic flexible airfoil;
FIG. 11 is a schematic representation of a flight dynamics and aeroelasticity coupling modeling process for use with the present invention;
fig. 12 shows the relationship of the respective physical quantities in the expression of the coupling equation;
fig. 13-16 show the dynamic response of a small unmanned aerial vehicle with different types of wings in sharp-edged gust in terms of longitudinal velocity, pitch rate, pitch angle and longitudinal displacement, respectively. Wherein: (1) showing the corresponding situation for configuring a rigid wing; (2) representing the corresponding situation of the initially designed wing; (3) showing the corresponding situation of the optimally designed wing.
Detailed Description
For a better understanding of the objects and advantages of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings and examples.
Example 1:
the initial design index input of this embodiment is: cruising speed u of small unmanned aerial vehicle015m/s, span b 1.7m, and total mass 2.5 kg. The longitudinal gust velocity in the simulation is designed as wg＝3m/s。
As shown in fig. 1, the method for optimizing the flight dynamics of the small unmanned aerial vehicle based on the bionic flexible wing disclosed in this embodiment includes the following steps:
the method comprises the following steps: and inputting and designing the aerodynamic shape of the bionic flexible wing according to the initial design index of the small unmanned aerial vehicle. The aerodynamic profile designed in this example is shown in fig. 2 (b). Wherein the design of the thin film airfoil of the wing was inspired by the bird wing airfoil shown in fig. 2 (a).
Step two: and on the basis of designing the aerodynamic appearance of the film wing in the step one, the overall design of the small unmanned aerial vehicle is completed, wherein the overall design of the small unmanned aerial vehicle comprises the design of the internal structure of the body and the aerodynamic appearance of the body and the empennage. The aerodynamic profile of the small unmanned aerial vehicle designed and completed in the embodiment is shown in fig. 3. The overall parameters of interest are shown in table 3.
Table 3 main general parameters of the small and medium sized unmanned aerial vehicle in this embodiment
Step three: and finishing modeling and calculation of the full aerodynamic force on the basis of the full aerodynamic shape designed in the step two, wherein the modeling and calculation comprise the spanwise aerodynamic force distribution of the machine. The pneumatic modeling uses a conventional vortex lattice method, as shown in fig. 4. The length of the vortex is ten times the length of the fuselage. The spanwise distribution of the key aerodynamic parameter found is shown in figure 5.
Step four: and (3) obtaining a mass characteristic according to the overall design of the small unmanned aerial vehicle in the step two and a pneumatic characteristic obtained by the full-aerodynamic modeling calculation in the step three, establishing a small-disturbance linear model of longitudinal flight dynamics, and converting the small-disturbance linear model into a state space form shown in a formula (1).
The state space matrix in this embodiment is as follows:
GR＝[0.413 -0.344 -5.390 0]T
solving for ARRThe short period eigenvalue can be obtained as-5.571 + -8.893 i and the long period eigenvalue as-0.002 + -0.782 i.
Step five: and (3) designing a bionic flexible wing structure based on the carbon fiber material according to the design requirements of the small unmanned aerial vehicle and the aerodynamic shape of the bionic flexible wing obtained in the first step. The preliminarily designed bionic flexible wing is shown in fig. 6 (a). Wherein the carbon fiber layer is 1mm in uniform thickness, and the direction of the adjacent two layers of carbon fiber material layers forms 90 degrees, so that the overall mechanical property is isotropic. The wing mass and the elastic shaft are both positioned close to the middle chord.
Step six: the wing ribs are arranged in the wing chord direction, and the change of the thin film airfoil caused by elastic deformation is reduced, so that the nonlinear change of the quasi-constant aerodynamic force of the wing is reduced. The wing ribs are also made of carbon fiber materials, the thickness of the wing ribs is 3mm, the width of the wing ribs is 5mm, the wing ribs are evenly distributed along the spanwise direction, and the distance between every two adjacent wing ribs is 85 mm.
Step seven: and carrying out shell element-based finite element modeling on the wing, and carrying out modal analysis. As shown in equation (2), the wing structure dynamics after modal analysis can be expressed as a linear superposition of the first orders of mutually decoupled modal motion.
The first three modes were taken in this analysis example. The relevant modal characterization parameters are shown in table 4.
TABLE 4
Step eight: and checking the mode shapes of the first orders of modes obtained in the seventh step to ensure that obvious wing section deformation distortion cannot be generated in the mode shapes. The first-order mode shape and the corresponding torsional deformation cloud chart are shown in fig. 7. It can be seen that the mode shape twist portion has little change in the spanwise direction at each spanwise position (y coordinate value), i.e., the designed airfoil shape of the film remains unchanged at all spanwise positions under first order modal deformation. The second and third order modes correspond to similar first order modes.
Because no wing-type distortion phenomenon exists, the vibration modes of the first three-order mode can be divided into a torsional part
And a curved portion
Is expressed in linear combinations of (a). The torsion and bending parts of the first three-order modes are shown in FIGS. 8-10.
Step nine: and establishing a coupling model of aeroelasticity and longitudinal flight dynamics of the wing. FIG. 11 is a schematic diagram of parameter transfer during coupled flight dynamics modeling. Integrating the flight dynamics model of the small unmanned aerial vehicle obtained in the fourth step, the natural frequency, the mode shape and the generalized mass of the wing mode obtained in the seventh step and the wing spanwise aerodynamic force distribution obtained in the third step, and simultaneously combining the flight dynamics equation set shown in the formula (1) and the structural dynamics equation set shown in the formula (2), adding a coupling term to obtain the mutually coupled structural dynamics equation set shown in the formula (3) and the flight dynamics equation set shown in the formula (4); and (3) establishing a wing aeroelasticity equation set as a formula (3) and a flight dynamics equation set as a formula (4) in parallel to obtain a state space form of a longitudinal flight dynamics model coupled with wing aeroelasticity as a formula (5).
The specific composition of each submatrix is shown in formula (6):
GE＝[H1α ... Hnα]T/u0
the definition of each element of each sub-matrix in formula (6) is detailed in tables 1 and 2. The relationships between the various physical quantities as they appear are illustrated in detail in fig. 12. Neglecting the damping of each order of structural mode, each sub-matrix can be obtained as follows:
GE＝[-710.8 3.11e3 -2.58e3]T
step nine: solving for the critical generalized aerodynamic derivative in the coupling State space as shown in equation (5)
And the characteristic root of the stability matrix A, according to the generalized aerodynamic derivative
And determining the influence of the aeroelasticity of the wings on the flight dynamics of the small unmanned aerial vehicle. In the present embodiment, the matrix a is extractedEEIn
Can obtain the value of
Solving the eigenvalue of the A matrix to obtain the characteristic root of the short-period simulating mode of-5.44 +/-8.49 i. It can be seen that in the negative
Under the influence of the value, the damping of the short-cycle motion of the small unmanned aerial vehicle provided with the initially designed wing having a uniform film thickness as shown in fig. 6(a) is increased and the frequency is decreased, as compared with the case where the rigid wing is provided. But because of
Too small, short-period motion damping and frequency variations are not significant.
Step ten: on the basis of defining the influence of the wing aeroelasticity on the flight dynamics of the small-sized unmanned aerial vehicle through the step nine, under the condition of not changing the structural arrangement and the overall aerodynamic layout of the fuselage, the distance e between the wing elastic axis and the aerodynamic center and the bending-torsion coupling form of the wing (namely the bending-torsion coupling form of the wing) in the formula (6) are changed by adjusting the thickness of the wing film at different chord-wise positions of the wing
Relative size and sign) of the first and second channels, let
Is directed toAnd (4) more negative direction change, repeating the fifth step to the ninth step until the aims of enhancing the flight stability and reducing gust disturbance are achieved under the condition that the structural arrangement and the overall aerodynamic layout of the airplane body are not changed.
The bionic flexible wing after the optimized design is shown in fig. 6 (b). The optimization direction of the film is based on the structural characteristics of the bird wing, the rigidity and the mass of the part close to the front edge are increased by locally modifying the thickness and the material of the film, and the rigidity and the mass of the part close to the rear edge are reduced. The above-mentioned local adjustment of stiffness and mass may change the value of e in equation (6) and
relative size and sign of (2), so that the final
Changing towards a more negative direction. Repeating the fifth step to the ninth step can obtain the small unmanned aerial vehicle with the optimized wing in the embodiment
The values and main short period characteristic values are shown in table 5.
From table 5, it can be seen that the short-cycle frequency of the small unmanned aerial vehicle can be significantly reduced and the damping ratio thereof can be increased by using the optimized wing with bird wing structural characteristics as reference.
TABLE 5 comparison of short-period characteristics of UAVs under different wing configurations
In order to further confirm the improvement effect of the bionic flexible wing on the flight dynamics, the design simulation example is as follows:
the flying speed is 15m/s, and the vertical speed of sharp-edged gust at the time of 0.25s of simulation time is 3 m/s. The simulation results of the longitudinal gust disturbance of the small-sized unmanned aerial vehicle respectively provided with the initial wing and the optimized wing are shown in fig. 13-16, and as can be seen from fig. 13-16, the small-sized unmanned aerial vehicle can achieve the purposes of enhancing flight stability and reducing gust disturbance under the condition of not changing the structural arrangement and the overall aerodynamic layout of the vehicle body.
The above detailed description is intended to illustrate the objects, aspects and advantages of the present invention, and it should be understood that the above detailed description is only exemplary of the present invention and is not intended to limit the scope of the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (6)
1. A flight dynamics optimization method of a small unmanned aerial vehicle based on bionic flexible wings is characterized by comprising the following steps: comprises the following steps of (a) carrying out,
the method comprises the following steps: designing the aerodynamic shape of the bionic flexible wing with a thin-film wing section according to the design requirement of the small unmanned aerial vehicle;
step two: on the basis of designing the aerodynamic shape of the film wing in the first step, the overall design of the small unmanned aerial vehicle is completed, wherein the overall design of the small unmanned aerial vehicle comprises the design of the internal structure of the vehicle body and the design of the aerodynamic shapes of the vehicle body and the empennage, namely the overall design of the full-aerodynamic shape of the small unmanned aerial vehicle is completed;
step three: on the basis of the full-aerodynamic shape of the overall design of the small unmanned aerial vehicle, complete full-aerodynamic modeling and calculation, including the distribution of aerodynamic force in the wing span direction;
step four: according to the mass characteristics obtained by overall design of the small unmanned aerial vehicle in the step two and the aerodynamic characteristics obtained by modeling calculation of the full aerodynamic force in the step three, establishing a small-disturbance linearization model of longitudinal flight dynamics, and converting into a state space form as shown in a formula (1);
wherein: the subscript R represents the rigidity case; x is the number ofR＝[u α q θ]TIs a state quantity of four orders of rigid motion; the four state quantities are respectively forward speed, attack angle, pitch angle speed and pitch angle; w is agVertical airspeed due to wind gusts; u. of0Is the cruising speed; g is the acceleration of gravity; matrix ARRAnd GRThe specific expressions of the elements therein are found in tables 1 and 2;
TABLE 1 definition of the respective derivatives in the matrices A and G
TABLE 2 Definitions of the generalized aerodynamic derivatives with respect to structural modalities
Wherein: s is the wing area, b is the wingspan, c is the local chord length,
is the average aerodynamic chord length, Q is the dynamic pressure, m is the total mass of the aircraft, IyRepresenting the pitch moment of inertia, x, of the small unmanned aerial vehicleEDenotes the distance between the elastic axis and the center of mass in the x direction, e denotes the distance between the elastic axis and the center of pneumatics, mi、
Respectively representing the generalized mass of the ith order mode, the torsion part in the mode shape and the bending part in the mode shape; cl0(y) and Clα(y) are each CL0And CLαIn the spanwise direction;
step five: designing a bionic flexible wing structure according to the design requirements of the small unmanned aerial vehicle and the aerodynamic shape of the bionic flexible wing obtained in the first step;
step six: the wing ribs are arranged in the wing chord direction, so that the change of the thin film airfoil caused by elastic deformation is reduced, and the nonlinear change of the quasi-constant aerodynamic force of the wing is reduced;
step seven: carrying out shell unit-based finite element modeling on the wing, carrying out wing root fixing modal processing to obtain natural frequency, modal shape and generalized mass of the first order modes, analyzing the main torsion characteristic of the first order modal shape, and the torsion part in the first n order modal shape which is the first n order modal shape; the dynamics of the wing structure after the modeling treatment is expressed by an equation (2);
wherein:
representing the amount of elastic deformation of the wing; etai，
ζiAnd ωiRespectively representing the generalized coordinate, the mode shape, the mode damping and the mode frequency of the ith order mode;
step eight: checking the mode shapes of the first orders of modes obtained in the seventh step to ensure that obvious wing section deformation distortion cannot be generated in the mode shapes; if obvious wing profile deformation and distortion phenomena occur, modifying and reinforcing the distribution and design of the wing ribs, and repeating the fifth step to the seventh step until the main torsional mode vibration mode is ensured not to generate obvious wing profile deformation and distortion;
step nine: integrating the flight dynamics model of the small unmanned aerial vehicle obtained in the fourth step, the natural frequency, the mode shape and the generalized mass of the wing mode obtained in the seventh step and the wing spanwise aerodynamic force distribution obtained in the third step, and simultaneously combining the flight dynamics equation set shown in the formula (1) and the structural dynamics equation set shown in the formula (2), adding a coupling term to obtain the mutually coupled structural dynamics equation set shown in the formula (3) and the flight dynamics equation set shown in the formula (4); establishing a wing aeroelasticity equation set as a formula (3) and a flight dynamics equation set as a formula (4) in parallel to obtain a state space form of a longitudinal flight dynamics model coupled with wing aeroelasticity as a formula (5);
wherein:
represents a generalized aerodynamic force with respect to a j-th order mode; subscript E represents the elastic case; quantity of state xE＝[η1 η2... ηn]TIs a state vector characterizing elastic deformation; the specific composition of each submatrix is shown in formula (6);
step nine: solving for the coupled State space as shown in equation (5)Critical generalized aerodynamic derivatives of
And the characteristic root of the stability matrix A, according to the generalized aerodynamic derivative
Determining the influence of the aeroelasticity of the wings on the flight dynamics of the small unmanned aerial vehicle;
step ten: on the basis of determining the influence of the wing aeroelasticity on the flight dynamics of the small-sized unmanned aerial vehicle through the step nine, under the condition of not changing the structural arrangement and the overall aerodynamic layout of a fuselage, the distance e between a wing elastic shaft and an aerodynamic center in the formula (6) and the bending and twisting coupling form of the wing are adjusted by adjusting the thickness of wing films at different chord-wise positions of the wing
The sign and size of (d); repeating the fifth step to the ninth step until the purposes of enhancing flight stability and reducing gust disturbance are achieved under the condition that the structural arrangement of the airplane body and the overall pneumatic layout are not changed;
the bending-twisting coupling mode is that
And
relative size and sign between.
2. The method for optimizing the flight dynamics of the small unmanned aerial vehicle based on the bionic flexible wing as claimed in claim 1, wherein the method comprises the following steps:
the design requirements include small unmanned aircraft base load requirements, cruise speed, and size limitations;
the maximum thickness of the thin film airfoil does not exceed two percent of the chord length;
the small unmanned aerial vehicle is a fixed wing unmanned aerial vehicle with the wingspan of less than three meters and the weight of less than 10 kilograms.
3. The method for optimizing the flight dynamics of the small unmanned aerial vehicle based on the bionic flexible wing as claimed in claim 2, wherein the method comprises the following steps: and seventhly, obtaining the former n-order mode according to the system accuracy, wherein the highest natural frequency of the mode obtained in the step seven is not more than ten times of the longitudinal short-period mode frequency of the flight dynamics model in the step seven in order to ensure the system accuracy under the condition of not increasing the total order of the system.
4. The method for optimizing the flight dynamics of the small unmanned aerial vehicle based on the bionic flexible wing as claimed in claim 3, wherein the method comprises the following steps: the specific implementation method of the nine steps is as follows,
solving for the critical generalized aerodynamic derivative in the coupling State space as shown in equation (5)
And the characteristic root of the stability matrix A; according to the generalized aerodynamic derivative
Determining the influence of the aeroelasticity of the wings shown in the formula (3) on the flight dynamics of the small unmanned aerial vehicle shown in the formula (4), wherein the influence of the flight dynamics of the small unmanned aerial vehicle is as follows: more negative
The value can obviously reduce the frequency of the short-period mode of flight dynamics and increase the damping of the short-period mode, so that the gust response of the small unmanned aerial vehicle is slowed down; and is more positive
The value would significantly increase the frequency of its short-period mode of flight dynamics and decrease its damping; wherein the main torsional mode of the wing is favorable for generating larger
Value for the coupling effect of wing aeroelasticity to unmanned aerial vehicle flight dynamics is showing more.
5. The method for optimizing the flight dynamics of the small unmanned aerial vehicle based on the bionic flexible wing as claimed in claim 4, wherein the method comprises the following steps: the detailed implementation method of the step ten is as follows,
the bionic flexible wing is designed by taking the geometric and structural characteristics of the wings of the large gliding birds as reference, the rigidity and the mass of the part close to the front edge are increased and the rigidity and the mass of the part at the rear edge are reduced by locally modifying the thickness and the material of the film; the local adjustment of stiffness and mass changes the value of e in equation (6) and
relative size and sign of (2), so that the final
Changing towards a more negative direction.
6. The method for optimizing the flight dynamics of the small unmanned aerial vehicle based on the bionic flexible wing as claimed in claim 1, 2, 3, 4 or 5, wherein the method comprises the following steps: the bionic flexible wing structure is characterized in that carbon fibers are selected as materials for designing the bionic flexible wing structure.
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| CN113761811B (en) * | 2021-09-09 | 2022-05-20 | 北京航空航天大学 | Unmanned aerial vehicle flexible flight dynamics analysis method considering complex wind field |
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