A method for calculating strain-combined fatigue spectrum of flight measured load of helicopter blade

Abstract

The application belongs to the field of helicopter fatigue strength, and relates to a strain synthesis fatigue spectrum calculation method for helicopter blade flight measured load. The method comprises the following steps: obtaining the flapping and edgewise bending moment time domain curve data of each characteristic section of the blade in the section coordinate system; determining the shape characteristics and stiffness characteristics in the section coordinate system; dividing the airfoil coordinate into sections, and discretizing the airfoil coordinate into a certain number of airfoil coordinate points; discretizing the flapping and edgewise bending moment time domain curves into a certain number of flapping and edgewise bending moment time domain curve data points according to time; converting the airfoil coordinate points, the flapping and edgewise bending moment data of each time point of the section from the section coordinate system of the section to the centroid principal axis coordinate system; and synchronously calculating the strain time domain curve of each airfoil coordinate point at each time point through the airfoil coordinate points, the stiffness characteristics, the flapping and edgewise bending moment of the section in the centroid principal axis coordinate system.

Description

Technical Field

[0001] This invention belongs to the field of helicopter fatigue strength and relates to a method for calculating the strain synthesis fatigue spectrum of helicopter rotor blades under measured flight loads. Background Technology

[0002] Helicopter rotor blades are simultaneously subjected to centrifugal force, flapping shear force, flaring shear force, torque, flapping bending moment, and flaring bending moment during flight. The flapping and flaring bending moments are the primary contributors to fatigue damage. These moments are obtained through flight load measurements and compiled into a fatigue load spectrum for rotor blade fatigue life analysis. Currently, there are two methods for compiling rotor blade bending moment fatigue load spectra: the life curve synthesis method and the fatigue damage equivalent fundamental frequency load synthesis method. Both methods involve rainflow counting of the flapping and flaring bending moments separately before synthesis. The life curve method is an engineering method incorporating design experience, but lacks sufficient theoretical basis. The fatigue damage equivalent fundamental frequency load synthesis method must meet the following conditions: 1) The load must fall within the high-frequency segment of the SN curve to be equivalent; 2) Both the flapping and flaring loads must be dominated by fundamental frequency loads or have comparable frequency amplitude distributions; 3) The synthesis assumes that the maximum values ​​of the flapping and flaring bending moments occur at the same time points. These conditions are difficult to satisfy simultaneously.

[0003] Both of the above methods involve counting rainflows separately for the flapping and swaying bending moments of the propeller flight measurements. While this makes spectral compilation simple, the time sequence is lost due to the separate processing, and the corresponding relationships between the alternating loads are lost, making it very difficult to accurately merge them back together. Summary of the Invention

[0004] The purpose of this invention is to overcome the difficulty of merging the processed wavering and swaying moment composite spectra for fatigue calculation. This invention changes the time-domain processing of wavering and swaying moments to time-synchronized merging of rainflow counts, compiles strain-synthesized fatigue spectra, and generates a table of material parameters α-fatigue limit f-airfoil coordinate Y-fatigue life L for accurate fatigue life analysis.

[0005] The technical solution of the present invention:

[0006] A method for calculating the strain synthesis fatigue spectrum of a helicopter rotor blade under measured flight loads includes:

[0007] For each characteristic profile of the blade, obtain the time-domain curve data of flapping and swaying bending moments in the profile coordinate system of that profile;

[0008] Determine the airfoil data and stiffness characteristics of this section in the section coordinate system;

[0009] The airfoil representing the profile is partitioned and discretized into a certain number of airfoil coordinate points; the flapping and swaying bending moment time-domain curves are discretized into a certain number of curve data points MBi and MTi according to time.

[0010] Transform the airfoil coordinate points of this profile, the flapping moment MBi and the oscillation moment MTi at each moment from the profile coordinate system to the centroid principal inertial axis coordinate system;

[0011] By transforming the airfoil coordinate points, stiffness characteristics, and time-domain curve data points of flapping and swaying moments MBi and MTi at each moment into the centroid principal inertial axis coordinate system, the micro-strain time-domain curve of each airfoil coordinate point at each moment is calculated.

[0012] Rainflow counting was performed on the strain-time domain curve data of each airfoil coordinate point to compile the fatigue strain spectrum of all airfoil coordinate points under the airfoil coordinate system of that profile, and then the fatigue limit f of different strains for different materials was calculated. k Fatigue life L k ;

[0013] For airfoil coordinate point j(Yj, Zj) at time i, its microstrain ε ij The calculation formula is:

[0014]

[0015] FC represents centrifugal force, and ES, EIT0, and EIB0 represent tensile stiffness, oscillation swing stiffness, and principal axis swing stiffness, respectively, characterizing stiffness properties. 0j Z 0j ) represents the coordinates of point j in the centroidal principal inertial axis coordinate system; MB0i represents the swinging moment at time i in the centroidal principal inertial axis coordinate system; MT0i represents the oscillation moment at time i in the centroidal principal inertial axis coordinate system.

[0016] The formula for calculating fatigue life L under different material fatigue limits f is as follows:

[0017] α represents the material parameters of the blade composite material, n represents the spectrum number, and N represents the number of failure cycles;

[0018] For the airfoil coordinate point j, the fatigue limit f for different materials and strains is calculated. k The fatigue life includes:

[0019] Based on the fatigue strain spectrum at point j, the SN curve equation is used to calculate the fatigue limit f under different strains for different material parameters α. k Fatigue life L k .

[0020] Calculation of different strain fatigue limits f for different material parameters α k The method further includes: [Fatigue life values ​​are specified below]

[0021] The different strain fatigue limits f of different material parameters α will be calculated. kFatigue life L k This forms a table of material parameters for the profile: α - strain fatigue limit, f - airfoil coordinates, Y - fatigue life, L, to facilitate table lookup or interpolation calculation of fatigue life.

[0022] The formulas for calculating MB0i and MT0i are as follows:

[0023] MBO1=MBi×cos(θ)+MTI×sin(θ);

[0024] MTOl=MT1×cos(θ)-MBl×sin(θ);

[0025] θ is the angle between the centroid principal inertial axis coordinate system and the section coordinate system.

[0026] The formula for calculating the coordinates (Y0j, Z0j) in the centroid principal inertial axis coordinate system is as follows:

[0027] Y0j=(Yj-YN0)×cos(θ)+(Zj-ZN0)×sin(θ);

[0028] Z0f=(Z)-ZN0)×cos(θ)-(Yj-YN0)×sin(θ).

[0029] The beneficial effects of this invention are as follows: The method for real-time synchronous synthesis and compilation of helicopter rotor blade flapping and tumbling bending moments using this invention overcomes the problem of difficulty in resynthesizing loads after separately processing flapping and tumbling bending moments, which leads to a loss of load synchronicity. This improves the accuracy of strain spectra and the resulting lifespan analysis. Based on the ply coordinate range and fatigue limit, the lifespan can be quickly found from the table of strain fatigue limit f-airfoil coordinate Y-fatigue life L. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the blade and a cross-section of a certain airfoil.

[0031] Figure 2 This is a discrete boundary diagram of the airfoil profile.

[0032] Figure 3 This is a cross-sectional swing moment curve.

[0033] Figure 4 This is a cross-sectional bending moment curve.

[0034] Figure 5 The strain curve is shown for airfoil coordinate point j. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0036] This invention provides a method for calculating the strain synthesis fatigue spectrum of helicopter rotor blades under measured flight loads. Unlike previous methods that separately processed flapping and flaring moments, leading to a loss of synchronization and difficulty in synthesis, this method is based on time-domain curves of flapping and flaring moments from flight tests, utilizing... Figure 1 The blade profile stiffness parameters, Figure 2 The blade profile is discretized into a series of data points. The flapping and flaring moments are synchronously synthesized into the strain at these data points. At a specific point on the profile, the two time-domain curves of flapping and flaring moments are combined into a single time-domain strain curve. Rainflow counting and spectrum compilation are then performed to obtain a true and accurate strain spectrum at all airfoil points on the profile. Several typical blade profiles were tested, and the spectrum compilation method was consistent for each profile; therefore, the following description focuses on one specific profile.

[0037] The steps are as follows:

[0038] [1] Determine the time-domain data of flapping and oscillating bending moments in the cross-sectional coordinate system of a certain section of the blade: Determine that the flapping and oscillating bending moments of each section are in phase and have the same sampling rate, and obtain the time-domain curve data of the flapping bending moment MBi (unit Nm) and oscillating bending moment MTi (unit Nm) at each time point i. The centrifugal force FC (unit N) adopts the theoretical value. According to the test load direction, the flapping bending moment is positive when it is bent downwards, positive when it is oscillating forwards, and positive when the centrifugal force points to the blade tip.

[0039] [2] Determine the shape characteristics and stiffness of a certain section of the blade: Determine the centroid coordinates (YN0, ZN0) (unit mm), principal inertial axis flapping stiffness EIB0 (unit Nm2), oscillation flapping stiffness EIT0 (unit Nm2), tensile stiffness ES (unit daN) and the angle α (unit degree) between the centroid principal inertial axis coordinate system and the section coordinate system.

[0040] [3] For a certain section of the blade, the airfoil coordinates are divided into a certain number of airfoil coordinate points (Yj, Zj): For each section of the blade, the discrete airfoil coordinate points are obtained, and the coordinates of each point in the section coordinate system are determined. The coordinates of the j-th point are (Yj, Zj) (unit mm).

[0041] [4] Transform the airfoil coordinate points (Yj, Zj) of a certain section into the coordinate points (Y0j, Z0j) of the centroid principal inertial axis coordinate system:

[0042] Y0j=(Y)-YN0)×cos(θ)+(Zj-ZN0)×sin(θ);

[0043] Z0j=(Z)-ZN0)×cos(θ)-(Y)-YN0)×sin(θ).

[0044] [5] such as Figure 3-4 As shown, the swing moment time-domain curve data points MBi and oscillation moment time-domain curve data points MTi in the section coordinate system are transformed into the swing moment time-domain curve data points MB0i and oscillation moment time-domain curve data points MT0i in the centroid principal axis coordinate system.

[0045] MB0i=MBl×cos(θ)+MTi×sin(θ);

[0046] MTOi=MTl×cos(θ)-MBi×sin(θ).

[0047] [6] Calculate the micro-strain at airfoil coordinate point j(Yj, Zj) at time i:

[0048]

[0049] [7] Repeat steps [5] to [6] to calculate the micro-strain εj at all times at the j-th point (Yj, Zj) of a certain profile airfoil coordinate, and generate the strain time-domain data curve of the j-th point as shown in the figure. Figure 5 As shown.

[0050] [8] Rainflow counting is performed on the time domain data of the j-th point εj of a certain profile, and the fatigue strain spectrum of the j-th point (Yj, Zj) of the airfoil coordinate of a certain profile is compiled.

[0051] [9] Based on the fatigue strain spectrum at point j, using the SN curve and Miner damage theory, calculate the fatigue limit f of a certain material with different strain parameters α. k Fatigue life L k ,

[0052]

[0053] α is the material parameter, N is the number of failure cycles, and n is the number of spectra.

[0054] The calculation results are shown in Table 1.

[0055] Table 1

[0056]

[0057]

[0058]

[10] Repeat steps [4] to [9] to calculate the fatigue limit-life of all coordinate points of the airfoil with a certain profile material parameter α, and generate Table 2.

[0059] Table 2

[0060] Serial Number strain fatigue limit Y1 point lifespan Y2 point lifespan … Yj point lifespan 1 <![CDATA[f1]]> <![CDATA[L 11 ]]> <![CDATA[L 12 ]]> … <![CDATA[L 1j ]]> 2 <![CDATA[f2]]> <![CDATA[L 21 ]]> <![CDATA[L 22 ]]> … <![CDATA[L 2j ]]> … … … … … … k <![CDATA[f k ]]> <![CDATA[L k1 ]]> <![CDATA[L k2 ]]> … <![CDATA[L kj ]]>

[0061] The above description is merely a specific embodiment of the present invention, providing a detailed description of the invention. Parts not covered herein are conventional techniques. However, the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. The scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for calculating the strain synthesis fatigue spectrum of a helicopter rotor blade under measured flight loads, characterized in that, include: For each characteristic section of the blade, determine the shape and stiffness characteristics of that section in the section coordinate system; The airfoil coordinate partitions that characterize the shape of the profile are discretized into a certain number of airfoil coordinate points. Obtain the time-domain curve data of the flapping and swaying bending moments of the flight test in the profile coordinate system; discretize the time-domain curves of the flapping and swaying bending moments into a certain number of curve data points MBi and MTi according to time. The airfoil coordinate points of the profile, and the MBi and MTi curve data points at each time i, are transformed from the profile coordinate system to the centroid principal inertial axis coordinate system. Using the airfoil coordinate points, stiffness characteristics, and MBi and MTi curve data points at each time i in the centroid principal inertial axis coordinate system, the micro-strain time-domain curve data of each airfoil coordinate point at each time i are calculated synchronously. Rainflow counting is performed on the micro-strain time-domain data of each airfoil coordinate point, fatigue strain spectrum of that coordinate point is compiled, fatigue life of different strain fatigue limits at that coordinate point for each material is calculated, and fatigue limit-fatigue life table of each material at that coordinate point is generated. Repeatedly calculate the fatigue life under different strain fatigue limits at all airfoil coordinate points for each material, and generate a fatigue limit-airfoil coordinate-fatigue life table for each material. For airfoil coordinate point j (Yj, Zj) at time i, the flapping and swaying bending moments are synchronously synthesized into micro-strain. The time-domain curve is calculated using the following formula: ； FC represents centrifugal force, and ES, EIT0, and EIB0 represent the tensile stiffness, swinging stiffness, and oscillation stiffness, respectively, along the principal inertial axes of the centroid, characterizing stiffness properties. Y0j , Z0j Let ) be the coordinates of point j in the centroid principal inertial axis coordinate system; MB 0 i The swinging moment of point j at time i in the centroid principal axis coordinate system; MT 0 i Let be the oscillation moment of coordinate point j at time i in the centroid principal inertial axis coordinate system; For airfoil coordinate point j, generate composite material parameters. α -Strain fatigue limit-fatigue life table, including: Based on the microstrain at point j Time-domain curves and strain spectra were compiled using rainflow counting. SN Using curve equations and Miner's damage theory, calculate parameters for each material. α Different strain fatigue limits f k fatigue life L k ; For a specific characteristic profile of the blade, calculate the strain fatigue limit-airfoil coordinate-fatigue life table for each composite material parameter α, including: The different strain fatigue limits of all airfoil coordinate points for each material parameter α were calculated. f k fatigue life L k The material parameters that form this profile α -Strain fatigue limit f -Airfoil coordinates Y - A fatigue life L table is provided for easy reference or interpolation to find fatigue life.

2. The method according to claim 1, characterized in that, MB 0 i and MT 0 i The calculation formula is: ； ； θ is the angle between the centroid principal inertial axis coordinate system and the section coordinate system.

3. The method according to claim 1, characterized in that, Coordinates of points in the centroid principal axis coordinate system ( Y0j , Z0j The formula for calculating ) is: ； 。 4. The method according to claim 1, characterized in that, For a specific profile of the blade, based on the Y-coordinate range of the ply length of a certain material along the chord direction of the profile and on the lower airfoil, the strain fatigue limit of that material parameter is determined. f -Airfoil coordinates Y - Find the corresponding airfoil coordinate Y-point range in the fatigue life L table, and then find its fatigue life by searching or interpolating based on the material's fatigue limit.

5. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the method as described in any one of claims 1-4.

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