Aircraft modal test method

Abstract

The invention relates to the technical field of aircraft modal tests, in particular to an aircraft modal test method. Comprising the following steps: S1, firstly, selecting a plurality of groups of suspension positions on an aircraft, sequentially suspending the aircraft at the plurality of groups of suspension positions by using a rigid cable, respectively measuring and recording modal frequencies and vibration modes of the aircraft, and selecting the suspension position corresponding to the lowest modal frequency as a test suspension position; s2, sequentially using elastic cables with different rigidities at the test suspension position for suspension, respectively measuring and recording the modal frequency and the vibration mode of the aircraft, and selecting the elastic cable with the corresponding rigidity when the modal frequency is the lowest as a test suspension rope; and S3, carrying out suspension at the test suspension position by using the test suspension rope, and repeating the test to obtain modal test data of the aircraft. According to the method provided by the invention, the interference of the suspension rigidity and the suspension position on a test result can be avoided, the test result is more accurate, and test result standardization is easier to establish.

Description

Technical Field

[0001] The present invention relates to the technical field of aircraft modal testing, and in particular to an aircraft modal testing method. Background Art

[0002] In the process of studying the dynamics of some high-altitude and long-flight UAVs, it is necessary to conduct modal tests on such aircraft. The traditional method is to conduct modal tests on aircraft prototypes, such as Figure 1 As shown, the aircraft is fixed by a suspension rope, and then the modal test is performed by tapping. The existing modal test method has the following problems:

[0003] (1) When using a suspension to conduct modal tests on such aircraft, the stiffness of the suspension is close to the structural frequency, which often results in higher measurement results.

[0004] (2) When modal tests on this type of aircraft are conducted using a suspension method, the test results vary greatly due to differences in suspension positions and suspension stiffness selected for different structures or tests, which is not conducive to conducting subsequent experimental research on this type of aircraft.

[0005] Therefore, the present invention is proposed. Summary of the invention

[0006] The purpose of the present invention is to provide an aircraft modal test method, which can determine the optimal suspension position and suspension stiffness of the aircraft, the test results are more accurate, and the test standardization of the overall aircraft dynamic characteristics can be achieved.

[0007] The present invention provides an aircraft modal test method, comprising the following steps:

[0008] S1. First, multiple groups of suspension positions are selected on the aircraft, each group of suspension positions includes two symmetrical nodes, and rigid cables are used to suspend the aircraft in the multiple groups of suspension positions in sequence. Each time the suspension position is changed, the modal frequency and vibration mode of the aircraft are measured and recorded. The suspension position corresponding to the lowest modal frequency of the aircraft is selected as the test suspension position;

[0009] S2. Using elastic ropes of different stiffnesses to suspend the aircraft at the test suspension position in sequence, measuring and recording the modal frequency and vibration mode of the aircraft each time the elastic rope is replaced, and selecting the elastic rope with the corresponding stiffness when the modal frequency of the aircraft is the lowest as the test suspension rope;

[0010] S3. Use a test suspension rope to suspend at the test suspension position selected in step S1, repeat the test, and obtain modal test data of the aircraft.

[0011] Furthermore, the method for selecting multiple groups of nodes of the suspension positions in step S1 is as follows: first determine a symmetry axis of the aircraft, then extend vectors from the symmetry axis to both ends of the aircraft with the same length a to obtain a group of two nodes of the suspension positions, and change the length a multiple times to obtain multiple groups of nodes of the suspension positions.

[0012] Furthermore, in step S1, when the aircraft is in a suspended state, the deformation of the wing tip under gravity is not less than 10% of the full wingspan.

[0013] Furthermore, the length a initially selected in step S1 is one eighth of the full wingspan of the aircraft, and then gradually increases to one quarter of the full wingspan.

[0014] Furthermore, in step S1, the full wingspan of the aircraft is not less than 20 m, and the first-order modal frequency is not higher than 1.5 Hz.

[0015] Furthermore, the length a in step S1 is changed no less than 5 times.

[0016] Furthermore, in step S1, the hanging width of the rigid cable at the node is 10-30 cm.

[0017] Furthermore, when elastic cables with different stiffness are used for suspension in step S2, the elastic cable with the largest stiffness is selected first, and then the stiffness is reduced in sequence; at least three elastic cables with different stiffness are selected.

[0018] Furthermore, the test in step S3 is repeated no less than three times, and then an average value is taken to obtain final modal test data.

[0019] Furthermore, the difference between each test result in step S3 should not be higher than 10% of the average value, otherwise the test result of this time is discarded and the test is repeated.

[0020] In summary, compared with the prior art, the present invention has the following advantages:

[0021] The present invention confirms the suspension position when the modal frequency of the aircraft is lowest by repeatedly changing the suspension position of the rigid cable, and determines the suspension stiffness when the modal frequency of the aircraft is lowest by repeatedly changing the stiffness of the elastic cable, thereby avoiding the interference of the suspension stiffness and the suspension position on the test results to the greatest extent, and making the test results more accurate; the method provided by the present invention sets the selection principle of the suspension stiffness and the suspension position, and is more likely to establish the standardization of the test results, so that the test results obtained by different institutions and different test personnel are consistent. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0023] Figure 1 It is a schematic diagram of the principle of the traditional suspension method in the prior art;

[0024] Figure 2 It is a principle diagram of the test method in the embodiment of the present invention;

[0025] Figure 3 Flow chart of the modal test method in an embodiment of the present invention.

[0026] Explanation of the reference numerals: 1-suspension fixing surface; 2-suspension rope; 3-aircraft. DETAILED DESCRIPTION

[0027] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0029] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "multiple" means two or more, unless otherwise clearly and specifically defined. In addition, the terms "installed", "connected" and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0030] Example

[0031] A modal test method for an aircraft, such as Figure 3 As shown, the specific process is as follows:

[0032] S1. Select the test suspension position

[0033] S11. First, determine a symmetry axis on the aircraft;

[0034] S12. Extend the vectors at both ends of the symmetrical axis of the aircraft to the same length a, and select two nodes as a set of suspension positions, such as Figure 2 As shown (where Figure 2 The suspension rope 2 in step S1 can be represented as the rigid rope in step S1 or as the elastic rope in step S2);

[0035] S13. Use rigid cables to suspend the aircraft at the two selected nodes so that the aircraft is completely airborne; the suspension width of the rigid cables at the nodes is 10-30 cm, and after suspension, it is necessary to ensure that the aircraft does not collide with the ground and the suspension equipment during vibration;

[0036] S14. Measure and record the modal frequencies and vibration shapes of the aircraft using the hammer vibration method or the sweep frequency method;

[0037] S15. Changing the length a multiple times, repeating steps S12-S14, the length a is changed at least 5 times, the length a selected for the first time is one eighth of the full wingspan of the aircraft, and then gradually increases to one quarter of the full wingspan;

[0038] S16. From multiple groups of test results, select two nodes corresponding to the lowest first-order frequency of the aircraft as test suspension positions.

[0039] S2. Select the stiffness of the test suspension rope

[0040] S21. Use elastic cables instead of rigid cables to suspend the aircraft at the determined test suspension position so that the aircraft is completely airborne; during the test, the elastic cables do not undergo plastic deformation, and the aircraft does not touch the ground during vibration;

[0041] S22. Use hammer vibration method or sweep frequency method to measure and record the modal frequency and vibration shape of the aircraft;

[0042] S23. Sequentially reduce the stiffness of the elastic cable, repeat steps S21 and S22, and select at least three elastic cables with different stiffnesses;

[0043] S24. Compare the modal frequencies of the aircraft in each group of tests, select the elastic cable stiffness corresponding to the lowest first-order modal frequency as the test suspension stiffness, and select the elastic cable as the test suspension rope;

[0044] S3. Perform a modal test at the test suspension position determined in step S1 using the elastic cable corresponding to the test suspension stiffness determined in step S2. Repeat the test for no less than three times and take the average value. The difference between each measurement result should not be higher than 10% of the average value. Otherwise, discard the test result and retest to obtain the final modal test data of the aircraft.

[0045] The rigid cable used in the present invention is a steel cable, and the elastic cable is a spring rope. The full wingspan of the aircraft in the present invention is not less than 20m, the first-order modal frequency is not higher than 1.5Hz, and the deformation of the wing tip under gravity should be less than 10% of the full wingspan when the aircraft is suspended.

[0046] A test specimen model was selected, and the model parameters are shown in Table 1. The modal test was carried out using the above method. The specific parameters of the selected model are as follows: length 3000 mm, width 40.3 mm, thickness 1.8 mm, weight 1.77 kg, material steel, density 8133 kg / m 3 .

[0047] The specific test process is as follows:

[0048] The model is defined as a large flexible aircraft. Then two nodes are symmetrically selected on the large flexible aircraft, and steel cables are used to suspend the aircraft structure at different positions, such as Figure 2 As shown, the distances between the selected node positions and the symmetry axis are shown in Table 1 below, and modal tests are carried out using the hammer method under different length a values. The test results are shown in Table 1.

[0049] Table 1 Modal test results at different lengths a

[0050] a value (m) 0.7 0.75 0.84 0.85 0.92 0.95 Modal frequency (Hz) 1.082 1.068 1.046 1.056 1.069 1.092

[0051] It can be seen from Table 1 that when the value of a is 0.84, the first-order mode of the model is the smallest, so a = 0.84 is selected for subsequent tests.

[0052] Spring ropes with different stiffness are used to suspend the large flexible aircraft. The specific dimensions of the spring ropes and the test results are shown in Table 2:

[0053] Table 2 Test results of spring ropes with different stiffness

[0054] Spring rope number Wire diameter(mm) Outer diameter(mm) Length(mm) Experimental phenomenon Frequency (Hz) 1 0.6 4 300 Plastic deformation of spring rope Unable to measure 2 0.8 15 300 Plastic deformation of spring rope Unable to measure 3 1.2 30 300 Elastic deformation of spring rope 1.044 4 1.5 30 300 Elastic deformation of spring rope 1.045 5 2 30 300 Elastic deformation of spring rope 1.048 6 2 22 300 Elastic deformation of spring rope 1.050

[0055] It can be seen from Table 2 that the spring rope No. 3 has the lowest stiffness for elastic deformation and is also the lowest frequency that can be measured. Therefore, the spring rope No. 3 is used to suspend at the node a=0.84 for subsequent modal tests. The test is repeated many times and the average value is taken. The first-order mode of the final test result model is 1.044Hz.

[0056] The present invention provides a novel modal test method suitable for a large flexible aircraft. The method can measure the modal frequency of a large flexible aircraft in a "free-free" state by a suspension method, and adopts a test selection method to avoid the interference of the suspension stiffness and the suspension position on the test result to the greatest extent, so that the test result is more accurate; the selection principle of the suspension stiffness and the suspension position is set, which makes it easier to establish the standardization of the test results, so that the test results obtained by different institutions and different testers are consistent.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An aircraft modal test method, characterized in that: The following steps are involved: S1. First, multiple groups of suspension positions are selected on the aircraft, each group of suspension positions includes two symmetrical nodes, and rigid cables are used to suspend the aircraft in the multiple groups of suspension positions in sequence. Each time the suspension position is changed, the modal frequency and vibration mode of the aircraft are measured and recorded. The suspension position corresponding to the lowest modal frequency of the aircraft is selected as the test suspension position; S2. Using elastic ropes of different stiffnesses to suspend the aircraft at the test suspension position in sequence, measuring and recording the modal frequency and vibration mode of the aircraft each time the elastic rope is replaced, and selecting the elastic rope with the corresponding stiffness when the modal frequency of the aircraft is the lowest as the test suspension rope; S3. Use a test suspension rope to suspend at the test suspension position selected in step 1, repeat the test, and obtain modal test data of the aircraft.

2. The aircraft modal test method according to claim 1, characterized in that: The method for selecting multiple groups of nodes at the suspension positions in step S1 is as follows: first determine a symmetry axis of the aircraft, then extend vectors from the symmetry axis to both ends of the aircraft with the same length a to obtain a group of two nodes at the suspension positions, and change the length a multiple times to obtain multiple groups of nodes at the suspension positions.

3. The aircraft modal test method according to claim 2, characterized in that: In the step S1, when the aircraft is in a suspended state, the deformation of the wing tip under gravity should be less than 10% of the full wingspan.

4. The aircraft modal test method according to claim 2, characterized in that: The length a initially selected in step S1 is one eighth of the full wingspan of the aircraft, and then gradually increases to one quarter of the full wingspan.

5. The aircraft modal test method according to claim 2, characterized in that: In step S1, the full wingspan of the aircraft is not less than 20 m, and the first-order modal frequency is not higher than 1.5 Hz.

6. The aircraft modal test method according to claim 2, characterized in that: The length a is changed no less than 5 times in step S1.

7. The aircraft modal test method according to claim 1, characterized in that: In the step S1, the hanging width of the rigid cable at the node is 10-30 cm.

8. The aircraft modal test method according to claim 1, characterized in that: When elastic ropes of different stiffness are used for suspension in step S2, the elastic rope selected for the first time has the largest stiffness, and then decreases in sequence; at least three elastic ropes of different stiffness are selected.

9. The aircraft modal test method according to claim 1, characterized in that: In step S3, the test is repeated no less than three times, and then an average value is taken to obtain final modal test data.

10. The aircraft modal test method according to claim 9, characterized in that: The difference between each test result in step S3 should not be higher than 10% of the average value, otherwise the test result will be discarded and retested.

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