Seesaw type tail rotor hub fatigue test method

Abstract

The invention belongs to the field of helicopter structural strength design, and relates to a seesaw type tail rotor hub fatigue test method. The method comprises the steps that test pieces of a seesaw type tail rotor hub shaft assembly and a shaft sleeve assembly are pasted and calibrated; determining a fatigue test static load and a dynamic load; the static load is centrifugal force, and the dynamic load is flapping bending moment and shimmy bending moment; determining a flapping load and a shimmy load applied to the process joint on the blade false piece through loading debugging, and determining a centrifugal force applied to the process joint on the blade false piece; and carrying out a teeterboard type tail rotor hub fatigue test. According to the seesaw type tail rotor hub fatigue test method, the seesaw type tail rotor hub fatigue test can be guided to be completed, the fatigue dangerous part and the fatigue failure mode of the seesaw type tail rotor hub are determined, and the blank in the field of seesaw type tail rotor hub fatigue test in China is filled.

Description

Technical Field

[0001] This invention belongs to the field of helicopter structural strength design and relates to a fatigue testing method for a seesaw-type tail rotor hub. Background Technology

[0002] The seesaw tail rotor is a tail rotor configuration that is relatively simple in structure, easy to maintain, and lightweight. Its core feature is that the entire rotor hub, together with the two tail rotor blades, can swing as a whole in the direction of the flapping motion, like a seesaw. It is mainly used in light helicopters.

[0003] However, its inherent characteristics bring certain challenges to the fatigue test verification of this type of tail rotor hub. For example, the integrity and symmetry of the tail rotor hub must be constrained and loaded as a whole, which increases the difficulty of load application coordination and control. Furthermore, the fact that its flapping direction can swing freely within a certain range makes it difficult to apply load. In addition, the size of this type of tail rotor hub is often small, which makes it difficult to load the fatigue test of each component evenly. Therefore, the seesaw tail rotor hub fatigue test method is a key technology to ensure the successful development of seesaw tail rotor. Summary of the Invention

[0004] Objective of the Invention: This invention proposes a fatigue testing method for a seesaw-type tail rotor hub. Based on the configuration characteristics of the seesaw-type tail rotor hub, this method derives a reasonable constraint and loading method from previous fatigue tests on this configuration. It can comprehensively assess the fatigue performance of various components of the tail rotor hub (mainly the tail rotor hub shaft and tail rotor hub bushing), providing key technical support for fatigue life assessment and component structural optimization design of this tail rotor hub configuration.

[0005] The technical solution of the present invention: To achieve the above-mentioned objectives, a seesaw-type tail rotor hub fatigue test method is proposed. The seesaw-type tail rotor hub is used as the test piece. The test piece includes a journal assembly and two bushing assemblies. A single bushing assembly and a single-side support arm of the journal assembly are connected by two self-lubricating bushings to achieve radial rotation of the bushing assembly on the journal support arm, thereby realizing the variable pitch motion of the blade. A tension bar assembly is installed in the axial internal space of the journal assembly. The two ends of the tension bar are connected to the two bushing assemblies by two bolts. The center position of the tension bar is connected to the center hole of the journal assembly by a long bolt. It mainly bears the centrifugal force load transmitted from the tail rotor blade end.

[0006] The fatigue testing method includes: Test piece patching and calibration of seesaw-type tail rotor hub shaft assembly and bushing assembly; Determine the static and dynamic loads for the fatigue test; the static load is centrifugal force, and the dynamic loads are swinging moment and oscillation moment. By loading and debugging, it was determined that the process joints on the blade dummy connected to the lugs of the two bushing assemblies were subjected to flapping loads and oscillation loads, and that the process joints on the blade dummy were subjected to centrifugal force. A fatigue test was conducted on the seesaw-type tail rotor hub.

[0007] Furthermore, the specific process for patching the test specimens of the seesaw-type tail rotor hub shaft assembly and bushing assembly includes: Bridge plates were attached to the symmetrical cross-sections of the two bushing assemblies and the symmetrical cross-section of the journal assembly, and the swing and sway bending moments of the journal and bushing assemblies were measured respectively.

[0008] Furthermore, the calibration process includes: Calibration is performed using the overall calibration method: During calibration, the test piece is installed on the calibration platform. After the test piece is connected to the blade dummy, a standard load is applied at the specified section of the dummy by weights. During calibration, the influence of the self-weight of the test piece and the blade dummy is eliminated by clearing the zero point. The calibration was repeated three times to determine the calibration coefficient matrix. After calibration, the calibration results of each patch profile are linearly verified.

[0009] Furthermore, the static and dynamic loads for the fatigue test are determined, including: The centrifugal force is taken as the theoretical static load value; Determine the bending moment M of the patch section of the test specimen journal (with the journal as the primary evaluation target). b M t The target number of cycles N under dynamic load is determined so that the bending fatigue performance obtained after N cycles meets the journal life index requirements. At the same time, the bending fatigue performance of the bushing is predicted based on the bending moment distribution law, taking into account the bushing life index requirements. M is calculated based on the target total number of cycles N and the estimated material fatigue performance. b and M t Bending stress generated at the critical section of the journal; Based on bending stress, and M b and M t Theoretical calculation of load ratio relationship to determine M b and M t The value of .

[0010] Furthermore, the bending stress is obtained through the following calculation formula: ; S aj For the j-th test load M b M t The bending stress generated under the action of the journal patch section, j takes the value of a positive integer from 1 to m, S ∞To predict the fatigue performance of the journal material, A and α are the shape parameters of the SN curve of the journal material. .

[0011] Furthermore, fatigue tests were conducted on the seesaw-type tail rotor hub, including: Using a centrifugal force loading actuator, centrifugal force is slowly applied to a predetermined value at the farthest end of the blade dummy connected to the bushing assembly; At the loading point, which is 4.5 times the radius of the tail rotor hub, the flapping loading actuator load is adjusted using the flapping process joint of the blade dummy component, and the flapping bending moment value is slowly increased until the predetermined value is reached. At the loading point, which is 3.6 times the radius of the tail rotor hub at a distance from the center of the tail rotor hub, the load of the oscillation loading actuator is adjusted using the blade dummy oscillation process joint, and the oscillation bending moment value is slowly increased until the predetermined value is reached. The flapping load actuators at both ends of the tail rotor hub test piece are applied in phase, while the oscillation load actuators are applied in opposite phase, i.e., the phase difference is 180°. Fatigue test loading: If no cracks or damage occur in the test piece after every 500,000 cycles, the static load of centrifugal force is kept constant, and the dynamic load of the journal patch section bending moment Mb and Mt is increased by 10% to 20% to carry out the next level of load test. Test termination condition: The test piece develops cracks or is damaged.

[0012] Furthermore, the method also includes: Determining the validity of test results: After the test, visually inspect the test piece or perform non-destructive testing. If no cracks are found, the number of test cycles completed is valid.

[0013] This invention starts with a fatigue test method for integrated loading of a seesaw-type tail rotor hub. By controlling the phase relationship between flapping and oscillation loading, it solves the interference caused by the inherent oscillation characteristics of the seesaw-type tail rotor hub to the test. By controlling the loading distance between the flapping and oscillation loads and the center of the tail rotor hub, it solves the problem of synchronizing the fatigue performance assessment of the journal assembly and the bushing assembly. By reasonably determining the loading load, it solves the problem of the sufficiency of the fatigue performance assessment of the seesaw-type tail rotor hub.

[0014] Beneficial effects: 1) The method proposed in this invention can guide the completion of fatigue tests on seesaw-type tail rotor hubs, determine the fatigue-prone parts and fatigue failure modes of seesaw-type tail rotor hubs, and fill the gap in the field of fatigue testing of seesaw-type tail rotor hubs in China. 2) Compared to traditional tail rotor hub fatigue testing methods, this invention proposes a method of applying dynamic loads in the fatigue test of a seesaw-type tail rotor hub. This solves the problem of integrated loading and testing of the seesaw-type tail rotor hub, and can effectively assess the fatigue performance of the journal assembly and bushing assembly, avoiding the increased testing cost and cycle of testing them separately. The fatigue test design of this method has reference value in the development of future tail rotor models. Attached Figure Description

[0015] Figure 1 This is a flowchart of the overall scheme of the present invention; Figure 2 This is a schematic diagram of a seesaw-type tail rotor hub patch and its loading. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] The features and illustrative embodiments of various aspects of the present invention will now be described in detail. Numerous specific details are set forth in the following detailed description to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without requiring some of these specific details. The following description of embodiments is merely intended to provide a better understanding of the invention by illustrating examples of the invention. The invention is by no means limited to any specific setups and methods set forth below, but covers any improvements, substitutions, and modifications to structures, methods, and devices without departing from the spirit of the invention. Well-known structures and techniques are not shown in the drawings and the following description to avoid unnecessarily obscuring the invention.

[0018] It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other, and the various embodiments can be referenced and cited in each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0019] This invention relates to a seesaw-type tail rotor hub fatigue testing method, including aspects such as test piece patch calibration design, determination of static and dynamic loads, test piece load application, test process control requirements, error analysis, and test validity determination. Figure 1 As shown, the main steps of the method are as follows: Step 1: Patch application and calibration of the test specimen for the forward-convex and backward-swept blade tip section a) Measure the swaying and rolling moments of the entire bridge at the symmetrical sections AA and aa of the bushing and the symmetrical sections BB and bb of the journal. The patch arrangement of the test piece is as follows. Figure 2 As shown; b) Perform swing and pendulum bending moment calibration on each measurement profile of the test specimen. Calibration is performed using the overall calibration method. During calibration, the test specimen is mounted on the calibration platform, and after connecting the test specimen to the blade dummy, a standard load is applied at the specified profile of the dummy using weights. The influence of the self-weight of the test specimen and the blade dummy is eliminated by clearing the zero point. The calibration is repeated three times to determine the calibration coefficient matrix. After calibration, the linearity of the calibration results for each profile needs to be verified.

[0020] Step 2: Determining Static and Dynamic Loads for Fatigue Testing a) The fatigue test loads are shown in Table 1, which are divided into static loads and dynamic loads. The swinging moment and oscillation moment are controlled by the journal patch section, and the output of each moment of the bushing patch section is recorded. Table 1

[0021] b)F c Take the theoretical static load value.

[0022] c)M bd M td Determination method: Determine the bending moment M of the test specimen journal (with the journal as the main evaluation target) at the patch section. bd M td The bending fatigue performance obtained from the target number of cycles under dynamic load meets the journal life index requirements, while the bending fatigue performance of the bushing is estimated based on the bending moment distribution law, taking into account the bushing life index requirements. M bd M td Take the theoretical calculation load ratio relationship, S aj For the j-th test load M b M t The bending stress generated under the action of the journal patch section, j takes the value of a positive integer from 1 to m, S ∞ To predict the fatigue performance of the journal material, A and α are the shape parameters of the SN curve of the journal material. aj It can be obtained through the following calculation formula:

[0023] In one possible embodiment, the journal material is known to be a titanium alloy, and its safe fatigue limit S under the no-scraping failure mode is taken based on material theory fatigue performance and past experience. ∞Given a bearing strength of 180 MPa, journal material SN curve shape parameters A = 0.205, α = 0.49, and after 500,000 cycles under the initial load, the dynamic load is increased by 10% to complete another 500,000 cycles, with the expected failure rate of the test piece, then m = 2, N1 = N2 = 0.5MC. Based on the above formula, S can be calculated. a1 =205.1MPa, S a2 =225.6MPa. Assuming the ratio of flapping and oscillation bending moment loads on the journal is 1:1, the initial flapping and oscillation bending moment loads that should be controlled on the journal assembly patch section are both 145.1MPa. Using this as the control target, the output loads of the flapping and oscillation actuators on both sides of the tail rotor hub test piece are adjusted.

[0024] Step 3: Applying load to the test specimen Loading such as Figure 2 As shown, the distances from the flapping force loading point to the rotor hub center and the oscillation force loading point to the rotor hub center were determined through debugging, so that the bending moment distribution of the journal and bushing patch sections was more reasonable. Generally, the dynamic bending moment of the bushing patch section is not less than 80% of the dynamic bending moment of the journal patch section. In order to prevent the tail rotor hub from not swinging in the flapping direction during the fatigue test, the flapping loading actuators at both ends are applied in the same phase, and the oscillation loading actuators are applied in opposite phase, that is, the phase difference is 180°. Step 4: Conduct a seesaw-type tail rotor hub fatigue test a) Apply centrifugal static load. Slowly apply centrifugal force to the static load F in Table 1 using a centrifugal force loading actuator. c ; b) Apply swinging and swaying dynamic loads. Adjust the output force of the swinging load actuator and slowly increase the swinging moment value of the journal patch section until the dynamic load requirement value M in Table 1 is reached. bd Adjust the output force of the oscillation loading actuator, and slowly increase the oscillation bending moment value of the journal patch section until it reaches the dynamic load requirement value M in Table 1. td Record the actuator load output and the dynamic load values ​​of swinging and oscillating bending moments of the bushing patch section at this time; c) Conduct fatigue testing. After every 500,000 cycles, if the test piece does not show cracks or failure, keep the centrifugal static load constant and increase the journal patch section bending moment M by 10%–20%. bd M td The dynamic load is then applied to the next level of load test, and the dynamic load values ​​of the swing and oscillation bending moment of the bushing patch section after the load upgrade are recorded. d) Test termination condition: The test piece develops cracks or is damaged.

[0025] Step 5: Analyze experimental errors a) Test control error Main sources: displacement sensor error, control system error, loading error; b) Test measurement error Main sources: measurement system error, strain gauge calibration error; c) Total test error No more than 3%.

[0026] Step Six: Determine the validity of the test results After the test, visually inspect the test piece or perform non-destructive testing. If no cracks are found, the number of test cycles completed is valid.

[0027] The key point of this invention is: 1) The method for patching the cross-section of the helicopter seesaw-type tail rotor hub; 2) The method for determining the fatigue load of the helicopter seesaw-type tail rotor hub and the load phase control requirements, as well as the method for determining the distance between the flapping vibration loading point and the center of the rotor hub. 3) The process control requirements for the fatigue test of the helicopter seesaw-type tail rotor hub.

Claims

1. A fatigue testing method for a seesaw-type tail rotor hub, characterized in that, include: Test piece patching and calibration of seesaw-type tail rotor hub shaft assembly and bushing assembly; The static and dynamic loads for fatigue testing are determined; the static load is centrifugal force, and the dynamic loads are swinging moment and oscillation moment. By loading and debugging, it was determined that the wagging load and the oscillation load applied to the process joint on the blade dummy were applied, and the centrifugal force applied to the process joint on the blade dummy was determined. A fatigue test was conducted on the seesaw-type tail rotor hub.

2. The method according to claim 1, characterized in that, Patches are applied at appropriate symmetrical cross-sectional positions of the two bushing assemblies and at symmetrical cross-sectional positions of the journal assembly, including: The swing and sway bending moments of the bushing assembly are measured by the patch bridge in the swing and sway direction. The swing and sway bending moments of the journal assembly are also measured by the patch bridge in the swing and sway direction.

3. The method according to claim 2, characterized in that, Calibration methods include: Calibration is performed using the overall calibration method: During calibration, the test piece is installed on the calibration platform. After the test piece is connected to the blade dummy, a standard load is applied at the specified section of the dummy by weights. During calibration, the influence of the self-weight of the test piece and the blade dummy is eliminated by clearing the zero point. To prevent the tail rotor hub from moving along the flapping direction during calibration, the flapping forces at both ends of the test piece are applied in the same phase when calibrating the flapping moment. The calibration was repeated three times to determine the calibration coefficient matrix. After calibration, the calibration results of each patch profile need to be linearly verified.

4. The method according to claim 2, characterized in that, Determine the static and dynamic loads for the fatigue test, including: The centrifugal force is taken as the theoretical static load value.

5. Determine the bending moment M of the patch section of the test specimen journal (with the journal as the primary evaluation target). b M t The target number of cycles N under dynamic load is determined so that the bending fatigue performance obtained after N cycles meets the journal life index requirements. At the same time, the bending fatigue performance of the bushing is predicted based on the bending moment distribution law, taking into account the bushing life index requirements. M is calculated based on N and the estimated material fatigue performance. b and M t Bending stress generated at the critical section of the journal; Based on bending stress, and M t and M b Theoretical calculation of load ratio relationship to determine M t and M b The value of .

6. The method according to claim 4, characterized in that, Bending stress can be obtained using the following formula: ； S aj For the j-th test load M b M t The bending stress generated under the action of the journal patch section, j takes the value of a positive integer from 1 to m, S ∞ To predict the fatigue performance of journal materials, A and α are the shape parameters of the SN curve of the journal material.

7. The method according to claim 1, characterized in that, Fatigue testing of the seesaw-type tail rotor hub was conducted, including: Use the centrifugal force loading actuator to slowly apply centrifugal force to the predetermined value; Adjust the displacement of the swing loading actuator and slowly increase the swing bending moment value until it reaches the predetermined value; Adjust the displacement of the oscillation loading actuator and slowly increase the oscillation bending moment value until it reaches the predetermined value; Fatigue testing loading: After every 500,000 cycles, if the test piece does not show cracks or failure, keep the centrifugal static load constant and increase the journal patch section bending moment M by 10% to 20%. b M t The dynamic load is then used to conduct the next level of load test; Test termination condition: The test piece develops cracks or is damaged.

8. The method according to claim 7, characterized in that, The method further includes: Determining the validity of test results: After the test, visually inspect the test specimen or perform non-destructive testing. If no cracks are found, the number of test cycles completed is valid.

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