A method for evaluating fatigue damage of a helicopter hub arm

Abstract

The application belongs to the technical field of structure fatigue, and discloses a helicopter hub arm fatigue damage evaluation method, step one: collecting hub arm flapping bending moment Mb and oscillation bending moment Mt time history data curve of evaluation cycles; step two: determining hub arm lug force conversion structure geometric parameters; step three: calculating upper lug force and lower lug force time history curve of hub arm; step four: respectively performing rain flow counting on the upper lug force and the lower lug force time history curve, obtaining upper lug force each level alternating load and load frequency of hub arm, lower lug force each level alternating load and load frequency; step five: determining hub arm flapping bending moment fatigue limit and corresponding oscillation bending moment fatigue limit; step six: calculating fatigue life cycle number N of each level fatigue alternating load S; step seven: calculating each level damage of the upper lug, calculating each level damage of the lower lug, and calculating total damage of the upper and lower lug of the hub arm.

Description

Technical Field

[0001] This invention belongs to the field of structural fatigue technology, and specifically relates to a method for assessing fatigue damage of a helicopter rotor hub arm. Background Technology

[0002] Among the major components of a helicopter, the rotor hub arm is extremely important. The rotor hub arm has a complex structure, connecting the rotor blades at one end and other components such as the rotor hub center unit at the other. During flight, the helicopter rotor is simultaneously subjected to centrifugal force, flapping shear force, flaring shear force, flapping moment, flaring moment, and hinge moment. All these loads must be transmitted through the rotor hub arm, making fatigue damage assessment extremely complex. Since the fatigue loads causing fatigue damage to the rotor hub arm are mainly flapping moment and flaring moment, previous fatigue damage assessments of helicopter rotor hub arms were based on ground-based bench fatigue tests using the flapping moment / flaring moment ratio under typical flight conditions. This yielded the fatigue limit of the rotor hub arm with the flapping moment as the characteristic load, and fatigue damage calculations were then performed based on the measured flapping moment. This method requires that the flapping load and flaring load frequencies of the actual flight fatigue damage conditions of the helicopter rotor hub arm be comparable, and that the actual flight flapping moment / flaring moment ratio not differ significantly from the pre-set typical flight condition flapping moment / flaring moment ratio. The loads of non-rigid rotor hub arms, such as articulated and star-shaped flexible types, basically meet this characteristic.

[0003] With the application of rigid rotors in modern helicopters, the ratio of rotor flapping moment to swaying moment varies greatly under different flight conditions. Using only a single typical flapping ratio with flapping moment as the characteristic load for fatigue damage assessment of rotor hub arms results in a large error and is difficult to meet the needs of fatigue damage assessment of rigid rotor hub arms. Summary of the Invention

[0004] Purpose of the invention: This invention uses the rotor hub arm / blade connecting lug force, which is synthesized by flapping moment and swaying moment in time, as the characteristic load for fatigue damage assessment. This overcomes the deviation caused by the inaccuracy of the corresponding swaying moment when only flapping moment is used as the characteristic load for fatigue damage assessment, and achieves a more accurate fatigue damage assessment of helicopter rotor hub arms.

[0005] Technical solution

[0006] A method for assessing fatigue damage to a helicopter rotor hub support arm includes the following steps:

[0007] Step 1: Collect time-domain data of flapping moment and oscillation moment at the hub arm / blade mating surface to obtain the time history data curves of the flapping moment Mb and oscillation moment Mt for the evaluated flights.

[0008] Step 2: Determine the geometric parameters of the force conversion structure of the hub arm lugs, including the horizontal distance d between the centers of the arm / blade connection lugs and the vertical distance h between the centers of the arm / blade connection lugs.

[0009] Step 3: Take the swinging moment Mbi and the oscillation moment Mti at time i, calculate the upper lug force and lower lug force of the propeller hub arm, and iterate through the time history data curves of the swinging moment Mb and the oscillation moment Mt of the propeller hub arm to obtain the time history curves of the upper lug force and the lower lug force.

[0010] Step 4: Perform rainflow counting on the time history curves of the upper lug force and the lower lug force respectively, set the counting threshold with reference to the fatigue limit of the outrigger, and obtain the alternating load and load frequency of each stage of the upper lug force and the alternating load and load frequency of each stage of the lower lug force of the propeller hub outrigger.

[0011] Step 5: Determine the fatigue limit of the propeller hub arm's swing moment by conducting a full-size ground-based fatigue test. and the corresponding oscillation moment fatigue limit The fatigue limit of the propeller hub arm, characterized by lug force, was calculated and converted. ;

[0012] Step 6: Calculate the number of fatigue life cycles N for each stage of fatigue alternating load S according to the SN curve formula;

[0013] Step 7: Based on the linear cumulative damage theory, calculate the damage of each level of the upper lug, calculate the damage of each level of the lower lug, calculate the total damage of the upper and lower lugs of the propeller hub arm, and determine the total damage of the propeller hub arm.

[0014] Furthermore, in step one, data collection begins when the helicopter is powered on from the ground and ends when the power is turned off upon landing. The time-domain data collection for the flapping moment and oscillation moment starts at the same time and has the same sampling rate.

[0015] Furthermore, in step two, the geometric parameters of the ear plate force calculation are converted into the horizontal distance between the centers of the ear holes of the support arm / blade connection and the vertical distance between the centers of the ear holes of the support arm / blade connection.

[0016] Furthermore, in step three, the calculation process for the upper ear plate force F1i is as follows:

[0017]

[0018] The calculation process for the lower ear plate force F2i is as follows:

[0019]

[0020] d is the horizontal distance between the centers of the support arm / blade connection ear holes, and h is the vertical distance between the centers of the support arm / blade connection ear holes.

[0021] Furthermore, in step five, the fatigue limit of the propeller hub support arm is characterized by the lug force. The calculation process is as follows:

[0022]

[0023] Furthermore, in step six, the calculation formulas for the fatigue life cycle count N1j of the upper ear piece and the fatigue life cycle count N2j of the lower ear piece are the same, as follows:

[0024]

[0025] A and α are the shape parameters of the SN curve determined by fatigue tests on small specimens of the same material as the support arm.

[0026] Furthermore, in step seven, the calculation process for each level of damage to the upper and lower earpieces is the same, and the formula is as follows:

[0027]

[0028] Where n is the load frequency, N is the fatigue life cycle number, the total damage of the upper ear piece is equal to the sum of the damage of each level of the upper ear piece, and the total damage of the lower ear piece is equal to the sum of the damage of each level of the lower ear piece.

[0029] In summary, the beneficial effects of the present invention are as follows:

[0030] Using this invention for fatigue damage assessment of helicopter rotor hub arms can overcome the deviation caused by the fact that the corresponding time is not completely consistent with the flapping moment / swaying moment ratio of the typical flight state selected by the ground fatigue test, which is the characteristic load of previous fatigue damage assessment methods for rotor hub arms that only use flapping moment as the characteristic load. This improves the accuracy of fatigue durability design and assessment of helicopter rotor hub arms. Attached Figure Description

[0031] Figure 1 Typical time history data curve for the swing moment Mb.

[0032] Figure 2 The curve shows the typical time history of the oscillation bending moment Mt.

[0033] Figure 3 This is a schematic diagram of a typical propeller hub support arm structure.

[0034] Figure 4 The data curve shows the typical time history of the force F1 on the lug of the propeller hub arm.

[0035] Figure 5 The data curve shows the typical time history of the force F2 on the lower lug of the propeller hub arm. Detailed Implementation

[0036] A fatigue damage assessment method for helicopter rotor hub arms that integrates the combined effects of flapping and flaring moments is proposed. Unlike previous methods that used flapping moment as the characteristic load, this method uses the rotor hub arm / blade connection lug force, synthesized from time-domain data of flapping and flaring moments, as the characteristic load for fatigue damage assessment. This method has significant application value for the fatigue durability design of rigid rotors with highly variable flapping-flaring ratios, and can also provide more accurate fatigue damage assessment for non-rigid rotor hub arms with relatively small flapping-flaring ratios under typical operating conditions. The specific steps are as follows:

[0037] Step 1: By calibrating the target helicopter rotor hub arm patch with measured flight loads, the time-domain data of flapping moment and oscillation moment at the rotor hub arm / blade mating surface are obtained.

[0038] The flapping moment and shimmy moment were determined to have the same start time and sampling rate. Data collection began when the helicopter was powered on at ground level and ended when power was turned off upon landing. This yielded the time history curves of the rotor hub outrigger flapping moment Mb (in Nm) and shimmy moment Mt (in Nm), as shown below. Figure 1 and Figure 2 As shown, it is used for fatigue damage assessment of the propeller hub arm.

[0039] Step 2: Refer to the design drawings of the propeller hub support and measure the actual structure to obtain the structural geometric parameters used to calculate and convert the flapping moment Mb and oscillation moment Mt at the propeller hub support / blade mating surface into the upper lug force F1 and lower lug force F2 of the propeller hub support.

[0040] A typical helicopter rotor hub arm connects to the rotor blade using a double-pin structure. The flapping and yaw moments at the rotor hub arm / blade mating surface are transferred to the rotor hub arm via shearing through the arm lugs using the double pins. Figure 3 As shown, d -- horizontal distance between the centers of the support arm / blade connection ear holes (unit: m), h -- vertical distance between the centers of the support arm / blade connection ear holes (unit: m).

[0041] Step 3: The swing moment Mb and oscillation moment Mt of the propeller hub arm are calculated and converted into the upper lug force F1 and lower lug force F2 of the propeller hub arm.

[0042] Take the flapping moment Mbi and the oscillation moment Mti at time point i. Calculate the upper lug force F1i and lower lug force F2i of the propeller hub arm according to equations (1) and (2) respectively. Traverse the time history curves of the flapping moment Mb and the oscillation moment Mt of the propeller hub arm to obtain the time history curves of the upper lug force F1 and the lower lug force F2 respectively. Figure 4 and Figure 5 As shown.

[0043] (1)

[0044] (2)

[0045] Step 4: Use the rainflow counting method to process the load time history curve into a load spectrum of a series of fatigue alternating loads S and their corresponding frequencies n, which can be calculated for fatigue damage accumulation according to the linear damage theory.

[0046] Rainflow counting was performed on the time history curves of the upper lug force F1 and the lower lug force F2 of the propeller hub support. The counting threshold was set with reference to the fatigue limit of the support arm to obtain the fatigue alternating loads S1j and load frequencies n1j of each stage of the upper lug force of the propeller hub support, and the fatigue alternating loads S2j and load frequencies n2j of each stage of the lower lug force.

[0047] Step 5: Obtain the swing moment fatigue limit and oscillation moment fatigue limit of the propeller hub arm through a full-size ground test bench, determine the fatigue performance of the propeller hub arm, and calculate and convert them into the corresponding arm lug force, so as to perform fatigue damage accumulation calculation on the propeller hub arm according to the linear damage theory.

[0048] Based on the calculated load spectrum of the target helicopter rotor and the flapping moment / swaying moment ratio under typical flight conditions, centrifugal force, flapping shear force, swaying shear force, flapping moment, swaying moment, and torque were applied during fatigue tests on a ground bench for the rotor hub outrigger. The fatigue limit of the rotor hub outrigger flapping moment, verified by fatigue tests, was then obtained. (unit: Nm) and the corresponding fatigue limit of the oscillation bending moment (Unit: Nm), the fatigue limit of the propeller hub outrigger characterized by the outrigger lug force is obtained by calculating according to formula (3). (Unit: N)

[0049] (3)

[0050] Step 6: Calculate the number of fatigue life cycles N corresponding to each level of fatigue alternating load S according to the SN curve formula, so as to use it for subsequent calculation of fatigue damage of each level of alternating load of the measured load.

[0051] Calculate the fatigue life cycle number corresponding to the upper ear force and lower ear force according to formula (4), and substitute any first-level upper ear force load S1j or lower ear force load S2j obtained in step four into the formula, where S is the fatigue life cycle number. The fatigue limit of the propeller hub arm lug force obtained in step five is given by A and α, which are the SN curve shape parameters determined by fatigue tests on small specimens of the same material as the arm. The fatigue life cycle number N1j corresponding to each level of fatigue alternating load on the upper lug force is calculated. The fatigue life cycle number N2j corresponding to each level of fatigue alternating load on the lower earpiece (times) and the fatigue life cycle number N2j. (times), see Table 1 and Table 2.

[0052] (4)

[0053] Step 7: According to the linear cumulative damage theory, the load frequency n divided by the fatigue life cycle number N is the corresponding damage D. The damage value of each alternating load is numerically summed to obtain the total damage value.

[0054] The fatigue damage values ​​D1j corresponding to each alternating load of the upper ear piece and D2j corresponding to each alternating load of the lower ear piece are calculated according to formula (5). The total damage D1 of the upper ear piece and the total damage D2 of the lower ear piece are obtained by accumulating the damage at each level, as shown in Table 1 and Table 2.

[0055] (5)

[0056] Table 1 Alternating loads / frequency and damage values / total damage on the upper lugs of the propeller hub support.

[0057]

[0058] Table 2 Alternating Loads / Frequency and Damage Values / Total Damage on Lower Lug of Propeller Hub Support

[0059]

[0060] The method of the present invention will be further described in detail below with reference to embodiments. A method for assessing fatigue damage of a helicopter rotor hub support arm is characterized by the following steps:

[0061] Step 1: Calibrate the target helicopter rotor hub support patch to measure the flapping moment and teetering moment at the rotor hub support / blade mating surface. The sampling rate is 1024Hz. Data collection begins when the helicopter is powered on on the ground and ends when the power is turned off upon landing. The time history curves of the rotor hub support flapping moment Mb (in Nm) and teetering moment Mt (in Nm) are obtained, as shown below. Figure 1 and Figure 2 .

[0062] Step 2: Referring to the design drawings of the propeller hub support and measuring the actual structure, the flapping moment Mb and oscillation moment Mt at the propeller hub support / blade mating surface are calculated and converted into structural geometric parameters of the upper lug force F1 and lower lug force F2 of the propeller hub support, as shown in the figure. Figure 3 The horizontal distance between the centers of the outrigger / blade connection ear holes is d=0.150m, and the vertical distance between the centers of the outrigger / blade connection ear holes is h=0.180m.

[0063] Step 3, according to the formula Japanese style The time history curves of the flapping moment Mb and the oscillation moment Mt of the propeller hub outrigger are calculated and converted into the time history curves of the upper lug force F1 and the lower lug force F2, respectively. Figure 4 and Figure 5 .

[0064] Step 4: Use the rainflow counting method to process the time history curves of the upper ear plate force F1 and the lower ear plate force F2 into a series of fatigue alternating loads S and their corresponding frequencies n that can be calculated for fatigue damage accumulation according to the linear damage theory. The counting threshold is set to 10000N, as shown in Tables 3 and 4.

[0065] Step 5: Obtain the fatigue limit of the flapping moment of the propeller hub arm through a fatigue test on a full-size ground test bench. =4635 Nm and the corresponding fatigue limit of the oscillation bending moment =618 Nm, calculate the fatigue limit of the rotor hub outrigger characterized by the outrigger lug force. =14935N

[0066] Step 6: Substitute the upper ear plate force load S1j and the lower ear plate force load S2j into the SN curve formula respectively. Calculate the corresponding fatigue life cycles N1j and N2j, as shown in Tables 3 and 4, and the fatigue limit of the propeller hub arm lug force. =14935N, SN curve shape parameters A=0.01 and α=0.805.

[0067] Step 7: Divide the load frequency n by the fatigue life cycle number N to calculate the fatigue damage value corresponding to each level of alternating load. The total damage is obtained by accumulating the damage at each level, as shown in Tables 3 and 4.

[0068] Table 3 Alternating loads / frequency and damage values / total damage on the upper lugs of the propeller hub support arm

[0069]

[0070] Table 4 Alternating Loads / Frequency and Damage Values / Total Damage on Lower Lug of the Hub Support

[0071]

Claims

1. A method for assessing fatigue damage to a helicopter rotor hub arm, characterized in that: Includes the following steps: Step 1: Collect time-domain data of flapping moment and oscillation moment at the hub arm / blade mating surface to obtain the time history data curves of the flapping moment Mb and oscillation moment Mt for the evaluated flights. Step 2: Determine the geometric parameters of the force conversion structure of the hub arm lugs, including the horizontal distance d between the centers of the arm / blade connection lugs and the vertical distance h between the centers of the arm / blade connection lugs. Step 3: Take the swinging moment Mbi and the oscillation moment Mti at time i, calculate the upper lug force F1i and the lower lug force F2i of the propeller hub arm, and iterate through the time history data curves of the swinging moment Mb and the oscillation moment Mt of the propeller hub arm to obtain the time history curves of the upper lug force and the lower lug force. Step 4: Perform rainflow counting on the time history curves of the upper lug force and the lower lug force respectively. Set a counting threshold based on the fatigue limit of the outrigger to obtain the alternating load and load frequency of each stage of the upper lug force of the propeller hub outrigger, and the alternating load and load frequency of each stage of the lower lug force. Step 5: Determine the fatigue limit of the propeller hub arm's swing moment by conducting a full-size ground-based fatigue test. and the corresponding oscillation moment fatigue limit The fatigue limit of the propeller hub arm, characterized by lug force, was calculated and converted. ; Step 6: Calculate the number of fatigue life cycles N for each stage of fatigue alternating load S according to the SN curve formula; Step 7: Based on the linear cumulative damage theory, calculate the damage of each level of the upper lug, calculate the damage of each level of the lower lug, calculate the total damage of the upper and lower lugs of the propeller hub arm, and determine the total damage of the propeller hub arm.

2. The method according to claim 1, characterized in that: In step one, data collection begins when the helicopter is powered on from the ground and ends when the power is turned off upon landing. The start time and sampling rate of the time-domain data collection for the flapping moment and oscillation moment are the same.

3. The method according to claim 2, characterized in that: In step three, the calculation process for the upper ear plate force F1i is as follows: The calculation process for the lower ear plate force F2i is as follows: d h is the horizontal distance between the centers of the arm / blade connection ear holes, and h is the vertical distance between the centers of the arm / blade connection ear holes.

4. The method according to claim 3, characterized in that: In step six, the formulas for calculating the fatigue life cycle count of the upper earpiece and the lower earpiece are the same, as follows: A and α are the shape parameters of the SN curve determined by fatigue tests on small specimens of the same material as the support arm.

5. The method according to claim 4, characterized in that: In step seven, the calculation process for each level of damage to the upper and lower earpieces is the same, and the formula is as follows: Where n is the load frequency, N is the fatigue life cycle number, the total damage of the upper ear piece is equal to the sum of the damage of each level of the upper ear piece, and the total damage of the lower ear piece is equal to the sum of the damage of each level of the lower ear piece.

Images