Frequency measurement method applicable to star-shaped flexible hub helicopter blades

Abstract

This invention provides a method for measuring the frequency of a star-shaped flexible hub helicopter rotor blade. The method includes the steps of moving a hook 3 above the tip of the blade to be tested, supporting the blade tip surface with a sling 2 and attaching it to the hook, so that the hook suspends the blade tip, measuring and adjusting the sling length to ensure that the natural frequency of the suspension system is less than 1 / 5 of the first-order bending frequency of the blade; the natural frequency of the suspension system, g, is the acceleration due to gravity, and L is the sling length; multiple striking points are arranged in a matrix from the blade root to the blade tip, and each measurement involves striking at each striking point. The natural frequency of the blade is obtained by measuring using a triaxial vibration sensor. This invention can accurately simulate the constraint at the blade root, and simultaneously simulate the centrifugal force during blade operation through spring rope suspension, allowing for a more accurate and realistic measurement of the blade's natural frequency in a non-rotating state. The test results and calculation results are shown in Table 1.

Description

Technical Field

[0001] This invention relates to the field of aerospace testing and measurement, and to a method for measuring the frequency of helicopter rotor blades with a star-shaped flexible hub, which improves the accuracy and reliability of measurements during rotor blade characteristic tests. Background Technology

[0002] The rotor is the primary source of vibration in helicopters. The alternating blade loads generated under alternating aerodynamic loads not only affect the fatigue life of the blades, but the resulting hub vibration load at the rotor hub center is also a major cause of helicopter vibration. By rationally configuring the blade frequencies and adjusting their vibration modes, the natural frequencies of the blades can be optimized to ensure a certain difference between the natural frequencies and aerodynamic excitation frequencies within the normal operating speed, maximum rotational speed, and ground speed range. This avoids resonance or excessive vibration caused by aerodynamic excitation forces and simultaneously controls the dynamic stress level of the rotor blades generated by alternating rotor loads, ensuring that the rotor's fatigue life requirements are met. Therefore, accurately obtaining the natural frequencies of the blades is the foundation for rational blade frequency configuration and a prerequisite for optimal blade dynamics design. Verifying and analyzing the results through blade natural frequency experiments is a commonly used technical method for engineers.

[0003] Currently, there are three main installation methods for measuring the natural frequency of a propeller blade in a non-rotating state: root-fixed support, vertical suspension, and horizontal suspension. The root-fixed support method affects the stiffness of the workpiece clamping, which in turn affects the measured frequency. The vertical suspension method requires a large installation space and presents significant difficulties for both installation and measurement. While the horizontal suspension method offers limited installation space, its constraint method cannot simulate the actual installation state of the propeller blade, necessitating further refinement of the analysis model. This invention not only eliminates the influence of the root-fixed support method on the measured frequency but also simulates the propeller blade's installation state in a non-rotating condition, resulting in reliable and accurate measurement results. Summary of the Invention

[0004] The purpose of this invention is to propose a method for measuring the natural frequency of rotor blades in a star-shaped flexible rotor hub helicopter, which realistically simulates the blade constraint form and solves the problem that traditional test methods cannot realistically simulate the constraint state at the root of the rotor blade, thus ensuring that the measurement results of the natural frequency of the rotor blade in the non-rotating state are true and reliable.

[0005] The technical solution of this invention is:

[0006] A method for measuring the frequency of a star-shaped flexible hub helicopter rotor blade is provided, comprising the following steps:

[0007] Step 1: Install rotor blade 1 onto rotor hub 4 via blade pin. The rotor hub is installed on the main rotor shaft. Brake the rotor to ensure that the rotor cannot rotate.

[0008] Step 2: Move the hook 3 above the tip of the blade to be tested. Support the blade tip with the sling 2, attaching it to the hook so that the hook suspends the blade tip. Measure and adjust the sling length to ensure that the natural frequency of the suspension system's oscillation is less than 1 / 5 of the blade's first-order bending frequency. g is the acceleration due to gravity, and L is the length of the sling;

[0009] Step 3: Adjust the height of the propeller tip by manipulating the hook to keep the propeller blade and hub plane horizontal. Use a rangefinder to measure the distance h1 from the center of gravity of the propeller blade to the ground, the distance h2 from the tip of the propeller blade to the ground, the distance R1 from the center of the hub to the center of gravity of the propeller blade, and the distance R2 from the center of the hub to the tip of the propeller. Use the formula... and Calculate the included angles θ1 and θ2 respectively. When both included angles θ1 and θ2 are 4.5°, the requirement that the blade and the hub plane are horizontal is met. If not, continue to manipulate the hook to adjust the height of the blade tip and change the distances h1 and h2 until the requirement is met.

[0010] Step 4: Keep the hub inertial weight separate from the limiter support, ensuring that the distance between the measuring inertial weight and the limiter support is greater than 3mm, so as to realistically simulate the root constraint state of the blade in the working environment.

[0011] Step 5: Arrange multiple striking points in a matrix from the blade root to the blade tip. Each measurement involves striking each striking point once, and the natural frequency of the blade is obtained by measuring using a triaxial vibration sensor.

[0012] Furthermore, the triaxial vibration sensors are installed at the blade root, blade airfoil section, and blade tip respectively for each measurement.

[0013] Furthermore, multiple triaxial vibration sensors are installed at the blade root, blade airfoil section, and blade tip, respectively.

[0014] Furthermore, the natural frequency of the blade obtained from each strike is averaged.

[0015] Furthermore, by using straps to bind and tighten the base of the rotor hub, the rotor hub inertial weight is kept separate from the limiter support.

[0016] Furthermore, from the blade root to the blade tip, on the equivalent plane of the airfoil, with the wingspan direction as the lateral direction and the perpendicular intersecting wingspan direction as the longitudinal direction, draw longitudinal and lateral matrix points, with at least two rows in the lateral direction and at least ten columns in the longitudinal direction. Even further, the longitudinal and lateral matrix points total 24.

[0017] Furthermore, the triaxial vibration sensor is installed by adhesive bonding.

[0018] The advantages of this invention are: it can accurately simulate the constraint method at the root of the blade, and at the same time, it simulates the effect of centrifugal force when the blade is working by suspending it with a spring rope, which allows for a more accurate and realistic measurement of the natural frequency of the blade in a non-rotating state. The test results and calculation results are shown in Table 1. Attached Figure Description

[0019] Figure 1 This is a schematic diagram illustrating the implementation of the method of the present invention;

[0020] Figure 2 This is a schematic diagram of the hammer impact point;

[0021] Wherein: 1-blade, 2-sling, 3-hook, 4-hub, 5-helicopter. Detailed Implementation

[0022] The disclosed examples will be described more fully with reference to the accompanying drawings, in which some (but not all) of the disclosed examples are shown. In fact, many different examples may be described, and these examples should not be construed as limited to those set forth herein. Rather, these examples are described so that this disclosure will be thorough and complete, and will fully convey the scope of this disclosure to those skilled in the art.

[0023] A method for measuring the frequency of a star-shaped flexible hub helicopter rotor blade is provided, comprising the following steps:

[0024] Step 1: Install the rotor blades onto the rotor hub using blade pins. The rotor hub is then mounted onto the main rotor shaft. Apply the rotor brake to ensure that the rotor cannot rotate.

[0025] Step 2: Move the hook above the tip of the blade to be tested. Support the blade tip with a sling attached to the hook, ensuring the hook suspends the blade tip. Measure and adjust the sling length to ensure the natural frequency of the suspension system is less than 1 / 5 of the first-order bending frequency of the blade. g is the acceleration due to gravity, and L is the length of the sling;

[0026] Step 3: Adjust the height of the propeller tip by manipulating the hook to keep the propeller blade and hub plane horizontal. Use a rangefinder to measure the distance h1 from the center of gravity of the propeller blade to the ground, the distance h2 from the tip of the propeller blade to the ground, the distance R1 from the center of the hub to the center of gravity of the propeller blade, and the distance R2 from the center of the hub to the tip of the propeller. Use the formula... and Calculate the included angles θ1 and θ2 respectively. When both included angles θ1 and θ2 are 4.5°, the requirement that the blade and the hub plane are horizontal is met. If not, continue to manipulate the hook to adjust the height of the blade tip and change the distances h1 and h2 until the requirement is met.

[0027] Step 4: Keep the hub inertial weight separate from the limiter support, ensuring that the distance between the measuring inertial weight and the limiter support is greater than 3mm, so as to realistically simulate the root constraint state of the blade in the working environment.

[0028] Step 5: Arrange multiple striking points in a matrix from the blade root to the blade tip. Each measurement involves striking each striking point once, and the natural frequency of the blade is obtained by measuring using a triaxial vibration sensor.

[0029] The triaxial vibration sensor was installed at the blade root, blade airfoil section, and blade tip for each measurement.

[0030] The natural frequency of the blade is averaged based on the measurements from each strike.

[0031] By using straps to bind and tighten the base of the rotor hub, the rotor hub inertial weight is kept separate from the limiter support.

[0032] From the blade root to the blade tip, on the equivalent plane of the airfoil, with the wingspan direction as the lateral direction and the perpendicular cross-wingpan direction as the longitudinal direction, draw longitudinal and lateral matrix points. There are at least two rows in the lateral direction and at least ten columns in the longitudinal direction, for a total of 24 longitudinal and lateral matrix points.

[0033] The triaxial vibration sensor is installed by adhesive bonding.

[0034] Table 1 Comparison of experimental results and calculated results (HZ)

[0035]

[0036] As can be seen from Table 1, the error between the measured results and the calculated results of this method is controlled within 3%, and its experimental accuracy meets the requirements of engineering applications.

[0037] Descriptions of various advantageous arrangements have been shown for illustrative and descriptive purposes, but such descriptions are not intended to be exclusive or limited to the disclosed forms. Many modifications and variations will be apparent to those skilled in the art. Furthermore, different advantageous examples may describe different advantages compared to other advantageous examples. One or more examples have been selected and described in order to best illustrate the principles and practical application of the examples, and to enable those skilled in the art to understand that this disclosure contains various examples with various modifications suitable for the particular intended use.

Claims

1. A frequency measurement method applicable to star-shaped flexible hub helicopter rotor blades, characterized in that: This method includes the following steps: Step 1: Install the rotor blades onto the rotor hub using blade pins. The rotor hub is then mounted onto the main rotor shaft. Apply the rotor brake to ensure that the rotor cannot rotate. Step 2: Move the hook above the tip of the blade to be tested. Support the blade tip with a sling attached to the hook, ensuring the hook suspends the blade tip. Measure and adjust the sling length to ensure the natural frequency of the suspension system is less than 1 / 5 of the first-order bending frequency of the blade. g is the acceleration due to gravity, and L is the length of the sling; Step 3: Adjust the height of the propeller tip by manipulating the hook to keep the propeller blade and hub plane horizontal. Use a rangefinder to measure the distance h1 from the center of gravity of the propeller blade to the ground, the distance h2 from the tip of the propeller blade to the ground, the distance R1 from the center of the hub to the center of gravity of the propeller blade, and the distance R2 from the center of the hub to the tip of the propeller. Use the formula... and Calculate the included angles θ1 and θ2 respectively. When both included angles θ1 and θ2 are 4.5°, the requirement that the blade and the hub plane are horizontal is met. If not, continue to manipulate the hook to adjust the height of the blade tip and change the distances h1 and h2 until the requirement is met. Step 4: Keep the hub inertial weight separate from the limiter support, ensuring that the distance between the measuring inertial weight and the limiter support is greater than 3mm, so as to realistically simulate the root constraint state of the blade in the working environment. Step 5: Arrange multiple striking points in a matrix from the blade root to the blade tip. Each measurement involves striking each striking point once, and the natural frequency of the blade is obtained by measuring using a triaxial vibration sensor.

2. The frequency measurement method for helicopter rotor blades with a star-shaped flexible hub as described in claim 1, characterized in that: The triaxial vibration sensor was installed at the blade root, blade airfoil section, and blade tip for each measurement.

3. The frequency measurement method for helicopter rotor blades with a star-shaped flexible hub as described in claim 1, characterized in that: Multiple triaxial vibration sensors were installed at the blade root, blade airfoil section, and blade tip, respectively.

4. The frequency measurement method for helicopter rotor blades with a star-shaped flexible hub as described in claim 1, characterized in that: The natural frequency of the blade is averaged based on the measurements from each strike.

5. The frequency measurement method for helicopter rotor blades with a star-shaped flexible hub as described in claim 1, characterized in that: By using straps to bind and tighten the base of the rotor hub, the rotor hub inertial weight is kept separate from the limiter support.

6. The frequency measurement method for helicopter rotor blades with a star-shaped flexible hub as described in claim 1, characterized in that: From the blade root to the blade tip, on the equivalent plane of the airfoil, with the wingspan direction as the lateral direction and the perpendicular cross-wingpan direction as the longitudinal direction, draw longitudinal and lateral matrix points, with at least two rows in the lateral direction and at least ten columns in the longitudinal direction.

7. The frequency measurement method for helicopter rotor blades with a star-shaped flexible hub as described in claim 6, characterized in that: There are 24 points in the vertical and horizontal matrix.

8. The frequency measurement method for helicopter rotor blades with a star-shaped flexible hub as described in claim 1, characterized in that: The triaxial vibration sensor is installed by adhesive bonding.

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