Pipeline vibration analysis of aircraft    

The modern aircraft hydraulic system mostly adopts variable piston pump, pulsating flow output is its inherent characteristics, the pressure pulsation resulting often makes the energy pipeline system was severely damaged, endanger the life safety of the passengers, the flight normal completion effect. The problem of pipeline vibration in aircraft hydraulic energy system has been puzzling the designers and accident analysts of aircraft hydraulic system for many years: with the high pressure of aircraft hydraulic system, this problem is more prominent.

       1.Vibration of aircraft hydraulic energy system

       The energy part of the aircraft hydraulic system consists of a hydraulic pump, an oil filter and a pipeline between them. The main causes of vibration are the rotary movement of hydraulic pump and the process of reciprocating suction and discharge, as well as the resonance of fluid in the pipeline. The main engine through the accessory gearbox to the hydraulic pump to provide power, in the normal processing and use conditions, the hydraulic pump spindle, bearings and cylinder rotating parts can be in good working condition for a long time, stable mechanical properties, less vibration. In the process of piston reciprocating movement to complete the suction and discharge of oil, the possible cavitation and cavitation problems can be solved by adopting the reasonable design of the pressurized fuel tank and the inner flow passage of the pump. The plunger hole cavity and the valve plate on the low pressure cavity suction when the plunger is in the bottom, the plunger cavity volume is very small, and the triangular groove are pressure structure, two cavity pressure quickly balance, weak flow pulsation can be neglected; the plunger cavity and the valve plate pressure cavity is connected with oil, although has taken the triangular groove and the damping hole and pre compression and other measures, but still higher than the high pressure cavity pressure plunger cavity pressure in two cavity connected, there will be traffic flow, and the flow pulsation, the hydraulic pump as a source of fluid vibration. The flow pulsation produced by the hydraulic pump passes through the pipeline to form pressure pulsation, and the vibration of the fluid is transmitted to the system through the pipeline.

       The vibration of the pipeline is complicated. The pulsating flow output of the hydraulic pump causes the forced vibration of the fluid pipeline and the solid pipe. The distribution parameter characteristic of the fluid pipeline makes the self-excited resonance occur when the pipeline structure satisfies the resonance condition. When the pulsation frequency of the hydraulic pump is close to the resonant frequency of the fluid, the vibration will be further strengthened. If the natural frequency of the solid pipe is close to the resonant frequency of the fluid, or close to the pulsation frequency of the hydraulic pump, the fluid solid resonance will occur. The strong vibration of the mechanical structure near its natural frequency often results in structural damage and accidents.

      2.General design of hydraulic pipeline system

       According to the fluid mechanics theory, the resonant frequency, amplitude and other distributed parameter characteristics of fluid pipeline can be obtained by formula (3-2). Cooperation with very complex solid fluid and the pipeline flow disaster, because the modern precise analysis of the theory and engineering application there is still a certain distance, the vibration characteristics of solid pipeline and its supporting structure, combined with the general estimate is determined by experiments. The plane of the guide tube is thin and long, the flow rate of the fluid is not affecting the velocity of solid natural frequency of pipeline are usually negligible, considering the pressure and the quality of the fluid in the pipeline, the pipeline to estimate the minimum natural frequency for F1

       M -- unit mass of pipe and fluid;

       Length of pipe between L and supporting points.

       The natural frequency of the actual pipeline needs to be tested and determined on the aircraft ground simulator and the test machine. In addition to static strength calculation and experiment, the supporting structure is also calculated and tested. Design of aircraft hydraulic system according to the above method, should be 1.5~2 times the minimum natural frequency can ensure the pipeline is higher than the highest frequency of pulsation of hydraulic pump, the pipe layout ensures that the fluid pipeline self-excited resonance, and the engine speed range 70%~100%, pulsation frequency of fluid pipe road 1~3 resonance frequency with hydraulic pump coupling does not occur he.

      3.Characteristics of slowly varying parameters of support structures

The key frequency of the pipeline system is basically staggered, which can ensure the long-term stable and reliable work of the hydraulic system. But the fault energy pipeline system still occur, most of which the cause of the malfunction of the natural frequency and the pulsation frequency of pipeline hydraulic pump coupling Co occur. In addition to the pipeline fracture, fixed structure tear and other forms of destruction, the pipe clamp contact surface has obvious wear, but the tube clamp does not reach the degree of fatigue failure. The pipe is clamped on the main structure of the aircraft through the clamping of the pipe clip, and the clamping relation between the pipe clamp and the pipe is the contact problem of the structure. The main characteristics of this kind of contact problems, a contact area is constantly changing, the relationship between the force and the displacement caused by external force is nonlinear; two is in the contact zone of stress is very high; the three is the structure parameters with slowly varying characteristics. The so-called slow varying parameters, refers to the cycle parameters and equations compared to change very slowly, which is analyzed in a short period of time, the parameters can be regarded as constants, surface analysis in a long time, to consider the significant change of parameters, as shown in Figure 3-6 by the pipeline and pipe contact structure clamp, pipe clamp in the body structure Bu, interacts with the pretightening force and the friction coefficient between the pipe clamp and the pipeline, the pipeline as a unit mass force, pipeline by preloading force, elastic deformation force and periodic force excitation effect, the power balance equation

       In the initial stage of aircraft equipment, contact structure in pre tightening force, contact area has no relative displacement in pipeline supporting fixed state ideal, pulsating pipe caused by the hydraulic pump pressure in the pipeline always encourage forced vibration. With the flight time increasing, the contact structure by tearing wear, even loose contact zone have small relative displacement, but did not cause direct damage, but the type (3-4) of K and N decrease, namely the structure deformation of elastic force and pretightening force reducing pipe clamp. This means that the stiffness of the supporting structure of the pipeline decreases, the support of the pipeline is not fixed, the lambda 1 in the formula (3-3) decreases, and then the F1 decreases. When F1 is reduced to FP in the close form (3-1), the fluid solid coupling vibration occurs. K, N, lambda 1 and F 1 are slowly varying parameters, and the slow changing parameter characteristic of the stiffness of the pipeline supporting structure is the real cause of the failure of the aircraft hydraulic energy pipeline and the related fixed structure.

       4.An engineering method for vibration analysis of fluid solid coupling

It is a practical engineering method to combine the theoretical analysis with the experimental test to design and analyze the hydraulic system of the aircraft. The hydraulic energy pipeline system is the weakest link in the aircraft hydraulic system. The following steps can be taken in the design and analysis:

1) according to the overall design requirements of aircraft and hydraulic system design specifications, materials with desired performance are selected, and pipeline layout is carried out.

2) according to the formula (3, 1) and formula (3, 2), the pulsation frequency of the hydraulic pump and the distribution parameters of the fluid pipeline are calculated.

3) the influence of the support structure on the lowest order natural frequency of the pipeline is experimentally tested. According to the formula (3-3), the lowest order natural frequency of the pipe under ideal supporting condition is calculated. Through the simulation of the wear process, several experimental curves with 1 preload changes are obtained. According to the strength of materials to determine the size of preload, and predict the maintenance life of the pipeline system. In the material treatment, attention should be paid to the same surface hardness of pipe clamp and pipe, which is a powerful guarantee to reduce the wear of contact structure.

4) analyze the key frequency range, adjust the relevant parameters, so that the key frequency is basically staggered.

5) and other components of aircraft ground simulator and test machine on the whole solid test, hydraulic system design finalized.