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Dynamic response analysis process of helicopter improvement

for many industrial fields since the first half of this year, such as aerospace, automobile, machinery and electronics, structural dynamic simulation is an important part of the product development process. Nx NASTRAN batch solver can complete typical dynamic analysis. Nastran is famous for its accuracy and extensive dynamic solution functions. It is the standard of dynamic analysis. However, these solutions are non interactive, require input files and professional knowledge, so they can only be used by finite element analysis experts. Because almost all dynamic simulations require additional pre - and post-processing tools to interpret the calculation results, the solution process is more complex

nx response simulation is a new generation of integrated simulation tool, which makes dynamic analysis easier and more efficient. Based on nx pre/post structure, it uses graphical and interactive user environment to drive NX NASTRAN solver for dynamic simulation. Through the close integration of advanced function processing tools and NX NASTRAN, NX response simulation can perform all dynamic analysis in a single environment. The graphical and interactive environment can enable analysts to understand the dynamic behavior of the involved structure more deeply, and then improve the product design process more obviously

this paper describes the function of NX response simulation through helicopter dynamics simulation. The helicopter is subjected to many types of dynamic loads, including harmonic vibration of the rotor, pressure fluctuation caused by turbulence, unbalanced load and transient vibration caused by landing and payload. Many loads are periodic and occur in the form of resonance of the rotor system. Other loads are random, and some are completely transient or impulsive. These loads often occur at the same time, so the analysis needs to evaluate the combined effects of these loads. Nx response simulation supports complex interaction of different dynamic loads and can be used for all forced response analysis and function processing processes. This paper describes these technologies through helicopter systems and subsystems

1: introduction

many companies develop products that need to simulate the dynamic performance of products to verify the service performance and survival performance of products. One of the great challenges is the efficient simulation process, which can make their simulation results quickly and effectively aided design decisions. At present, all dynamic simulation processes include the use of a variety of software tools. The core of all processes is the pre/post processor and the batch finite element solver NASTRAN

nx response simulation is a new generation of integrated tool, which makes dynamic analysis easier and more efficient. Based on nx pre/post and 15 years of application experience in I-DEAS response analysis, NX response simulation uses NX NASTRAN solver in NX graphical interactive environment

interaction is the key to improve efficiency, which is realized through effective use of modal analysis. In modal analysis, the system mode is calculated first, and then the forced response is calculated by using the system mode. The modal analysis step is a typical batch process, which usually takes several minutes to several hours to complete. After obtaining the modal, the forced response analysis can be completed in a few seconds, so it is suitable for interactive processing. Nx response s in people's daily life, the simulation can use the modes solved by NX NASTRAN to effectively and interactively calculate the forced response

nx response simulation has an integrated function toolkit that can plot and process function data. Users can define loads graphically and perform mathematical operations, such as complex leaf transformation, interpolation, mathematical combination of multiple functions, and so on. It can also import the test data in the standard test data format as the input of the forced response simulation, or compare with the simulation results

2: helicopter dynamics application

analysis engineers are concerned about the vibration level of the structure under these loads, because these loads will affect the operational performance and durability of aircraft structures

occurs when the rotor speed is one or two times of the vertical and lateral load, which is caused by the unbalanced force of the rotor and the lift difference between the rotor blades. Higher frequency loads occur at multiple blade passing frequencies. For example, for a double blade system, the excitation of 4 and 6 times the rotor speed is the most critical. Other loads are random, such as engine vibration and vibration caused by aerodynamic loads

load definition comes from actual flight measurement or multi-year flight test manual. Mil-std-810f provides a large number of acceptable data for helicopter vibration levels, which are often cited as the necessary environment for helicopter components or payloads. This load definition allows the subsystem to be analyzed independently of the entire system. This can improve the analysis efficiency. The vibration tolerance index in mil-std-810f includes the load effects described above, so it includes the definitions of harmonic and random acceleration loads

figure 1: mil-std-810f helicopter vibration exposure (fig 514.5c-10) provides insight into helicopter operating vibration environments

complete helicopter system analysis is used to evaluate the response of rotor and frame structures. Mil-std-810f is used here to predict the load of helicopter system

the actual flight data is composed of periodic and random loads, which is called sine on random. The assumptions of random vibration analysis and harmonic response analysis are different. Therefore, it is not appropriate to use the combined data for frequency domain analysis of random response or harmonic response. The common method is to decompose the pure sinusoidal load, which is still at the level of foreign countries in the early 1990s, from the random vibration load, so that the separated load can be used for frequency domain simulation of response. This is the method recommended by mil-std-810f. They provide a means of load separation. Sinusoidal and random analysis responses, such as stresses, can be superimposed. The NX response simulation tool provides this method

nx function tool can easily generate PSD, sine or pulse load expressions. The same tool can also transform the actual flight time history into any number of frequency domain forms, such as PSD, harmonic or vibration response spectrum

transient flight loads can be expressed as sinusoidal loads and random loads

Figure 2 shows the modal of the helicopter system used for simulation. The illustration is an instrument panel for vibration withstand load analysis in mil-std-810f. The purpose is to predict whether the acceleration response level of the instrument panel during helicopter operation is within the test range

additional analysis is used to predict the response of random aerodynamic loads and harmonic non-equilibrium loads. The response of each load is first analyzed separately. Then the sine on random responses in time domain are solved, and their combined effects are predicted

figure 2: helicopter system and instrument panel subsystem fine element models used for example results

step 1: use NX NASTRAN to solve the system mode. (SOL 103) solve both constrained and free modes

calculate two static modes. One is static constrained mode, and the other is additional mode. Additional modes are used for modal acceleration calculation. It can give more accurate results, such as stress or strain

to calculate the static mode, the user needs to define the basic motion DOF for the constrained mode and apply the unit load for the calculation of additional modes. Static mode load conditions can be set completely in NX pre/post environment

after modal calculation, NX response simulation can reference these modes by attaching op2 files. User defined dynamic events such as transient, frequency, random PSD, or response spectrum. The function toolkit allows users to create load functions. The response can be calculated anywhere in the model and can be displayed by post-processing

the foundation of the instrument panel is connected through a rigid unit, and its center point is the input position of the foundation acceleration

the modal solution of the instrument panel consists of four regular modes. The first-order mode is the lateral bending mode of 44hz, up to 500Hz. The three constrained modes of the unit deformation of the center point of the rigid body element are calculated, so the response of the foundation can be predicted in the three translational directions

mil-std-810f defines that harmonic load comes from main rotor, tail rotor and drive system. The position of the subsystem determines which of the above loads and the resonant frequency of the impact specimen notch special broaching machine used to process the specimen used for the experiment is dominant. The magnitude of harmonics depends on the type of subsystem being analyzed. For the instrument panel, the main rotor input is dominant, so the frequency based on the main rotor is selected. For a two blade system, the main rotor frequencies used for analysis are 1p, 2p, 4P and 6p times the rotor frequency, typically 6Hz. Thus, the frequency of the main resonant load is 6, 12, 24, 36Hz. The frequency tolerance of the main rotor is ± 5%

since the load spans all frequencies, the analysis process analyzes the frequency response of the structure by applying an envelope. The function process can filter the response according to the rotor period. Figure 3 shows the lateral response at the top of the panel. The thick dotted line is the envelope response. The responses of all 2n times the harmonic frequency are plotted within 500Hz. Usually only 6 times less frequency is the range of concern

1 and 2 times response amplification are very small, and 4 and 6 times amplification are very large. At 6 times frequency, it is about 5g. By plotting the envelope and harmonic response, the influence of arbitrary variation of rotor frequency on the response can be obtained

figure 3: instrument panel lateral vibration from harmonic loads showing discrete sine tone responses from filtering of enveloped response

the instrument panel shall also be subject to random excitation evaluation. Figure 4 shows the lateral PSD response under basic input conditions. The RMS lateral response is 1.92g and the input is 1.79g. Assuming that the typical maximum response is 3 times the standard deviation, the maximum acceleration will be close to 6G. This will approach the harmonic response

figure 4: instrument panel lateral vibration from random loads is regulated by primary mode at 44 hz

the response of helicopter system caused by random aerodynamics and harmonic rotor vibration is calculated. The analysis result of helicopter under free condition is equivalent to the response under flight condition. The canonical mode of the system is solved to 500Hz. The additional modes at the loading position are calculated. The dynamic stress level of the rotor connection system is studied

since the load defined in mil-std-810f is a forced foundation acceleration load and the helicopter is a free condition, this load cannot be directly applied. However, the load can be derived by applying a forced load and scaling the load to a value close to the acceleration specified in the standard

for example, assuming that the distributed aerodynamic load is at the center of gravity of the aircraft, the PSD load of the force will be applied near this position. The PSD load of the force is given according to the PSD of the acceleration in the same frequency range and shape. The load is then proportionally increased to the predicted aeronautical structure acceleration PSD. x. The loads in Y and Z directions are applied at the same time. Figure 5 shows the comparison results between the acceleration spectrum of aircraft center position and that of mil-std-810f. If the comparison results are close, it indicates that the applied load is reasonable

in order to find out the position of the maximum stress in the connection system under random load, the von Mises root mean square stress within this range is calculated. Figure 6 shows the location of the maximum stress. Nx response simulation can directly calculate von Mises stress of random analysis

figure 5: predicted acceleration PSD is repr

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