The Dynamic Behaviour Analysis Of Helicopter In Case Of Dynamic Load

The vibration level of helicopter body is an important index to estimate the performance of the helicopter, and the fierce vibration will strictly influence the helicopter's functions, safety and longevity. This thesis has studied the vibration frequency and mode shape of the airframe in condition of the change of location and mass as well as the excitation of other exterior load. The content and conclusion are as follows:First, we choose the way of calculating the eigenvalue and eigenvector (that is Lanczos standard and SINV), choose the materials used in various parts of the helicopter, and confirm the parameters of the materials.Second, we establish the FEM model of the airframe with the software of finite element analysis MSC.Nastran and MSC.Patran, then pick up the natural frequency of the mainframe'first 15 orders and analyze the vibration mode shape of each step to get the following conclusions:The first mode shape is that the whole airframe makes rigid retortion vibration circling the horizontal axis and the greatest swing occurs at the front upper part of the empennage;The mode shape of the second,fourth,eighth,eleventh and thirteenth order is that the whole airframe makes flexural vibration circling the vertical axis and the greatest swing occurs at the parts of the horizontal surface of the empennage and the empennage;The mode shape of the third,seventh,tenth and fifth order is that the whole airframe makes flexural vibration circling the vertical axis against the horizontal level with the greatest swing in the empennage;The mode shape of the fifth,sixth,ninth,twelfth and fourteenth order is complicated, and the airframe makes both rigid retortion vibration circling the horizontal axis and flexural vibration circling the vertical axis with the greatest swing in the middle and rear part of the engine room and the top part of the empennage.This 15-order mode within the scope of 15～65 Hz is a dangerous area for the helicopter. The excitation for this frequency segment will cause the fierce vibration of the airframe, and influence the stability of flying, at last will result the destruction by fatigue.Finally, we make simulating computing for the load on countershaft,side girder and axes symmetry respectively. The results show that the vibration frequency of airframe will decrease a little when the axes are loaded with excitation. The vibration frequency of the airframe changes with the different location where the excitation loads; the vibration frequency increase a little when the mass is located in the middle of the engine room. When the side girder is loaded with excitation, the structure of the airframe is anisomerous. The effect of moving mass load excitation for the vibration frequency of airframe is bigger than that of axes, especially for the low step frequency. The rule of change is the same as the loading condition for the axes. The axes symmetry mass excitation can reduce the vibration frequency of the airframe, but by a little margin, so it doesn't have much influence for the stability and security of the FEM modal. The effect of mass load in the engine room for the vibration frequency of unsymmetrical airframe is bigger than symmetrical airframe,the airframe is so unsteady that it is prone to occur accident,Therefore,we should avoid to design the unsymmetrical airframe.The analysis results for the natural frequency and mode shape of the helicopter'framework show that some parts of the framework will make great swing and partial resonance under some frequency, which means the harmony of the framework stiffness is imperfect, so we need to further modify the parameter and improve the modal. Besides, we need to undertake actual test for the structure of the airframe. The analysis results of this thesis provide experience for the dynamics analysis of the airframe structure, and provide conference for the distribution of the testing points when undertaking actual test for the helicopter.