Research On Nonlinear Dynamics Of Rotor Systems In Manuevering Flight Environment

The maneuvering flight of aircraft refers to the flight process during which the flight parameters of the aircraft(including velocity, acceleration, angular velocity and angular acceleration) are changing. Over the years, many fighter planes crashed accidentally in maneuvering flights. In most cases the accidents were attributed to mechanical failures of the aircraft engine, which are generally caused by vibration problems of the rotor systems. To the rotor system of the engine, the maneuvering flight of the aircraft makes extremely complicated operating environment, which may destroy the security and stability of the system. Therefore, it is very urgent to carry out the investigation of rotor dynamics in maneuvering flights.In recent years, some researches on the dynamic characteristics of aircraft rotor systems in maneuvering flights, especially on the nonlinear dynamics, have been carried out. In most of these studies, only numerical methods were used, but the deeply theoretical study works were rarely seen. In this subject, however, both the numerical methods and the theoretical methods are used to detect the nonlinear dynamics of rotor systems in maneuvering flight environment. On the one hand, discover some special nonlinear phenomena of different rotor systems occu rring only in maneuvering flight environment. On the one hand, reveal the mechanisms of the found nonlinear phenomena. The primary obtained results are as follows:Firstly, taking a nonlinear rotor system with cubic stiffness as an example, the modeling method of rotor system in maneuvering flight environment is presented. The motion equations of the system are formulated by using Lagrange equation, in which, the effects of maneuvering flight are formed by the flight parameters of the aircraft, namely the maneuver load. Then taking Herbst maneuver as an example, the dynamic responses of the rotor system in real maneuvering flight environment are simulated in real-time, in order to test the formulated system model. It is found that the maneuver load may lead to the vibration jump of the rotor system, which is harmful to the security and stability of the system. Since the real maneuvering flight test results are confidential data, it is almost impossible to carry out the experimental studies on the rotor dynamics in maneuvering flight environment. But we have proposed the dynamic simulation strategy by introducing the simulated flight parameters obtained by using some flight control method, and this is very meaningful to carry out the virtual experiments on the rotor dynamics in maneuvering flight environment.Secondly, the vibration response characteristics and the local bifurcation characteristics of the nonlinear rotor system are investigated, in order to reveal the mechanism of the vibration jump phenomenon induced by the maneuvering flight. It is found that the maneuver load makes “enhanced stiffness” effects to the nonlinear rotor system. As a result, the critical rotation speed increases, and the amplitude-frequency response curve moves to the right, as well as the r esonant hysteresis region narrows. Suppose that only the horizontal direction has the maneuver load, then the “enhanced stiffness” effects in the horizontal direction is more significant than that in the vertical direction. Therefore, the r esonant hysteresis regions of the responses for the two directions will be separated from each other when they are moving to the right, and they will disappear one after another. Accordingly, one double limit set line and two hysteresis set lines are derived in the parameter plane of maneuver load-rotor eccentricity. The parameter plane is separated into four regions by the three lines, and it indicates that there are four types of bifurcation modes in the system. In the bifurcation analysis, the two state variable singularity method recently proposed in employed, since the general single state variable singularity method can’t deal with the present problem. The obtained results show the advantages of the two state variable singularity method, and generate a great engineering application example for the method.Thirdly, the bifurcation behaviors of the nonlinear rotor system in turni ng flight are investigated, and it is found that the maneuver load may induce the occurrence of the 1/2 sub-harmonic resonance of the system, and then lead to the rub-impact of the rotor and the stator, namely sub-harmonic resonance rub-impact phenomenon. By using the multiple scales method to solve the motion equations of the system, the mechanism of the 1/2 sub-harmonic resonance is detected. It is shown that the solutions for the sub-harmonic resonance exist only when the maneuver load increases across a critical value. As the maneuver load continue to increase, the rotation speed region with respect to the sub-harmonic resonance solutions will widen. Moreover, the bifurcation behaviors of a rotor-ball bearing system in turning flight are investigated as well. In this system, there also exists the sub-harmonic resonance phenomenon. Generally, the rub-impact is considered as a damaged type of failure caused by the structural damage induced large rotor unbalance. In this subject, however, the presented sub-harmonic resonance rub-impact phenomenon is an environmental fault instead of a damaged type of failure. So this is a new type of rub-impact and is a special one, which should be paid significant attention to.Finally, considering the climbing and diving flight environment, the dynamics of the Jeffcott rotor system, the nonlinear rotor system with cubic stiffness and the rotor-ball bearing system with a shaft crack fault are all investigated. It is found that the maneuver load can amplify the super-harmonic responses of cracked rotor systems significantly. Thus a new idea for early detection of rotor shaft crack faults is proposed, that is providing p roper environmental excitations to amplify the weak signals of crack faults, in order to make it more easier to detect them.