Nonlinear Stability Analysis of Tiltrotors in Transition

倾转旋翼飞机的用途早在实现之前就已经被想象出来了。倾转旋翼飞机的设计不仅结合了两个关键功能--固定翼飞机的高巡航速度和直升机垂直起飞和降落的能力--而且关键是以一种功能高效的方式做到了这一点，在这两种飞行状态下，都使用了相同的推进系统，由控制系统引导。随着理论和材料科学的进步最终使倾转旋翼机的设计成为现实，许多原型设计如雨后春笋般涌现，最终产生了V-22鱼鹰和AW609等作战实例。随着世界各地的机场越来越难以处理对它们提出的交通要求，通过类似载客量和性能的倾转旋翼将支线飞机交通转移到现场直升机停机坪可以获得很多好处，尽管这些设计比现有型号大得多，而且由于许多有问题的现象的非线性，目前的设计工具可能无法升级。一个这样的问题是涡动颤振，在前飞中，旋翼发动机产生的空气动力和陀螺力与弹性机翼模式耦合，使整个机舱围绕转子的旋转轴在圆锥形内旋转。此外，结构和转子动力学对用于飞行控制设计的频率范围有很大影响，需要陷波滤波器才能达到1级操纵质量，而且这个问题如何随尺寸变化尚不清楚。另外一个问题是转子本身的可伸缩性：由于刚度不随尺寸成比例增加，因此预期大小的结构对动态非常敏感。此外，叶片可能会发生很大的位移，因此非线性效应不能被忽视。一些现有的和理论上的模型目前正在用于分析，每个模型的一个例子分别是XV-15和NASA的大型民用倾斜器。在这些模型中，气动轴、结构轴、材料轴和几何轴可以用与物理上相互作用相一致的方法同时分析。在存在非线性(结构、材料、空气动力学等)的情况下，旋转结构和静止结构之间的动态相互作用。是一个复杂的问题。在旋翼飞行器中，这一问题仍然导致对系统部件的模态特性、它们的动态响应(振动和载荷)以及它们的稳定边界(叶片颤振、地面共振)的预测很差。传统上，为了研究旋转-静止耦合系统(如旋翼/机身)的稳定性，控制z：\cmxsj的动力学方程被变换到旋转或非旋转坐标系中。然而，当在旋翼水平(例如滞后阻尼器或后缘襟翼)或在机身水平(例如塔架刚度和阻尼)存在非线性时，完全变换是不可能的。虽然一些现有的数学方法利用了一些假设来简化分析，但这些方法存在各种缺陷和局限性，并降低了所获得的任何结果的准确性。最后，很少有文献涉及悬停-巡航转变过程中的涡动。这主要是因为只有在高速下才会出现涡动，尽管机舱旋转的陀螺效应和高速过渡的可能性使这一领域成为一个值得研究的重要领域。因此，本项目将利用动力系统(分叉)理论来研究倾转旋翼飞机发动机吊舱旋转-静止耦合系统的非线性动力学，重点研究悬停和前飞过渡阶段的涡振稳定性。

The usefulness of tiltrotor configuration aircraft was imagined long before it was realised.The tiltrotor design does not just marry two key capabilities - the high cruising speed of afixed wing aircraft with the helicopter's ability to take off and land vertically - but cruciallydoes so in a functionally efficient way, where the same propulsion systems are used for bothflight regimes, being vectored by a control system. As the improvement of theory andmaterial science eventually allowed tiltrotor designs to become a reality, a number ofprototype designs sprang up, eventually yielding operational examples such as the V-22Osprey and the AW609. With airports worldwide struggling more and more to handle thetraffic that is demanded of them, there is a lot to be gained from moving regional jet traffic toon-site helipads via tiltrotors of similar passenger capacity and performance, although thesedesigns are considerably larger than existing models and current design tools might not bescalable due to nonlinearity of many of the problematic phenomena. One such problem iswhirl flutter, where, in forward flight, aerodynamic and gyroscopic forces generated by arotor couple with elastic wing modes to make the entire nacelle whirl in a cone around itsrotor's axis of rotation. Additionally structural and rotor dynamics have large effects in thefrequency ranges used for flight control designs, necessitating notch filters to achieve Level1 handling qualities, and it is yet to become clear how this issue scales with size. Additionalissues lie with the scalability of the rotors themselves: since stiffness does not increaseproportionally with size, structures of the intended magnitude are severely dynamicallysensitive. Moreover, very large displacements of the blades may occur, and for this reasonnonlinear effects cannot be ignored.A number of existing models, both existing and theoretical, are currently being used foranalysis, an example of each being the XV-15 and NASA's Large Civil Tiltrotor,respectively. Within these models, the aerodynamic, structural, material and geometric axescan be analysed simultaneously using methods that are true to the interactions that link themphysically.The dynamic interaction between rotating and stationary structures in the presence ofnonlinearities (structural, material, aerodynamic, etc.) is a complex problem. In rotorcraft,this problem still leads to poor predictions of the modal characteristic of the systemcomponents, their dynamic response (vibration and loads) and their stability boundaries(blade flutter, ground resonance). Traditionally, to investigate the stability of coupledrotating-stationary systems (e.g. rotor/fuselage) the dynamical equations, governing the z:\cmxsj\outside funded\jesinforequest2012.docbehaviour of the coupled system, are transformed to either a rotating or non-rotatingcoordinate system. However, when nonlinearities exist either at the rotor level (e.g. lagdampers or trailing edge flaps) or at the airframe level (e.g. pylon stiffness and damping)then a full transformation is not possible. Although some available mathematical techniquesmake use of a number of assumptions to simplify the analysis, these methods suffer fromvarious shortfalls and limitations and degrade the accuracy of any results achieved.Lastly, very little literature covers whirl flutter in the context of hover-cruise transition. Thisis mainly due to whirl flutter occurring only at high speed, although the gyroscopic effects ofnacelle rotation and the possibility of high speed transition make this an important area tolook into. This project will therefore investigate the nonlinear dynamics of the coupledrotating-stationary system of the tiltrotor aircraft engine nacelle through dynamical system(bifurcation) theory, focusing on whirl flutter stability during the transition phase of flightbetween hovering and forward flight.