Nonlinear Stability Analysis of Tiltrotors in Transition

倾转旋翼构型飞机的实用性早在它被实现之前就被想象出来了。倾转旋翼机的设计不仅结合了两种关键能力——固定翼飞机的高巡航速度和直升机垂直起降的能力——而且关键是以一种功能有效的方式做到了这一点，即在控制系统的矢量控制下，两种飞行模式都使用相同的推进系统。随着理论和材料科学的进步，倾转旋翼设计最终成为现实，许多原型设计涌现出来，最终产生了V-22Osprey和AW609等可操作的例子。随着世界各地的机场越来越难以处理他们所要求的交通量，通过具有类似载客量和性能的倾转旋翼机将支线喷气机流量转移到现场直升机停机坪有很多好处，尽管这些设计比现有模型要大得多，而且由于许多问题现象的非线性，目前的设计工具可能无法扩展。其中一个问题是旋涡颤振，在向前飞行时，由旋翼产生的气动和陀螺力与弹性机翼模式相结合，使整个机舱围绕旋翼旋转轴呈锥形旋转。此外，结构和旋翼动力学在用于飞行控制设计的频率范围内有很大的影响，需要陷波滤波器来达到一级处理质量，并且还不清楚这个问题如何随尺寸缩放。另外的问题在于转子本身的可扩展性：由于刚度不随尺寸成比例地增加，预期大小的结构对动态非常敏感。此外，叶片可能发生很大的位移，因此非线性效应不可忽视。许多现有的模型，无论是现有的还是理论的，目前都被用于分析，分别是XV-15和NASA的大型民用倾转旋翼机的一个例子。在这些模型中，空气动力学、结构、材料和几何轴可以同时进行分析，使用的方法是真实的相互作用，将它们物理联系起来。在存在非线性（结构、材料、气动等）的情况下，旋转和静止结构之间的动力相互作用是一个复杂的问题。在旋翼飞行器中，这个问题仍然导致系统组件的模态特性、动态响应（振动和载荷）和稳定边界（叶片颤振、地面共振）的预测很差。传统上，为了研究耦合旋转-静止系统（如旋翼/机身）的稳定性，需要动力学方程来控制z:\cmxsj\外部资助\jesinforequest2012。耦合系统的行为，被转换为旋转或非旋转坐标系。然而，当非线性存在于旋翼水平（如阻尼器或后缘襟翼）或机身水平（如挂架刚度和阻尼）时，则不可能进行完全转换。尽管一些可用的数学技术利用一些假设来简化分析，但这些方法存在各种缺陷和局限性，并降低了所获得结果的准确性。最后，很少有文献涵盖旋涡颤振在悬停-巡航过渡的背景下。这主要是由于旋涡颤振只发生在高速下，尽管机舱旋转的陀螺效应和高速过渡的可能性使这成为一个重要的研究领域。因此，本项目将通过动力系统（分岔）理论研究倾转旋翼飞机发动机舱耦合旋转-静止系统的非线性动力学，重点研究悬停和前飞过渡阶段的旋涡颤振稳定性。

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.