Nonlinear Stability Analysis of Tiltrotors in Transition

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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.