Nonlinear Stability Analysis of Tiltrotors in Transition

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 a
fixed wing aircraft with the helicopter's ability to take off and land vertically - but crucially
does so in a functionally efficient way, where the same propulsion systems are used for both
flight regimes, being vectored by a control system. As the improvement of theory and
material science eventually allowed tiltrotor designs to become a reality, a number of
prototype designs sprang up, eventually yielding operational examples such as the V-22
Osprey and the AW609. With airports worldwide struggling more and more to handle the
traffic that is demanded of them, there is a lot to be gained from moving regional jet traffic to
on-site helipads via tiltrotors of similar passenger capacity and performance, although these
designs are considerably larger than existing models and current design tools might not be
scalable due to nonlinearity of many of the problematic phenomena. One such problem is
whirl flutter, where, in forward flight, aerodynamic and gyroscopic forces generated by a
rotor couple with elastic wing modes to make the entire nacelle whirl in a cone around its
rotor's axis of rotation. Additionally structural and rotor dynamics have large effects in the
frequency ranges used for flight control designs, necessitating notch filters to achieve Level
1 handling qualities, and it is yet to become clear how this issue scales with size. Additional
issues lie with the scalability of the rotors themselves: since stiffness does not increase
proportionally with size, structures of the intended magnitude are severely dynamically
sensitive. Moreover, very large displacements of the blades may occur, and for this reason
nonlinear effects cannot be ignored.
A number of existing models, both existing and theoretical, are currently being used for
analysis, an example of each being the XV-15 and NASA's Large Civil Tiltrotor,
respectively. Within these models, the aerodynamic, structural, material and geometric axes
can be analysed simultaneously using methods that are true to the interactions that link them
physically.
The dynamic interaction between rotating and stationary structures in the presence of
nonlinearities (structural, material, aerodynamic, etc.) is a complex problem. In rotorcraft,
this problem still leads to poor predictions of the modal characteristic of the system
components, their dynamic response (vibration and loads) and their stability boundaries
(blade flutter, ground resonance). Traditionally, to investigate the stability of coupled
rotating-stationary systems (e.g. rotor/fuselage) the dynamical equations, governing the
z:\cmxsj\outside funded\jesinforequest2012.doc
behaviour of the coupled system, are transformed to either a rotating or non-rotating
coordinate system. However, when nonlinearities exist either at the rotor level (e.g. lag
dampers 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 techniques
make use of a number of assumptions to simplify the analysis, these methods suffer from
various 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. This
is mainly due to whirl flutter occurring only at high speed, although the gyroscopic effects of
nacelle rotation and the possibility of high speed transition make this an important area to
look into. This project will therefore investigate the nonlinear dynamics of the coupled
rotating-stationary system of the tiltrotor aircraft engine nacelle through dynamical system
(bifurcation) theory, focusing on whirl flutter stability during the transition phase of flight
between hovering and forward flight.