(Dr. Aldo Ferri, advisor)

"An Aeroelastic Study of the Conversion Maneuver of Tiltrotor Aircraft"

Abstract

Tiltrotor aircraft such as the XV15 have been a subject of considerable interest because of its ability to combine vertical take-off and landing capability with efficient long-range and high-speed cruise flight capability. The conversion maneuver is one of the most important dynamic and aeroelastic issues of a tiltrotor configuration. In this research, first, a simple model for the system nacelle-rotor combined with thin airfoil 2D aerodynamic theory is studied. The dynamic and aerodynamic derivation as well as the linearization of equations of motion is performed. The state space model is simulated to obtain the response quantities of interest including deflections, rotations, moments, vibratory loads and aerodynamic loads. Second, a numerical analysis using the DYMORE computer code is undertaken. This code is a finite element based tool for the analysis of nonlinear elastic multibody systems. The nonlinear equations of motion are solved with a robust time integrating scheme. Different multibody, flexible models including the wing, the shaft, the hub, the rotor and the joints are studied. Four varieties of rotor models are examined: non-gimbaled and gimbaled rigid disk and non-gimbaled and gimbaled fully articulated blades. Several simulation cases are analyzed to determine the trends of natural frequencies and the system stability as well as the dynamic (in vacuum) and aeroelastic (in air) responses of the system during conversion. Finally, a correlation between the simple model results and the more comprehensive multibody flexible models ones is established.

One of the key findings of this research is that a tiltrotor experiences relatively high frequency and high amplitude vibrations during conversion compared with hover and cruise modes. Furthermore, good qualitative agreement is shown between the dynamic results of the simple model and the multibody ones. However, discrepancies are noticed when aerodynamic loads are included. The absence of wing airloads in the simple model and the different aerodynamic theories used for each case are the main sources of these discrepancies.