Tiltrotor Stability Using Coupled Models

Abstract

This work investigates determining the stability of tiltrotor aircraft through coupled models between flight mechanics and aerodynamics. Because of the configuration of tiltrotor aircraft, there are complex interactions of the wake with the aircraft surfaces, the effects of which are difficult to capture in the classical methods used to analyse aircraft stability. These classical methods are well established for fixed-wing aircraft, and have been extended for application to helicopters. However, due to the wake interaction it is likely that the assumption that the response of the aircraft can be modelled by the terms of a Taylor series is not sufficient at capturing the dynamic behaviour. The flight mechanics and aerodynamics coupled model developed in this thesis allows for the full wake dynamics to be retained in the stability analysis.

The aerodynamics of the tiltrotor has been simulated using the Unsteady Vortex Lattice Method (UVLM), with a free-wake approach to modelling the vorticity in the fluid computational domain. However, the main results of this research are independant of the solver used. Improvements to the UVLM include a particle deflection scheme and viscous corrections to improve the physics
of the aerodynamics, as well the fast multipole method to improve the computational cost.

Reduced Order Models (ROMs) are constructed from the UVLM to generate the model of the aerodynamics, which when coupled with the flight mechanics produces a coupled model that has an appropriately small size, whilst retaining the dominant dynamics of the system. Tiltrotor aircraft in hover and horizontal flight produce a periodic system, therefore a periodic ROM method is introduced to determine the applicability of a time-averaged approximation to the periodic system using a well established ROM method (Eigensystem Realisation Algorithm). A further ROM methodology is introduced in an attempt to improve convergence of the system behaviour at lower model sizes due to difficulties found in finding consistent stability behaviour as the ROM size was changed. It has been found that the time-averaged approximation correlates with the periodic system well. A significantly greater degree of instability has been found when the stability is determined using the coupled model rather than the classical method, indicating that the classical approach must remove important dynamics.

Date of Award	13 May 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Dorian P Jones (Supervisor) & Ann L Gaitonde (Supervisor)