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The development of civil tilt rotor aircraft over the past decade has brought with it new possibilities in terms of mission flexibility and operational capability for both the search and rescue and civil transport roles. However, in their unaugmented state, tilt rotor aircraft exhibit rather poor handling qualities. This research presents potential solutions to these deficiencies. The work has been conducted using simulation models of three differently sized tilt rotor aircraft. Predictive analysis combined with real-time, piloted simulation of the models has allowed detailed assessments of the handling qualities to be conducted. The three models represent the Bell/NASA XV-iS and the Eurocopter Eurotilt and Eurofar design concepts. A full analysis of the unaugmented handling qualities of the aircraft was conducted to inform the subsequent control design. Deficiencies such as poor agility, especially in the roll and yaw axes; low bandwidth; pitch instability at low speeds and a pitch 'dropback' phenomenon were identified as contributing to piloted ratings generally within the Level2/Level 3 region. An investigation of methods by which performance across the whole tilt rotor flight envelope could be improved was conducted. It was determined that, if optimised performance was to be achieved, a full authority controller would be required, especially with a large aircraft such as Eurofar that exhibits very poor agility. 'Unified handling qualities' controllers have been developed for helicopter mode and for the conversion mode, offering identical responses in each model. 'Model following' processes have allowed the desired, Levell, handling qua.ities to be specified. In both cases,the controller was capable of achieving the specified handling qualities for all three aircraft. Piloted simulation confirmed that, in most cases,the handling qualities were as expected. In the conversion mode, a H«J synthesis process was adopted that, in addition to proving the required improvement in handling qualities, simultaneously allowed the controller to act to suppressstructural loads. Operations in a degraded visual environment typically require an attitude response type rather than a rate response type. A controller was therefore developed for Eurofar that conferred an attitude command response, and, taking the level of stabilisation one stage further, a translational rate command response. It is shown that the required level of stability can be achieved through the use of a partial authority controller, but that the improvement in agility is limited, as would be expected. Piloted simulation identified that generally good handling qualities have been achieved for low aggression tasks, with low workload even in severely degraded visual environments. Even with an augmented aircraft, the conversion and reconversion processes remain demanding. This has been addressed through the development of an automatic system that controls the nacelle angle so as to maintain the aircraft at the centre of the conversion corridor. This is complemented by a tracking system to guide the aircraft towards its destination, and so allows a completely automated approach and reconversion as the aircraft returns to its base. An investigation of flight path control of the XV-iS in the conversion mode using H«J synthesis revealed possibilities for simultaneously offering airspeed and flight path response types in the longitudinal/heave axes, despite the inherent cross-coupling difficulties associated with this regime. Overall, tilt rotor handling qualities have been improved across the flight envelope, the aircraft being improved to the extent that Level 1 performance has been attained for the vast majority of low to moderate aggression test manoeuvres in both good and poor visual environments. For more aggressive tasks, the achieved performance is somewhat less satisfactory, although this is primarily limited by the inherent poor agility of the aircraft conferred by their. configuration. |
