Structural filters preliminary design for the aeroservoelastic decoupling of a next generation civil tiltrotor technology demonstrator

Abstract

The paper discusses the methodology used to design the structural notch filters used to guarantee the aeroservoelastic (ASE) stability of the NGCTR-TD tiltrotor. The complex configuration of a tiltrotor aircraft from one side, and the fly-by-wire control architecture from the other side, imply that several structural elastic modes can be sensed and excited by the Flight Control System (FCS), which on the contrary is intended to affect only the rigid aircraft motion. This control spill over can potentially affect the stability properties of the aircraft and a set of notch filters is then introduced to remove any adverse ASE coupling. The paper describes the numerical model used to assist the design of the notch filters and the optimization procedure used to get a configuration that achieves the best trade-off between ensuring ASE robust stability and keeping adequate (FCS) performances, that are inevitably affected by the introduction of the notch filters. The ASE stability assessment and the notch filters design need to be performed as early as possible during aircraft development, in particular this activity should be performed in conjunction with FCS design to allow the best compromise between aircraft handling qualities and stability. For this reason the analysis and filter design methodology described here allowed the management of several design iterations and will also accommodate further design adjustments that will be performed after prototype ground and flight tests.