Load Alleviation in Tilt Rotor Aircraft Through Active Control; Modelling And Control Concepts

This paper presents the first results from research into active control of structural load alleviation (SLA) for Tiltrotor aircraft carried out in the European ‘critical technology’ RHILP project. The importance of and the need for SLA in Tiltrotors is discussed, drawing on US experience reported in the open literature. The paper addresses the modelling aspects in some detail; hence forming the foundation for both the FLIGHTLAB simulated XV-15 and EUROTILT configurations. The primary focus of attention is the suppression of in-plane rotor yoke loads for pitch manoeuvres in airplane mode; without suppression these loads would result in a very high level of fatigue damage. Multi-variable control law design methods are used to develop controller schemes and load suppression of 80-90% is demonstrated using rotor cyclic control, albeit at a 20-30% performance penalty. However, rotor flapping transients tend to increase by the action of the SLA system. A dual-objective control design approach demonstrates the effectiveness of suppressing both loads and flapping simultaneously.