Identification of Aeroelastic Parameters for Cormorant Tail Rotor Structures based on Vibration Measurement in the Stationary Frame

Abstract

The teetering tail rotor of the Canadian CH-149 Cormorant helicopter fleet experienced limit cycle oscillation (LCO) phenomenon in certain adverse flight conditions in which the transient high amplitude vibration resulted in significant increase of dynamic strain to the tail rotor structures. This could contribute to pre-mature damage of the composite half hub among other causes. Therefore it is required to evaluate the integrity of the tail rotor structure using realistic aeroelastic parameters of the tail rotor in the rotating frame. This paper presents a novel center frequency scaling factor theory and related parameter identification methodologies in order to estimate and track the variation of the critical aeroelastic parameters in the rotating frame during LCO events based on vibration information measured exclusively in the stationary frame. This information would enable update of the tail rotor stability diagram in order to confidently evaluate the impact of LCO events to the structural integrity of the Cormorant tail rotor structures. Based on the developed procedures and identified parameters, recommendations are provided for data analysis and techniques to improve the fidelity of results from aeroelastic simulation analysis for the Cormorant tail rotor structures without the need to install sensors in the rotating frame.