Helicopter drive train modelling for manoeuvre load alleviation

Abstract

Helicopters are complex and expensive aircraft with a level of technology that is immature compared to fixed- wing counterparts. Helicopters suffer from vibratory loads stemming from the main rotor and exhibit control and stability problems in the low-speed flight regime. Operating a helicopter near the limits of its flight envelope may result in unacceptable high structural loads which adversely affects wear and tear of drive train components. Conservative safety regulations lead to high operating cost where a significant part can be attributed to maintenance. Manufacturers and operators therefore strive to make helicopters more capable and reliable in an effort to reduce operating cost. Structural load alleviation offers an attractive option to achieve this goal by reducing component damage accumulation and subsequent required maintenance.

This thesis investigates structural load alleviation in the tail rotor drive train of the UH-60 Black Hawk. The Black Hawk provides a compelling case for load alleviation research because of its ever growing operational weight and resulting increase of drive train load levels. Furthermore, the lifetime of the UH-60 is to be extended so that it will fly for many years to come. Current research and applications of rotorcraft structural load alleviation focus on the main rotor but less attention is given to tail rotor drive train components. This project seeks to address this knowledge gap by investigating manoeuvres that result in critical dynamic loads in the UH-60 tail rotor drive train. A survey of pertinent literature and interviews with helicopter pilots indicate that pedal inputs for left-hand turns in hover lead to high dynamic loads in the UH-60 tail rotor drive train.

A flight simulation model is constructed that offers the novel capability to predict dynamic loads in tail rotor drive shafts. This model consists of an available high fidelity engine model and existing rotor models coupled by a multi body dynamics tail rotor drive train model with properties that are based on component measurements and CAD drawings. Experiments are conducted to determine the relation between manoeuvre aggressiveness and dynamic loads in tail rotor drive shafts. Based on the results a manoeuvre load alleviation control strategy is devised to reduce dynamic loads while ensuring applicable Level 1 handling quality requirements. Application of this control strategy will decrease dynamic loads during left-hand yaw manoeuvres in hover. Furthermore, the results highlight what reduction in loads can be achieved for varying levels of manoeuvre aggressiveness. These findings may aid in the design of flight control systems that incorporate tail rotor drive train load alleviation objectives.