Control loads reduction through control system architecture optimization – application to a conventional rotor on compound helicopters

Abstract

A kinematic study of a helicopter main rotor control system is carried out to investigate loads in servo actuators and non-rotating scissors during high speed and high load factors maneuvers. The kinematic model is then used to optimize the servo-actuators placement and pre-inclination in order to minimize static and dynamic loads in the three servo-actuators and in the non-rotating scissors. The inputs for the model (blade pitch link loads and pilot input to trim the aircraft) are taken from flight tests measurements, current rotor computations being unable to predict blade root torsion moments vs azimuth with enough accuracy. The analysis is based on X3 demonstrator flight tests, which showed high control system loads that used to reduce flight envelope during the first flight test campaign. Flight tests measurements are used to validate the kinematic model used for the optimization. Computations made for X3 case at 220kts showed a reduction of 40% of maximum static load and 45% of maximum dynamic load on servo-actuators compared to the initial placement of the servo actuators. With appropriate servo actuators pre-inclination, dynamic loads in the non-rotating scissors are decreased by 95% at high speed trim flight. This paper shows how it is possible to keep a conventional rotor control system for compound helicopters. The optimization algorithm presented in this paper can be used for conventional helicopters to reduce loads in the control system and then limit command reinjection because of control system flexibility, and on compound helicopters to expand the flight envelope and to remove control system loads as the first limit factors at high speed.