The Effects and Analysis of Backlash on Rotorcraft Torsional Stability

This paper discusses a means of analyzing the torsional stability of a closed loop rotorcraft propulsion system consisting of the rotor drive train, the turboshaft engine(s) and their fuel control system(s). Specifically the analysis of the rotor speed control loop at low power conditions during which mechanical components (i.e. gears, splines, etc.) can oscillate across gaps, or backlash, is discussed. Linearized rotorcraft propulsion system models have been widely used in the past for stability analyses and the associated design of propulsion governing systems. However, with the advent of electronic control systems the need to incorporate propulsion system non-linearities has become more prevalent due to the quicker response time of the fuel control systems as compared to the highly damped analog systems. The dynamic characteristics of backlashing systems can be significantly different from the non-backlashing linear system. Thus, if these non-linear effects are unaccounted for in the design of the fuel control system, torsional stability can be compromised at such conditions. This phenomenon has been observed on several rotorcraft equipped with electronic fuel control systems. Thus, the means to incorporate the associated non-linearities into models of propulsion systems has been developed and implemented into the analysis of a particular rotorcraft. The revised propulsion system model has been used to fully evaluate design changes for the fuel control system, allowing torsional stability design criteria to be satisfied for all operating conditions. This methodology can now be used in the development of new rotorcraft designs thereby eliminating a potential time consuming and costly control system redesign.