Investigation of Drive Train Coupled Torsional Stability Analysis Methodology for Tiltrotor: the Helicopter Mode case

Coupled powerplant and rotorcraft flight dynamics simulations are commonly carried out in the non-linear time-domain framework (e.g. for pilot-in-the-loop handling qualities assessments), although these integrated models are generally not fully accurate from drivetrain dynamics perspective. Nevertheless, there is interest to verify that usual assumption of decoupled torsional stability (including rigid drivetrain analysis) and aircraft rigid body stability is valid, and up to what extent. The process described in the paper entails the automatic assembly of relevant subsystems (bare aircraft flight dynamics, Flight Control System including fly-by-wire actuation, sensors, and Control Laws software, drivetrain dynamics, powerplant dynamics) state space matrices through a Company developed Matlab toolbox. The proposed approach is control system design oriented, i.e. it does not require detailed flexible multibody modelling of the entire aircraft including dynamic systems and it is a natural extension of the process generally carried out by Control Laws and Flight Mechanics discipline for rigid-body stability assessment. The paper specifically addresses, as case study, the coupled torsional dynamics analysis of Next Generation Civil Tiltrotor Technology Demonstrator (NGCTR-TD) in Helicopter configuration. The NGCTR-TD project is developed under the EU Clean Sky 2 program for defining novel technology for future civil Tiltrotor platforms. The proposed coupled torsional stability modelling process is described, and the main outcomes valid for NGCTR-TD trimmed in hover and near-hover conditions presented. It is shown that the proposed methodology provides consistent outputs against the original NGCTR-TD linear stability results (from individual flight dynamics and drive system models). Analysis of Modal Participation Factors is used to quantify the coupling existing between drive system and flight dynamics state vectors, in nominal and failure conditions (Inter Connect Drive Shaft failure).