CFD/CSD STUDY OF INTERACTIONAL AERODYNAMICS OF A COAXIAL COMPOUND HELICOPTER IN HIGH-SPEED FORWARD FLIGHT

Abstract

This work presents a computational study of the aerodynamic interactions that arise between the components of a high-speed lift-offset coaxial compound helicopter in forward flight. The objective of this study is to develop a computational methodology that would enable fundamental understanding of the complex aeromechanics of a modern lift-offset coaxial compound rotorcraft configuration in it's entirety. The modeling of a helicopter is a coupled aeroelastic problem, in which the aerodynamics is highly dependent on the structural dynamics, and vice versa. Therefore, the prediction of the rotorcraft airloads and blade deformations must be performed with sufficient fidelity to accurately model both aspects of the problem.

A high-fidelity computational fluid dynamics framework, HPCMP CREATE$^{TM}$-AV Helios, was used in conjunction with an in-house comprehensive analysis solver, to simulate a lift-offset coaxial compound helicopter in forward flight. A notional X2TD helicopter consisting of a lift-offset coaxial rotor, airframe and an aft-mounted propeller, was modeled in this work. An in-house comprehensive analysis solver, PRASADUM, performed trim calculations and the structural modeling using low order aerodynamics.

Conventionally, the comprehensive analysis rotor airloads that are computed from the built-in low order aerodynamic models, would be corrected with the high-fidelity CFD airloads using delta coupling procedure. In this study, the conventional rotor delta coupling methodology was used to study the interactional aerodynamics of a coaxial rotor system in forward flight at a range of flight speeds (50 knots to 225 knots). This study also focused on extending this methodology to perform high-fidelity airloads corrections for airframe and the propeller. The low order rotor, airframe and propeller aerodynamic loads were corrected with the high-fidelity CFD airloads, using a full vehicle loose delta coupling methodology. The two CFD/CSD coupling approaches, rotor and full vehicle, were compared. The results showed that correcting the low fidelity CSD airframe airloads with high-fidelity CFD airloads affects the rotor trim solution.

The converged trim state from the full vehicle delta coupling procedure was utilized to study the fundamental interactional aerodynamics between various components of the coaxial compound helicopter. The CFD simulations were performed for isolated helicopter components and component combinations.