Coaxial contrarotating twin rotor aerodynamics

Abstract

The modelling of a Coaxial Contrarotating Twin Rotor (CCTR) system has been developed from single rotor modelling techniques. Particular attention has been paid to the characteristics of the tip vortex, namely, vortex strength, structure and decay. Equations are presented which describe both the vortex core and the complete structure of a rolled-up tip vortex. These equations are utilised with., conventional strip theory to enhance the predictive accuracy, and replace the arbitrary application of a tip loss factor. Using this combined approach, named Vortex-Strip theory, the full interference effects between the two rotors of a hovering CCTR have been successfully modelled. By a similar approach, a Vortex-Glauert theory has been developed to model the wake of a CCTR in forward flight. Comparisons between theory and experimental results from model and full scale rotors are shown over a variety of hovering and forward flight conditions. Both wake models have been written in Fortran IV computer language with a flexible input procedure. This feature allows a user to Investigate a wide range of rotor parameters including different rotor radii, number of blades, CCTR rotor spacing, with variations in blade twist, taper and effective hinge offset. The computer program, named ROTOR, was used in a preliminary optimisation study of a hovering CCTR. The results show that a conventional CCTR is a more efficient layout in hover than an equivalent single rotor; a finding which has been endorsed by experimental results. Although the presented models have been developed for a CCTR, it should not be overlooked that they are equally applicable to single rotor configurations.

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