Computational Investigation of Coaxial Rotor Interactional Aerodynamics in Steady Forward Flight

Abstract

This paper examines the aerodynamic environment of a high speed coaxial compound helicopter in steady level forward flight using CFD-CSD coupling. Numerical simulations have been performed to study trim configurations of an eight-bladed coaxial compound configuration (four blades per rotor) using comprehensive analysis coupled to a vortex wake model. The model is validated using public-domain information and flight test data obtained for the Sikorsky X2 Technology Demonstrator. Rotor-rotor interference follows the expected momentum theory type model, with the lower rotor experiencing larger induced inflow. Thrust sharing between rotors remains nominally constant, with at most ±5% bias over the speed range studied. CFD analysis for the rotors is then introduced at select forward flight speeds to identify key aerodynamic modeling refinements. In particular, impulsive normal force due to rotor blade crossings are not captured accurately by the vortex wake model. The impulsive 8/rev pitching moment is completely missed by the reduced order model. While significant, this aerodynamic pitching moment does not excite a noticeable torsion response from the stiff rotor considered. For future designs with lower stiffness blades, these impulsive airloads may induce additional vibratory hub loads and drive the design of the rotor pitch control system.

CFD/CSD Study of Interactional Aerodynamics of a Coaxial Compound Helicopter in High-Speed Forward Flight

“…Passe (2015) conducted a CFD/CSD study, utilizing CREATE-AV Helios and an in-house comprehensive analyzing code PRASADUM, to study interactional aerodynamics of a notional X2TD coaxial rotor system at 55, 100 and 150 knots [37].…”

“…[6]. The equivalent flat plate surface area of the combined horizontal tails is 34 feed squared[37]. The actual aircraft included ground adjustable horizontal tail pitch incidence, as described in Ref [1],.…”

“…However, the additional horizontal tail was not modeled in this study. The equivalent flat plate surface area of the vertical tail is 15.4 feet squared[37].…”

Unsteady loads for coaxial rotors in forward flight computed using a vortex particle method

“…A model of the X2 Technology Demonstrator (X2TD) is put together in the present work based on data from public-domain information (13-16) . This main rotor blade was designed to mitigate the large drag experienced by the inboard sections in reversed flow using double-ended elliptic sections, while a high-lift cross-section is used at mid-span, which transitions to a transonic aerofoil at the tip to reduced compressibility effects.…”

“…Therefore, this method was adopted by tools such as CHARM (2) , to simulate the performance of a coaxial rotor, and was also coupled with comprehensive tools, such as CARMRADII (11) , UMARC (1) , RCAS (12) , PRASADUM (4) to investigate the vibratory loads of coaxial rotors in forward flight. However, there are significant factors to be investigated, such as blade-wake interactions, reversed flow, and vortex shedding from the leading edge (13) .…”