Aerodynamic issues in the design of a torque-balanced, contrarotating coaxial proprotor are discussed. A blade element momentum theory (BEMT), formulated to account for the interfering flows in a coaxial rotor system, was validated against performance measurements for a contrarotating coaxial propeller. The BEMT was then used to develop an initial series of blade shapes for a coaxial proprotor, with the goals of maximizing aerodynamic efficiency both in hover (as a pure lifter) and in forward flight (as a pure propulsor). Initially, separate blade performance optimization in hover and in axial flight was undertaken. The BEMT suggested that, besides the normal limiting problems of blade stall and compressibility losses, the overall performance of a coaxial proprotor can be limited by its inability to reach a torque-balanced state. Performance estimations were also supported by results from a free-vortex method, which provided information on the spanwise loads and downstream wake boundaries. A hybrid blade design for the coaxial proprotor was then obtained. The optimum blade twist for the upper proprotor was shown to be of conventional hyperbolic form, whereas the optimum twist on the lower proprotor must have a multipart hyperbolic form. The break points in the twist distributions are related to the average positions of where the wake boundaries from the upper proprotor are assumed to impinge upon the lower proprotor at the specified design conditions in hover and axial flight. The hybrid blade design was shown to give the proprotor good propulsive efficiency while still retaining relatively good hovering performance. The need for a unified composite efficiency metric to properly describe and compare the performance of coaxial proprotors over their entire operational envelope is also discussed.


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    Title :

    Aerodynamic performance considerations in the design of a coaxial proprotor


    Additional title:

    Aerodynamische Leistungsbetrachtung zu einem koaxialen Propellerrotor


    Contributors:

    Published in:

    Journal of the American Helicopter Society ; 54 , 1 ; 012005/1-012005/14


    Publication date :

    2009


    Size :

    14 Seiten, 28 Bilder, 39 Quellen




    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English





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