The concept ofa coaxial rotor system that is optimized for most efficient hovering and/or axial flight performance is discussed. A blade-element momentum theory (BEMT) for a coaxial rotor system was developed and implemented numerically. The BEMT was found to agree well with performance measurements (thrust and power) made on coaxial helicopter rotors and on single and contrarotating propellers. Good correlation was also found with performance and spanwise loadings predicted by a free-vortex wake method, thereby helping to validate the assumptions made with the BEMT and better assess its overall predictive capabilities at the blade-element level. Furthermore, the BEMT was shown to give better performance predictions than using the simple momentum theory. The BEMT predicts that the optimum or 'ideal' performing condition for a coaxial rotor in axial flight occurs when both the upper and lower rotors operate at uniform disk loadings at a torque-balanced condition. This operating condition corresponds to linear distributions of thrust over both the upper and lower rotors. The ideal case also corresponds to uniform inflow over the upper rotor, but a double-valued variation of uniform inflow over the lower rotor. The optimum blade twist for minimum induced losses on the lower rotor is of double hyperbolic form, with the break point in the twist being at the radial location where the outer edge of the wake boundary from the upper rotor is assumed to impinge on the lower rotor. The results also show that at the most efficient torque-balanced operating condition, the upper and lower rotors of the coaxial operate at different mean lift coefficients, so that an optimum performing coaxial rotor in hover or axial flight will require different blade planforms (i.e., solidities) on both rotors to minimize profile losses.
An optimum coaxial rotor system for axial flight
Ein optimales Koaxialrotorsystem für axialen Flug
Journal of the American Helicopter Society ; 53 , 4 ; 366-381
2008
16 Seiten, 27 Bilder, 36 Quellen
Article (Journal)
English
An Optimum Coaxial Rotor System for Axial Flight
Online Contents | 2008
|AERODYNAMIC OPTIMIZATION OF COAXIAL ROTOR IN HOVER AND AXIAL FLIGHT
British Library Conference Proceedings | 2010
|Optimal Aerodynamic Design of a Coaxial Rotor System in Hover and Axial Flight
British Library Conference Proceedings | 2010
|