Comparison of Tail Rotor Performance with a Slotted, Natural-Laminar-Flow Airfoil

Abstract

The slotted, natural-laminar-flow (SNLF) airfoil is a low-drag, high-lift airfoil concept that has been explored for commercial and general aviation applications. This work seeks to determine the performance benefits of using an SNLF airfoil on the tail rotor of a small helicopter. Blade element momentum theory (BEMT) and Reynolds-Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) results for a rotor in hover are compared, including with an SNLF rotor that has the rotational speed reduced to generate the same amount of thrust as the baseline rotor. The BEMT analysis uses polar data generated from two-dimensional RANS CFD with methods and grids validated against wind tunnel data taken from high-quality facilities. Sectional data for the baseline and SNLF airfoils are presented and discussed, such as the higher maximum lift coefficient of the SNLF airfoil and subsonic and transonic Mach numbers and the SNLF airfoils' performance with high levels of freestream turbulence. Skin friction coefficient contours from the rotor CFD indicate the blade operates as intended and show regions of high skin friction on the tip closure geometry, highlighting a region of potential improvement. Integrated results, such as dimensionless and dimensional thrust and power, demonstrate that the SNLF rotor with a reduced rotational speed outperforms the baseline blade in terms of power requirements; however, the magnitude of the gain is substantially different between the BEMT and rotor RANS CFD. Lastly, BEMT results with various freestream turbulence levels and completely turbulent boundary layers indicate that the reduced rotational speed SNLF rotor requires the same amount of power as the fully turbulent baseline.