Helicopter Vibration and Rotor Power Reduction Through Horizontal Tail Incidence Angle Control

This paper examines reductions in rotor hub vibratory loads and rotor power that can be obtained through the introduction of auxiliary pitching and rolling moments in the fixed system generated by a modified horizontal tail. The horizontal tail design used for this study incorporates independently controllable incidence angles in the left and right halves of the tail. The auxiliary moments produced by this tail configuration allow changes in vehicle attitude and rotor pitch control inputs, such that vibrations and rotor power can be reduced, while vehicle equilibrium is still maintained. This concept was examined using a comprehensive aeroelastic rotor analysis using two helicopters (i) a light-weight, hingeless rotor helicopter and (ii) a medium-weight, articulated rotor helicopter. An in depth analysis of the light-weight design at an advance ratio of 0.35 supports previous results that the onset of stall on the horizontal tail can limit its vibration reduction potential. In general, the light-weight helicopter analysis shows that a horizontal tail controlled in this manner can generate modest reductions in vibration throughout a range of flight speeds (0.2 ≤ μ ≤ 0.4), while the medium-weight helicopter analysis shows that horizontal tail control can provide up to 93 percent reductions in some components of the hub loads throughout the range of flight speeds examined (0.1 ≤ μ ≤ 0.3). Using the same flight speed ranges, the analysis predicts that by using the horizontal tail incidence control, both aircraft are capable of large (up to 20 percent) reductions in rotor power. A validation study using CAMRAD II reveals that the inclusion of a free wake model diminishes these power reductions (4 percent reduction versus 15 percent reduction at μ = 0.35).