Performance and Hub Vibrations of an Articulated Slowed-Rotor Compound Helciopter at High Speeds

Abstract

For a compound helicopter with an articulated main rotor, this study focuses on the high advance ratio, low disk loading environment of high speed flight at 225 kts. It seeks to understand the predominant phenomena which contribute to the main rotor power and hub vibrations, and demonstrate the most sensitive design and control criteria used in minimizing them for steady level flight - specifically the use of redundant controls and the effect of blade twist. The compound model is based on a UH-60A with 20,110 lbs of gross weight, and simulations are performed by Rotorcraft Comprehensive Analysis Sytem. The results show that for a -8° twisted rotor, the minimum power and minimum vibration states exist for two distinct sets of redundant controls and main rotor behavior. The low power state occurs at lower RPM and higher auxiliary thrust, with lower flapping and thrust at the main rotor. At a state with reduced vibrations, main rotor forward tilt, thrust, and average inflow ratio are higher, such that the rear of the rotor encounters less of the wake from the preceding rotor blades. For an untwisted rotor, it is shown that the regions for minimum power and minimum vibrations coincide with each other, with almost no flapping or pitch inputs and reduced rotor thrust from the twisted blade. By untwisting the rotor and providing proper control settings, the power requirements can be reduced by about 4-7%, with potential reductions in hub vibrations ranging from 50-80%.