Computational Study of the Advancing-Side Lift-Phase Problem

Abstract

The prediction of airloads and the corresponding structural response in high-speed forward flight of rotors poses a significant challenge to predictive rotorcraft aeromechanics methods. One of the issues identified in the flight test data of the Puma and Black Hawk aircraft is the phase difference between the minimum lift coefficient and the minimum of the blade pitch on the advancing side of the rotor during high-speed forward flight. This is commonly referred to as the advancing-side lift-phase delay. In the present work, the unsteady three-dimensional flowfield on the advancing side of a helicopter rotor is analyzed using computational fluid dynamics in an attempt to quantify contributions to the preceding effect. Time-dependent two-dimensional computational fluid dynamics simulations of blade sections with combined pitch/freestream Mach number oscillation were carried out to isolate the contribution to the phase difference of pitch angle and Mach number variations in the absence of the complex rotor-induced flowfield, sideslip, and rotor blade dynamics. The results for the freestream Mach number oscillations show that the lift coefficient lags the Mach changes at outboard stations, but this effect is reduced for combined pitch/Mach number oscillations. Finite span and sideslip contributions to the phasing were quantified by investigating the chordwise extent of supersonic flow on the advancing side for two nonlifting rotors in high-speed flight. Finally, the UH-60A rotor in high-speed forward flight was considered. By comparing results for rigid blades with results for a prescribed blade torsional deflection, the contribution of the blade torsion to the advancing-blade lift phasing was also quantified. Furthermore, rigid-blade simulations with different flapping schedules demonstrated the sensitivity of the lift phasing to trim-state variations. It was found that Mach number effects are dominant and the lift phasing depends primarily on the encountered Mach number and pitch schedule. Further, the elastic torsional deflection of the blades effectively changes the pitch schedule of the blade sections and also plays a role in the phasing of the lift and pitching moment coefficients.