Prediction of Helicopter's Rotor Airloads in Transition Flight Regime

Abstract

A computational method is developed to predict helicopter's rotor blade airloads and examine the cause of the high vibratory airloadings acting on the rotor hub in the transition flight regime. The rotor wake model describes the wake by potential vortex filaments that are free to move with the local velocity and focuses on the modeling of the vortex core size and strength changes as a result of close blade-vortex interaction. Also, a maximum limit on vortex-induced lift coefficient of a blade section is considered. Several flight conditions of the fourbladed rotor system of the H-34 helicopter in and near the transition flight regime are chosen as test cases. The agreement of the computed blade sectional. airloadings with the measured data is good. By computing the blade airloading distributions and wake geometries, the influence of the close rotor vortex wake on the blade airloading variation with forward speed is studied, and the cause of the high vibratory airloadings acting on the rotor hub during transition flight is investigated and better understood. Nomenclature c blade chord C, constant, = vt / v r radial position along a blade r ~ ~ l b ade-vortex interaction distance rc core size of a blade tip vortex element r,o initial blade tip vortex core size r vortex core size at the wake age cpl rd distance between a blade-bound vortex center and center of the interacting vortex element rv radial distance from the vortex centerline R blade radius t vortex wake age parameter Vt vortex tangential velocity Vf forward flight speed vortex-induced incremental lift coefficient vortex wake age, cp = a t wake age at the beginning of a close bladevortex interaction strength of a blade tip vortex element bladebound circulation maximum bound circulation along a blade advance ratio, p = Vf / SZR rotor rotational speed air kinematic viscosity eddy kinematic viscosity rotor blade azimuth angle Introduction When a helicopter passes through the transition flight regime where the advance ratio p 0.1, the vibrations transmitted to the rotor hub fiom the main rotor system become very high. This phenomenon of transition vibrations is a common problem for most helicopters. It is assumed that this high level of transition vibrations is mainly caused by the close interaction between the rotor and its vortex wake.lv2 However, despite longtime research on this topic, the mechanism through which the rotor-vortex wake interaction causes the high level of vibrations is yet not fully clear and requires more investigation. The flow field around a rotor wake is dominated by the strong blade tip vortices. After the blade tip vortices are generated, they are convected with the local velocities and form a distorted, helical, vortex wake geometry. At high forward flight speeds (p > 0.2), the blade tip vortices are convected away from the rotor disk and the effect of the blade tip vortices on the blade airloading is small. However, at lower forward speeds (0 < p < 0.15), the blade tip vortices are convected slowly downstream and remain close to the rotor disk. Then, the influence of the rotor vortex wake on the blade airloading becomes important. Rotor wake experiments show3-' that at low forward flight speeds the rotor vortex wake geometry is very complex and ' Professor, AIAA Associate Fellow Ph.D., AIAA member Assistant Professor, AL4A member Copyright O 2004 by Zvi Rusak and Wen-King Chen. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. 1 42nd AIAA Aerospace Sciences Meeting and Exhibit 5 8 January 2004, Reno, Nevada AIAA 2004-1277 Copyright © 2004 by . Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.