AIAA 2000–2031 Noise Prediction For Maneuvering Rotorcraft

This paper presents the initial work toward firstprinciples noise prediction for maneuvering rotors. Both the aeromechanical and acoustics aspects of the maneuver noise problem are discussed. The comprehensive analysis code, CAMRAD 2, was utilized to predict the time-dependent aircraft position and attitude, along with the rotor blade airloads and motion. The major focus of this effort was the enhancement of the acoustic code WOPWOP necessary to compute the noise from a maneuvering rotorcraft. Full aircraft motion, including arbitrary transient motion, is modeled together with arbitrary rotor blade motions. Noise from a rotorcraft in turning and descending flight is compared to level flight. A substantial increase in the rotor noise is found both for turning flight and during a transient maneuver. Additional enhancements to take advantage of parallel computers and clusters of workstations, in addition to a new compact-chordwise loading formulation, are also described. INTRODUCTION The ability to predict rotorcraft noise has advanced greatly in the past two decades. In particular, deterministic noise sources are well understood and the accurate prediction of non-impulsive noise (i.e., thickness and loading noise) can be accomplished routinely and with great confidence—for steady rectilinear flight. The prediction of impulsive noise, such as blade-vortex-interaction (BVI) noise and highspeed-impulsive (HSI) noise, is more difficult, but only because the prediction of the rotor flow field and dynamical state is still challenging. The problem of predicting noise for a rotorcraft in a maneuver, however, still poses a daunting and largely untackled challenge. In both civil and military operations, maneuvering flight is essential for a rotorcraft to perform its intended mission. Nevertheless, current rotor-noise-prediction methods do not fully address even the simplest maneuvers, such as accelerating and decelerating flight in rectilinear motion. The real-world situation, however, is considerably more complex, involving unsteady, nonperiodic conditions and the transient effects associated with control inputs and short-period maneuvers (e.g. a pull-up). Unsteady aircraft motions—such as pitch, roll, or yaw motions—cause significant time-dependent shifts in the noise directivity, much like a flashlight sweeping across a scene during a nighttime search. Furthermore, transient maneuvers can generate a significant increase in the noise radiation due to both kinematic and aerodynamic effects. Recently Gopalan et al. has developed a method to predict the first-order effects of acceleration (deceleration) parallel to the flight path. JanakiRam and Khan have performed a detailed state-of-the-art prediction and validation for a helicopter in descending and decelerating flight. In both of these works, quasisteady analyses (i.e., predictions for a series of steady trim states made at several points along the flight path) were used for flight conditions where the aircraft state varied along a straight flight path. Aircraft acceleration was included in the trim computation, but it was not represented in the noise prediction directly. This paper addresses some of the unique aspects, and difficulties of predicting the rotor noise of a maneuvering rotorcraft, and the significant impact maneuver has on rotor noise. The primary goal of this paper is to outline the development of a new version of the rotor-noise-prediction code WOPWOP that is capable of computing the noise due to a transient * Senior Research Engineer, Computational Modeling and Simulation Branch, Senior Member AIAA. † Research Scientist, Subsonic Aerodynamic Branch, Senior Member AIAA. Copyright ©2000 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U. S. Government has a royalty-free license to exercise all rights under the copyright herein for Governmental Purposes. All other rights are reserved by the copyright owner.