Loading schedule design for rotor thickness noise reduction

The concept of generating a loading noise that can cancel thickness noise, originated by Gopalan and Schmitz, is considered and a preliminary strategy for developing point control force schedules is presented. While the approach could, in principle, be used for cancellation of different noise at a target observer location, it is investigated here in the context of rotor thickness noise reduction. First, some simple loading schedules are considered and results confirm that a rapid increase of loading around 90 deg. azimuth is necessary for thickness noise cancellation. Next a point control force governing equation, based on Farassat's Formulation 1A, is developed without any significant approximations. The loading solution for force on the blade in the direction of the observer, L_r, is solved numerically and a window function is used to modify the solution to ensure periodicity and tailor the results. Loading schedules for different radial and chordwise locations are obtained. The analysis shows that larger rate of increase and amplitude of loading are required when the point control force is moved from the blade tip to the root. Subtle differences are also required if the control force is moved to different chordwise positions. The placement of point control forces and the implication on power requirement are examined briefly. Finally, an initial consideration is given to using a control force on one blade to cancel the noise generated by another blade.