A theoretical and experimental study was conducted to develop a validated first principles analysis for predicting noise generated by helicopter main-rotor shed vortices interacting with the tail rotor. The generalized prediction procedure requires a knowledge of the incident vortex velocity field, rotor geometry, and rotor operating conditions. The analysis includes compressibility effects, chordwise and spanwise noncompactness, and treats oblique intersections with the blade planform. Assessment of the theory involved conducting a model rotor experiment which isolated the blade-vortex interaction noise from other rotor noise mechanisms. An isolated tip vortex, generated by an upstream semispan airfoil, was convected into the model tail rotor. Acoustic spectra, pressure signatures, and directivity were measured. Since assessment of the acoustic prediction required a knowledge of the vortex properties, blade-vortes intersection angle, intersection station, vortex stength, and vortex core radius were documented. Ingestion of the vortex by the rotor was experimentally observed to generate harmonic noise and impulsive waveforms.

/pdf/rotor-vortex-interaction-noise-47g1xdgtsd.pdf

Refraction of sound by aircraft wing tip vortices

Rotor-vortex interaction noise

As we look to the future, increasingly stringent civilian aviation noise regulations will require the design and manufacture of extremely quiet commercial aircraft. Indeed, the noise goal for NASA's Aeronautics Enterprise calls for technologies that will help to provide a 20 EPNdB reduction relative to today's levels by the year 2022. Further, the large fan diameters of modem, increasingly higher bypass ratio engines pose a significant packaging and aircraft installation challenge. One design approach that addresses both of these challenges is to mount the engines above the wing. In addition to allowing the performance trend towards large, ultra high bypass ratio cycles to continue, this over-the-wing design is believed to offer noise shielding benefits to observers on the ground. This paper describes the analytical certification noise predictions of a notional, long haul, commercial quadjet transport with advanced, high bypass engines mounted above the wing.

https://ntrs.nasa.gov/api/citations/20000057037/downloads/20000057037.pdf

Noise Reduction Potential of Large, Over-the-Wing Mounted, Advanced Turbofan Engines

Classical Mechanics

An integration technique for the automatic solution of an initial value problem for a set of ordinary differential equations is described. A criterion for the selection of the order of approximation is proposed. The objective of the criterion is to increase the step size so as to reduce solution time. An option permits the solution of “stiff” differential equations. A program embodying the techniques discussed appears in Algorithm 407.

The automatic integration of ordinary differential equations

Structure of trailing vortices.

Ray Theory and a New Method for Ray Tracing

Angular deflection and the formation of a line caustic are examined for acoustic rays passing through a cylindrical vortex with a viscous core. Ray paths are displayed in normalized form and appear to be applicable to the aircraft‐wake problem as well as to long‐distance sound propagation through synoptic weather features.

Refraction of sound by aircraft wing tip vortices

Citations

Rotor-vortex interaction noise

Noise Reduction Potential of Large, Over-the-Wing Mounted, Advanced Turbofan Engines

References

Classical Mechanics

The automatic integration of ordinary differential equations

Structure of trailing vortices.

Ray Theory and a New Method for Ray Tracing

Acoustic Ray Paths through a Model Vortex with a Viscous Core

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