Engineering topics

Sim, Ben WEL-C.

Publications and source records attributed to Sim, Ben WEL-C..

3 records

NASA NTRS1993

Rotorcraft noise: Status and recent developments

This paper briefly reviews rotorcraft noise mechanisms and their approximate importance for different types of rotorcraft in different flight regimes. Discrete noise is due to periodic flow disturbances and includes impulsive noise produced by phenomena which occur during a limited segment of a blade's rotation. Broadband noise results when rotors interact with random disturbances, such as turbulence, which can originate in a variety of sources. The status of analysis techniques for these mechanisms are reviewed. Also, some recent progress is presented on the understanding and analysis of tilt rotor aircraft noise due to: (1) recirculation and blockage effects of the rotor flow in hover; and (2) blade-vortex interactions in forward and descending flight.

George, Albert R.↗

NASA NTRS1993

Propfan noise propagation

The unconventional supersonic tip speed of advanced propellers has led to uncertainties about Propfan's noise acceptability and compliance with Federal Aviation Noise Regulation (FAR 36). Overhead flight testing of the Propfan with an SR-7L blade during 1989's Propfan Test Assessment (PTA) Program have shown unexpectedly high far-field sound pressure levels. This study here attempts to provide insights into the acoustics of a single-rotating propeller (SRP) with supersonic tip speed. At the same time, the role of the atmosphere in shaping the far-field noise characteristics is investigated.

George, Albert R.↗

NASA NTRS1993

A survey of propfan noise propagation at cruise

Acoustic measurements collected during the Propfan Test Assessment (PTA) program in 1989 had demonstrated that cruising Propfans have undesirably high far-field noise level. This study attempts to investigate the source of these noise disturbances generated by a single-rotating propeller with supersonic tip speed. Various atmospheric and propagation effects responsible for shaping the far-field acoustics will be examined. A methodology for analyzing Propfan propagation noise is proposed based on the theory of geometrical acoustics. This approach allows acoustic rays to be traced into the far-field via Snell's law. Amplitude variation along the rays are evaluated by applying the Blokhintzev energy invariant principle. Additional terms are appended to the energy invariant principle to account for propagation and atmospheric effects. A FORTRAN program is written to analyze the rays emitted from a helicoidal source path traced by supersonic helical speed blade tip. Earlier papers by Myers and Farassat have demonstrated the presence of Mach envelopes that lead to formation of caustics. Linear geometric acoustics is used to ascertain the existence of these nonlinear caustics in the far-field. The importance of caustics to Propfan noise generation is then studied.

Sim, Ben WEL-C.↗