I am also VERY interested in this subject. Below is a brief NASA report(not the full 231 page text) about experimental mufflers utilizing ceramic foam product called Ultramet. Dave Ander's plane used one(or two?) of the prototype mufflers on his plane when he competed the NASA sponsored efficiency contest. He handily won the quietest airplane award and mentioned the experimental mufflers using Ultramet ceramic foam as a good part of why his plane was so quiet. Ceramic foam is said to be VERY lightweight and has the ability to withstand extreme temps. I am assuming that the mufflers would resemble tube style mufflers with Ultramet foam as the sound absorbant material inside rather than fiberglass or stainless mesh. Of course, the mufler shell(321 stainless?) would constitute most of the overall weight. In my searches, I have not found a single decent photo so far of this elusive muffler design. Apparently, the overall dimensions and layout of the muffler using Ultramet is very important and has a large effect on the overall effectiveness. The NASA report does say that its best prototype has achieved a 14 db drop in exhaust noise compared to a "stock system". I'm sure that for a very specific rpm/situation, but 14db is HUGE. Of the many questions I would like to ask Dave Anders, the Ultramet mufflers one is on the top of my list.
TITLE AND SUBTITLE:
Noise Reduction System for General Aviation Aircraft, Phase II
AUTHOR(S):
Heng, Sangvavann; Stankiewicz, Edwin, P.; Sherman, Andrew, J.
REPORT DATE:
2005-11-xx
FUNDING NUMBERS:
WBS-22-781-30-75
PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES):
Ultramet 12173 Montague Street Pacoima, California 91331
PERFORMING ORGANIZATION REPORT NUMBER:
E-15310
SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES):
National Aeronautics and Space Administration Washington, DC 20546-0001
REPORT TYPE AND DATES COVERED:
Final Contractor Report
SPONSORING/MONITORING AGENCY REPORT NUMBER:
NASA/CR-2005-213987; ULT/TR-98-6989
SUPPLEMENTARY NOTES:
Work funded by NASA Small Business Innovation Research contract NAS3-27633. Project Manager, Richard P. Woodward, Propulsion Systems Division, NASA Glenn Research Center, organization code RTA, 216-433-3923.
ABSTRACT:
In this project, durable, high temperature ceramic foams were evaluated as potential passive broadband noise absorber/baffle materials for reciprocating piston general aviation aircraft engines. In the Phase I project, a ceramic foam-based combined dissipative/reactive muffler design proved its potential for successfully reducing the size, weight, induced backpressure, and noise of general aviation aircraft engines. However, tuning the combined muffler design for specific engine noise reduction proved highly complex and difficult, requiring analytical tools that did not as yet exist. In Phase II, numerous test methods were developed to screen various newly developed ceramic foam-based muffler designs and evaluate their acoustic characteristics. More than 30 prototypes representing actual muffler designs and containing Ultramet ceramic foams were fabricated and characterized. Methods for acoustic evaluation included insertion loss bench, dynamometer, ground, and flight testing. Based on the results of these tests, Ultramet ceramic foams were shown to be generally effective as broadband noise absorbers at frequencies above 800 Hz, particularly for larger general aviation engines. The most promising ceramic foam-based muffler prototype reduced the noise emitted by a Continental O-200 engine by up to 14 A-weighted decibels (dBA) relative to the stock exhaust system (a short, straight pipe). Varying the ceramic foam design parameters yielded variations of as much as 5 dBA in the induced sound pressure levels, but did not affect the frequencies reduced. However, the backpressures induced by the majority of the ceramic foam muffler prototypes were well below maximum allowable levels. Given their light weight and compact size (including required canning and inlet/outlet pipes), these mufflers can be retrofitted under the cowlings of general aviation aircraft.
SUBJECT TERMS:
Aircraft engine performance; Engine noise; Noise reduction; General aviation aircraft; General aviation; Aircraft engine; Muffler; Open-cell foam; Ceramic; Silicon carbide
NUMBER OF PAGES:
231
George
