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How to Reduce RC Engine Noise
by Ron Scott
Here are some things I've learned over the years that may help you reduce noise levels.

Use a good muffler system. The original muffler should provide a good level of sound dampening - if the baffles haven't been removed. There are several after market mufflers available in various model magazines.

Add a rubber exhaust deflector. This can reduce noise from 1 - 3 dB in certain cases. It also extends the exhaust gas residue out a few inches from the airplane.

Use a larger prop. Increasing the prop diameter by ½ inch to one inch can have a 2 - 5 dB reduction in noise. Basically this slows down the engine rpm.
[Tech. Ed. note: Important! If you have a damaged prop, it should be used for paint stirring. Only if there are very minor dings or surface flaws on a prop should the following suggestion be done. Clean and smooth, do not remove material.]

Clean and smooth prop. Clean edges will cut the air smoothly with less noise, whereas cuts and dings on the edges can cause friction and drag in the airstream--thus causing noise. I use 150 grit sandpaper for the rough areas, then 400 grit for finishing work. When you consider that a 10 inch prop at 10,000 rpm is traveling at 297 mph the aerodynamic profile of the prop makes a difference. A cut in the outer edge can cause a hissing sound and add to the engine noise.

Lower the Fuel's nitro level. For every 5% increase in nitro, expect a 1 dB increase in noise level. Reducing the nitro in the fuel from 15% to 5% can reduce the noise by 2 dB.

Keep the airframe tight. Often times, noises develop in the fuselage and/or wing areas due to mechanical resonances due to engine vibrations. This is often caused by loose hardware, wing braces, etc. Make sure all hardware is secure and tight (no loose wing ribs, etc.). Soft engine mounts can reduce or eliminate these noises by de-coupling the engine from the fire wall. This essentially lowers the resonant frequency and partially damps the high frequency vibration.

De-tune (richen) the engine's fuel mixture to reduce rpm. This obviously reduces power but the engine is quieter.

Go from a two-stoke engine to a four-stroke engine. The four-stroke engines are usually quieter. The added benefits are worth the additional money. A four-stroke engine is quieter, more fuel efficient, more reliable, and idles more smoothly. It also leaves a lot less oil residue all over the airplane.

from
Update
Santa Barbara Radio Control Modelers
Peter Nickel, editor
Santa Barbara CA