| EXHAUST SILENCERS |
| What makes a good silencer? There are now several effective exhaust silencers available on the market for a range of engine sizes. These come in two basic types: an integrated primary silencer, or an add-on secondary silencer. All good silencers incorporate internal structures to break up the sharp pressure wave that is produced by rapidly expanding hot exhaust gases. It is this pressure wave that is responsible for the vast majority of exhaust noise. The flow of exhaust gas is usually very quiet; this can be illustrated by exhaling very quickly. The noise produced by this simple act is far less than that produced by any model aircraft engine exhaust, despite the volume of gas moved per second by your lungs being more than by a 0.90 cu.in 2 stroke running at 10000 RPM ! Silencers and power loss An add-on silencer should not cause any power loss when attached to a primary silencer. Power loss is caused by back pressure - a result of exhaust gas not being able to exit the silencer quickly enough. If all the holes in a silencer are large enough, back pressure does not occur to an appreciable extent and power is not lost. The best way to measure power is by using maximum RPM as a surrogate measure. The maximum RPM should be measured both before and after fitting the silencer, with no other changes, except that a minor degree of retuning may be needed after fitting the silencer. If the RPM has remained within 200 RPM (allowance for experimental error), no power loss or gain has occurred. Tuned pipes Tuned pipes can be used very effectively as silencers, provided that they have a suitable internal structure ('quiet' tuned pipes), and that any increase in power provided by using such a device is not used to increase the RPM. Instead, a propeller with increased pitch should be used to absorb the increased power whilst maintaining a constant RPM. Mounting silencers A couple of other points are worth making on the subject of silencers. Firstly, silencers should not be hard mounted to the fuselage or wing of the aircraft. This increases the amount of vibration transmitted to the fuselage and hence airframe noise (see flexible mounts and engine noise). Use of the noise meter Secondly, the noise meter can be a useful tool in determining the comparative effectiveness of silencers. A series of tests should be carried out, all at the same RPM with the same propeller, changing nothing but the silencer. In this way, meaningful comparisons of silencer effectiveness can be achieved. You will often notice that the reading most affected by adding a silencer is not that taken with the exhaust facing the meter, but that taken with the starboard side of the aircraft facing the meter (on an aircraft with an upright or side-mounted engine with side exhaust) This fact can provide a useful pointer as to whether the exhaust is the dominant source of noise. If the starboard reading is higher than the others, the exhaust is usually the dominant source. |
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| Silencer designs: S13a S13h S35 (NRD-1000) S60, S62 |