| FLEXIBLE MOUNTS AND ENGINE NOISE |
| Flexible mounting systems An important point to remember about flexible engine mounting systems is that airframe noise is not usually the most important noise source. Propeller and exhaust noise should be adequately tackled first. Having said this, flexible mounting systems can make a significant difference, and often turn a reasonably quiet aircraft into a very quiet aircraft. The quality of the sound is often improved markedly, being perceived as much less harsh. There are many different systems available, with some evidence that a soft rubber is preferable to a hard grade of rubber in damping vibrations transmitted to the fuselage. The idea behind all the systems used is to prevent vibration from being transmitted from the engine to the fuselage and thus causing the fuselage to act like a drum, amplifying the vibration and emitting it as noise. A successful system must ensure that no part of the engine or silencer is in contact with the fuselage or wing without a layer of rubber in between. 'Lord' mounts are a good option, and are readily available, as are other fixings to be found in DIY outlets. Consider retrofitting existing aircraft with rubber mounts, and always plan to include them when constructing new aircraft. Engine Noise. The mechanical noise produced by an engine in relatively good condition is relatively small compared to the other components discussed above. Note however that out of balance propellers can lead to a lot of vibration - always ensure propellers are balanced prior to use - and worn bearings can similarly generate a lot of noise. Mechanical noise is difficult to tackle, and the most practical solution is usually to encase the engine in a decent cowling; even a cheap plastic cowling can have an effect on reducing noise emission. Large Engines and Four strokes. The same principles of limiting RPM and fitting effective silencing and flexible engine mounts work for four strokes and larger 2 strokes. Four strokes are easier to silence in some ways, as the active pumping of exhaust gases by the exhaust stroke makes them resistant to the effects of back pressure when a silencer is added, and the number of exhaust strokes is less. Thus a smaller silencer can be used, e.g. a silencer used for a 0.51 cu.in 2 stroke can be used on a 0.90 cu.in 4 stroke. Large engines, although producing more noise than a small engine, will be run at much lower RPM's. This offsets the higher noise levels usually associated with such engines, and may provide a qualitative bonus, as low frequency sounds tend to be less irritating. Larger engines therefore provide an opportunity to be exploited. It is a mistake to think that moving to smaller engines must automatically reduce the noise problem and lead to fewer complaints. |
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| Data and pictures of flexible mounting systems |
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