Guest Editorial: Noise—and Your Aircraft
Noise Committee, RC Flying Club of Toronto
Case Study: A Flying Field Saved
Just about everyone is concerned about pollution, be it waste, water, air, or noise. Flying sites have been lost, are being lost, and will be lost unless we can come up with some answers. Once, flying sites were easy to find around urban areas. Now, with population growth, increased activity in our hobby, and urban sprawl, it is almost impossible to find and keep a site within reasonable driving distance. If we are to continue enjoying our hobby, we must learn to quiet our operation.
Armed with sound-level meters, we set out to find what we were up against. Tests showed that a .40-size engine with its standard, factory-supplied muffler, measured at 7 meters distance and 1 meter off the ground, registered from 86 to 93 dBA. A .60-size engine with its muffler registered 89 to 96 dBA.
Why choose 7 meters distance and 1 meter off the ground? FAI has a "spot check" of 105 dBA measured at 1 meter on the ground. We found that there was too much room for error in that method: only the slightest variance in distance or angle of the meter would give vastly different readings. Also, the FAI check is supposed to be made on concrete or blacktop, which many fields don't have. The 7 m and 1 m measurement is simple, quick, and has been used successfully in several European countries.
Exhaust gases are the major source of noise. But what about prop noise? intake noise? the construction of the aircraft? All of these contribute to the overall noise level. How much? Let's look at them one at a time.
Contributors to Noise
- Exhaust gases
- Prop noise
- Intake noise
- Aircraft construction
Exhaust gases
Mufflers quiet the sound by allowing the gases to slowly expand and cool. The bigger the muffler, the better it works. Most mufflers in use are not large enough to be effective beyond 12,000 rpm. At that point, the gases are being pushed through the engine so fast that they have little time for expansion or cooling.
Prop noise
From around 12,000 rpm and up, prop noise becomes a contributing factor, but not to the degree many people think. Much of the sharp increase in noise level at higher rpm has often been blamed on the prop. In tests using electric motors, we swung props up to 14,000 rpm and found that, although the noise is definitely there, it is a fairly low-pitched noise and does not carry more than a few hundred feet at most.
Pitch (frequency) versus volume
Noise nuisance is comprised of two parts: volume (loudness) and pitch (frequency). For example, driving a car at 60 mph with a window open produces a noise level in excess of 90 dBA, but it is not uncomfortable because of the low pitch. On the other hand, microphone feedback in a PA system is unbearable because of its high pitch.
A good example is the short range of prop noise on four-stroke engines. When a four-stroker flies by, you can hear the prop noise; as soon as it is a few hundred feet away, the prop noise disappears, whereas the engine noise continues.
Intake noise
We were unable to prove that intake noise is a significant contributing factor until the aircraft approaches an 80 dBA noise level or lower.
Aircraft construction
Construction does have an effect on overall noise. If you take a dBA reading on a particular engine and muffler mounted on an open-structured aircraft, then mount the same combination in an aircraft with solid sides and a foam-core wing, the dBA reading on the latter will be quieter. The "drum effect" of fabric- or heat-shrink-covered open-structured aircraft is a low-pitched sound and only a small contributor to the overall scene.
It is also clear that how and where an aircraft is flown is a large factor in the nuisance value. Even a particularly noisy aircraft, if taken off at full power but then throttled back and flown low and close in, will cause less annoyance than a quieter aircraft flown from the field at full throttle and at high altitudes. It is enlightening to stand a half-mile from the flying site and note which aircraft are the offenders; altitude is a large factor.
Our tests showed that even a noisy craft (over 92 dBA) could not be heard at one-half mile if it stayed below 200 ft. However, 200 ft. isn't much, and we don't expect everyone to fly that low all the time. So let's look at other solutions.
Goals and solutions
First, we needed a goal. Since many .40-size and smaller engines will pass an 88 dBA level with little or no adjustment, we chose 88 dBA as our initial target. This would cut current average noise levels considerably. Since exhaust is the prime cause of noise, we attacked that first.
- Double mufflers: If using one muffler works, then using two works better — quite dramatically. Aircraft producing well in excess of 88 dBA were reduced to near 80 dBA. With this method there is very little, if any, loss of rpm. The second ("add-on") muffler need not be fancy: a butane-lighter refill can or a similar small can with a pipe soldered or epoxied at one end, coupled to the primary muffler with silicone tubing, works fine. The only drawback is aesthetics — it's hard enough to hide one muffler in a scale ship, let alone two.
- Small add-on with steel wool: A small add-on made from a couple inches of brass tubing, a 3/56 clamp, and a small amount of 00-size steel wool can be used. This method will not be as effective as the double-muffler system, but it has the advantage of being adjustable. You can regulate the dBA level by the amount of steel wool used, but there is a proportionate drop in rpm. Very little steel wool is needed; using too much can drop rpm in excess of 1,000 and may cause overheating.
- Baffled mufflers: Some fliers have cut their mufflers in half and added a divider in the middle to make a dual-chambered muffler. This is not as efficient as the double-muffler system but will get most engines under the 88 dBA mark.
3- or 4-blade props
For those who want higher rpm, a three- or four-bladed prop has been shown to cut the noise level considerably. Also, when used in conjunction with a small add-on, even a tuned pipe can pass the test with little or no performance loss.
Where does all this lead? In our case, it got us back a flying field. Our club lost its field last summer due to noise complaints from local residents. We met with the residents and field administration and did noise-level tests. It was decided that our aircraft were too noisy, so we could not use the field. This loss prompted our investigation into the noise problem.
A subsequent meeting with the same group demonstrated that we had dramatically reduced noise-emission levels. The residents agreed that, within the confines of an 88 dBA limit, we would be allowed to use the flying site again.
Editor's Note
Accompanying the foregoing was a letter from Joe Murray (vice-president of the Radio Controlled Flying Club of Toronto, Canada) saying that it had originally been produced for the club's newsletter after about a year's study of the problem. Later, they ran a noise symposium with participation from organizers of the area clubs. There is now a proposal from some to cut down overall noise levels, which has produced a dramatic decrease in the number of complaints, Joe was pleased to say.
Transcribed from original scans by AI. Minor OCR errors may remain.
