Poor quality silencers threatening flying sites ?

[cymaz]

I have found this....not used them though

[Martin McIntosh]

Not read all of this by any means, but many years ago I did a lot of research with others into getting F3A models under the limit. The first discovery was that it is a complete waste of time measuring at 7m since the wind noise is generally far more than the model is making. It was agreed at the time that measurements should be made at 1m from the cylinder head to nullify the wind effect. I cannot now remember what the maximum figure was under the new ruling. Props. were changed from 11x7 two blade to 9 3/8x7 three blade which got way under but with reduced power.

This was done after adding butane canisters to the tuned pipes which reduced the exhaust noise to a negligible amount.

I currently have a couple of 50cc petrol two strokes. They are very noisy and due to the model designs are not capable of having canisters fitted so they are grounded at the moment.

The problem is that when measuring on the dBa scale, which replicates the human ear, the higher the revs., the less it reads, so the lower revving large motor noise travels much further.

If manufacturers would concentrate on producing four stroke petrols the problem would disappear due to the inherently quieter exhaust note. The only one I tried recently turned out to be complete rubbish despite rave reviews though.

[MattyB]

Posted by Martin McIntosh on 29/09/2015 16:09:36:

If manufacturers would concentrate on producing four stroke petrols the problem would disappear due to the inherently quieter exhaust note. The only one I tried recently turned out to be complete rubbish despite rave reviews though.

Which one was it - a Saito, NGH or OS? Personally I am waiting for the Laser 180 - it should be a cracker, and we know unlike others it won't be released before it's ready...

Edited By MattyB on 29/09/2015 17:26:25

[iqon]

Fuji make a four stroke

[Andy Symons - BMFA]

Posted by Martin McIntosh on 29/09/2015 16:09:36:

Not read all of this by any means, but many years ago I did a lot of research with others into getting F3A models under the limit. The first discovery was that it is a complete waste of time measuring at 7m since the wind noise is generally far more than the model is making.

In that case it will have been far too windy to fly and the model should have been tested in calmer conditions. 7m is practical and sensible.

[cymaz]

I have a general rule of thumb , 10mph or less to do a noise test.

[Andy Symons - BMFA]

Posted by Martin McIntosh on 29/09/2015 16:09:36:

The problem is that when measuring on the dBa scale, which replicates the human ear, the higher the revs., the less it reads, so the lower revving large motor noise travels much further.

It doesn't quite work like that the sound level reading at a fixed point is not dependent on revs, it is whatever the reading is, if an A weighting filter is used the value the the meter reads is adjusted slightly according to correspond to how the ear works, the total sound level reading is comprised of the sound level in each of the frequency octaves that when added together (logarithmically) give you the total, each of those sound energy levels is adjusted (weighted) before being added. The table below shows the values. The ear is more sensitive to frequencies between 1kHz and 5kHz (frequencies of speech, all to do with ear canal length and harmonics) so as you can see the measured value in those frequencies is increased slightly.

Typically in my investigations model aircraft noise contains energy in frequency octave bands from 60Hz to 20kHz the amount of energy in each ocatve is dependent on rpm, prop size etc etc. Typically large petrol engined aircraft have more energy in the lower frequency bands.

Lower frequency noise energy dissipates over much greater distances than the higher frequency noise energy.

 

Having said all that, as some others have mentioned, it isn't the poor quality silencers that are the problem as such, it's unreasonably loud aircraft that are the problem.

We should all ask ourselves the question, (whatever aircraft you fly , whatever the power source and no matter where you fly) Is my model aircraft as quiet as it can reasonably be made to be?

Edited By Andy Symons - BMFA on 29/09/2015 18:53:48

Edited By Andy Symons - BMFA on 29/09/2015 18:54:48

[Andy Symons - BMFA]

Going back to the effects of wind, lets assume you are testing your aircraft which say for arguments sake is going to be about 82dB. Lest say the wind is creating 60dB of sound.

Who would like to hazard a guess at what the combined Sound level will be?

Here is an online calculator to help **LINK**

[Erfolg]

Your last comment is almost a verbatim quote of our club chairman when considering noise.

I had understood that the method of taking a reading was based on a Code of Practise, including set distance etc, is it not?

[Andy Symons - BMFA]

Posted by Erfolg on 29/09/2015 18:59:19:

Your last comment is almost a verbatim quote of our club chairman when considering noise.

I had understood that the method of taking a reading was based on a Code of Practise, including set distance etc, is it not?

It is a DOE code of practice.

[Andy Symons - BMFA]

I had an email in the office a week or two ago regarding the COP. I was asked a few questions which where as below, with my answers.

1. Is the COP document on model noise a fixed guideline based established/documented qualities of noise and its known acoustic transfer of sound, or is it a document open to interpretation by anyone choosing to use it?

Section 71 of the Control of Pollution Act (1974) gave the secretary of state the power to introduce codes of practice “For the purpose of giving guidance on appropriate methods (including the use of specified types of plant or machinery) for minimising noise” the code contains guidance that a lay person (non acoustician) could follow with the minimum of equipment or knowledge. There is no interpretation required, you either are compliant with the code or not. The code does not carry the force of law and local authorities do not have to take it in to consideration but most do and by complying with the code you are demonstrating a responsible attitude and complying with the only guidance available.

2. Is there an established rate/table of sound decay over distance, so that at a point/distance, a source of 82db would reach/decline to the ambient db? If there is such a document would you please advise me where I can refer to it.

There are many factors that affect the decay of sound pressure over distance, theoretically for a fixed “point source” of sound of a single frequency (source that radiates sound as if from a single point) in a free field (airborne as opposed to on the ground) sound decays according to the inverse square law in all directions, this equates to a 6dB reduction for each doubling of distance from the source, so for example if you have 82dB at 7m , theoretically you will have 76dB at 14m 70db at 28m, 64db at 140m, 58dB at 280m, 52dB at 560m and so on. However the fact that a model aircraft is moving, has sound pressure across many different frequency bands and geography, ground cover, wind, temperature, humidity and air pressure all have an effect the only way this can be established is by gathering empirical data at specific sites.

3. Is there recognition of a downwind/upwind calculation/adjustment that can be applied.

Too technical, the COP is supposed to be a tool that a non acoustician can use to give reasonably predictable results.

4. Is there an established countryside ambient db?

No. Usually can only be determined by measurement and be site specific and would often be considered to be the L90 measurement taken over a minimum of an hour, the L90 being the sound pressure level that is exceeded for 90% of the measurement period. From my own measurements in the field this L90 measurement is often between 30 and 40dB.

[Martin McIntosh]

Matty, it was the NGH 38. Have looked at the O.S. one and it seems quite impressive, with the usual price tag to suit.

I have a Laser 180 glow and see no reason to change to a petrol version, but look forward to a 300 single if that ever happens.

[Martin McIntosh]

I see where you are coming from with the wind speed. The problem is the wind noise created when blowing over a microphone no matter how well it is muffled, rather than the background wind noise which should be comparatively negligible.

The GBRCAA / BMFA competition measurements I was referring to were taken at every event. If we had waited for 10mph or less, none would have taken place!

A few years ago YT International did a demo at my local club. Their 50cc warbirds were very quiet and had what appeared to be standard silencers with the end bunged up and a hole of about 8mm drilled in the bung. I suspect that there was more to it because when I tried this I just lost an unacceptable amount of power as expected

[MattyB]

Posted by Martin McIntosh on 30/09/2015 10:17:43:

Matty, it was the NGH 38. Have looked at the O.S. one and it seems quite impressive, with the usual price tag to suit.

I have a Laser 180 glow and see no reason to change to a petrol version, but look forward to a 300 single if that ever happens.

My Dad has the NGH38, which is destined for his Seagull Spitfire. It has needed a lot of running in and has now loosened up considerably, though it still vibrates a fair bit as with all big petrols. It is no powerhouse, but seems well suited to the model based on flight videos from others who have used it in the same airframe. Based on the price he has no complaints so far, but time will be the true test of that!

[john stones 1 - Moderator]

Clubmate has one of the NGH 38's in a Corsair, the model weighs a fair bit and the engine fly's it well, he also has the Seagull Spitfire the NGH will fly it no problem, looks a decent motor to me

John

[Martin McIntosh]

You two must be very lucky then. Mine was so tight that a geared starter would not turn it from cold. It would only do 6200 on the smallest recommended prop, an 18x8, and even then after careful running in, instead of the quoted 8000 and would not even fly a Seagull Edge 540. Upon examination by a model motor manufacturer the camshaft was found to be chipped and worn away due to a hardening problem. Don`t touch one with a barge pole.

[Martin Whybrow]

There are some serious misunderstandings about noise measurement; one of our senior members took our SPL meter to the nearest housing estate to our field to measure the noise levels and claimed to have measure in excess of 82dba; the nearest point of the estate is 450m away from the closest part of our flying field and the other side of a main dual carriageway; I pointed out that the measurements he was taking was most definitely not our models because the sound level at 7m would have been unbearably loud to read 82dBa at 450m, but he claimed that it was because he 'could hear the models'!

[Gordon Brown]

I'm unsurprised at Martin's tale, noise measurement and interpretation seems to baffle many people and misunderstandings abound. Even Andy Symons makes the occasional mistake; for "dissipates" read "propagates"

[Andy Symons - BMFA]

Thanks Gordon. Yes should be propagates.

[Erfolg]

I accept that the measurement of noise is not as easy as measuring volts with a muti-meter. I also accept that it is possible to take totally useless measurements, by not understanding the instrument in use and also the code of practise. That does not however mean that measurement is impossible or produce meaningful results.

I do accept that the process is not definitive, in an absolute sense, but rather like taking the temperature in a small room, some variation in the room, although one measurement taken at an optimum point is sufficient to state the temperature.

I think that is why our club made use of the services of a member to take measurements, of various model types before formulating our clubs policy, which is electrics only. Any model deemed to be noisy is not permitted to fly in the offending condition.

Our club is very light in rules and diktats from the club hierarchy or members, although noise is one area where there are definite policy. That is if the model could cause a noise complaint, or be noticed, then it will not be permitted to fly.

I was thinking about noise and where i live, and remember that at one time all the local manufacturing companies would sound Hooters to signify the beginning, and intervening periods of the working day. The final company was finally stopped from using the system, just prior to it closing. There was a time when every Sunday the peace of the day would be broken by all the local churches calling by peels of bells, their congregations to prayer, now there is not a one. It does seem that noise that was once accepted and seen as part of life have now been silenced.

[john stones 1 - Moderator]

Yep Clarksons gone now Erf

[Gordon Brown]

Without going into too much detail, it is possible to make measurements that have some degree of meaningfulness, but to do so with any great degree of confidence requires a fair degree of training and understanding, and - not least - a meter that is capable of measuring accurately. The CoP gives a reasonably simple protocol for checking noise emissions but it's not a complete guide and if you read it carefully you'll find that it specifies the use of a Class 1 (or Type 1) meter, as follows;

The noise meter shall be in accordance with BS5969:1981 (Type 1)(Specification for sound-level meters),set to· the A frequency weighting and to the S time weighting. The meter should be calibrated prior to use. Some local authorities may be willing to assist model flying clubs with noise measurements in order to establish the noise levels of aircraft being flown by club members.

The BMFA has a suitable meter and calibrator but in this day and age you'd be hard pressed to persuade the local EHO to do spot measurements. My own club is fortunate in that we have an EHO as a member and I am a consultant with five Class 1 meters that I can bring down to the club if needed.

What difference does the meter make? If you look at Class 1 meters they have an accuracy at 1000Hz of + or - 1.1dB, Class 2 meters specify + or - 1.4dB. The meters in use at most clubs will not be type or class approved so GOK what their accuracy is but I suspect it is nowhere near as good as even a Class 2 meter. Confusingly, the meter will almost certainly have a display that reads in tenths of a decibel, fooling you into thinking it's really accurate. No way! The 82dBA you may think you have measured with your Radio Shack meter could be anywhere between 80 and 84, and I'm probably being generous in assuming + or - 2dB accuracy.

If you add to this the other uncertainties in noise measurement, such as how you hold or mount the ruddy thing and you'll increase that figure very rapidly. Just to explain, if you hold the meter a few inches from your chest while you are doing the measurement - and most folk will do just that - then you are not only measuring the noise coming from the model, but also the same noise being reflected from your body. This is likely to add a decibel or two to the result! Whenever I do measurements, I mount the meter on a tripod, tilted over so I can read the screen from a distance, just to avoid this phenomenon.

I'm not suggesting that clubs should invest heavily in noise measurement equipment (it's appallingly expensive) , but if your measurements are taken to task by the local authority, don't be surprised if the EHOs measurements are wildly different to yours!

As for church bells, they still ring every Sunday round here, but the factory hooters have long gone, as have the factories...

[Allan Bennett]

Posted by Erfolg on 30/09/2015 16:55:45:

... I think that is why our club made use of the services of a member to take measurements, of various model types before formulating our clubs policy, which is electrics only. Any model deemed to be noisy is not permitted to fly in the offending condition. . . .

Just as a matter of interest, have you come up with a suitable figure for electric models yet? Our club has two fields, one of which was for "silent" models, which was originally bungee-launched gliders and then electric-powered gliders. If there was any doubt, our rules said it was up to a club committee member to decide whether any particular model was too noisy to be deemed to be "silent". Eventually, of course, members wanted some actual numbers because more and more people wanted to fly electric non-glider models, so the committee one day listened to a few common models, decided which were quiet enough, and measured their dB levels. The figure we arrived at was 72dB(a), which is now the rule for that particular site.

[Erfolg]

Alan

I personally was not involved in the process. Although the results were made known, I cannot say what they were, or with any confidence provide ball park figures.

Our situation was perhaps a little different than many clubs. As a club we were acutely aware that we could not afford any complaints. A true example of what we potentially had to manage, was an incident that occurred with me (some +25 years ago). I was happily flying my model, having bungeed the model up. I became aware that some one was crossing the field to me, it was a woman. She was demanding that I land the plane. I asked why? The answer was because you are breaking an agreement with the council. By now I was down. She stated that the issue was the horrific noise from the model that ......... and so on. I said it is a glider, there is no noise. The discussion went on for some time.

Far more pertinently, a number of members had started flying IC RC models (some time later). We had always had manly diesel FF models, where the noise lasted seconds rather than 10-20 minutes. Also there were a number of people starting to fly, some form of F3b, with a hundred weight of Nicads,, pus electro slot and Zagis.

The committee decided that to reach any considered decision, facts were required. Particularly as the usual claims that electric models were as , or even noisier than IC engines. The measurements demonstrated that the view that electric models rivalled IC engines, was without foundation, the difference in noise levels very different, even when testing the noisiest Zagi.

In essence that is how the committee based there decision. Some members were lost. But our field had become more secure. It was a nuisance for me, as I flew a powered sailplane and was planning a vintage type IC model.

At my other club, that flies IC, I have been surprised how much noisier some small 4 strokes can be compared with modern glo engines, although it is big petrol models that are by far the noisiest. Although I have suspected that it is the airframe, being so light, that the noise is not damped, attenuated by the mass of the model as well as the engine itself. In some cases I suspect the airframe is acting more like a drum, than I would have thought.

In my opinion, IC engines in general are much better than they were some 15 years back. Although I do not care if the noise is from an electric, IC of any type, or gas turbine, we should consider if it is perceived as a noise nuisance and act appropriately.

Edited By Erfolg on 30/09/2015 23:48:56

Edited By Erfolg on 30/09/2015 23:50:15

[bert baker]

Posted by IanN on 11/09/2015 08:26:04:

If anyone wants a scan of the Sid King article and plan, mentioned above, for his diffuser silencer for the Z38, i'll be more than happy to send that on. Just PM me your email addy

My attempt at building the Sid King muffler is slowly coming along, I must get round to take some pics before I go to far with it.

I do hope it works,I have thought about stuffing the outlets slightly with some wadding but not 100% sure if it would be of any use.

Bert