Get Noise Down To Acceptable Limits

Keep Your Site With Quiet (& Low RPM) Flight!
By Norman Carter- Chairman- Felbridge Flyers RMFC

Felbridge Flyers LogoI know what you are thinking "Oh no, not another article about noise - it does not affect me as our Club do not have a problem", well read on, you and your Club may be part of the problem.

The Club I belong to, Felbridge Flyers RMFC, fly in the crowded S.E. of England where finding a flying site that meets DoE guidance on distances from houses is none too easy. When you do find one you have to do all you can to keep it. For many years now we have noise tested models and been very insistent (some say dictatorial!) that models are within the DoE code of practice limits. We actively encourage doing much better than the maximum figures, where possible using large props to keep engine speed down.

Before any new model is allowed to be flown it had to be noise tested, if it passed it was issued with a "pass" sticker giving details of model, engine, prop and silencer arrangement, this would be affixed to the airframe to prove to other members that it is "legal". We also record details in the Club's computer system, actually my PC - what did we do before we had these? What we discovered was that most older engines with standard silencers got nowhere near the standards required, anything .40 sized or above needing additional silencing by using after silencers. Later engines being much better only needing the excellent OS mute kits inserted in the silencers to pass. The latest generation of engines, i.e. engines like the OS LA series, are much better, getting well inside the noise limits with no additional silencing.

Contrary to popular belief it is not only 2 stokes that are the difficulty, 4 strokes pose a different problem. Their exhaust note tends to be louder but can be effectively silenced by simple modifications. Again modern engines have more effective silencers, e.g. the Laser Quiet silencer does what it says on the label......

Where 4 strokes can be better, providing the exhaust is silenced, is in the annoyance factor. A 4 stroke has only one "bang" every 2 revolutions but a 2 stroke one "bang" for every revolution. The perceived frequency buzz from the 2 stroke at 10,000 rpm is 166 Hz while the 4 stroke at the same speed is 83 Hz. Admittedly the "bang" of a 4 stroke can be louder as the combustion pressure is higher. You may say this is a vast over simplification of a complex problem and you would be right, it is however our Club's practical appreciation of the problem.

Noise Testing EquipmentWith the threat of our current field being sold we did find another field, meeting all DoE requirements, and determined we must do all we could to retain it's use. Even though the nearest house was almost 700m away noise (or annoyance) was , the Committee decided, going to be a major factor. We needed to be on top of this from the start so swallowed hard and paid 600 for a new noise meter and calibrator meeting the appropriate British Standard as contained in the code of practice, BS 5969 type 2. It is a Dawe D-1422C and calibrator D-1411E, Radio Spares supplied ours.

We proceeded to retest all our members models and issued them with new style pass stickers and kept our records updated. It was encouraging to note that 95% of members models were below 80dBa with significant numbers being in the 75 - 76 range. It was at about this time that the Club started to arrange some fun fly events inviting other Clubs. We tested their models and found that the majority did not get through the standard noise test, often by large margins. Questioning these people the normal reaction was "I don't have any problem at my Club, why here?", "surely it's the average reading you take not the highest?" (this from the Chairman of a visiting Club!).

We were very strict and only allowed people to fly if they got within the noise level. Some people were unwilling even to change a prop and annoyed that they could not fly. We did point out in our invitations that noise limits would be strictly applied; it seems many Clubs do not have a noise meter or access to one. This is something that the S.E. Area Committee of the BMFA has taken on board, they now have a noise meter and a call to them should have one of the Committee along to your Club.

This brings me onto another significant area, prop noise and rpm. Our tests had indicated that different props can have a significant effect on noise emissions with some props being noisy and others less so, even at the same size. This is understood by manufacturers and props with better characteristics are on the market (i.e. APC). These props sometimes improve the situation and sometimes don't so are not the universal panacea. Experimentation here is the key, try a different one and test again.

Once satisfactory compliance with noise levels is achieved it is not the end of the story, you have to deal with the "annoyance factor", that is much harder to achieve. A noisy model can be clearly heard a considerable distance from the take off point but the noise can seldom be measured at any sort of distance, what we have to contend with now is the annoyance factor. We invited the Local Council's Environmental Health officer to our new site and his comments in a letter to us after observing at 400m from our take off point were "the noise of the model aircraft engines were audible from time to time but for the most part were drowned out by other environmental noise" . No, the secondary problem is the annoyance factor of 2 stroke engines at high rpm. A mosquito buzzing around your bedroom at 3am has no measurable sound output but it sure is annoying.

Quieter models won't even be heard however and so the annoyance factor should be less. Having said that one Club local to us was the subject of a noise complaint while they were flying gliders off towlines, clearly the sight of model aircraft was something the complainant did not like and they assumed they were making a noise! Unfair though this seems we have to realise that our pastime is seen as boys with toys by a lot of people and we will have to make exceptional efforts to keep our cherished flying sites. The Not In By Back Yard (NIMBY) culture is alive and well. As an aside, have you ever noticed the amount of noise emanating from a local league football match on your playing fields - they surely need some form of silencing!!!!!!!!!

Keep Your Site With Quiet FlightOur Club Committee decided that action was required to keep our new site so proposed a radical new rule at the April AGM, not only did we propose a reduction of the maximum sound output (only to 81dBa) but also a maximum ground run rpm of 10,000 for i/c powered aircraft. As can be imagined there was much debate over this proposal. The Committee explained that over the months leading up to the AGM lots of tests had been conducted and the 10,000 rpm figure was seldom exceeded by members models in any case, as we had encouraged large props to keep noise output down. This was the case with small engines of .12 capacity upwards. Again more money needed to be spent on an electronic tachometer to enable readings to be taken from behind the prop (no one wants to be 30mm in front of a 1.50 engine at full power!).

As can be imagined there were sections of the membership that were sceptical, "If the manufacturer says use a 10 x 6 on this .51 at 16,000 rpm who are we to argue?" This was primarily from new members unused to our ways and evolving ideas. We have always argued that it is THRUST you need not rpm, if a larger prop turning slower gives more thrust it is the way to go. Engine manufacturers are in the game of selling engines, and it's power that seems to sell. They need to quote the best power rpm - usually above the practical rpm range in any case. What we need is torque to turn props effectively and this happens at lower rpm, thank goodness. We have found little problem in running i/c engines of most sizes below 10,000 rpm at full throttle. I suppose an analogy could be your car, it gives maximum power at around 6,000 rpm which would be 100+ mph in top, but it PULLS more strongly at around 3,500 - 4,000 rpm, the peak of the torque curve. It is driven most of the time in the peak torque regime - think about it. It needs a culture change to encourage people to use these more neighbour friendly lower rpms together with lower dBa outputs. Getting modellers to use that throttle control during the flight, not tear around with the stick nailed to the full speed position, will also help. Members who have changed to larger props to get below the rpm limits have been surprised that the performance and handling of the models have been improved. OK, you may not get the high speed missile models but you will still have somewhere to fly.

It is also good to see manufactures building engines for lower rpm operation with better torque characteristics. Modellers are also beginning to appreciate that less noise and rpm is the way to go and are demanding engines to suit. It was good to see effective noise control at a local BMFA scale day at Ardingley - some models were presented registering 90 dBa at 7 metres, not allowed to fly thank goodness.

Click here for a chart showing a selection of recent noise and rpm checks on a range of engines from .12 two stroke to 1.50 four stroke, it can be done.

Keep Your Site With Quiet (and low rpm) Flight.

This page has been contributed by Norman Carter
Web Site: Felbridge Flyers RMFC
This Article was first published in BMFA News,
and has been reproduced with their kind permission