Topic: Practical Noise Reduction dor Large Gas Powered Models Part 1

Noise Reduction Results For Big Petrol’s

Given that excessive noise is the most popular reason that flying sites are lost, it is a constant source of amazement that the modelling magazines do not publish noise levels when they review a kit/engine/silencer/prop combination.  I recently asked an editor of one of the big 2 magazines why this was and got the reply “ we don’t own a noise meter”!  The hobby, of model aircraft engineering is governed by the same noise law as Burglar Alarms and Ice Cream vans.  In that act it states that a model aircraft shall not exceed 82dB when measured in a wind of less that 5 mph at a distance of 7 meters when placed in a cradle 1 meter off the ground.  (I paraphrase not quote).  So like it or not your club has a noise limit.


I have jotted down some of my experiences in trying to get Zenoah 38, 45 and 62cc engine models down to or below 82db.  My experiments where carried out on the well known plan build plane, The Greenley which started life many years ago with a US (Quadra) 41 with standard silencer.  This and other models where noise tested using the same process that the Environment Agency would use if they where to turn up at your club site following a noise complaint.  EG Model secured on a stand 1 meter off the ground pointing into a wind of less that 5 mph, in open space and measured at exactly 7 meters from the centre of the engine at front rear and both sides.  The noise meter was a digital unit as available at Maplins for about £60 and calibrated by the local Council EA.  The complete test schedule can be found on the BMFA web site , the BMFA handbook and at the EA web site.

  It is most important that you know exactly how the measurements must be taken as it is very likely that the person sent out to do the test will never have seen a large model close to let alone carried out the test.  For example, if the wind is over 5 MPH you can refuse the test and reschedule.

 By using the methods below I have managed to get the Zenoah 45 powered Greenley down to a measured 8odB at 7 meters, and a Zenoah 62 powered 30% Glens CAP down to 82dB so it can be done.

Firstly let me state that I am not affiliated with any of the manufacturers named in the following article.  I did, however, get a great deal of assistance from Glen Fletcher of Glens Models and Toni Clarke.

Assuming there is some kind of muffler on the engine, the noise from a model plane engine comes from the following areas.

40% from the propeller
30% from the induction carburettor
20% from the exhaust
10% from the airframe

I found that the worst starting point for quieting down a model is to start with the muffler.  The first and cheapest action is to make sure that the airframe is as deadened as much possible.  This can be done by sticking foam pads along the inside of the fuselage and cowl.  Also make sure that the join between the wing and fuselage in similarly insulated.  I use the sticky backed foam that you find on the stalls at most model shows.  I tried soft engine mounts and found that with big petrol’s it is generally a lot of effort and expense with very little reward.  The engine power output is greatly reduced and in my experience starting the engine can be harder unless you have an Easy Start system or a good electronic ignition.  Also make sure that all flying surfaces are free from sloppy hinges and a great move is to seal the hinge gap with a tape.  Another tip is to cut short sections of silicone tube and super glue them into the cooling fins of the engine.  This will help absorb the high pitch frequency vibration from the head.

I tried soft engine mounts and found that with big petrol’s it is generally a lot of effort and expense with very little reward.  The engine power output is greatly reduced and in my experience starting the engine can be harder unless you have an Easy Start system or a good electronic ignition.

Tim, I've read all 3 parts of your thread and there's some interesting facts I didn't know about silencing, many thx.
However I'm puzzled by the above statements that power is reduced and starting is difficult? I've used soft mounts and consider them to be essential, especially on modern ARTF airframes as these are so light and fragile that the vibration can literally shake the airframe to pieces. Secondly these ARTF airframes lack any mass to absorb the vibrations (unlike plan/kit built models with solid balsa construction, foam wings etc) and hence act as resonators to amplify the noise. The use of soft mounts is of course de rigeur in F3A.

Paul.

ps. do you know what the formula is for converting from 2 blade to 3 blade prop?

Tim, I've read all 3 parts of your thread and there's some interesting facts I didn't know about silencing, many thx.
However I'm puzzled by the above statements that power is reduced and starting is difficult? I've used soft mounts and consider them to be essential, especially on modern ARTF airframes as these are so light and fragile that the vibration can literally shake the airframe to pieces. Secondly these ARTF airframes lack any mass to absorb the vibrations (unlike plan/kit built models with solid balsa construction, foam wings etc) and hence act as resonators to amplify the noise. The use of soft mounts is of course de rigeur in F3A.

Paul.

ps. do you know what the formula is for converting from 2 blade to 3 blade prop?

Hi Paul, sorry for the late reply:
Regarding soft mounts: My statement was made on the basis that if you cannot get the first 3 elements of noise reduction sorted then there is no point in spending time and money on soft mounting the engine.  I know there is a benefit to be had by softmounting, in F3A the benefit is more points for quiet running!!.  However I would say that for the average clubman a Toni Clarke Hydramount,  or similar, is the last part of the puzzle.  Most folks I come across go about the problem the wrong way round (arse backwards.  For example a club mate of  mine has a DA50 in an 80 inch  Yak 53.  The first sound check carried out in a 5mph wind with a calibrated sound meter on a tripod at 7 meters, gave a reading of 89db, a fail at my club.  The next week the model was presented with "a new in line silencer".  Sound check gave 93 db.  Inspection proved he had not listened and had purchased a can which on inspection was proved to be a carbon tuned pipe which increased the top end by 1500 rpm.  So we start on the tail chasing I write about.  This is typical of the way the majority attack noise.  Another mistake is to take the intake inside the fuselage and leave it open.  This in effect makes the large body of the fuselage a megaphone.  Simply boxing the intake in kills the noise straight away.  So, sure softmounts make a difference but it is the last part of the jigsaw.


With regard to 2 blade to 3 blade conversion:  The object is to reduce the tip speed so the starting point is to take the max RPM on a 2 blade prop.  Compare that to the graph and see how many RPM's you need to drop by.  Say it is 500 rpm on the graph.  Next fire up the engine and with the meter and plane in the correct place, increase the revs until the sound level you are aiming for is on the meter, then measure the RPM.  Then compare the 2 figures.  In practice they are never the same as wind speed and design of model has an effect.  Then by using the manufacturers performance figure (this where Zenoah are best), choose a prop that will drop the RPM and keep the thrust.  In practice the starting point for Fiala props is to go down 1 inch in diameter and up 2 in pitch.  For example on by 30% Glens CAP with a Zenoah 62 standard Toni Clarke drum silencer, I  went from 24 x 10 to a 23 x 12 to go from 90 db to 84 db (my club level is 85db).  After fitting the 90 degree bend etc, the RPM went up by 600 rpm so I went to 22 x 12 and got down to 83 db.  The secret is that the torque curve flattened out giving the same performance.  A good starting point is down 1 inch in diameter and up 2 in pitch.   

The problem is that a 22x12 is not going to perform like a 23x8?

I have had a very different experience to reducing noise than you seem to have... I find that with DL, DLE and 3W that the prop (within reason) has very little effect on the noise of the model.  It is the exhaust that has by far the most effect in my experience.  The stock exhausts with the above engines are earsplitting  ~90dBA at 7m.  Fitting an ALUMINIUM header and canister along with a carb intake filter have brought the noise down to ~80-82dBa and still allowed me to fly my 23x8 mejzliks.

I agree the carbon fiber pipe you refer to is not a quiet option!

CM

The problem is that a 22x12 is not going to perform like a 23x8?

I have had a very different experience to reducing noise than you seem to have... I find that with DL, DLE and 3W that the prop (within reason) has very little effect on the noise of the model.  It is the exhaust that has by far the most effect in my experience.  The stock exhausts with the above engines are earsplitting  ~90dBA at 7m.  Fitting an ALUMINIUM header and canister along with a carb intake filter have brought the noise down to ~80-82dBa and still allowed me to fly my 23x8 mejzliks.

I agree the carbon fiber pipe you refer to is not a quiet option!


http://www.rcmf.co.uk/4um/index.php?action=dlattach;topic=87378.0;attach=113572;image
CM

No single item will achieve the desired aim of a quieter overall noise output.  The point is that an holistic approach has to be taken.  It is not possible for the tips of a 23 inch propeller running over 6000 rpm not to exceed 600 mph and generate noise as it approaches the speed of sound.  You must have limited the top rpm of your throttle curve.   The original point of the article was to demonstrate, with practical examples, how most modellers approach silencing a model the wrong way round.  For the record my measurements are taken with a £450 digital meter that is calibrated by the EA to within +/- .005 .  A  comparison with a cheap analogue meter and an I phone App showed an under reading of these two of 4 db.  As Decibels are a logarithmic measurement, that is obviously not an error of 4x1.  For example the difference between 80 and 90 db is not 10 but 30. I use a wind speed meter to ensure that if the wind speed is over 5 mph I can use the correct algorithms to factor the result.  The tests are carried out in accordance with EA directive e.g. model measured 7 meters from the center of the engine, downwind and rotated to present head on, port side, starboard side and tail, (with the exception of supporting the model 1.5 meters above the ground on a cradle as when this was attempted with models of 2 and 3 meter wingspan it resulted in conditions that where risk assessed as unacceptable to the model and operatives.  Models under 1 meter wingspan where supported on a cradle)  In short, it's not just about pacing off about 7 meters and having a couple of mates hold it down and a quick look at the phone. 

Another factor of reducing the prop size apart from reducing the tip speed is found when carrying out the test behind the model.  This is most often where the highest reading s are taken.  My conclusion is that the prop wash from a large diameter prop can greatly increase the decibels.  The EA test stipulates that it is the loudest reading that counts not the average.  I would be glad to test your plane under these conditions 

Tests carried out exactly as you describe, using the same calibrated digital sound meter as the leicestershire environment agency use.  loudest reading from the front of around 83dBA, 80 from the rear and sides, no atv on top end.  your welcome to test any of my models at any time.

CM