How to safely noise test large models

[Nik Harrison]

I would like suggestions and/or approved method of noise testing larger models.  The model in question is a B-25 with 2x22cc petrol engines, span 2.5m and weight around 10Kg.  By modern standards this is not a particularly large model but it does mean the simple "hold it up on WOT" is a bit optimistic.  Any suggestions welcome.

[PeterF]

When I was looking after noise tests at my club, larger models were tested restrained on the ground with the props and engines above grass, not a hard surface such as paving or tarmac to dampen reflected sound from the ground. Based on measurements of several planes both on the ground and held at waist height I found that measuring on the ground increased the measurement by 2-3 dBA. Therefore, we allowed an extra 2dBA for planes measured on the ground.

 

Another factor is being in front of a prop at WOT, even 7m away. A few years ago I recall an incident report from the BMFA where a prop on a large petrol engine had come off during a noise test (can not recall the engine capacity). From memory there was a recommendation to have netting between the model and the tester or have the meter on a tripod with maximum value recorded. How netting impacts the noise measurement I do not know.

[Paul De Tourtoulon]

Maybe a flypast would be better, I do track days with my Aprilia rsv4-r, and the track regulation it's 95dB on week ends and 100dB in the week, my bike is street legal at 104dB at 3875 rpm and 109dB at 6125rpm, they let me on the track, and as I pass by the sonometer when I go up the straight at 200 klm/h it passes, so what my point is, why do a ground test, it's like model 4 strokes a few make more noise than a 2 stroke but are less offensive on the ear.

Edited August 22, 2023 by Paul De Tourtoulon

[Ron Gray]

2 hours ago, Nik Harrison said:

the simple "hold it up on WOT" is a bit optimistic

Dangerous on any model IMO! @PeterFhas the right idea, on the (grass) ground and knock 2dB off the results and don't stand in front of the prop!

[Paul De Tourtoulon]

I still don't see the point of dB testing on the ground, when I did helicopter competitions they were tested while hovering at 1m50.

[Ron Gray]

Couldn't agree with you more Paul!

[Jon H]

I wouldnt bother. Its a B25 so shouldnt be screaming round at full chat anyway. Just fly it and see if it sounds excessively noisy. 

 

Having noise tests completed will mean nothing if there is a complaint so just forget it. We all know when a model is loud, and a model may be louder in the air than on the ground if the prop starts howling so its all a bit pointless to be honest. 20cc class petrol 2 strokes, 17x8 props @ 8500-9000rpm? Assuming a half decent exhaust it wont be that bad for noise in db terms. 

[Dickw]

45 minutes ago, Paul De Tourtoulon said:

I still don't see the point of dB testing on the ground, when I did helicopter competitions they were tested while hovering at 1m50.

Just trying to imagine someone prop hanging the B25 in the OP at 7m upwind of me while I noise test it 🤣

 

Dick

 

 

[Paul De Tourtoulon]

2 minutes ago, Dickw said:

Just trying to imagine someone prop hanging the B25 in the OP at 7m upwind of me while I noise test it 🤣

 

Dick

 

 

Easier than holding Wren's B52 back while testing for dB's,,, 40 kilos of hot air,,,🥳

 

[Martin McIntosh]

Difficult question. I think that you would need to be midway between them from the front since they are probably cowled. Next thing is how far away and over what sort of surface? Long ago in my F3A days we soon discovered that at 7 or 10m the wind noise was usually more than the motor produced so somebody calculated what the equivalent at 1m would be, negating the wind. I cannot now remember the figure but it could have been 92 or 94 dBa. This was over grass or the more usual tarmac, with the model on the ground because flight time was at a premium at a competition.

Petrols are a real problem because the dBa scale favours the higher frequency of a 10cc glow at 12-14000 rpm.

[Nik Harrison]

Thanks for all the replies.  I would have to say that given that the replies were "opinions" it seems that nobody was able quote a definitive BMFA  method for the larger models.  I agree that the views expressed are all very sensible and no doubt that is how we will proceed.  Our site has a 80dBA limit which does not help.  Maybe it's about time the BMFA did have a definitive method for larger models.  Just a thought!

[GrumpyGnome]

Does your club not have 'rules' or guidance of how to measure the 80db limit?

[Martin Harris - Moderator]

Just bear in mind that it isn’t a BMFA noise test but as laid down as guidance for good practice by the DfE.  The BMFA recommendation to use it is because it’s the only “official” procedure available to ordinary clubs/flyers and it may bear a little weight in a dispute. 
 

A full noise test after a formal noise complaint involves taking a number of readings at various locations and comparing the collated results with the ambient noise level. 

[Nik Harrison]

Martin,

Thank you for the clarification on the noise test parameters.  We will take it into account at our club.  Our 80dBA limit is a result of a council imposed limit and we do our best to ensure we get no complaints.  GrumpyGnome will be pleased to hear we have been following the BMFA guidelines on how to do the noise test with a calibrated noise meter, but that guidance was not safe when testing larger multi-engined models hence my query.  Thanks for all the replies.

Nik

[Zflyer]

We recently completed  noise (acoustic) testing at our site when making our planning application, in part because of complaint. It was conducted by an acousric engineer. We used an assortment of i.c. and electric,no edf. All the aircraft were below or on the 80 limit on the ground. They were also tested in the air.

Suffice to say the report was fine and no doubt assisted our successful application.

 

 

[Paul De Tourtoulon]

I would like to see what set up your 10cc methanols and your petrol range to get below 80dB, and how you went about testing it, a video would also help.

Edited August 23, 2023 by Paul De Tourtoulon

[Martin McIntosh]

PDT, the methanols were set up with tuned pipes which enabled a silencer (alloy can) to be alloy soldered to the rear then either a pepperpot end from several 1/16th holes or a small bore tube fitted. Because the gasses had cooled considerably by that time then the outlet could be very small. Also, changing from an 11x7 wood to a 9 3/8th three blader had a huge effect, albeit with a large reduction in power, but I am talking of 15,000 plus rpm. A figure of <79 dBa or the equivalent of whatever it was could be achieved (79 is half the noise level of 82).

Video? We are talking of the mid 1970`s.

 

Nik, the LMA should be able to help.

[Paul De Tourtoulon]

Ok, I had a couple of those screaming 60's with pipes in the fuselage, my 60 powered helicopter has one but more like 90dB ( I will test it on its next flight)

I don't like 3 bladed props, and even with all that piping, mine aren't exactly quiet.