What are the limits?
The noise limit for the Club has been defined by the Council ByLaws which is 82dB(A) on all four faces at 7mts and 1 mtr above ground. This value shall be the maximum value and NOT the average of the 4 noise readings. For example, a model reading 85dB(A), 80dB(A), 79dB(A) and 76dB(A), fails the test because the maximum noise output is 85dB(A).
Note that an increase of 3dB(A) doubles the noise level; the dB(A) scale is logarithmic, not linear.
Can I fly my model?
Yes and no. If your model has failed the noise test you may be allowed to fly for a short period whilst investigating ways to further reduce the noise level. It would be wise however to throttle back whenever possible to minimise the noise output. You must, however, make some modifications to reduce the noise level before bringing the model to fly at the Club again. The member should arrange to meet a Committee member to check the noise output of the plane before it can be re-flown. If a Committee member deems your model too noisy, then you cannot fly.
How can I quieten down my model?
Propeller A very easy fix is to increase the pitch or diameter or change the make of your propeller. Some props are quieter than others; APC props produce less noise than flexible or "unbreakable" props or those which have square tips such as Master Propellers. Graupner grey props are also quiet. A well-balanced propeller and spinner will produce less vibration and therefore less noise, so check this aspect before progressing.
Tip Speed Propellers get noticeably louder as the tip speed approaches 400 mph. This speed can easily be calculated by multiplying the prop diameter by the RPM then dividing this by 336.
Engine Speed Try and keep the max revs down to below 10k. A high revving engine is ALWAYS noisier than a lower revving one. A quick and simple change which almost always makes a difference. See Propeller section.
Mount It is not always possible, but soft mounting or rubber mounting your engine (even a petrol engine) can often reduce a lot of resonant airframe noise. This is always good practice as it reduces the vibration received by the radio gear and normally reduces the probability of things shaking loose or failing. Apache Aviation mounts are excellent in reducing transmitted engine noise to the airframe, as well as being cheap.
Silencer/Pipe With two-stroke engines you can add an extra chamber to some standard exhausts or affix an after silencer - e.g. an Irvine mouse. Some tuned pipes can reduce noise but some can increase it, so check before you buy. With four-stroke engines, you can replace the standard silencer with a tuned pipe, and/or add a "pepper pot" type attenuator to the pipe outlet. Petrol engines almost always need their silencers changing to a larger capacity one to reduce emitted noise down to Club levels. A big silencer is always better than a small one.
Airframe Check your airframe for any loose components. Check the silencer and engine mounting bolts for tightness. Wheels can often resonate; sleeving the axle or just adjusting the collets can fix this. Some unsupported pushrods can vibrate against formers down the fuselage and cause a rattling sound: these can be changed to run in guides or be replaced with snakes. Short pushrods are preferred, but the longer pushrods commonly used should be supported at least twice along its length.
The Club will do all it can to help you pass the noise test, so please ask for advice and help. Almost all models can be made to meet Club noise levels with some effort and with no loss of performance. In fact, in many cases your model will fly better following changes, especially to the propeller!
EDMAC Committee View
The Committee has discussed this issue at great length and it may well be the case that if an aircraft passes a noise check on the ground but is still perceived to be noticeably noisy in the air the Committee will ask the pilot to make further improvements in noise reduction. Noisy aircraft will lead to complaints and put at risk our rights to fly on the Downs so it is in everyone’s interest that members ensure that their aircraft are as quiet as possible.
Author: Roger Marples, July 2012