Keepin' it Quiet
If a club has a noise complaint made about it to the local authorities then
they are obliged to investigate. If any model is found to be over the BMFA
guideline of 82dB(A)
at 7m, then it is possible the club will loose it´s flying site. See "How
to Noise Test a Model Aircraft
(The BMFA way)"
A way to stop these complaints arising in the first place is to make sure our
models are as quiet as possible.
Although it is not possible to make every model quiet here are some ideas.
In my experience this is one one of, if not the biggest causes of noise. When noise testing a model the BMFA stipulate that the meter should always be down wind of the model. This means that when an aircraft is being tested in it's noisiest direction, i.e. with the nose of the aircraft pointing towards the meter a small gust of wind will all ways go through the back of the prop, this has caused a jump of up to 6dB (A) this just shows how much noise a prop generates. To keep under the 82dB(A) BMFA guideline a propeller's tip speed should be kept to 350mph or less. Old fashioned flat ended props are less efficient and make more noise then the modern narrower rounded end props from manufacturers such as APC or Bolly. It has been found that prop tip speeds can be raised to as high as 400mph and still be within the 82dB(A) limit. Please see the propeller tip speed chart which gives a good indication as to the RPM you should be aiming for, for set diameters.
To lower the RPM an engine has to be loaded more. There are 3 way to do this...
For example a .60 size engine using an 11x7 prop may do 13,500rpm and have a tip velocity of approx. 450mph. If you change the prop to a 12x8 the rpm would drop to approx. 11,000rpm and a reduction of the tip speed to approx. 400mph, therefore a reduction of around 50mph in tip speed is roughly a reduction of 2dB(A). The conclusion is "choose your propeller carefully". See Choosing a prop for your engine for more information on suggested propellers.
Using a prop with 3 or 4 blades puts more load on an engine and therefore reduces the RPM, the prop tip speed, and hence noise.
Vibration as well as being damaging to your expensive and critical radio gear is also a source of noise.
The most common way of reducing vibration is to fit an anti-vibration mount.
A good quality anti-vibration mount (the Hyde mount for my YS140L in my ZN line Majestic cost £120.00!) can reduce noise by up to 3dB(A). Noise is measured on a logarithmic scale (I bet when you were in school you thought you'd never use log scales in "real life") so a reduction of 3dB(A) is roughly half the noise!
Apache Aviation make a good range of anti-vibration engine mounts.
An airframe with big open spaces such as "fun fly's" act as a drum, so it is especially important on this type of airframe to pay attention to noise.
A non or badly balanced propellor as well as being dangerous (puts excessive loads on the prop) will do 3 things...
So invest in a prop balancer and balance those props!
Engine vibration also causes cowls and wheels to generate a lot of noise. When testing a friends model the noise level suddenly increased by 3dB(A) without the model moving, I looked up to see a cowl screw come loose and fall to the ground. So make sure your cowl is secure and your wheels aren't loose.
Most people think this is the the only source of noise, it's not. Modern silencers have a baffle inside to reduce noise and you should never run the engine without a silencer. There are 5 main types of silencer...
A full length tuned pipe such as Just Engine's "Purple Pipe" range are very good at "keepin' it quiet" as they have been specially designed to be quiet. In my experience the "BCM" in-cowl sometimes known as a "Pitts style" silencer, although sound nice, tends to be very noisy. If an in-cowl silencer is required then your best bet is to go for the "Aviation Alloys", they are expensive, but you do get what you pay for. Another additional measure you could use is the grey rubber exhaust expansion extensions at around £4.00 each, a worth while investment. All of these silencers and accessories are available from Just Engines.
Carburettors make a certain amount of noise sucking in air. Fitting an air filter helps to stop this and stops debris getting in your expensive engine and damaging it. Air filters can be fitted to nearly all 2 and 4 strokes.
Reducing the "Nitro" content of your fuel will also reduce noise as "Nitro" reduces the temperature at which fuel ignites and makes it burn rather than explode so burning is still taking place when the exhaust port opens.