Keepin' it Quiet
The main complaint about our hobby is... NOISE!
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.
1. Prop Noise
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...
- Increase the diameter (but this can sometimes lead to an increase
in tip speed)
- Increase the pitch of the prop
- Increase the number of blades i.e. 3 or 4 bladed prop
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.
2. Vibration
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 propeller as well as being dangerous (puts
excessive loads on the prop) will do 3 things...
- Sap power from your engine
- Increase the wear on your engine
- Creates vibration which in turn creates noise
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.
3. Exhaust Noise
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...
- Standard
- Quiet
- Super quiet
- Mini tuned pipe
- Full length tuned pipe
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. There are 'add on' silencers
called 'dustbins' or 'mouse's' Another additional measure you could
use is the gray rubber exhaust expansion extensions at around £4.00
each, a worth while investment. All of these silencers and accessories
are available from Just
Engines.
4. Carburetor or Induction Noise
Carburetors 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.
5. Fuel
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.
In conclusion
- Select the correct prop for the engine to keep the rpm below
350-400mph
- Use rounded tip props like APC
or Bolly
- Use anti-vibration mounts for your engine
- Make sure your cowl
is secure and your wheels aren't loose
- Invest in a different exhaust system if necessary
- Use air filters
- Use fuel with a lower 'Nitro'
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