How do you test Engine DB's Properly
09-28-2007, 06:52 PM
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How do you test Engine DB's Properly
My local club has a noise level max of 100 DB. Thats not bad I know but I want to know how to properly test??? Do I stand 90 Deg and point the bird toward the wind (if any) and how many feet away is the correct way?
I beleive that the guys at the club are just walking up anywhere between 5 and 10 feet away around 90Deg at the side with the meter pointed directly at the engine and have the throttle full open while sitting on asphault??? Is this right? If I were on the grass and had wind to help, the DB's would measure much different.
I hope that I posted this in the right area as I have had no luck finding answers to this in existing threads nor on AMA's website. I found some stuff but it pertained to certain competitions.
Thanks guys
Shayne
I beleive that the guys at the club are just walking up anywhere between 5 and 10 feet away around 90Deg at the side with the meter pointed directly at the engine and have the throttle full open while sitting on asphault??? Is this right? If I were on the grass and had wind to help, the DB's would measure much different.
I hope that I posted this in the right area as I have had no luck finding answers to this in existing threads nor on AMA's website. I found some stuff but it pertained to certain competitions.
Thanks guys
Shayne
09-29-2007, 05:42 AM
#2
RE: How do you test Engine DB's Properly
Extract from the British Model Flying Association (BMFA) Hand book in the UK.
METHOD OF MEASUREMENT OF NOISE
EMITTED BY A MODEL AIRCRAFT
The model, working at maximum rpm, should be
held between 1 and 2 metres off the ground in a
bracket or by one person standing upwind of the
model. The microphone of the noise meter should,
where necessary, be fitted with a windshield. It
should be positioned downwind of the model, 7
metres away from it (measured horizontally), and
1.2 metres vertically from the ground. The wind
speed should not be over 3 metres per second
(force 2).
Measurements should be taken in the open air and
away from reflecting surfaces such as buildings.
The noise level emitted, for the purposes of this
Code, shall be the maximum value of four readings:
(I) model pointing directly at the meter,
(ii) model pointing directly away from the meter,
(iii) model pointing 90o to left of meter,
(iv) model pointing 90o to right of meter.
The noise meter shall be in accordance with
BS5969:1980 (‘Specification of sound-level
meters’), using the A-weighing response, and with
the meter set to ‘slow' response. The meter should
be calibrated prior to use.
Hope this helps.
MIke
METHOD OF MEASUREMENT OF NOISE
EMITTED BY A MODEL AIRCRAFT
The model, working at maximum rpm, should be
held between 1 and 2 metres off the ground in a
bracket or by one person standing upwind of the
model. The microphone of the noise meter should,
where necessary, be fitted with a windshield. It
should be positioned downwind of the model, 7
metres away from it (measured horizontally), and
1.2 metres vertically from the ground. The wind
speed should not be over 3 metres per second
(force 2).
Measurements should be taken in the open air and
away from reflecting surfaces such as buildings.
The noise level emitted, for the purposes of this
Code, shall be the maximum value of four readings:
(I) model pointing directly at the meter,
(ii) model pointing directly away from the meter,
(iii) model pointing 90o to left of meter,
(iv) model pointing 90o to right of meter.
The noise meter shall be in accordance with
BS5969:1980 (‘Specification of sound-level
meters’), using the A-weighing response, and with
the meter set to ‘slow' response. The meter should
be calibrated prior to use.
Hope this helps.
MIke
09-29-2007, 08:18 AM
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RE: How do you test Engine DB's Properly
All commercially available hobby dB meters (hobby shack etc.) I tested were not off more than 0.5 dB. This is fine for our purpose, even without calibration.
Beware:
some meters reqire you to point the meter at 90° to the noise source, so read the instructions carefully.
Beware:
some meters reqire you to point the meter at 90° to the noise source, so read the instructions carefully.
09-29-2007, 08:28 AM
#4
RE: How do you test Engine DB's Properly
ORIGINAL: bzyguy01
My local club has a noise level max of 100 DB. Thats not bad I know but I want to know how to properly test??? Do I stand 90 Deg and point the bird toward the wind (if any) and how many feet away is the correct way?
I beleive that the guys at the club are just walking up anywhere between 5 and 10 feet away around 90Deg at the side with the meter pointed directly at the engine and have the throttle full open while sitting on asphault??? Is this right? If I were on the grass and had wind to help, the DB's would measure much different.
I hope that I posted this in the right area as I have had no luck finding answers to this in existing threads nor on AMA's website. I found some stuff but it pertained to certain competitions.
Thanks guys
Shayne
My local club has a noise level max of 100 DB. Thats not bad I know but I want to know how to properly test??? Do I stand 90 Deg and point the bird toward the wind (if any) and how many feet away is the correct way?
I beleive that the guys at the club are just walking up anywhere between 5 and 10 feet away around 90Deg at the side with the meter pointed directly at the engine and have the throttle full open while sitting on asphault??? Is this right? If I were on the grass and had wind to help, the DB's would measure much different.
I hope that I posted this in the right area as I have had no luck finding answers to this in existing threads nor on AMA's website. I found some stuff but it pertained to certain competitions.
Thanks guys
Shayne
What is your purpose? Do you simply want to measure your plane for compliance with the club rule? Or are you concerned about noise complaints from neighbors?
If you have concerns about neighbor complaints, the place to measure sound levels is in the neighbor's yard. The noise heard there should not interrupt conversations.
Good luck,
Dave Olson
09-29-2007, 10:05 AM
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RE: How do you test Engine DB's Properly
Baldeagel....Great info....I'm not sure if those are the same specs that I need to abide by Tho...Regardless that was a 100% reply to my thread...Thank you very much!
pe reivers...Thank you....I have already purchased that Digital Radio shack meter and am currently learning to use it properly.
Scar.....Good point...and this is exactly why I purchased my own meter as a few club members feel that they are above the law and test in so many different fashions as to how and who and when they want....I want the Proper method to be used on all of us....not just certain individuals they want to pick ....we all know that the method of DB testing will have great varying results. I want to comply....But I want to comply on a fair feild if you get my drift. Its kinda funny how 2 feet and 90Degrees will effect the DB's huge! Thank you for your reply.
pe reivers...Thank you....I have already purchased that Digital Radio shack meter and am currently learning to use it properly.
Scar.....Good point...and this is exactly why I purchased my own meter as a few club members feel that they are above the law and test in so many different fashions as to how and who and when they want....I want the Proper method to be used on all of us....not just certain individuals they want to pick ....we all know that the method of DB testing will have great varying results. I want to comply....But I want to comply on a fair feild if you get my drift. Its kinda funny how 2 feet and 90Degrees will effect the DB's huge! Thank you for your reply.
09-29-2007, 11:10 AM
#6
RE: How do you test Engine DB's Properly
Just as an update, the BMFA have now said that the DB rating is not an average of the four readings, but the maximum reading obtained i.e. if one reading is over that specified then it has failed, our own limit is 97DB and there is talk of this comming down too 90DB in the near future. Glad I could help.
Noise and even stray light is pollution after all.
Mike
Noise and even stray light is pollution after all.
Mike
09-29-2007, 03:50 PM
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RE: How do you test Engine DB's Properly
We measure at 7 meters distance, officially the mean of four, plane and meter at 1.2m height.
Max wind speed three m/sec for valid measurements, no reflecting objects near the site, and measure over short grass.
At this distance we have 80 dB(a) max, and a few centimeters more or less than 7 meters do not affect the measurements much.
Max wind speed three m/sec for valid measurements, no reflecting objects near the site, and measure over short grass.
At this distance we have 80 dB(a) max, and a few centimeters more or less than 7 meters do not affect the measurements much.
09-29-2007, 04:47 PM
#8
RE: How do you test Engine DB's Properly
ORIGINAL: pe reivers
We measure at 7 meters distance, officially the mean of four, plane and meter at 1.2m height.
Max wind speed three m/sec for valid measurements, no reflecting objects near the site, and measure over short grass.
At this distance we have 80 dB(a) max, and a few centimeters more or less than 7 meters do not affect the measurements much.
We measure at 7 meters distance, officially the mean of four, plane and meter at 1.2m height.
Max wind speed three m/sec for valid measurements, no reflecting objects near the site, and measure over short grass.
At this distance we have 80 dB(a) max, and a few centimeters more or less than 7 meters do not affect the measurements much.
-the meter as noted -is a RS I have had for many years and is for all intents and purposes "good enough".
On this hard packed salt/mud - we performed the tests from all points and met (barely) the published requirements
to do this:-- The ZDZ40 F3A was running a 20x12 ZDZ prop -wooden - in low 6000rpm the exhaust system is a Carbon fibre full tuned type (Ed Skorepa) and the all composite fuselge was carefully taped and tightened and shimmed etc -to eliminate all buzzing and rattling --the fuselage sounds--sounded awful in the air and had to fixed to meet the sound test
the test -by the numbers -is a POS .
But -part of the rules - so we did it -by the rules.
None of the in cowl gassers have a chance of meeting this test done correctly.
Power wise- this plane is a rocket but appeals to a very limited audience - the entire setup is expensive.
09-30-2007, 09:19 AM
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RE: How do you test Engine DB's Properly
I believe this is the FAI "quick test", and if I am informed right they too specify grass. Over a hard surface you get about 3dB more than over grass. A pity grass won't grow on the salt of your field, but maybe a coconut fibre mat on the floor between plane and meter could be used as substitute?
09-30-2007, 11:24 AM
#10
RE: How do you test Engine DB's Properly
what is grass?
Here's an excerpt from the FAI Sporting Code.
The maximum noise level will be 94 dB(A) measured at 3 m from the centre line of the model aircraft with the
model aircraft placed on the ground over concrete or macadam at the flying site. With the motor running at full
power measurement will be taken 90 degrees to the flight path on the right hand side and downwind from the
model aircraft. The microphone will be placed on a stand 30 cm above the ground in line with the motor. No
noise reflecting objects shall be nearer than 3 m to the model aircraft or microphone. The noise measurement
will be made prior to each flight. If a concrete or macadam surface is not available then the measurement may
be taken over bare earth or very short grass in which case the maximum noise level will be 92 dB(A).
In the event a model aircraft fails the noise test, no indication shall be given to the pilot, and/or his team, or the
judges and both the transmitter and the model aircraft shall be impounded by the flight line official
immediately following the flight. No modification or adjustment to the model aircraft shall be permitted (other
than refueling). The model aircraft shall be retested by a second noise steward using a second noise meter and
in the event that the model aircraft fails the retest, the score for the preceding flight shall be zero.
The flight time will be interrupted while the noise check at the flying site is being made. The competitor shall
not be delayed more than 30 seconds for the noise check.
Here's an excerpt from the FAI Sporting Code.
The maximum noise level will be 94 dB(A) measured at 3 m from the centre line of the model aircraft with the
model aircraft placed on the ground over concrete or macadam at the flying site. With the motor running at full
power measurement will be taken 90 degrees to the flight path on the right hand side and downwind from the
model aircraft. The microphone will be placed on a stand 30 cm above the ground in line with the motor. No
noise reflecting objects shall be nearer than 3 m to the model aircraft or microphone. The noise measurement
will be made prior to each flight. If a concrete or macadam surface is not available then the measurement may
be taken over bare earth or very short grass in which case the maximum noise level will be 92 dB(A).
In the event a model aircraft fails the noise test, no indication shall be given to the pilot, and/or his team, or the
judges and both the transmitter and the model aircraft shall be impounded by the flight line official
immediately following the flight. No modification or adjustment to the model aircraft shall be permitted (other
than refueling). The model aircraft shall be retested by a second noise steward using a second noise meter and
in the event that the model aircraft fails the retest, the score for the preceding flight shall be zero.
The flight time will be interrupted while the noise check at the flying site is being made. The competitor shall
not be delayed more than 30 seconds for the noise check.
09-30-2007, 11:58 AM
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RE: How do you test Engine DB's Properly
OK, That is clear enough. With the meter at three meters from fuselage centerline, a vibrating wingtip will easily spoil the measurement because of it's very closeness to the meter with large models. They do seem to allow for the hard ground by adding about 3 dB though. 94 dB over macadam equals about 91 over grass (not very short, but normally cut).
I like our industry standard distance of 7 meters better.
I wonder how your salt dampens the noise. I imagine it not being as closed a surface as macadam?
I like our industry standard distance of 7 meters better.
I wonder how your salt dampens the noise. I imagine it not being as closed a surface as macadam?
09-30-2007, 12:37 PM
#12
RE: How do you test Engine DB's Properly
the salt flat is quite interesting - it self levels due to water table rising slowing and essentially flooding and receeding - -it is like a huge billiards tabe in flatness and almost as hard--- in summer ,it is always slightly damp,cold and hard
09-30-2007, 03:08 PM
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RE: How do you test Engine DB's Properly
Hmm.. I better rethink my idea of what you fly on. Kinda hard for a simple Dutchman. If there are no open pores, there is no damping at all and total surface reflection will be a probable. Your readings may well be higher than they would be over concrete or macadam.
Do you have significant comparison material from your vast experience; like when flying in other locations? It does not need be scientifically published
Do you have significant comparison material from your vast experience; like when flying in other locations? It does not need be scientifically published
09-30-2007, 05:37 PM
#14
RE: How do you test Engine DB's Properly
No salt flats in Netherlands??
actually - it is unique -not like concrete as it is deep
some years it is absolutely FLAT for miles n miles
that is why speed runs are done here --- 200 mph feels like cruising along at 60-- your only referrence is the black line to keep you going to/thru the traps the reflection is also unique - a light meter reads same - up or down
If you ever want a full even tan (burn) this IS the place. the purple line on map ,is 100 miles long--almost halfway, is our flying field
actually - it is unique -not like concrete as it is deep
some years it is absolutely FLAT for miles n miles
that is why speed runs are done here --- 200 mph feels like cruising along at 60-- your only referrence is the black line to keep you going to/thru the traps the reflection is also unique - a light meter reads same - up or down
If you ever want a full even tan (burn) this IS the place. the purple line on map ,is 100 miles long--almost halfway, is our flying field
09-30-2007, 09:53 PM
#15
RE: How do you test Engine DB's Properly
I almost accepted a job in Salt Lake City with Evans & Sutherland. It was too much of a move for us at that time. Hindsight says that it was the right decision to not move there, but I sometimes wish I had given it a chance.
I've seen some of Holland, and the flat fields are drop dead beautiful. It looked like a great place to fly if you had access to those places.
I've seen some of Holland, and the flat fields are drop dead beautiful. It looked like a great place to fly if you had access to those places.
10-01-2007, 10:14 AM
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RE: How do you test Engine DB's Properly
You guys should hire an actual audio engineer to get your DB ratings. Its much more complex then you might thinking. There are even different types of weightings you must take into account.
Read and learn!
http://www.phys.unsw.edu.au/jw/dB.html
Read and learn!
http://www.phys.unsw.edu.au/jw/dB.html
10-01-2007, 11:27 AM
#18
RE: How do you test Engine DB's Properly
Digesting all the info on that site will take years to really understand it
a simple Radio Shack meter will register all the model noises--close enough for folk music
those who have a formal musical education ,at one time labored thru the basics of first getting a sound -then a recognizable sound - then a pleasant sound --from their first musical instrument
Been there since age 7 -and I started late!
I learned to detect rpm -by simply hearing the pitch of the engine noise.
absoute rpm readings -no - but relative rpm yes.
I was doing tests one day and a friend noted I said a particular rpm was about so much higher than the last one.
He asked" how can you tell?"
I said "because the pitch was relatively so much higher ."
He did not know that pitch and rpm went hand in hand-and frankly - with him having no background in music -I understood the question.
Anyway - that site has lots of info -but spend a little time with a Radio Shack meter - you will soon see the relationship of noises /frequencies etc..
This is one place where experience REALLY counts
a simple Radio Shack meter will register all the model noises--close enough for folk music
those who have a formal musical education ,at one time labored thru the basics of first getting a sound -then a recognizable sound - then a pleasant sound --from their first musical instrument
Been there since age 7 -and I started late!
I learned to detect rpm -by simply hearing the pitch of the engine noise.
absoute rpm readings -no - but relative rpm yes.
I was doing tests one day and a friend noted I said a particular rpm was about so much higher than the last one.
He asked" how can you tell?"
I said "because the pitch was relatively so much higher ."
He did not know that pitch and rpm went hand in hand-and frankly - with him having no background in music -I understood the question.
Anyway - that site has lots of info -but spend a little time with a Radio Shack meter - you will soon see the relationship of noises /frequencies etc..
This is one place where experience REALLY counts
10-01-2007, 02:32 PM
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RE: How do you test Engine DB's Properly
ORIGINAL: dick Hanson
Digesting all the info on that site will take years to really understand it
a simple Radio Shack meter will register all the model noises--close enough for folk music
those who have a formal musical education ,at one time labored thru the basics of first getting a sound -then a recognizable sound - then a pleasant sound --from their first musical instrument
Been there since age 7 -and I started late!
Digesting all the info on that site will take years to really understand it
a simple Radio Shack meter will register all the model noises--close enough for folk music
those who have a formal musical education ,at one time labored thru the basics of first getting a sound -then a recognizable sound - then a pleasant sound --from their first musical instrument
Been there since age 7 -and I started late!
Important for us is:
Halving or doubling the noise level is 3 dB(A)
Just hearable difference is 1 dB
The A means A-weighted curve (adapted to the non-linear characteristic of the human ear) This curve is used to evaluate the way people hear noise (or music)
A noise meter for our purpose must be set to slow, and A curve. It need not be very accurate. Most hobby shack meters are within 0.5 dB without ever calibrating them.
Doubling the distance reduces noise by 6 dB (that is why measuring at 3 meters is prone to large error)
10-01-2007, 03:32 PM
#21
RE: How do you test Engine DB's Properly
not a large error -- a HUGE error
we did in air sound tests -looking for sound which were irritating and sounds which were of the type which were simply "background noise".
some frequences or undulating sounds were horrible -attention getters
an undulating siren -as bad as it gets ( the German ambulance sound)
it is difficult to pick n choose which sounds will offend-hence the worthless sound tests presently used .
My 160 on huge cans and a 30x14 prop - really easy to tolerate - a wretched, in cowl small glow engine diving and revving upwards of 20000rpm - intolerable - to me .
Personally -I would look at 100 ft as a minimum test distance and also record frequencies .
we did in air sound tests -looking for sound which were irritating and sounds which were of the type which were simply "background noise".
some frequences or undulating sounds were horrible -attention getters
an undulating siren -as bad as it gets ( the German ambulance sound)
it is difficult to pick n choose which sounds will offend-hence the worthless sound tests presently used .
My 160 on huge cans and a 30x14 prop - really easy to tolerate - a wretched, in cowl small glow engine diving and revving upwards of 20000rpm - intolerable - to me .
Personally -I would look at 100 ft as a minimum test distance and also record frequencies .
10-01-2007, 06:31 PM
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RE: How do you test Engine DB's Properly
I know what you mean.
At one moment in time I was involved in noise specifications for car electric motors to operate sunroofs.
We were an international team, and soon got ourselves entangled in psycho-noise and adjacent tertz-bands. I did lots of tests on motor vibration, using a setup with a phonograph needle placed on a small lump of wax on the motor housing. Presto HiFi recordings directly on the noise generator. It took sophisticated gear a factor thousand more expensive to better that.
Using Wave software I could do frequency analisys (Fast Fourrier Transform) and changes in time. I was soon able to isolate bad motors just by looking at the diagrams. This was however not very suitable for production, so in the end everybody agreed to use simple dB mesurements on contact vibration again. Often you have to make a decision fast without taking too many variables into account. Not the best way, but the way everybody can agree on.
At one moment in time I was involved in noise specifications for car electric motors to operate sunroofs.
We were an international team, and soon got ourselves entangled in psycho-noise and adjacent tertz-bands. I did lots of tests on motor vibration, using a setup with a phonograph needle placed on a small lump of wax on the motor housing. Presto HiFi recordings directly on the noise generator. It took sophisticated gear a factor thousand more expensive to better that.
Using Wave software I could do frequency analisys (Fast Fourrier Transform) and changes in time. I was soon able to isolate bad motors just by looking at the diagrams. This was however not very suitable for production, so in the end everybody agreed to use simple dB mesurements on contact vibration again. Often you have to make a decision fast without taking too many variables into account. Not the best way, but the way everybody can agree on.
10-01-2007, 08:28 PM
#23
RE: How do you test Engine DB's Properly
memories--
there are sounds which associate with certain marques.
I could always tell by sound --a Buick transmission (3speed )
A Ford V8 starter motor
A Chev V8 dual exhaust .,
A Buick Dual exhaust
etc..
Once knew a guy who worked on Chrysler starter motors -trying to quiet the very distinctive sound of the gear/flywheel engagement -
I could pick out that sound a block away.
Model engines are more difficult -being two strokers -the size determines primary difference (operating rpm)
Basically -after futzing around - I found that stiff props -with pitch high enough to prevent tip rip-- plus LARGE expansion mufflers were the most practical fixes.
there are sounds which associate with certain marques.
I could always tell by sound --a Buick transmission (3speed )
A Ford V8 starter motor
A Chev V8 dual exhaust .,
A Buick Dual exhaust
etc..
Once knew a guy who worked on Chrysler starter motors -trying to quiet the very distinctive sound of the gear/flywheel engagement -
I could pick out that sound a block away.
Model engines are more difficult -being two strokers -the size determines primary difference (operating rpm)
Basically -after futzing around - I found that stiff props -with pitch high enough to prevent tip rip-- plus LARGE expansion mufflers were the most practical fixes.
10-02-2007, 08:36 AM
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RE: How do you test Engine DB's Properly
Since Dick started this;
Although it is a sidestep from the actual noise measurement, how to keep the noise down is equaly important
Dick already mentioned two, but there is a scope of measures that can be taken. One should bear in mind, that if any one sound source exceeds the other one by three dB, it blankets the other source. For example, if prop noise is predominant, there is no use in improving the exhaust system. Tackle that prop noise first, and then maybe there is still a need to tackle exhaust noise or something else.
Sources of noise:
1) Exhaust noise: The loudest one.
Order of power loss by muffling:
1a tuned pipe with aftermuffler(s) - no power loss even at best muffling levels (drawback: very large systems)
1b canisters (2-3-4-chamber designs) slight power loss as chambers increase (size is within reason)
1c side nufflers have serious power loss if any form of damping is present (size is small but losses are huge)
2) Prop noise.
Tip speed should stay below Mach 0.7, or the prop may start showing flow speeds approaching speed of sound. "ripping" like the crack of a whip is the result. Thinner foils with thickest foil section far back rip later and at higher tip speeds. Thin foils can be had with carbon or metal props
Lower tip speeds can be realized by keeping the pitch up. Pitches of half prop diameter or less easily enter the noise danger zone. The sound waves may set the prop in vibration, which endangers the prop and operator as well.
3) engine vibration (caused by prop unbalance, or whatever. This will set the airframe and wings in rapid oscillating motion, and the plane becomes a noise radiator. Especially the light hovering planes are bad offenders here. They just love to vibrate worse than the engine itself. To prevent this vibration, auxiliary mass dampers can be located in the tail structure and wing tips. To prevent engine and prop vibration reaching the frame, the engine can be vibration isolated by rubber mounts etc.
4) Carb noise. Some engines produce quite a lot of carb noise. Worst offenders seem the reed engines, followed by side carb port systems. Rotary valve systems are the most quiet.
Carb noise can be dampened by ducting the air inlet from the fuselage and fitting air filters or damping chambers. (plenum chamber on the carb mouth.
5) Prop buffeting. If the prop is at any point of rotation near an airframe part, that is sure to generate quite some noise, depending on the distance from the prop hub.
Although it is a sidestep from the actual noise measurement, how to keep the noise down is equaly important
Dick already mentioned two, but there is a scope of measures that can be taken. One should bear in mind, that if any one sound source exceeds the other one by three dB, it blankets the other source. For example, if prop noise is predominant, there is no use in improving the exhaust system. Tackle that prop noise first, and then maybe there is still a need to tackle exhaust noise or something else.
Sources of noise:
1) Exhaust noise: The loudest one.
Order of power loss by muffling:
1a tuned pipe with aftermuffler(s) - no power loss even at best muffling levels (drawback: very large systems)
1b canisters (2-3-4-chamber designs) slight power loss as chambers increase (size is within reason)
1c side nufflers have serious power loss if any form of damping is present (size is small but losses are huge)
2) Prop noise.
Tip speed should stay below Mach 0.7, or the prop may start showing flow speeds approaching speed of sound. "ripping" like the crack of a whip is the result. Thinner foils with thickest foil section far back rip later and at higher tip speeds. Thin foils can be had with carbon or metal props
Lower tip speeds can be realized by keeping the pitch up. Pitches of half prop diameter or less easily enter the noise danger zone. The sound waves may set the prop in vibration, which endangers the prop and operator as well.
3) engine vibration (caused by prop unbalance, or whatever. This will set the airframe and wings in rapid oscillating motion, and the plane becomes a noise radiator. Especially the light hovering planes are bad offenders here. They just love to vibrate worse than the engine itself. To prevent this vibration, auxiliary mass dampers can be located in the tail structure and wing tips. To prevent engine and prop vibration reaching the frame, the engine can be vibration isolated by rubber mounts etc.
4) Carb noise. Some engines produce quite a lot of carb noise. Worst offenders seem the reed engines, followed by side carb port systems. Rotary valve systems are the most quiet.
Carb noise can be dampened by ducting the air inlet from the fuselage and fitting air filters or damping chambers. (plenum chamber on the carb mouth.
5) Prop buffeting. If the prop is at any point of rotation near an airframe part, that is sure to generate quite some noise, depending on the distance from the prop hub.
10-02-2007, 09:09 AM
#25
RE: How do you test Engine DB's Properly
ORIGINAL: pe reivers
Since Dick started this;
Although it is a sidestep from the actual noise measurement, how to keep the noise down is equaly important
Dick already mentioned two, but there is a scope of measures that can be taken. One should bear in mind, that if any one sound source exceeds the other one by three dB, it blankets the other source. For example, if prop noise is predominant, there is no use in improving the exhaust system. Tackle that prop noise first, and then maybe there is still a need to tackle exhaust noise or something else.
Sources of noise:
1) Exhaust noise: The loudest one.
Order of power loss by muffling:
1a tuned pipe with aftermuffler(s) - no power loss even at best muffling levels (drawback: very large systems)
1b canisters (2-3-4-chamber designs) slight power loss as chambers increase (size is within reason)
1c side nufflers have serious power loss if any form of damping is present (size is small but losses are huge)
2) Prop noise.
Tip speed should stay below Mach 0.7, or the prop may start showing flow speeds approaching speed of sound. "ripping" like the crack of a whip is the result. Thinner foils with thickest foil section far back rip later and at higher tip speeds. Thin foils can be had with carbon or metal props
Lower tip speeds can be realized by keeping the pitch up. Pitches of half prop diameter or less easily enter the noise danger zone. The sound waves may set the prop in vibration, which endangers the prop and operator as well.
3) engine vibration (caused by prop unbalance, or whatever. This will set the airframe and wings in rapid oscillating motion, and the plane becomes a noise radiator. Especially the light hovering planes are bad offenders here. They just love to vibrate worse than the engine itself. To prevent this vibration, auxiliary mass dampers can be located in the tail structure and wing tips. To prevent engine and prop vibration reaching the frame, the engine can be vibration isolated by rubber mounts etc.
4) Carb noise. Some engines produce quite a lot of carb noise. Worst offenders seem the reed engines, followed by side carb port systems. Rotary valve systems are the most quiet.
Carb noise can be dampened by ducting the air inlet from the fuselage and fitting air filters or damping chambers. (plenum chamber on the carb mouth.
5) Prop buffeting. If the prop is at any point of rotation near an airframe part, that is sure to generate quite some noise, depending on the distance from the prop hub.
Since Dick started this;
Although it is a sidestep from the actual noise measurement, how to keep the noise down is equaly important
Dick already mentioned two, but there is a scope of measures that can be taken. One should bear in mind, that if any one sound source exceeds the other one by three dB, it blankets the other source. For example, if prop noise is predominant, there is no use in improving the exhaust system. Tackle that prop noise first, and then maybe there is still a need to tackle exhaust noise or something else.
Sources of noise:
1) Exhaust noise: The loudest one.
Order of power loss by muffling:
1a tuned pipe with aftermuffler(s) - no power loss even at best muffling levels (drawback: very large systems)
1b canisters (2-3-4-chamber designs) slight power loss as chambers increase (size is within reason)
1c side nufflers have serious power loss if any form of damping is present (size is small but losses are huge)
2) Prop noise.
Tip speed should stay below Mach 0.7, or the prop may start showing flow speeds approaching speed of sound. "ripping" like the crack of a whip is the result. Thinner foils with thickest foil section far back rip later and at higher tip speeds. Thin foils can be had with carbon or metal props
Lower tip speeds can be realized by keeping the pitch up. Pitches of half prop diameter or less easily enter the noise danger zone. The sound waves may set the prop in vibration, which endangers the prop and operator as well.
3) engine vibration (caused by prop unbalance, or whatever. This will set the airframe and wings in rapid oscillating motion, and the plane becomes a noise radiator. Especially the light hovering planes are bad offenders here. They just love to vibrate worse than the engine itself. To prevent this vibration, auxiliary mass dampers can be located in the tail structure and wing tips. To prevent engine and prop vibration reaching the frame, the engine can be vibration isolated by rubber mounts etc.
4) Carb noise. Some engines produce quite a lot of carb noise. Worst offenders seem the reed engines, followed by side carb port systems. Rotary valve systems are the most quiet.
Carb noise can be dampened by ducting the air inlet from the fuselage and fitting air filters or damping chambers. (plenum chamber on the carb mouth.
5) Prop buffeting. If the prop is at any point of rotation near an airframe part, that is sure to generate quite some noise, depending on the distance from the prop hub.
the exhaust system -all inside the fuselage -
in "cruise "mode -very quiet setup
under full power it exceeds the "test" -the 27" prop simply does not load enough to stop prop noise - this is a real problem if you are after rapid response at very low airspeeds. I have no fix for it.
Carb noise with a filter setup -not bad at all (rotary valve engine)
But as a compromise - not too bad The 40 cc pattern plane is quiet in the air as stop start flying is not part of the game.
160/150 cc models setup for really quick 3D flying -will have lots of prop noise- no way around it - the prop has to spool up fast -which means -lower loading.
For meeting stringent field noise rules - you really have to address HOW you fly the models.
My electrics solve all these issues -of course