prop versus engine noise.

JETLAG03

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Had a few comments about the noise of my BMW K1200RS at full throttle 8500rpm on take off, I have a sabre style propeller.

I have increased the AoA of the prop and hope to reduce to the engine revs to 6000rpm, yet to test.

I did a ground test of the noise using my iphone with the NIOSH app from app store. The dB test was NOT conducted as prescribed for a motorcycle, simply because a motorcycle does not have the big fan spinning at the rear. The iphone was placed on a stand 2 metres from the giro, just in front of the exhaust and well forward of the prop and its wash.


1. With propeller

LAeq 92.2dB
max 103,9 dB
LCpeak 116.6dB
TWA 63.9db
exchange rate 3dB
time weighting - fast

2. without propeller

LAeq 86,4 dB
max 93,5 dB
LCpeak 109.1dB
TWA 57.4db
exchange rate 3dB
time weighting - fast

I was unable to spin the propeller independant of the engine so could not confirm the noise purely from the propeller itself. Maybe some has such data?

Basic data suggests that with the propeller the noise is 10.4dB louder = X2 (ish) than motor only. Probably, I have misunderstood the raw data but suspect that there is little if any benefit to be gained from trying to silence the engine further when a higher level of noise is being produced by the propeller.

Am I right?

Does for example a noise level of 93.5dB get completely masked by another noise level of 103.9dB or do the two noise join together to create one noise greater than the two seperate.

The propeller is close to the gearbox/reducer about 100mm (4") but I cannot move the propeller further back due to the structure of the machine.

I look forward to your comments, please keep any suggestions as simple to understand as possible, I am not as academic as many of you.

thanks

phil
 

Chook

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Phil I have been doing some work in this area in Australia for pilot hearing protection specifically helmet types with electronic and passive noise reduction. The combined noise is not as loud as you may think. The decibel system is logarithmic so use log1 + log2.

For comparison see my unfinished table attached.
I am looking at it from the pilots point of view whereas the Europeon standards look at from a bystanders point of view and as you may have discovered its quite a complex reading.

Gyro typePropellerAmbient Tempprop pitchwind2000rpm4000rpm4400rpm5000rpm5800rpmNotes
Amax single seater Subaru EA81Warpdrive13⁰130dBANo pod. reading at pilots helmet
ELA 07S Rotax 912ULSDUC Windspoon14⁰nil87.9 dBA98.0 dBA110.2 dBAreadings from unoccupied rear seat @ helmet level
ELA 07S Rotax 912ULSBolly Boss24⁰14⁰nil87.7 dBA105.5 dBA107.9 dBA111.2 dBAreadings from unoccupied rear seat @ helmet level
ELA 07S Rotax 912ULS68" Warpdrive11.5⁰10.5⁰readings from unoccupied rear seat @ helmet level
Titanium TAGreadings from unoccupied rear seat @ helmet level
ELA 07S Rotax 914 turbo69" Warpdrive33⁰13⁰3kt E85dBA103dBA102dBA109dBA109dBAFront seat at helmet level LHS
90dBA107dBA109dBA113dBA114dBARear seat at helmet level LHSRear seat at helmet level LHS
 
Last edited:

JETLAG03

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Hi @Chook impressive data there. I will take some readings on mine from the pilot head position, simply so you have some more data comparable with your recording position.

Noise is difficult, it can be very subjective, one mans noise is another mans music. But when some guys at my airfield commented on the noise of my machine I decided to collect data rather than opinions, there is a heavy bias against non-rotax engines from some quarters.

p.s. your bottom line is missing some data ie giro type
 

schmoe90

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Props are pretty noisy - no data, but the motor on the e-Gull is pretty much silent as it's electric,
I heard one of these electric ultralights fly years ago, and was surprised at how noisy the prop was.
 

Vance

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Had a few comments about the noise of my BMW K1200RS at full throttle 8500rpm on take off, I have a sabre style propeller.

I have increased the AoA of the prop and hope to reduce to the engine revs to 6000rpm, yet to test.

I did a ground test of the noise using my iphone with the NIOSH app from app store. The dB test was NOT conducted as prescribed for a motorcycle, simply because a motorcycle does not have the big fan spinning at the rear. The iphone was placed on a stand 2 metres from the giro, just in front of the exhaust and well forward of the prop and its wash.


1. With propeller

LAeq 92.2dB
max 103,9 dB
LCpeak 116.6dB
TWA 63.9db
exchange rate 3dB
time weighting - fast

2. without propeller

LAeq 86,4 dB
max 93,5 dB
LCpeak 109.1dB
TWA 57.4db
exchange rate 3dB
time weighting - fast

I was unable to spin the propeller independant of the engine so could not confirm the noise purely from the propeller itself. Maybe some has such data?

Basic data suggests that with the propeller the noise is 10.4dB louder = X2 (ish) than motor only. Probably, I have misunderstood the raw data but suspect that there is little if any benefit to be gained from trying to silence the engine further when a higher level of noise is being produced by the propeller.

Am I right?

Does for example a noise level of 93.5dB get completely masked by another noise level of 103.9dB or do the two noise join together to create one noise greater than the two seperate.

The propeller is close to the gearbox/reducer about 100mm (4") but I cannot move the propeller further back due to the structure of the machine.

I look forward to your comments, please keep any suggestions as simple to understand as possible, I am not as academic as many of you.

thanks

phil
There is a lot to learn about sound and the perception of noise.

How did you load the engine without the propeller?

It is my observation that the exhaust and inlet sound changes substantially with different power settings.

The sound of The Predator changed when I moved some things out of the way of the inflow for the propeller and when I went from a two blade to a three blade propeller.

Good luck with your research Phil.
 

JETLAG03

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@Vance there was no load on the engine without the propeller. Just as if it was a BMW K1200RS stood on its stand revving the engine, I know it will not give the same sound/note as a loaded engine but what can a fella do when all he has is a big hammer, and in emergencies an even bigger one.
 

Chook

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Thanks Phil. Yes noise is very perceptive but from our point of view (Gyro pilots) fairly destructive to our hearing which is why i am doing this.
This thread could not accept an XL spreadsheet so I copied the data directly and pasted it. That last line is in fact the rear seat reading of the same flight as the next line up which shows a difference between the front and rear seat profiles of around 5 dBA. You are obviously aware of the doubling of sound effect every 3dB so the 5dBA is quite significant.

Regards......Chook.
 

JETLAG03

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@Chook Just when I think I am getting to grips .. another spanner re noise levels. Inever realised there is a known difference between "actual" and "perceived" noise levels. I thought 10dB was considered as double, but you are right 3dB is in fact double.

As explained in this bit found on google
"Doubling of the volume (loudness) should be sensed as a level difference of +10 dB − acousticians say. Doubling of sound intensity (acoustic energy) belongs to a calculated level change of +3 dB. +10 dB is the level of twice the perceived volume or twice as loud (loudness) in psychoacoustics − mostly sensed."

The other problems with on ground measurements is that as suggested by @Vance the data is somewhat distorted because an engine without a propeller is under zero load therefore less noise and the engine with the propeller at full revs and static is putting the propeller and engine under more load than normal as the giro would normally begin its ground roll changing the propeller loadings and charectoristics plus the movement of air towards the rear will, I suspect further reduce the perceived noise levels in flight.

To say, all I wanted to find out initially was "what makes the most noise the engine or the propeller and do the two noises combine to increase the overall noise or is the quietest noise simply masked (drowned out) by the loader noise. In my simple mind, I think of sound waves as waves in the ocean where some will combine to make a bigger wave and others will interfere to cancel each other out.

phil
 
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MilesW

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To say, all I wanted to find out initially was "what makes the most noise the engine or the propeller and do the two noises combine to increase the overall noise or is the quietest noise simply masked (drowned out) by the loader noise. In my simple mind, I think of sound waves as waves in the ocean where some will combine to make a bigger wave and others will interfere to cancel each other out.

phil

Phil

Unless you are spinning your propeller particularly fast (tip speed in excess of high mach 0.8), as heard on the ground, the engine is likely the source of most of your noise, though interference from the keel and struts etc can be problematic.
Sadly, the loud noise will not drown out the quieter. Sound is additive. You may no longer perceive the quieter noise, but the overall noise level will be louder. Think what you experience in a loud restaurant.
Chooky is doing good work to quantify the in cabin noise levels and it is not good news. Gyrocopters are really loud. Harmfully loud.
 

Chook

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One thing that is often overlooked is the noise from the intake, intake "honk" can be very loud.
Scott yes we have noticed that it is particularly loud on the 912 Sportcopter. Sadly this mostly affects the pilot.
Unfortunately I missed my opportunity to quantify this noise with my meter and record it but will hit Miles up for a session.

Regards........Chook.
 

Resasi

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The sound of The Predator changed when I moved some things out of the way of the inflow for the propeller and when I went from a two blade to a three blade propeller.
Have always had experience with three blades props, about to switch to a four bladed. Found this.

Cockpit noise can come from many sources, including the engine, exhaust system, airflow around the fuselage, and the propeller. High levels of cabin noise can cause stress and pilot fatigue, and may even lead to hearing problems among pilots. Increasing the number of blades on a propeller is one solution to reducing cabin noise. In most installations, increasing the propeller blade count also reduces noise. This is largely due to a reduction in vibration.

In a single-engine aircraft, the propeller blade wake beats on the windshield and produces cabin noise. A 2-blade propeller produces two pressure pulses per revolution, whereas a 3-blade propeller will produce three smaller pulses per revolution for the same amount of total thrust. As a result, the 3-blade prop will be inherently smoother and therefore quieter. In general, the 3-blade propeller will have a smaller diameter than the 2-blade propeller that it replaces, which also serves to reduce the tip speed and noise. In a twin-engine aircraft, the reduced diameter of the 3-blade propeller will result in less tip-generated noise and a greater clearance between the blade tip and the fuselage. Vibration is not only noisy, but it can also cause discomfort and potentially damage avionic systems. Adding more propeller blades can help reduce vibration, improving overall cabin comfort for pilots and passengers.

 

XXavier

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My gyro is now fitted with a six-blade prop, and I'm convinced that it's less noisy than the usual three-blade DUC prop.

Sin títulohhj.jpg
 

MilesW

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My gyro is now fitted with a six-blade prop, and I'm convinced that it's less noisy than the usual three-blade DUC prop.

View attachment 1151707


Is it less noisy, or have you just shifted the frequency so you perceive it as quieter?

It would be ineresting if you could do some testing in the same manner as Chooky. Would be a most valuable addition to the data.
 

wolfy

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I have mainly been using Bolly props both 70" 3 blades on single seat and 4 blade on a two seater. Bolly are nice and they also sound like an angry wasp at rpm. The new aeroprop I am using is 71.5" 3 blade wide chord and it is super quiet even though the rpm is the same.

wolfy
 

XXavier

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Is it less noisy, or have you just shifted the frequency so you perceive it as quieter?

It would be ineresting if you could do some testing in the same manner as Chooky. Would be a most valuable addition to the data.

I (and others in my airfield) think that this prop is less noisy (in audible frecuencies, of course...)

Measuring the individual contributions to the total noise of the engine (or the prop) is very difficult. You would need an adequate dummy load in place of the prop if you wanted to measure the engine contribution...
 

Brian Jackson

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The quietest prop I've ever heard in person was a Prince P-tip. So much so that the contrast with an identically powered plane was startling. It's also what's going on my gyro, along with an intake silencer. Rolling off the harsh, high-end and upper-mid frequencies makes a huge difference in perceived noise, though I do not know the actual measured values.
 

JETLAG03

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Hi @MilesW and @DavePA11 the propeller is 1700mm spinning at 2000rpm so a tip speed of about 641kph or 0.52 mach.
 

Mike G

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Guys

Noise is only vibration and as I keep saying every time you ask about vibration “what is the frequency of the vibration that bothers you?” If it’s 1/rev then track and balance, if it’s 2/rev there’s virtually nothing you can do about it see post 40 of this thread https://www.rotaryforum.com/threads/lessons-learned-by-a-superior-rotor-balancer.48218/page-2.

So you want to establish which makes the most noise, the engine or the prop. Well they rotate at different speeds and therefore create noise at the frequency that corresponds to their speed (rpm).

If a 4 stoke engine has X cylinders and it rotates at N rpm the exhaust noise will be at X.N/120 hz. If it has a gearbox ratio R:1 to the prop, then the prop rotates at N/(60.R)hz. If the prop has B number of blades the prop noise will be at the blade passing frequency of NB/(60.R) hz.

Example:

A Rotax 9xx series engine with a 3 bladed prop at 5000 rpm has an engine noise at

4 x 5000/120 = 167 hz

and the prop noise will be at

5000 x 3/(60 x 2.43) = 103 hz

Now a sound meter usually displays sound levels in octave bands and the ones that interest us here are the 63 hz, 125hz & 250 hz bands and the band limits are given here:

Lower Band Limit
(Hz)
Center Frequency
(Hz)
Upper Band Limit
(Hz)
44​
63​
88​
88​
125​
177​
177​
250​
355​


As you can see the two frequencies that interest us (103 & 167) are in the same 125 hz octave band and the average sound meter won't separate them out for us. So you could try running the engine at 5500 rpm which should (but I don't guarantee it) push the engine noise into the 250 hz octave band and keep the prop in the 125 hz band or you look for a noise meter that has the option to display 1/3rd octave bands.

By looking at the spectrum you should see two peaks at these frequencies and the highest is obviously the major culprit. If one is 10 dB or more higher than the other then reducing the lower one will have no effect on the total and if the difference is between 4 & 9 dB then reducing the lower one will have virtually no effect.
Mike G
 
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