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Old Mar 05, 2012, 03:04 PM
Joined Aug 2011
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Prop Slot Noise Reduction Design and Tests

My thanks to Robin Bennett for the following explanation that makes sense:

Originally Posted by RobinBennett
Noise is just alternating high and low pressure air. There's a low pressure area above an aerofoil (in the case of the prop, 'above' means 'in front') - there's also a high pressure area behind a prop, but it's not as powerful.

In free space, the low pressure region just moves around and can be averaged out to nothing, but the high and low pressure regions can't cross from one side of the obstruction to the other - they get broken up and have to reform, leading to the overall pressure going up and down over each revolution.


Added additional test video on 3/9/2012.

I fly next to an equestrian center and some of my prop slot planes are just too loud for the environment and the horses. I had read several threads on prop slot noise and saw no consistent solution so I set out to do my own experiments.

I have a plane, shaped as a duck, that has the motor in the head at the end of a long neck. This plane is quiet and has smooth airflow ahead and behind the propeller. I concluded this was my best platform for the experiments.

I cut shapes from 6mm Depron to test. I did baseline tests with a straight edge. What I found surprised me. There is noise due to prop slot proximity on both the front and the back sides of the prop. The back or downwind side will have minimum slot induced noise if the slot is at a constant distance to the prop, 13% of the prop diameter. The front of the prop has a little louder effect from slot proximity but the effect varies depending on the distance from the prop center. Near the center, the slot should be 10% of the prop diameter away. Over the outer 40% of the prop diameter the slot should be 22% of the prop diameter away from the prop. I did some tip experiments and the slot should be 5% of the prop diameter away from the tip.

I have included a video of my tests and a drawing of what I found to be the ideal shape that can be sized to any prop diameter. I have tested this shape on my large and small foamies and have verified its shape as a solution to prop slot noise. The reduction in noise also gave a boost to aircraft speed; prop pressure gradients are more smooth.

I replaced the original drawing to clarify the direction of airflow.

Prop Slot Noise Reduction by Otto Dieffenbach (1 min 42 sec)



Supplemental Test: The effects of slot thickness, angle and jagged edge.

Prop Slot Noise Reduction: Part 2 by Otto Dieffenbach (1 min 29 sec)


Some interesting questions have been asked about the effects of slot thickness and angle so I took the duck down off the wall again and ran a few tests to determine the effects. Like in the first tests, I ran the duck to full power and moved a straight piece in front and behind the prop. I tested 6mm Depron, 3mm Depron and thin aluminum sheet.

To my surprise, it did not matter how thick the straight slot edge was, it generated the same amount of noise at the same distances to the prop, whether from the front or behind the prop.

Angle also did not effect noise level nor did a jagged edge.

Conclusion: So making the area around the slot thinner or shaping it to an aerodynamic, low drag, shape will have no effect on slot noise.
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Last edited by Otto Dieffenbach; May 29, 2012 at 10:57 AM. Reason: New drawing
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Old Mar 05, 2012, 03:42 PM
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Romania, Bucuresti, Bucharest
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Otto, thanks for taking the time to study this, I'm sure I'm not the only one looking for a solution to excessive noise of the prop in slot.

Your video of the tests is showing very well what I think is a dramatic improvement in noise levels.

Thank you for sharing!
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Old Mar 05, 2012, 04:16 PM
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That is neat!!!
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Old Mar 05, 2012, 04:17 PM
skumgummi dave
Gresham, OR.
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Thank you Otto. Very practical work.

Dave-
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Old Mar 05, 2012, 04:31 PM
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Ky
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Cool, can't wait to try it out. I've avoided most prop in slot designs because of the awful noise they make.
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Old Mar 05, 2012, 04:48 PM
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Canada, BC, Williams Lake
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Another Gem of a thread Otto . Nice work as usual
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Old Mar 05, 2012, 05:23 PM
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Cool work Otto. Have you tested in-flight vs static yet?
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Old Mar 05, 2012, 05:28 PM
Addicted and loving it
United States, FL, Orlando
Joined Apr 2007
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Very cool info Otto. Thanks alot for posting. I would be very curious to know if there was an effect on current draw or on thrust. Seems like all that noise has to indicate a loss of power somewhere.
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Old Mar 05, 2012, 05:30 PM
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Quote:
Originally Posted by Petefoss View Post
Cool work Otto. Have you tested in-flight vs static yet?
Yep. I've tried it on my two mini wings and my huge Delta Dart. Now all three just give normal motor/prop noise. The Dart was interesting because the motor is in the middle of the X in the fuse. Now that both slots are shaped it is quite a good neighbor in the community.
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Old Mar 05, 2012, 05:34 PM
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Quote:
Originally Posted by billman View Post
Very cool info Otto. Thanks alot for posting. I would be very curious to know if there was an effect on current draw or on thrust. Seems like all that noise has to indicate a loss of power somewhere.
Do not have an answer on the current draw. The three planes I have modified all seem to fly smoother and have more pep. My 3s mini wing really seemed to benefit although it was already fast. The noise reduction on all three is very significant.
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Old Mar 05, 2012, 05:35 PM
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Quote:
Originally Posted by Otto Dieffenbach View Post
Yep. I've tried it on my two mini wings and my huge Delta Dart. Now all three just give normal motor/prop noise. The Dart was interesting because the motor is in the middle of the X in the fuse. Now that both slots are shaped it is quite a good neighbor in the community.
WOW, that is amazing

are you going to try this on the fly people as well?

Thanks
Doug
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Old Mar 05, 2012, 05:56 PM
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It's great to have someone like you on this forum to have so much knowledge and glad to share with everyone. Thanks again for all you do for all of us!
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Old Mar 05, 2012, 06:09 PM
E28
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Is the prop on that duck turning as a tractor, pulling away from the chair?
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Old Mar 05, 2012, 06:34 PM
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Nope its a mexican duck . they fly backwards
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Old Mar 05, 2012, 07:20 PM
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Good test Otto, clearly shows the noise source and how the bowtie is quieter.

A different noise reduction shape was reported about a year ago but I haven't seen any follow ups on showing it really works. Perhaps with your set up, you could verify the claim???? From this post.
"I do exactly the opposite, and make the slot a football shape. Mine was nearly silent, much quieter than the bowtie shape you tested in the youtube video you posted. Same prop. Maybe try that out as well."
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