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Old Sep 26, 2008, 04:48 PM
My plans are in my blog
Rusty-Gunn's Avatar
Kotzebue, Alaska
Joined May 2006
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I recently read (I can't seem to remember where) that using a jagged edge, like what them stealth fighters use (triangle teeth, sort of) may reduce the noise. I may give this a try, as i'll be building a prop in the slot jet this week end.
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Old Sep 26, 2008, 05:26 PM
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Columbus, Ohio
Joined Jun 2005
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I like the noise. nothing like a mid slotted prop doing 22k rpm!!!
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Old Sep 26, 2008, 05:31 PM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
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Quote:
Originally Posted by thewz
hey....wait a gosh darn second....if it only takes a minute why don't you do it now? jk
Takes ME more than a minute and hourly rates $250/hr so PM and I'll send Paypal address. Seriously, over a dozen planes need lotsa room and it's pouring outside (we're feeling Kyle). Left my SCUBA gear at home.

As far as serrated edge, been there done that. Nothing we did to the edges made any significant difference in thrust but made the sound change in some cases I recall. I think this trick is used by F3P guys on LE/TE to alter stall characteristics. I did try vortex generators on the Quicksilver but those wings don't stall anyway so no diff..
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Old Sep 26, 2008, 06:02 PM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,987 Posts
There...I'm soaking wet and ESC is sparking and arcing! I hope you're satisfied!

My client better not find out what I've been doing on his dime.


Quote:
Originally Posted by thewz
hey....wait a gosh darn second....if it only takes a minute why don't you do it now? jk
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Old Sep 26, 2008, 06:34 PM
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Ontario Canada
Joined Aug 2003
534 Posts
Hi Lead,

I usually taper the LE & TE of the slot, but I have never noticed much difference in speed or noise with my type of sport flying.
I am sure that there is an improvement but I have no means of measuring it.

Bob
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Old Sep 26, 2008, 07:16 PM
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Brisbane Queenland
Joined Jan 2008
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I think what we are seeing is an effect similar to prop cavition on a boat propellor and that the gap in front of the slot is just as inportant as the one behind and its shape and size will be in relation to the prop used. Maybe I am just imagining things but even pushers with the prop set close to any rear surface seem to be noisier that those with prop set back little.
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Last edited by jason23; Sep 26, 2008 at 07:18 PM. Reason: spelling
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Old Sep 26, 2008, 08:33 PM
"Hobby Apparatus" Flyer
Mikey C.'s Avatar
Oxford, PA
Joined Aug 2003
1,357 Posts
I just had a brain wave. Why not bevel opposing edges into the back of the slot (the edge that the prop "pushes" air over)? Instead of simply thinning it out or putting an edge, you bevel one side with, say, 45 degrees facing up, then the other side with 45 degrees facing down. You would do this in order to counter act the torque of the motor-- sort of like crude, fixed-in-place louvers.

I can't be the only one who's thought of this, no?

Here's a really crude representation of what I'm thinking. The wedges are on the back edge of the slot:

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Old Sep 26, 2008, 10:06 PM
Don't look at me like that....
62pilot's Avatar
United States, AR, McDougal
Joined Aug 2005
2,751 Posts
I always bevel the edge infront of the prop, and round the edge behind it.Don't know if it helps, but logic says it will a little.
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Old Sep 27, 2008, 06:50 AM
Winging it >
leadfeather's Avatar
Joined May 2006
9,171 Posts
Mikey, I think that is a good idea. Now how can we show that it works?

Prop clearance front and back seems to be important for reducing noise less noise which should mean more thrust.

What about the prop clearance at the sides of the slot. So far I've always kept it kinda close, but the pressure pulses from the tips of the propeller might be hitting there too and causing noise???
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Old Sep 27, 2008, 05:54 PM
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Brisbane Queenland
Joined Jan 2008
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I do not think that the sides of the slot are as important to the generation of noise as the pressure pulses will much less. That said i think a close fitting ring around the prop would increase the thrust by eliminating the radial loses ala a Korts nozzle. Then again all thrust benifits (if any) are likley to be negated by the extra drag of the devise. It was on he list of things to before i totalled my Dekan/Kosh F-22. Be intresting to see if anybody building something like the Sumo experianced this.

jason
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Old Oct 17, 2008, 12:27 AM
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Joined Jul 2006
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Thrust Stand Data

I have a homemade thrust stand and did a quick and dirty test to measure the thrust with and without a slot.

I used a GWS 5x3 DD prop with a small (10g) BL motor in pusher configuration. The slot was cut out of a 12"x6.5" sheet of cardboard that was about 6mm thick. The slot size was approx 1" x 5.5".

I ran 2 power levels with and without slot - here's the data:

Slot Volts Current Thrust
No 6.3V 2.3A 96g
Yes 6.3V 2.3A 92g (~4% less)

No 7.6V 3.0A 130G
Yes 7.6V 3.0A 122g (~6% less)

So... there is a measurable difference, but its not that significant. I suspect the thrust would be reduced further if I had more flat surface in front of the prop, but my test stand gets in the way of adding this.

Anyway, I hope people find this data useful.

Andrew
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Old Oct 17, 2008, 04:20 AM
Winging it >
leadfeather's Avatar
Joined May 2006
9,171 Posts
Andrew,

Very good work. Are you going to be able to continue testing and adding to the data?

From the data so far it appear that maybe the losses increase with power. It would be interesting to see if this trend continues with further power increases. On the KF ParkJet, the first slotted build had a 6x4 prop powered by the Grayson Hobby parkjet motor (about 150 Watts I think). It was very noisy and had a lot less thrust than did the pure pusher version. Based on you data and my experience, I'm guessing that the loss of thrust for a given slot size may become more pronounced as power increases.

In your test rig, could you add a fin right after the slot to see if a fin produces additional loss of thrust?

Can you test the effect of slot size?

Again great work, it is nice to have actual data from a controlled experiment, well done.
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Old Oct 17, 2008, 06:22 AM
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Joined Jun 2008
583 Posts
i would love to see data with the different types of slot sizes, and slot LE/TE dressings, along with a best guess on noise changes.

I have a EPP F22 made of elmers foam board(paper on) that i recently put a new motor on. I had the prop centered in the slot but at the last second noticed that the prop adaptor was wobbleing. it was 3.17mm and the motor shaft was 3mm. the replacement prop adaptor was shorter, putting the Le oft he prop very close to the slot. the re-maiden of the plane left me very dissapointed in my new motor. This post has made me rethink the situation. i may give it another chance with a reconfigured prop/slot relationship.
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Old Oct 17, 2008, 09:05 AM
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Hi Guys,

I will try to run a few more experiments with different slot sizes, and power levels. It may take a while, as I'm (unfortunately) busy with some business matters.

Stay tuned...

Andrew
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Old Oct 17, 2008, 09:30 AM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,987 Posts
Up to 30% reduction in thrust can be demostrated depending on slot size and shape. Generally in proportion to noise level.



Quote:
Originally Posted by achronox
I have a homemade thrust stand and did a quick and dirty test to measure the thrust with and without a slot.

I used a GWS 5x3 DD prop with a small (10g) BL motor in pusher configuration. The slot was cut out of a 12"x6.5" sheet of cardboard that was about 6mm thick. The slot size was approx 1" x 5.5".

I ran 2 power levels with and without slot - here's the data:

Slot Volts Current Thrust
No 6.3V 2.3A 96g
Yes 6.3V 2.3A 92g (~4% less)

No 7.6V 3.0A 130G
Yes 7.6V 3.0A 122g (~6% less)

So... there is a measurable difference, but its not that significant. I suspect the thrust would be reduced further if I had more flat surface in front of the prop, but my test stand gets in the way of adding this.

Anyway, I hope people find this data useful.

Andrew
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