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Optimal Way To Bevel Fins For Whistle Rockets

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bguffer

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What is the optimal way to bevel fins so that BSD Thor-like rockets whistle on the way up? A year ago i saw a post on this subject, but that post is long gone.

I ask because i have a unbuilt Binder Designs Velociraptor kit, and thought i would try bevel the fins the same way. Maybe it whistles, maybe it won't.

Bob
 
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blackjack2564

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Treat the 2 split fins as 1. Only bevel the top leading edge and the bottom trailing edge.

Leave the bottom of the top fin, and the top of the bottom fin square edged.

It will whistle like crazy. Also more pronounced whistle if you use high thrust short burn motors [reds, greens] longer burn motor noise covers the whistle.

3in Thor went up Sunday on a I 245 green and whistled very loud all the way to apogee.
 

FROB

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I tried that on my L2 rocket using 0.062" G10 fins and a 1/4" gap.
whistle was barely noticeable. i suspect you need thicker fins and a wider gap for it to work. i'm curious if anyone's ever worked out the science behind this to predictably design a good aerodynamic whistle.
 

FlyBack

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... i'm curious if anyone's ever worked out the science behind this to predictably design a good aerodynamic whistle.
Hmmmm, you mean like this: Chanaud, Robert C. "Aerodynamic Whistles." Scientific American (January 1970): 40-46

Regards,

Flyback
 
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Pantherjon

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Treat the 2 split fins as 1. Only bevel the top leading edge and the bottom trailing edge.

Leave the bottom of the top fin, and the top of the bottom fin square edged.

It will whistle like crazy. Also more pronounced whistle if you use high thrust short burn motors [reds, greens] longer burn motor noise covers the whistle.

3in Thor went up Sunday on a I 245 green and whistled very loud all the way to apogee.
And that would have been my Thor! :D She did sound sweet going up didn't she, Jim? ;) And that is even with the rear fins put on upside down!..LOL

I didn't do ANY shaping of the leading/trailing edges on the fins, and like Jim said, it whistled like crazy when it went up on that I245G Sunday..
 

FROB

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Great find guys!
LOL usually i'm the guy going off to do some quick web research to help some poor lazy or lost soul on the forums, but i was feeling lazy this time- nice to know it works the other way round:D:eek::):p

So according to what I've just read from those articles, i would expect that the way to get the best whistle from a fin "gap" is to bevel the top fin 's bottom edge on one side only, and do the same thing with the leading edge of the lower fin, but on the *opposite* side,
This would effectively create a channel slightly diagonal to the direction of flight, with what should be unstable air flow impinging directly on the sharp leading edge of the lower fin, which splits it in two and should pretty much maximize the effect.

I think my next mid-size rocket build (probably 3" dia) will have to use adjustable fins so i can experiment with this a little.
Or a design with lots of little fins with different thicknesses and gaps to be able to try several combinations per flight. I'm thinking 6 rows of small rectangular fins of varying thickness with 4-6 fins per row.
 

MKP

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My Deuce's Wild whistles like crazy, but I think it's the launch lugs.
 

Graham Orr

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Me thinks whistles can be classified in two forms:
1) Purely Aerodynamic - Vortex shedding, possibly in alternation sheets similar to von Karman oscillations occur in audible frequencies. As Reynolds number (speed) increases the pitch tends to increase.
2) Aero-Mechanical - Vortex shedding coupled with mechanical resonance (aka fin flutter). As Reynolds number increases, pitch can either increase or decrease depending on coupling parameters.

So yes, there is some substantial scientific stuff behind all of this. There may even be closed form solutions for particular boundary conditions... though it may require you to get a PhD in mathematics in order to solve...
 

powderburner

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I'm thinking 6 rows of small rectangular fins of varying thickness with 4-6 fins per row.
If I understand you correctly, you plan to test multiple fin configurations per flight? (Six rows?)

How will you know which fin arrangement is whistling?

Don't get me wrong, I would like to see an explanation for this phenomenon also, and whether it can be generated a "low power" speeds and fin sizes. I am not sure that I am following your test plan.
 

FROB

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If I understand you correctly, you plan to test multiple fin configurations per flight? (Six rows?)

How will you know which fin arrangement is whistling?

Don't get me wrong, I would like to see an explanation for this phenomenon also, and whether it can be generated a "low power" speeds and fin sizes. I am not sure that I am following your test plan.
Just thinking out loud- it may not be very scientific -
i just thought that would make an interesting looking rocket. Leaving small gaps and square edges, all the same - the fly it and see if it whistles.
if not, take and xacto knife, file, sandpaper, etc., and start reshaping one of the fin gaps. fly again. carve some more, fly again...
if you run out of material on the fin gap and still no noticeable effect, the start on the next pair of fins with a different strategy.
 

DMcCauley

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Just tape a few athletic whistles to your rocket. That will do the trick.
 

MrGneissGuy

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Nothing like reviving a really old thread and a bit of a tangent to the OP, but having searched the forums for creating whistles with fins, this seemed to have the most information so I thought instead of starting a new thread, I’d post in this one so that things already said don’t have to be said again.

I think it was the second club launch my daughters and I attended. There was a large hawk that was launched with whistling easily audible the entire flight. Since then, the musician part of me wanted to experiment. I’ve got several other kits and a min diam scratch I’m currently working on, so this is strictly in brain storming stage right now. I get, for the most part, the basics of what makes the whistle in a split fin. But does anybody know what might have the most effect on the pitch of the whistle? Is it the length of the leading edge of the fins, the fin thickness, the distance of the split, and/or something all together different?

Would the air flow/speed coming from a small (say 2 or 10 gallon) air compressor be sufficient to experiment with? If I used the right attachment, could I use the compressor to push air past the fins to see if a configuration whistles and what pitch might be generated? I’m thinking that it takes three (or more) notes to make a chord and rockets have three (or more) fins…

Or am I just completely off my rocker?
 

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