SpudFiles

I saw this new blade-less fan from Dyson, it uses the Coanda Effect to multiply the air going into it to up to 15 times the volume going out of it.

The design uses an airfoil shape wrapped around into a circle, and then uses pressurized air coming from a small opening around the rim of the circle to induce a low pressure in the center of the circle, thus producing air flow, once you see the YouTube video, you'd understand better.

Any ways, I was thinking of a projectile that could use this same shape, it could possibly be longer, and have low profile fins to stabilize it in flight. My hypothesis is that it will act like a ram jet in a way, or "glide" a bit further than a projectile with the same weight. To shoot it, a sabot would be used, one that would resemble a shot gun shell wadding.

What do y'all think? Do you think it might work?

Tubular bullets are an existing concept, like for example the PMC ultramag:





The ring airfoil grenade is also something that has been played with for a while: http://www.angelfire.com/art/enchanter/RAG.html

In terms of sectional density, it has the same effect of a saboted dart, reducing the frontal area of the round and therefore drag.

It would be interesting to see some experimentation of one were so inclined

Thanks for the link! I didn't even know that they made those! I really want to test this theory, but I don't have a lathe, or a mill... I could probably figure something out though.

I'm going to be working on a larger scale than bullets, I have a few pneumatic launchers, the barrel of one is 1", and I have two others that have 1 1/2" barrels.

Is there a way to get the rounds spinning? do you think that if I turned the fins a few degrees, it would cause the round to spin, or just wobble in the air?

inventorguy1000 wrote:Thanks for the link! I didn't even know that they made those! I really want to test this theory, but I don't have a lathe, or a mill... I could probably figure something out though.

Those tools would have been helpful but there are other options for the short on cash and resources but long on patience

Is there a way to get the rounds spinning? do you think that if I turned the fins a few degrees, it would cause the round to spin, or just wobble in the air?

If you're putting fins on, I would avoid trying to make it spin.

The fins you drew are pretty small though, I would make them much bigger, like so:

Those tools would have been helpful but there are other options for the short on cash and resources but long on patience Wink

I'll look into that, and I have a TON of experience with with patience

So, should I make the fin length as long as the body length, or like 1 1/2 times the body length?

I won't have time to work on this until the weekend, I have double shifts at work tomorrow, but i'll post on what i came up with!

inventorguy1000 wrote:So, should I make the fin length as long as the body length, or like 1 1/2 times the body length?

It's all a matter of where the centre of pressure and centre of gravity is, some recommended reading.

Here's another possible configuration, extending a rear tube with smaller fins which would also keep it centred in the barrel. As it's an "organic" shape I could see you making a wooden mold using a drill-clamped-in-a-vice "lathe" and file and sandpaper, then casting using a suitable material.

There are these throwing toys that might use the same principle.

[youtube][/youtube]

Some years ago I've build one myself. I cut the middle part out of a plastic bottle and weighted one end.
Worked like a charm.

Ok, Sooo... I made a prototype.

<table><tr><td><a href="https://picasaweb.google.com/lh/photo/A ... site"><img src="https://lh4.googleusercontent.com/-ouOR ... irfoil.jpg" height="54" width="144"></a></td></tr><tr><td>From <a href="https://picasaweb.google.com/1155567727 ... te">Potato gun</a></td></tr></table>

I Started with the basic Ring Airfoil shape. I have a 1 1/2" barrel, so I took a piece of 1 1/4" pvc, which doesn't fit inside 1 1/2" pvc pipe unmodified; I rigged it up to a simple drill Press, and used files to shave it down to the shape I wanted, then cut that piece off from the rest of the pvc piece.

I used an old pill bottle that fit perfectly in the barrel as a sabot...well more of a wadding. It flew pretty strait, very little wobbling, and hit the target, which was an old tarp draped over an old trebuchet skeleton. I was pretty happy with it then, being the first shot, and hitting the target.
But, that's where I should have stopped.

Some years ago I've build one myself. I cut the middle part out of a plastic bottle and weighted one end.

I saw this, and thought that weighting it would increase the range, so I hot melt glued a ring of solder onto the front, it did work, but it started to wobble in the air.

So, then I tried jackssmirkingrevenge's idea of putting a tube on the end to stabilize it, I got another old pill bottle that was bigger than the first, cut off the bottom, and the very top, and hot melt glued it to the back, creating the "tube extension".
When I shot it, it veered down into the ground the first time I shot it, and it made a wide spiral before hitting the ground the second time.

<table><tr><td><a href="https://picasaweb.google.com/lh/photo/p ... site"><img src="https://lh3.googleusercontent.com/-nYU2 ... CN0252.JPG" height="108" width="144"></a></td></tr><tr><td>From <a href="https://picasaweb.google.com/1155567727 ... te">Potato gun</a></td></tr></table>

I thought about putting fins on it, but there was no room on the top, it was too snug as-is. So I put fins inside the tube, thinking that that might help, but no. I don't know what I was thinking! Having the fins on the inside would direct the airflow inside the tube and not really the tube itself, I really needed to have them on the outside to direct the whole tube. It just kept on getting worse and worse!

You might be thinking; Hot melt glue? Really? No wonder it didn't work...
BUT, it is the industrial high temp stuff, and I used an old soldering iron to "smooth" out the lumps and bumps. I guess I just didn't have everything perfectly aligned, and it was probably off balance anyways, along with having the fins on the inside instead of the outside of the tube.

All in all, I think that there's something to this, I'm going to experiment with this a little more, and try to make a balanced, and stable projectile, if I do that, and do some calculations on the weight, length of the tail section, and things like that, I think I might be able to make a viable projectile.

Here's the next design that i'm gonna try:

<table><tr><td><a href="https://picasaweb.google.com/lh/photo/j ... site"><img src="https://lh4.googleusercontent.com/-706E ... vamped.jpg" height="99" width="144"></a></td></tr><tr><td>From <a href="https://picasaweb.google.com/1155567727 ... te">Potato gun</a></td></tr></table>

I based it off of jackssmirkingrevenge's designs with the fins extending behind the tube, but giving them a more "organic shape" as jack said.

Let me know what you think! and could I get some help on the calculations for the desired weight, length of tail, the CG and CP?

Thanks!

Ya know, I think I would be in favor of crowley doing the range testing of his hybrid with something of this design. It would have similar aerodynamic properties to the miniboy, be infinitely easier to see if made in a large diameter, and could be mass produced with little effort.
Not saying he should get out of the miniboy test, but this would at least yield a result:P

be infinitely easier to see if made in a large diameter, and could be mass produced with little effort.

speaking of which.. has anyone ever built a PVC projectile with extendable fins?

POLAND_SPUD wrote:

be infinitely easier to see if made in a large diameter, and could be mass produced with little effort.

speaking of which.. has anyone ever built a PVC projectile with extendable fins?

I recall from the movie "October Sky" there was mention of an aeronautical flare with extendable fins. Now that being a movie...

Fnord wrote: Not saying he should get out of the miniboy test, but this would at least yield a result:P

Did mrcrowley ever launch the miniboy? Last I heard he was making a smoke mix to better follow it's flight path.

Still haven't launched it. I have orders not to launch it unless a recovery system is in place and at the moment the ATV we were going to use to get us down range isn't in working order. Doesn't seem like smoke will help too much either; once you get to the shooting location, the distance is really put in to perspective and I don't see myself being able to follow a faint smoke trail that lasts less than half the flight time of the projectile.

be infinitely easier to see if made in a large diameter, and could be mass produced with little effort.

speaking of which.. has anyone ever built a PVC projectile with extendable fins?

I've been thinking about something like this for a long time! I haven't thought about it enough to start working on one, but I've been thinking a lot about the mechanism; Either spring loaded fins curved to the projectile that flip out to the side, or a spring loaded rod with a point inside the projectile that pushes the fins out.

I've been thinking about something like this for a long time! I haven't thought about it enough to start working on one, but I've been thinking a lot about the mechanism; Either spring loaded fins curved to the projectile that flip out to the side, or a spring loaded rod with a point inside the projectile that pushes the fins out

yes, but how to make a mechanism that's simple and light?

these are nice but seem a bit heavy

inventorguy1000 wrote:Ok, Sooo... I made a prototype.

Nice

So, then I tried jackssmirkingrevenge's idea of putting a tube on the end to stabilize it, I got another old pill bottle that was bigger than the first, cut off the bottom, and the very top, and hot melt glued it to the back, creating the "tube extension".
When I shot it, it veered down into the ground the first time I shot it, and it made a wide spiral before hitting the ground the second time.

It looks like the tube isn't long enough given the weight distribution - where is the centre of gravity? Also, it needs at least some small stubby fins to keep it centred in the barrel, if it's leaving the muzzle at unpredictable angles then you'll never get a decent shot out of it.

I thought about putting fins on it, but there was no room on the top, it was too snug as-is. So I put fins inside the tube, thinking that that might help, but no. I don't know what I was thinking! Having the fins on the inside would direct the airflow inside the tube and not really the tube itself, I really needed to have them on the outside to direct the whole tube. It just kept on getting worse and worse!

Fins inside the tube are completely useless if they are fully shrouded, what you would want for that to work is something like this:

Let me know what you think! and could I get some help on the calculations for the desired weight, length of tail, the CG and CP?

Since this principle is so relevant to model rocketry there is a lot to read on the subject online.

http://resources.yesican-science.ca/bot ... press.html

You can approximate the location of the center of pressure by making a cardboard cutout of the profile of the rocket and balancing it on a knife edge; the balance point is the center of pressure.

In terms of ideal location of each point, this is from a link I posted earlier:

CP--how far aft of the CG?

The exact location of the CP relative to the CG is rather a tradeoff. Having the CP too close to the CG risks having a rocket that is dynamically underdamped or even unstable. As well, since the CP for a typical rocket moves forward with increasing angle of attack, a sharp wind gust or other perturbation may result in loss of stability. Having the CP too far aft of the CG is also undesirable. This is because the rocket will experience significant or even sever weathercocking, which means its flight path will veer in a direction into the wind, rather than climb vertically. For model rockets, the rule-of-thumb is to have a rocket with one-calibre stability. Calibre refers to body diameter, so that the CP should be located one body diameter aft of the CG. Anything more than two-calibre stability leads to excessive weathercocking. Amateur rockets, which tend to be more massive (literally, have more mass for a given size) than model rockets, may require a CP further aft than this recommendation, owing to the much greater turning momentum that results from its mass. I generally aim for the CP to be 1.5 to 2 calibres aft of the CG of a fully loaded rocket.

The advice is for model rockets but it is relevant to spudgun projectiles too

speaking of which.. has anyone ever built a PVC projectile with extendable fins?

Only useful if you want a full bore projectile, in real terms this would be to carry a payload, otherwise the benefits of subcalibre projectiles are obvious.