Outward-angled ring fin

I've had this idea bouncing around and I'm not sure of the aerodynamics of it, and I figured someone here may have seen or even tried the idea before.

Basically, it's a ring finned rocket, but the ID of the aft end of the ring is less than the forward end by a small amount. I was thinking the ring would have a cant of about 15 degrees or so. It would be mounted right at the back end of the rocket. In essence, it's like a small rigid parachute that has an enormous spillhole.

My intuition about the stability tells me that would be that the CP of the overall rocket would be thrown way backwards by the parachuting effect, keeping the rocket stable.

Anyone have any experience with this sort of thing, or places where I might be off?

WW

Try searching for information on cone fin rockets (EMRR). In effect you will be producing a section of a cone that you will be using
as the fin, so your rocket should have a CP with caracteristics of both a ring fin and a cone fin rocket. Both of these designs are
much higher drag that flat fin models. Because it will also have an area of low pressure at the inside trailing edge of the ring fin,
it will probably also have an airfoil effect.

I've thought about a ring fin that is in essence a section of a cone, but never the inverted one. That sounds like a cool idea. I bet teflon chimes in on this one.

As long as the inverted cone is open, you should be fine. It will move the CP back, just like a ring fin, but probably not more than a ring fin of the same size.

A conical is stable if the point is aimed in the direction of air motion. It's unstable if you flip it the other way. Putting a spill hole in it makes it less unstable, but it's still unstable.

So, keep the CG about where you would for a ring fin rocket, or a little ahead.

That's why you want long suspension lines/shock cord on a parachute. The inverted cone (or bowl) is unstable, so you move the CG futher away (in the direction of travel) to slow down the oscilations. If you add a spill-hole, you increase the stability, but make the spill hole too big, and you increase the descent rate.

urbanek

If you have a funneled shaped ring I wonder if tehre would be any boatatil benefit effects? I also wonder if you would get some kind of ram air effect from going to a larger to smaller diameter? surely the pressure would increase?

And the drag will increase too

Well, I've got the thing half-built.

BT-50 based.
"fin" is 2.5 inches long
leading edge is 3.0" ID
trailing edge is 2.0" ID
4 3/32" basswood struts hold it in place.
Overall model length is 18"

No nose cone yet, nor a parachute attached. No swing tests yet, either.

Any of you rocsim folks want to plug in those numbers and let me know if it's going to be even close to stable?

But as it's coming into shape, I'm seeing a strong resemblance to a paper cup with a body tube coming out the middle of it!

WW

As it was configured, the model was unstable. After adding in 1.2 oz noseweight in the form of an eyebolt and 5 solid 5/16" zinc washers, it was still unstable with a D12-5 in the back.

Conclusion: design needs modification to be stable. Most likely, it will need to either be longer, or have a greater angle on the cone in back, or both.

The entire model empty only weighs 1.2 oz, so adding more noseweight, while it may bring stability, would be at too great a cost in performance, especially since the design is heavily reliant upon aerodynamic drag for stability.

WW

Originally posted by wwattles
Conclusion: design needs modification to be stable. Most likely, it will need to either
be longer, or have a greater angle on the cone in back, or both.

Click to expand...

How far forward (in relation to the end of the BT) is the cone? Maybe you could
move the cone rearward a bit to put the CP further back?

got pics? How about a screen shot of your sims?

Originally posted by brianc
How far forward (in relation to the end of the BT) is the cone? Maybe you could
move the cone rearward a bit to put the CP further back?

got pics? How about a screen shot of your sims?

Click to expand...

The cone is right down against the bottom of the rocket. I could move it further back, but I'd have to angle the fins holding it in place, which I didn't feel like messing with.

The sims were done manually, doing the old tried-and-true swing test. And unless someone's got a digital camera they want to give me, there won't be any pics of it anytime soon. It's not exactly a thing of beauty, anyway.

WW

Originally posted by wwattles
As it was configured, the model was unstable. After adding in 1.2 oz noseweight in the form of an eyebolt and 5 solid 5/16" zinc washers, it was still unstable with a D12-5 in the back.

Conclusion: design needs modification to be stable. Most likely, it will need to either be longer, or have a greater angle on the cone in back, or both.

Click to expand...

To me, it seems like the rocket has an oversize boat-tail. Or, it's a tube with an upside down saucer rocket stuck on the end. Either of those is going to affect stability adversely.

Lengthening the rocket, so that the CG is further forward, ought to help. I would guess that reducing, not increasing, the angle of the cone, thus making the ring more like a tube and less like a cone, might also help.

I'm afraid this kind of fin design is almost impossible to get stable. Like a finless rocket with a boattail in the end.

Look at this graphic to get an idea of how the location of the CP behaves on differently shaped rockets without fins. Ba means Barrowman, CC is cardboard cutout and the °-numbers are angles of attack at which the CP location was measured in a wind tunnel.

<img src="https://www.raketenmodellbau.org/attachment.php?postid=65947">

Your instability problem is easy to illustrate: If you have your rocket at a slight angle of attack, the aerodynamic forces on the outward/upper part of the cone fin increase whereas they decrease on the inward/lower part. This creates a moment turning the rocket even further outward, hence the instability. This may be solved by a CG a long way towards the front of the rocket, but I am not sure whether the necessary CG location is still inside the rocket... As Adrian already pointed out, lenghtening the cone or increasing its angle will only increase the turning moment.

Oliver