The "Venturi" Ring-Tail Thread

[Dotini]

I'm currently preparing these four model (built last year ) for comparative flights as soon as this week. The purpose is to determine if ring size and placement might have any significant difference in performance, i.e., a so-called Venturi effect. . A variety of motors will be used.

 

[GlenP]

A typical venturi consists of a converging and diverging nozzle. If you can accelerate the flow to sonic velocity at the narrowest part, ie the throat, and then it can continue to accelerate and go supersonic at the exit, but that is really fine tuned to a particular speed and area ratio, a model rocket does not operate at a single speed continuously. Here, you are really using the engine thrust to accelerate the air through the constant diameter "venturi" throat of the ring fin. You may get a short burst of extra thrust from that expansion, but only when the conditions are just right at a particular speed during the thrust/ascent. You could try a converging transition ahead of the engine, then a diverging transition aft of the engine, to see if you can get a boost in thrust, kind of like a SCRAM jet, not sure if a typical model rocket black powder motor burns long enough for sustained flight at a given speed for that to really work or not. You also get an increase in drag just before you hit that magic speed, so it also might get close, but start to slow before it gets fast enough to actually reach that sweet spot.

All of your struts are mounted on the outside of the main body tube. But you have some space in between the motor mount and body tube that you could conceal a good portion of the struts in that space, ie like TTW, or rather Behind-The-Wall fins that extend outward quickly to the ring fin. Just another idea, mount the struts on the inside of the body and notch the centering rings to fit.

Very nice looking rockets and paint jobs!

 

[BABAR]

A typical venturi consists of a converging and diverging nozzle. If you can accelerate the flow to sonic velocity at the narrowest part, ie the throat, and then it can continue to accelerate and go supersonic at the exit, but that is really fine tuned to a particular speed and area ratio, a model rocket does not operate at a single speed continuously. Here, you are really using the engine thrust to accelerate the air through the constant diameter "venturi" throat of the ring fin. You may get a short burst of extra thrust from that expansion, but only when the conditions are just right at a particular speed during the thrust/ascent. You could try a converging transition ahead of the engine, then a diverging transition aft of the engine, to see if you can get a boost in thrust, kind of like a SCRAM jet, not sure if a typical model rocket black powder motor burns long enough for sustained flight at a given speed for that to really work or not. You also get an increase in drag just before you hit that magic speed, so it also might get close, but start to slow before it gets fast enough to actually reach that sweet spot.

All of your struts are mounted on the outside of the main body tube. But you have some space in between the motor mount and body tube that you could conceal a good portion of the struts in that space, ie like TTW, or rather Behind-The-Wall fins that extend outward quickly to the ring fin. Just another idea, mount the struts on the inside of the body and notch the centering rings to fit.

Very nice looking rockets and paint jobs!

@Dotini , I am thinking the design feature is intended to use the nozzle jet to initiate a low pressure area inside the ring, the purpose of which is to direct more airflow through the ring, giving it more stability. Or maybe just to look cool. Both?

 

[BABAR]

I think you are definitely in the running for, “The Lord of the Rings.”

 

[dhbarr]

I'm currently preparing these four model (built last year ) for comparative flights as soon as this week. The purpose is to determine if ring size and placement might have any significant difference in performance, i.e., a so-called Venturi effect. . A variety of motors will be used.
View attachment 505696
View attachment 505697

None with the ring offset above the tail? Gut says it would be less overstable.

I look forward to your results, this is cool stuff!

 

[Dotini]

None with the ring offset above the tail? Gut says it would be less overstable.

I look forward to your results, this is cool stuff!

Last year I built and flew several models models with a ringtail. I noticed that all of them flew exceptionally stably with no sign of weathercocking, but lacked the altitude seen compared with other models of similar size and weight with conventional fins. However, the combined area of the rings and support fins amounted to considerably more area than my rockets with simple fins. So since then I've built some models with smaller rings and reduced support area in order to better compare apples with apples in term of total fin area. Unfortunately, weather and illness has prevented me from resuming my test program with my 4 new models. Things are better now, so I'm hoping for tests to resume within a week or two.

Left, 4 models previously flown with ringtails. On right, 4 new unflown models await tests. An effort has been made to reduce the size and area of the rings, with the 3 on the right having the rings entirely behind the tail, working toward reducing the rings to the absolute minimum size and area. None of these rockets carry ballast.

 

[jqavins]

I think that the aerodynamic action of ring tails is fundamentally different from that of "regular" fins, at least is some ways. And the pylons, I suspect, give little or no contribution if they are contained entirely inside the rings, since they are not in anything like the air flow that regular fins see. (Pylons that extend forward and/or aft of the ring would be a different story.)

What I'm getting at is that I think going by total surface area, and/or ring diameter vs. fin span, is not really an apples to apples comparison. Better, I think, would adjusting the rings to get the same CP as the same size rocket with regular fins, given whatever regular fin size you choose.

 

[Dotini]

I think that the aerodynamic action of ring tails is fundamentally different from that of "regular" fins, at least is some ways. And the pylons, I suspect, give little or no contribution if they are contained entirely inside the rings, since they are not in anything like the air flow that regular fins see. (Pylons that extend forward and/or aft of the ring would be a different story.)

What I'm getting at is that I think going by total surface area, and/or ring diameter vs. fin span, is not really an apples to apples comparison. Better, I think, would adjusting the rings to get the same CP as the same size rocket with regular fins, given whatever regular fin size you choose.

Could be. More tests will give more data. Lately, my fin preference is for those that are less than the diameter of the tube.

 

[lakeroadster]

... the pylons, I suspect, give little or no contribution if they are contained entirely inside the rings, since they are not in anything like the air flow that regular fins see.

So many variables... If the ring is large, in relation to the body tube, the pylons will see nearly the exact air flow that regular fins see.

 

[GlenP]

I think that the aerodynamic action of ring tails is fundamentally different from that of "regular" fins, at least is some ways. And the pylons, I suspect, give little or no contribution if they are contained entirely inside the rings, since they are not in anything like the air flow that regular fins see..

Yes, fundamentally different. But, no, they have an even greater contribution due to the end effects of the ring. A fin or wing lifting surface has zero pressure difference at the tip, hence the lift distribution goes to zero. If you put a wingtip device, like a winglet or fence, or a T-tail on a vertical stab, the wing becomes even more effective across the span near the tip. A grid or box fin is super effective because you have no loss of aero forces at the "tips" of all the component fins in the grid.

So, yes, different, but no, not less effective in the ring. The ring acts as a tip device that makes those fins even more effective than a fin of the same area with no ring around it. Likewise, the ring fin has no tip itself either. One of the reasons joined wings are highly efficient is the more efficient lift distribution over the full span of the wing, pylons inside a ring fin experience a somewhat similar effect.
For example:
https://en.wikipedia.org/wiki/Closed_wing

https://en.wikipedia.org/wiki/Grid_fin

 

[jqavins]

Thanks for that explnation.

 

[Funkworks]

... the aerodynamic action of ring tails ...

On this subject, I think a ring tail would be ideal for a spinning or spiralling rocket. I haven't seen that yet (a spiralling rocket with a ring tail) and it seems like something worth trying.

Just thought I'd drop that thought somewhere.

 

[Dotini]

On this subject, I think a ring tail would be ideal for a spinning or spiralling rocket. I haven't seen that yet (a spiralling rocket with a ring tail) and it seems like something worth trying.

Just thought I'd drop that thought somewhere.

Hopefully tomorrow I will launch this spinning ringtail together with the other ones.

Each support fin has a small flap rising from one side. Due to the end-plate effect, these should be powerful in generating spin.

 

[Dotini]

Today I flew all of my "Venturi" ring-tails at Dahl Field. Pending further testing under differing conditions, it did not appear that the far-aft location of the ring made any difference greater than +/- 5 or 10 % to my models with "normally" located rings.