Tube fin to Diameter lengths

[mjennings]

Is there any good rules of thumb for tube fin length to diameter ratio's. I'm working on a design that would have tube fins several inches long and would be 3/4" to an 1" in Diameter. I could pull out my fluids book and find out all kinds of info but I don't know if it will help me figure out if they will help or just add drag. I'm ok with the drag. I don't know if RockSim's calculations break down for large length to diameter ratios, but it has so far given me the thumbs up.

 

[TheAviator]

A good rule of thumb for stability's sake is about 1 to 2 times diameter. More than this can be added for aesthetics if desired, just don't put the fins too far up the airframe. The best rule of thumb is: if it looks right, it probably is right.

Edit: Oops, I forgot, this is assuming you use a tube that is the same size as the airframe. If the tube fins are smaller, you will have problems. If they are bigger, you should be fine, but 1-2 times the diameter of the airframe should be fine.

 

[mjennings]

Thanks Brian I had come to the minimum conclusion as you stated from some models I have.

I know from Fluid dynamics that in a long skinny tube fin the flow will trip and become turbulent, if it wasn't already turbulent when it got to the tube fin. Where in a shorter tube fin the flow may stay laminar. I don't know what this effect has on stability, and the associated drag rise with the skinny tube. I doubt RockSim takes this into account, otherwise it be a whole lot more expensive.

I'm looking for Maximum Length to Diameter. While my intent was not a "Starfleet look" at the start it does have sort of the appearance of ST warp nacelles. Attached is a quick sketch not really to any scale but just to give an idea.

 

[TheAviator]

My suggestion in that case would be to have the standard fins and standoffs make the rocket stable or very close to stable without the tube fins. At the very worst, you can build it (don't paint it yet), load it for flight, and do a swing test. If you don't get the stability you want, change things around until you do. It's not ideal, but there's really no way to simulate configurations like that in RockSim 8.0 or less.

 

[mjennings]

Yeah I'm running RS9 so that is a little help but I didn't think of checking the stab with out the tubes on. I think I'll build it in RS 9 with out the tubes, and check stab, then add BT's with blunt NC's to check stab, and then build and swing test the final config. Thanks for the advice.

 

[Lentamental]

Mjennings, I was wondering how you got your tube fins to be recognized as fins. I have RS 7.03, and cant figure out how. I am not trying to do anything as complicated as what you are doing, and the tube will be pretty huge, probably around 5-6" in diameter, so turbulence will not be such an issue.
Thanks a million!

 

[gregzo]

Not sure if the following will help or not...sorry if it is just dribble for what you need.
I have made quite a few tube fin rockets...even got my L3 on one.

This will answer the question...long way around the barn however. You just have to calculate the effective fin area first.
The trick I use (your mileage may vary) is to take the circumference of the tube fin in question. Multiply that by 90% to get an effective fin span height (takes into account fin area losses for attachment points and where the tubes touch one another). Divide that length by THREE and use that as the fin height of THREE fins at that location. If you just use the circumference...RockSim will not calculate the stability correctly as the fin hangs too far off the body.

So for a standard six tube fin rocket....the sim is 18 fins
6" diameter rocket
(6) x 6" tube fins
(6" x 3.14 x 0.90)/3 = 5.472" semi-span x (18) fins
Now just adjust the fin length until stabilty gets to where you want/need it to be. (I told you I would get around to answer the question of how long they should be!! )

Hope this helps
Good Luck

 

[mjennings]

Lentamental I have RS 9, the tube fin options got added in RS 8 i believe, if you use gregzo method that should work out for you.

gregzo not what I'm looking for but should help out Lentamental nicely, I may try playing with this method though to see how it tweaks the results.

Unfortunatly I haven't been able to work on this design since my earlier posts.

 

[Lentamental]

Great, thanks so much. I am currently running RS7.03 that I got back in 2002 or so, when I was last into rockets, in the 4th and 5th grade, but once we get our TARC grant money from school, I will make sure we upgrade to either 8 or 9 (8 runs better on WINE, for linux, which I and several others on the team mainly use.)

 

[gpoehlein]

An even quicker and dirtier approach for RS7 is to create a fin the same profile as the tube fin, then triple it for each tube. If memory serves, I believe you can even put the three fins in the same position for each tube and all will count as fin surface area. Using three fins kind of compensates for the attachment point (since pi is aprox 3.14). So if you are using six BT-50 tube fins on a BT-50 body tube, and each tube is 3 inches long, just make a set of six fins 3" root and tip chord and .976" span. Duplicate the set two more times and just leave them in the same position (if you want to be really finicky about it, offset each new set from the previous by 1 or 2 degrees.) This should put the CP pretty close to the correct position for equivalent tube fins.

 

[gregzo]

gpoehlein - I think we are saying the exact same thing...

I am also runnign RockSim 7.

Good luck to all with your Tubers!!!
Finless Rockets of ALL types are COOL !!!

 

[MarkII]

FWIW, I'll throw this into the discussion - my FSI Viking III clone (see attachment) flies arrow-straight with its short, swept beveled tube fins and stand-offs. The tube fins and the airframe are both 0.921" in diameter, but if you were to stand one of the tubes on one of its beveled edges, the distance to the other beveled edge is only 1.0". That is, the length of the tube fin, when measured from beveled edge to beveled edge, is only 108.58% of the diameter. The stand-offs are 0.5" wide. The tube fins have parallel bevels that are 30° from the horizontal (60° from the vertical), making it effectively a parallelogram. But if you measure from the tip of one bevel at one end to the tip of the bevel at the other end, the length of the tube becomes 3.0" (327.73% of the tube's diameter). The "fin tip" length of the tube fins is 1.5".

As you can see, the fin stand-offs are angled in such a way as to continue the 60° bevel all the way to the root at the leading edges, and are cut away at the trailing edges so that the length of the stand-off tip where the tube fin attaches to it is the same length as the tube's root and tip lengths.the amount of sweep below the tip edge of the stand-off is 1.5", and the sweep aft of the end of the airframe is 1.0" The overall length of this rocket is 16.0"

MarkII

 

[Lentamental]

All of your advice worked perfectly, as far as I could tell, when I used it in RS7. However, I just downloaded the trial of RS9, and when ever I try to put tube fins on the rocket, it cuts the altitude almost exactly in half, from 1000' to 500'. At first I thought that this might be a problem of turbulence through a skinny hole, but even when I increased the ring fin to a 6" tube around my BT70 body tube (I intend to use about a 4" ring fin in the final run) it had exactly the same effect. Next I thought it could be a case of serious over stability (it was around 7 calibers) but with super large fins, with aproximately the same over stability, the flight still went its normal height of 1000 feet.
A little more spicifics on the rocket, in case it helps you at all, it is flying on an E30, with about 100g of nose weight in payload. I attached the file, for any further information. Flight 1 is with normal fins, and flight 2 is with the attached tube fins.
Thanks

edit:
Ok, so I did a little more investigation, and it turns out that the rocket with the ring fin has a calculated Cd of ~1.6, while the one with similarly sized regular fins has a Cd of only about 0.3 Do you guys have any idea what is causing this, and/or how I can avoid it?

edit 2:
Another interesting tidbit, is that with tube fins of the same BT70, it ends up having a Cd of .6
Is there any way of having nonplanar fins without the Cd skyrocketing? (no pun intended) If not, that is really unfortunate, because tube and ring fins are so much better at dealing with cross wind.

View attachment TARC Tube Fin.rkt

 

[mjennings]

MarkII that's a cool looking bird

Lentamental, Tube and ring fins tend to have higher drag, I'm not sure how RockSim calculated the CD for tub/ring fins, but the .6 and 1.6 do seem high. Being that this is just an estimate from the software you may find in flight test they achieve similar altitudes. You could cheaply build a couple of rockets (one in each configuration) scaled down to fly on 18mm or even 13mm motors set up a couple of tracking stations and do altitude fly offs, and see if the drag penalty is really as bad as RockSim says

 

[gregzo]

My tubers with offset front and rear bevels (standard 6 tube...tube fin otherwise) have a real life Cd of approx 0.73 to 0.75 depending on which bird (2.5", 4.5" & 6").
This gets proven time and time again with altimeter data.

Your mileage may vary...
Good Luck