Rounding Card Stock Fins?

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lcorinth

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I'm about to start building the display model of the Estes Viking, which has thick card stock fins, for a class I'm teaching. I usually like to sand the leading and trailing edges of my fins round, but I've never worked with card stock fins. Anybody know if this works? Or does card stock shred when sanded?
 
You can sand them with 220/320 grit paper. One trick to keep them from "fuzzing out" is to treat the edges with thin CA.
 
Like sooner.boomer said, thin CA on edges. Don't apply it on the gluing edge that you attach to the body tube. I traced the fins on balsa wood and cut out wood fins for one of mine which also works. I made a booster and needed more fins and I figured that the cardboard on the booster would be better for the hard landings.
 
I decided not to bother. I don't want 11 year olds gluing their fingers together on my watch! Next week, we've got balsa fins - we can round those. We've got plenty of time to play around with these models.
 
I would suspect that dragging the edges that are to be rounded through thin CA (like you're squeegeeing a layer of grout onto a surface before applying a tile) would work (then do it again after sanding). To help keep the area clean, wipe the freshly glued edge with a dry paper towel or napkin to prevent the CA from creating "warts".
 
I would suspect that dragging the edges that are to be rounded through thin CA (like you're squeegeeing a layer of grout onto a surface before applying a tile) would work (then do it again after sanding). To help keep the area clean, wipe the freshly glued edge with a dry paper towel or napkin to prevent the CA from creating "warts".

I tell ya what, I wish I'd treated the display model with CA, just for strength. We have to launch in a parking lot, and with a streamer recovery, I got some pretty dinged up fins.
 
Just curiosity why do you want to round your card stock anyway?especially for the Viking I think the square fins edges actually look pretty sharp and I don't think you're going to get that much of a performance improvement if any
 
Just curiosity why do you want to round your card stock anyway?especially for the Viking I think the square fins edges actually look pretty sharp and I don't think you're going to get that much of a performance improvement if any

CA soaking with Thin or Med CA makes rounding Heavy Cardstock fins sand just like Styrene Plastic. using 220 to 360 grit make sanding them very smooth.

BARBAR I thought your worked for NASA. It's been common knowledge that the act of simply rounding the leading and trailing edges of any fin set will increase the achived altitude of that particular model by up to 10% over Square edged fins. Wind tunnel tests and tracked altitude Test provided evidence that this practice is sound.
While it may not be a huge gain in the grand scheme of things. The practice make is a good teaching tool at can add to the feeling of acomplishment particularly for new builders. Craftsmanship has to start with the basics. Simple fin round is a easy begining:)
 
I decided not to bother. I don't want 11 year olds gluing their fingers together on my watch! Next week, we've got balsa fins - we can round those. We've got plenty of time to play around with these models.

I shot off the first reply (first launch of the year!) saying that CA could be used. You can also sand the edges then rub in white or yellow glue, and after that dries, lightly sand off any "fuzzies" that remain. White/yellow glue is thicker than CA, so it might actually leave a line where it ends. It's also harder to sand because it's somewhat plastic after drying. It might take a coat or two of a high-build primer to help blend in the glue line around the edges. Or just replace the paper fins with balsa...
 
It's been common knowledge that the act of simply rounding the leading and trailing edges of any fin set will increase the achived altitude of that particular model by up to 10% over Square edged fins. Wind tunnel tests and tracked altitude Test provided evidence that this practice is sound.
That's incredible. Really; not credible. Sure, it's a good practice because it reduces fin drag, but to say it gives a substantial improvement* in any rocket's performance, no matter the fins' contribution to the total drag, does not ring true. Card stock, being really thin, should contribute very little to the total drag, so rounding those fins should make less difference than rounding say 1/8" balsa fins on the same airframe, and probably a trivial difference.

While it may not be a huge gain in the grand scheme of things. The practice make is a good teaching tool at can add to the feeling of acomplishment particularly for new builders. Craftsmanship has to start with the basics. Simple fin round is a easy begining:)
Craftsmanship starts with the basics, and so does engineering. Starting with card stock fins that are left square, then introducing the idea of rounding the fin edges on the second project, where it make a greater difference as well as being easier to do, gives you a chance to talk about evaluating when extra effort (or expense in other cases) is worthwhile and when it's not.

I, therefore, applaud your decision, Daniel.

* By writing "increase the achi[e]ved altitude of that particular model by up to 10%" you're implying an improvement of something somewhat near 10% in all cases.
 
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Just curiosity why do you want to round your card stock anyway?especially for the Viking I think the square fins edges actually look pretty sharp and I don't think you're going to get that much of a performance improvement if any

The main reason I wanted to round the fins (I'm using a kit with balsa this week) is that I'm with these kids for a ton of time, and so there's lots of time to fill. I'm not going into all the finer points of craftsmanship - fin filling, etc. - but I do mention (as I did on week one - this is week three) that rounded fins might look nicer, and might improve altitude. I show them how I do it and tell them "You can do this if you want to, but don't feel like you have to. The rocket will fly just fine if you don't."

In answer to the question as to why I wanted to round the card stock fins, it's because I was building the display model for that week, and I wanted to make it look more finished. Didn't end up doing it, and it didn't matter.

As a side note, due to the large amount of time I'm with these kids, I have been able to teach them a lot of technical information, and they've managed to retain it. Several parents have commented to me that they're surprised how much of the basic physics of rocketry the kids have learned. And now they want to go out and learn more, which was my goal. Lots of kids launch a few RTF models for a year or two, then get bored. I'm trying to give them an idea that there's more to it than that.

This weekend, in fact, I ran into one of my students in the hobby shop with his mom, buying more rocket stuff, because he really wanted to do more. I was happy about that.
 
This weekend, in fact, I ran into one of my students in the hobby shop with his mom, buying more rocket stuff, because he really wanted to do more. I was happy about that.
That's terrific! I do hope you said something to them. It must have felt great; I'm smiling just reading about it.
 
CA soaking with Thin or Med CA makes rounding Heavy Cardstock fins sand just like Styrene Plastic. using 220 to 360 grit make sanding them very smooth.

BARBAR I thought your worked for NASA. It's been common knowledge that the act of simply rounding the leading and trailing edges of any fin set will increase the achived altitude of that particular model by up to 10% over Square edged fins. Wind tunnel tests and tracked altitude Test provided evidence that this practice is sound.
QUOTE]

LOL, I said I build rockets for NASA, not that I worked for NASA. That would imply they actually PAID me for the rockets.

No doubt rounding can increase performance, as you said UP TO 10%. (As other poster mentioned, with such thin card stock to begin with, YMMV.) Even more perhaps if you aggressively taper the trail edges. For a SPORT rocket, question is whether the effort and time is worth the performance increase, or perhaps better put, whether it give you more FUN when you actually launch your noncompetition sport rocket.
Beauty is in the eye of the beholder. I am not convinced personally that rounded fins necessarily look better than squared fins on ALL rockets. Example is the Flutter Bye, definitely not a competition bird that personally I think looks better with somewhat "blocky" squared fins.

Your are correct of course. Well rounded fins certainly are a mark of good craftsmanship, no question. For those more experienced to imply that beginning builders "must", or "should" do certain things...... that I am less certain about. The "musts" for the beginner should regard building a rocket that is safe and recoverable. The additional craftsmanship touches (rounded or tapered fins, filled nose cones and spirals, smooth fillets, Future finish coatings, etc.) are great suggestions but particularly with sport rockets (translated: for FUN rockets) should be left to the discretion of the beginning builder.

Perhaps I've seen to many posts of beginners spending hours tapering fins on sport rockets (yes, I know, this post is ROUNDING, not tapering), to me a waste of time and actually makes the rockets more prone to fin damage on landing. If the builder is however specifically LOOKING to increase performance, it is a whole different story, but in most rockets that aren't designed to be competition birds to begin with (Viking--- 3, 4, ? 5 fins)..... well, you get my drift.
 
CA soaking with Thin or Med CA makes rounding Heavy Cardstock fins sand just like Styrene Plastic. using 220 to 360 grit make sanding them very smooth.

BARBAR I thought your worked for NASA. It's been common knowledge that the act of simply rounding the leading and trailing edges of any fin set will increase the achived altitude of that particular model by up to 10% over Square edged fins. Wind tunnel tests and tracked altitude Test provided evidence that this practice is sound.
QUOTE]

LOL, I said I build rockets for NASA, not that I worked for NASA. That would imply they actually PAID me for the rockets.

No doubt rounding can increase performance, as you said UP TO 10%. (As other poster mentioned, with such thin card stock to begin with, YMMV.) Even more perhaps if you aggressively taper the trail edges. For a SPORT rocket, question is whether the effort and time is worth the performance increase, or perhaps better put, whether it give you more FUN when you actually launch your noncompetition sport rocket.
Beauty is in the eye of the beholder. I am not convinced personally that rounded fins necessarily look better than squared fins on ALL rockets. Example is the Flutter Bye, definitely not a competition bird that personally I think looks better with somewhat "blocky" squared fins.

Your are correct of course. Well rounded fins certainly are a mark of good craftsmanship, no question. For those more experienced to imply that beginning builders "must", or "should" do certain things...... that I am less certain about. The "musts" for the beginner should regard building a rocket that is safe and recoverable. The additional craftsmanship touches (rounded or tapered fins, filled nose cones and spirals, smooth fillets, Future finish coatings, etc.) are great suggestions but particularly with sport rockets (translated: for FUN rockets) should be left to the discretion of the beginning builder.

Perhaps I've seen to many posts of beginners spending hours tapering fins on sport rockets (yes, I know, this post is ROUNDING, not tapering), to me a waste of time and actually makes the rockets more prone to fin damage on landing. If the builder is however specifically LOOKING to increase performance, it is a whole different story, but in most rockets that aren't designed to be competition birds to begin with (Viking--- 3, 4, ? 5 fins)..... well, you get my drift.

I agree with you that many kits look better with squared fins. Der Red Max, Big Bertha, Nautilus II (from 3D Rocketry) are three I've built where I prefer the look of square fins.

As I said above, I certainly didn't insist the kids round anything. I offered it as an option, and a lot of them seemed to want to try it (during week 1, when we all had balsa fin rockets). Toward the end of the week, the kids asked me to show them how I airfoiled fins, since a lot of my kits have those. I did a quick demo, just to show them. But I'd never insist on it.

My main goal is for them to build rockets that are sturdy enough to fly and recover. If I'd been too concerned with craftsmanship, their painting skills would have hurt my brain too much. XD

But I like to tell them that their building skills will get better with practice.
 
Beauty is in the eye of the beholder. I am not convinced personally that rounded fins necessarily look better than squared fins on ALL rockets. Example is the Flutter Bye, definitely not a competition bird that personally I think looks better with somewhat "blocky" squared fins.
Just to amplify this, if you're going by the "how it looks from 10 feet away" test (some do, some don't) and your fins are extremely thin, then rounded or square makes not difference at all.
 
Just to amplify this, if you're going by the "how it looks from 10 feet away" test (some do, some don't) and your fins are extremely thin, then rounded or square makes not difference at all.

Der MicroMeister's rockets are works of art. In this case however, given the scale he uses, you often can't SEE his rockets from 10 feet away! :wink:
 
Just to amplify this, if you're going by the "how it looks from 10 feet away" test (some do, some don't) and your fins are extremely thin, then rounded or square makes not difference at all.

Well, my rockets spend about 2 minutes in the air per month (if I'm lucky that month), and the rest of the time on a shelf, where I show them off to guests, so I'm going for the "looks good 2 feet away" test. I figure I have these things in my house, might as well call them "decor."
 
My rockets have to look good to. I sacrifice a little performance with many coats of sealers, primers, paints and clear coats.
Some of my rockets have been flown with just a primer coat just to make sure they fly well before putting nice paint jobs on them to never be flown again.
 
Der MicroMeister's rockets are works of art. In this case however, given the scale he uses, you often can't SEE his rockets from 10 feet away! :wink:

LOL! very true! but our launchers start at 50ft away! But like most folks our models regardless of size fly a couple minutes a month, but are Very closely inspected the rest of the time.
That said; ALL micro models in my fleet (that have fins) have rounded leading and tailing edges, quite a few are airfoiled. Not just for looks but for a Little better performance.
 
* By writing "increase the achi[e]ved altitude of that particular model by up to 10%" you're implying an improvement of something somewhat near 10% in all cases.[/QUOTE]

Up to 10% is exactly what it says; some models will get less depending on the practice and other factors but the majority average has been evidenced by decades of actual flight tracking experiments. Many of these were carried out back in the stone age 60's at MIT's rocket society, If I wanted to take the time, I'm sure I could run down copies of the old tech reports. Instead; If you don't believe what's been written look it up doubting Thomas.
 
Okay, how about a compromise.
You can get round carded fins edges without sanding through the laminations.

Yellow Jacket 3.jpg

Pic 1 -
On my carded fins the printed 110 lb. overlay skin is folded down the leading edge.
Emboss down the leading edge line before folding with the tip of a (dull) butter knife and straightedge.
Coat the inside with a glue stick and lay the fold tightly over the cereal box card board.
The fold is an automatic rounded leading edge.

Yellow Jacket 4.jpg

Pic 2 -
One side of the overlay fin skin print is oversize to insure ink coverage.
After drying in a heavy book cut out the fins on the printed line.
After cutting the outside edges may be a raised.
Burnish this down with something smooth and round.

Yellow Jacket 5.jpg

Pic 3_
To slightly round the outside and trailing edges, apply a bead of white glue.
This also seals the exposed sides. Wipe off the excess glue but leave the round glue bead.
After the glue dries lightly smooth with 400 grit.
This won't be a perfect half round edge but will round off the sharp edge a bit.

I have plenty of FREE carded downscale PDFs on my website:
www.modelrocketbuilding.blogspot.com
Click on the Free PDF Plans tab near the top of the homepage.
 
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* By writing "increase the achi[e]ved altitude of that particular model by up to 10%" you're implying an improvement of something somewhat near 10% in all cases.

Up to 10% is exactly what it says; some models will get less depending on the practice and other factors but the majority average has been evidenced by decades of actual flight tracking experiments. Many of these were carried out back in the stone age 60's at MIT's rocket society, If I wanted to take the time, I'm sure I could run down copies of the old tech reports. Instead; If you don't believe what's been written look it up doubting Thomas.[/QUOTE]

I'm with der MicroMeister on this one, the statement is correct but most people interpret it wrong. Advertisers depend on this. If they tested 100 cars with a new fuel additive, and 99 of them got 50% worse but 1 of them got 10% better! they could still advertise "up to 10% improvement" and be correct. This is also why they will usually put YMMV, LOL.

Kind of sad we don't exercise our thinking skills a bit more. What is the AVERAGE improvement, and what is the variance (or standard deviation, and no those aren't the same thing but now I don't remember the difference anymore.)

Back to the thread, kudos to the rocketry teacher for getting the students to think. Reasons to round fins:
either some potential improvement (up to 10%!)
and if the builder likes the look may make a purtier to the builder. Some builders may like square fins. Some may not care, in which case not worth the effort.

Card stock sure is nice in that doesn't require grain filling! makes me wonder why it isn't used more. What is the down side?
 
Up to 10% is exactly what it says; some models will get less depending on the practice and other factors but the majority average has been evidenced by decades of actual flight tracking experiments. Many of these were carried out back in the stone age 60's at MIT's rocket society, If I wanted to take the time, I'm sure I could run down copies of the old tech reports. Instead; If you don't believe what's been written look it up doubting Thomas.

I'm with der MicroMeister on this one, the statement is correct but most people interpret it wrong. Advertisers depend on this. If they tested 100 cars with a new fuel additive, and 99 of them got 50% worse but 1 of them got 10% better! they could still advertise "up to 10% improvement" and be correct. This is also why they will usually put YMMV, LOL.

But this was exactly my point. While the improvement is anything between 0 and 10% across all sorts of fins, using that figure in this discussion subtly implies that the 10% figure is relevant to cardstock fins in particular. Is it really? When fin drag is a small fraction of total drag as compared to its fraction with other, thicker materials, is the improvement ever that large with cardstock?

Kind of sad we don't exercise our thinking skills a bit more. What is the AVERAGE improvement, and what is the variance (or standard deviation, and no those aren't the same thing but now I don't remember the difference anymore.)
Standard deviation is the square root of variance.
 
With cardstock fins of any 'normal' size, I would suspect that they would flutter, which is probably a bigger factor than the amount of rounding. Ditto if they are bent or warped. Mine don't always start that way but have always ended up that way after numerous flights and landings.
 
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