A while back I accquired a large (4") diameter kit along with a bunch of other stuff. It appears to be complete except the fins are missing. I realize this is definately not a model rocket but thought I'd seek help here first as it's not really a kit related question.

I have read that your fin area should be 1 1/2 times the diameter of the body tube. So does that mean your total fin area for all fins should be 1 1/2 times the diameter or each fin should be? If each fin should be, why would having 4 fins be benefical over 3? I'm looking at making 3 fins out of 1/4 inch birch plywood and I'm trying to figure out a fin pattern that will work. I am notorious for "over finning" and was hoping some of you could provide a little insight.

Also, any suggestions on attaching these fins to the tube? The tube is NOT slotted, but the business end is 3 x 29mm and the overall length is about 6 1/2 '. I guessing that attaching the fins directly to the tube is going to result in less than desirable results very shortly after launch but I could be wrong. I'm not into HP but I was thinking maybe adapting it for 24mm and going that route but not sure I can enough punch to get it off the pad; nevertheless, it would make a really cool display. So, I guess what I'm getting at is, how big should the fins be to look proportionate to the rocket? Comments, toughts, accusations, and "it'll never work" most welcome!

Thanks

__________________
Dave
Fire is the leading cause of fire.
Was once SAM#0132

You can slot the tube yourself. Back in the olden days we all did.

I would say an area of diameter ^ 2 for each fin. Think of that as a 4" x 4" square. A simple choice would be a clipped delta, like the Patriot uses.

1/4" plywood is kind of overkill. 1/8" or 3/16" should be fine if you go "through the tube."


I would put in a 29mm mount. There are some 24mm reload kits that would get it off the pad, but not many.

This is where RockSim comes in very handy. You could try out various shapes and sizes until you reach a satisfying look with enough area to satisfy the margin requirements. You'd also see that five fins strums a happy chord, while six twangs a sour note. Strange but true.

As for your area-rule formula, I'd say each fin needs to have that 1.5x BT diameter area. Three fins would be the minimum without using outriggers of some sort (winglets, or fin-tip fins). Three fins are all you'd need, as you're not trying to align the model with the horizon (airplane mode), you're just trying to keep the nose in front of the tail.

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Craig McGraw

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SAM 0044
AMA 352635

How so, Craig?

__________________
John Thro, NAR #84553 SR
I was too old when I started! Now I'll *never* become a BAR!

I suppose it depends on the overall design, but I've found that (a set of) five matched fins tends to have a greater effect over (a set of) four than does six matched fins over five. The amount of improvement levels off at five. In other words, you get a certain percentage of improvement in the margin with each fin being added up to the fifth fin; but the percentage of improvement is outside the trend (on the low side) once you add that sixth fin.

I spotted this on several designs, line Nova and Loadmaster. With larger-diameter tubes, the percentage of improvement may get back into the trend; bit at least up to ST-10 size models, five is the optimum value for max-fins.

My initial observation suggests there is a degree of aerodynamic "blanking out" of the effect of the fins when they start getting too close to each other.

__________________
Craig McGraw

BARCLONE Rocketry -- http://barclone.rocketshoppe.com
BARCLONE Blogsite -- http://barclone.wordpress.com
BARCLONE Forum -- BARCLONE Forum

BARs helping BARs

SAM 0044
AMA 352635

But six fins are so darned cool .....

Here's six fins mounted in a 4" diameter tube. It's a modified LOC Ultimate with a single 54mm motor mount. If you are interested email or PM me and I'll send you the fin pattern.

It's a proven design, and the original uses 7x29mm motors. The stock kit will fly great on one, three, four or seven motors.



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__________________
Steve Naquin
TRA# 677 L2
NAR# 85518 L2
SAM# 0052

🚀 In Construction: Der Blue Maxx/Minie-Magg 5.5” & Vander-Burn MDRM Clone w/Stickershock23 Custom Decals
🚀 In Paint & Detail: USR Banshee
🚀 In Build Queue: Estes Doorknob w/Vander-Burn Rocketry Upgrade Kit [Sport Decor], Semroc Centurion-F, Semroc Egg Crate
🚀 In Repair: SLS Lil’ Hustler, SLS Aero-Dart 1969 Trim
🚀 Stay Tuned For Fall 2021 Launch Dates

How exactly do you slot your own tubes?

__________________
Dave
Fire is the leading cause of fire.
Was once SAM#0132

A sharp knife. Having a piece of aluminum angle stock to guide the cuts helps a lot too.

Thanks again for your input. It'll be interesting to see how things turn out.

__________________
Dave
Fire is the leading cause of fire.
Was once SAM#0132

I slot mine just as Stefan suggested. I have a piece of aluminum angle from Home Depot 1/2 x 1/2" x 72" that I cut down with a hack saw to about 24" long to have a more manageable length. I mark lines for the centers of my slots where the fins should go with the aluminum angle. If the fins are 1/8" thick, I go back and mark another line 1/16" on each side of the first marked line and extend the marks the length of the root edge of the fins. I do this to center the slots/fins and adjust accordingly depending on the fin thickness I'm using.

I use the aluminum angle and a sharp knife to score the lines where I'm cutting the slots. I prefer to use a heavy duty utility knife for this because it doesn't flex and break as much as a #1 blade will and works better in heavy walled cardboard tubing. You can get that Husky knife at Home Depot for around $10 and it's a great investment if you don't already have one. I don't like a knife with a retractable blade, this blade locks and replacing the blade is very easy and secure. Don't try to cut too deep in a single pass, score it a few times gradually increasing the depth of the cut into the cardboard. After a few passes I drop the aluminum angle and just keep scoring in the groove by hand until the blade cuts completely through. Notch out the ends of the slots with a #1 blade. Sand any flash out of the inside of the tube after so it doesn't interfere when you push the centering rings and motor mount up past the slots on the inside surface.

If your slots are a little sloppy, don't fret. Fillets on the fin to body tube joint will cover the small spots if you do happen to cut a little crooked here and there. The original 4" diameter BSD Little John had to have the fin slots cut. Notice on this model I didn't slot to the end of the tube. The fins on this model mount slightly forward of the rear of the body tube so as not to interfere with the centering rings. You may consider this on your design as well before you cut your slots, depending on how your fins will mount - so they don't interfere with centering rings, ect. The fins extend inside all the way to the 54mm motor mount on this model and the rear centering ring butts up against the rear of the root edge of the fins on the inside. Fillets finished it out well.

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__________________
Steve Naquin
TRA# 677 L2
NAR# 85518 L2
SAM# 0052

🚀 In Construction: Der Blue Maxx/Minie-Magg 5.5” & Vander-Burn MDRM Clone w/Stickershock23 Custom Decals
🚀 In Paint & Detail: USR Banshee
🚀 In Build Queue: Estes Doorknob w/Vander-Burn Rocketry Upgrade Kit [Sport Decor], Semroc Centurion-F, Semroc Egg Crate
🚀 In Repair: SLS Lil’ Hustler, SLS Aero-Dart 1969 Trim
🚀 Stay Tuned For Fall 2021 Launch Dates