Looking for advice on fin tip issue

[Bill S]

Situation: I have a potential situation that I may need to address. On my old Talon rocket, the fin tips have a large extension at the tip which goes forward. I had done a multi-part fin out of 1/8" balsa, and the fin tip was a separate piece with the balsa grain parallel to the body tube. A couple times the tip would break off upon landing, but my last flight, using an E12-6 (the largest motor this rocket can handle), I noticed the rocket "wiggling" near apogee, and when I recovered it noticed that the fin tip was broken clean off. I guess the tip was flexing too much and broke?

This is a side view of the fin in question:



I've since built an upscaled version of this rocket, using 1/8" plywood for the fins. The fin is one piece, without the fin tip being a separate piece. I've noticed that the fin tips seem to have a fair bit of "flex/wiggle" side to side when I was puttying/sanding them. I don't know if the plywood would stand up to flight better than the balsa, but I was thinking of trying to make the area where the fin tip joins the main body stiffer somehow. I hope I won't need to "paper" the fins at this late stage.

I"m using up to an G-80, max speed should be in the neighborhood of 300mph. Thoughts?

 

[David Schwantz]

How did you align the grain of the ply?

 

[Bill S]

How did you align the grain of the ply?

Parallel to the body tube, which I know is a big no-no with balsa. But I was concerned that if I did it like a balsa fin (angled), the fin tip would break or something.

 

[KenECoyote]

Plywood is definitely better since you no longer have to worry about the grain.

One way to further strengthen is to glue a strip of stock along that extension lengthwise on both sides (a rib?). Doesn't have to be big either.

 

[DigBaddy]

Could fill the gap with clear acrylic....

 

[Hobie1dog]

Balsa is such a weak material to use on a design like that

 

[Bill S]

Plywood is definitely better since you no longer have to worry about the grain.

One way to further strengthen is to glue a strip of stock along that extension lengthwise on both sides (a rib?). Doesn't have to be big either.

I had considered that, even though it would not look as "cool". But I wanted to ask around before I did that. I basically have the rocket completed, but not yet ready for primer, so if I'm going to do something to the fins, now is the time.

 

[David Schwantz]

Actually the grain direction is probably causing the flex. With the grain running parallel to the tube , you only have one ply running across the narrow span. A much better choice may have been to use "hard" aircraft ply. It has from 5 to 9 ply's. This would give you many more ply's across the narrow spot.

 

[Bill S]

Actually the grain direction is probably causing the flex. With the grain running parallel to the tube , you only have one ply running across the narrow span. A much better choice may have been to use "hard" aircraft ply. It has from 5 to 9 ply's. This would give you many more ply's across the narrow spot.

The plywood is 3-ply. I was unable to find nicer plywood, unfortunately.

I'm going to chance it and see how things go.

 

[neil_w]

Another possibility is to add an extra layer of wood to each side of the spike. This will make it visibly thicker, which I think might actually look really good. Of course it'll be more draggy then, and it'll add some weight to the rear, but maybe? Somewhat like the fin tips on the Estes Asteroid Hunter, it'll make the tips look more like pods than just fin shape.

Would be easy enough to add this to the OR model to see how it would look.

 

[Bill S]

I looked at using cardstock to paper the fins up, just the cardstock for all 5 fins will be close to 1oz, plus glue, total maybe 1.5oz or so, not helpful for stability.

I did mess up with the plywood grain, but I think it'll hold up. The fin flexing isn't a lot, so it may work okay.

Neil, that's not a bad idea, let me look at that.

 

[David_Stack]

Router bit in a Dremel to carve out a channel, and then embed a small diameter carbon rod, to serve as a spar. CWF or your filler of choice to hide it.

 

[Bill S]

I used some scrap balsa lying around to get an idea of what adding some balsa to the fin side and tip could look like. Not bad. I'd have to use 1/16" balsa to reduce the weight. Looks like about an ounce added to the rear, not quite as bad as cardstock. Still unsure if I want to mess with it or roll the dice.

 

[Bill S]

I decided to go with a modified cardstock template to reinforce the fin, plus a little bit of balsa to give the fin tip some "width". Should add about 3/4 of an ounce to the rear, which I Rocksimm'd and it shouldn't be an issue. I just have to go really light on the glue to keep the weight down.

 

[SolarYellow]

I suspect the problem is the forward projections are fluttering like a woodwind instrument's reed. Stiffen them up, make them 3D as proposed instead of flat. Might even work to just make pod noses at the forward tips, anything to shut down the aerodynamic oscillation.

 

[bjphoenix]

I suspect the problem is the forward projections are fluttering like a woodwind instrument's reed. Stiffen them up, make them 3D as proposed instead of flat. Might even work to just make pod noses at the forward tips, anything to shut down the aerodynamic oscillation.

When the tips vibrate that puts torsion into the shorter piece of fin where it necks down. The plywood with only the inner ply active has less torsional stiffness that way. Putting the card stock on the outside of the fins will increase the torsional stiffness quite a bit.

 

[Marc_G]

My technique for something like this is to use papering a single ply of computer paper (20 lb bond) on either side. The paper has a light coat of 77 Spray adhesive, then I soak with CA. Adds very little weight and stiffens the fins tremendously.

Still, aircraft ply is probably the best solution.

 

[Bill S]

Aircraft ply would have been the best solution, had I been able to find any. I messed up not orienting the 3-ply better, but I was under the impression that it wasn't very important with plywood.

I was also under the impression that using adhesive label paper or paper with spray adhesives was nowhere near as strong as paper/cardstock with a wood glue of some kind?

 

[lakeroadster]

I was also under the impression that using adhesive label paper or paper with spray adhesives was nowhere near as strong as paper/cardstock with a wood glue of some kind?

And that is correct. Wood glue bonds the wood fibers and the glue is stronger than the paper or the wood.

Clamping the papered fin between (2) thicker boards, as soon as the glue / paper is applied to the base fin will result in a stronger end product.

And finally, soaking the paper with CA after the wood glue has dried increases the strength, the CA prevents the paper from delaminating.

 

[Bill S]

Lakeroadster, that agrees with what I was thinking. Pity its going to be challenging to weight down/clamp the fins, now that they are attached to the rocket. Further complicating things is that there are 5 of them, so it doesn't leave a great deal of leeway to get books/clamps, etc in there.

 

[Zeta]

a composite sandwich of heavy paper, CA glue, and balsa wood is strong and light

 

[dr wogz]

my comments echo what Ken & David have suggested..

add a rib, or add an extra 'fin' at 90° along the bottom edge of the 'spike'. That is just too weak, and will flex & flap like a wind instrument's reed. You need to add a rib 90° to stop / limit any bending..

When cutting the fins, make sure they are 45° to the grain to maximize any torsional stiffness..

 

[SolarYellow]

Still, aircraft ply is probably the best solution.

I bought a pack of Midwest 3/32 five-ply aircraft ply in one of my Black Friday orders. Ate the $12 shipping to return it because it was warped worse than the worst China-ply on the shelf at Hobby Lobby.

 

[Marc_G]

I hat getting warped wood!

 

[Bill S]

Here is what I ended up doing to reinforce the fins: 65lb cardstock on the main fin, with a 2-part 1/8" + 1/6"
balsa fin tip piece to help keep the fin tip from flexing so much. The card stock helped quite a bit with the flexing. Cardstock reinforcements and the 2-part fin tip assemblies all told added 1.04oz to the weight of the rocket, minus glue. Stability should still be ok.

The rocket itself will be a metallic purple color, with the 2-part fin tip assemblies will be metallic blue, and they will be glued on after the main paintjob is done.

 

[BABAR]

Thinking outside the box, consider an external shock cord mount that allows the body tube and fin can to come down nose end first. Hide it on the launch lug side (or better yet, rail side) so it is partially hidden at launch. Can be a small Kevlar loop, with a snap swivel on the attachment end of the shock cord so you can tuck it inside rocket for display.

 

[Bill S]

Here is a picture of the completed fin assemblies on this rocket. I hope to fly it at the end of the month.