Fixing Floppy Fins

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Gunstar

Well-Known Member
Joined
Mar 2, 2018
Messages
140
Reaction score
62
I'm building a modified Broadsword, and one the changes was to give it plywood TTW fins. I used 1/8 inch plywood, but I find that the part of the fin the extends down past the end of the body is pretty floppy. I can easily push the bottom edge of the fin about an inch to the side with my finger. In hindsight I probably should have used 1/4 ply.

Should I be worried about this or not, and if so what would be a good way to fix it? Paint on a coat of slightly watered down wood glue? Glue cardstock or 1/16 ply to each side of the fins or what?
 
It is solidly glued in. The part of the fin that runs along the body tube is fine, very solid, it is the part that extends further down that flexes.

20200925_190211[1].jpg
 
I would doubt you'll have any problem on Ds or Es. However, skinning the fins with thin wood or even glued paper will stiffen them up if you want. 1/4" is excessive for a 24mm rocket; I'd go with 1/32" balsa, basswood, or just office paper.
 
Light the fires...

I did a scratch build rocket in high school and used cardboard for the fins... they were floppy also. It was a memorable launch as the fins fluttered, which nearly stopped the rockets upward flight... but when the rocket slowed down the flutter stopped and it sped back up. It did that a couple times. Pretty cool.

Give it a try... if it's an issue then address it at that point.

Maybe then do something out of the ordinary... like (4) 3/16" dia. dowels ran from fin to fin.... or a ring that captures all 4 fins?

Whatever you do... let us know. And please post up a flight video too!

Good Luck!
 
Light the fires...

I did a scratch build rocket in high school and used cardboard for the fins... they were floppy also. It was a memorable launch as the fins fluttered, which nearly stopped the rockets upward flight... but when the rocket slowed down the flutter stopped and it sped back up. It did that a couple times. Pretty cool.

Give it a try... if it's an issue then address it at that point.

Maybe then do something out of the ordinary... like (4) 3/16" dia. dowels ran from fin to fin.... or a ring that captures all 4 fins?

Whatever you do... let us know. And please post up a flight video too!

Good Luck!

I'm not worried much about the flight, the first 2/3 of the fin are glued to the body tube and that part of the fin is not floppy, it's only the part that extends past the bottom of the body tube that worries me. That is the part that will hit the ground first and I don't want a break.
 
There are essentially three types of 1/8" plywood for hobby use: 3-ply plywood, 5-ply lite plywood, and 5 or 6-ply aircraft-grade plywood. As you move from left to right, the weight, stiffness, and surface hardness increases. 3-ply is very flexible and can actually be used to wrap curved surfaces.
 
It's plywood, 3 layers with each layer's grain perpendicular to the ones beside it.
It is only three ply so it is quite anisotropic. There will be a stiffer and floppier direction. If you go the higher quality aircraft grade stuff with more layers it will be better but can still make a difference which way the grain goes. Basically it matters because the strength make more of a difference the further from the neutral axis (in this case the center of the ply) of the material it is.
https://en.wikipedia.org/wiki/Second_moment_of_area
Cantilevered fins (hanging out the back) are more susceptible to this because their support edge (root of fin) is so far away.

Given it will be flying on F motors I would expect no problems.
 
It is only three ply so it is quite anisotropic. There will be a stiffer and floppier direction. If you go the higher quality aircraft grade stuff with more layers it will be better but can still make a difference which way the grain goes. Basically it matters because the strength make more of a difference the further from the neutral axis (in this case the center of the ply) of the material it is.
https://en.wikipedia.org/wiki/Second_moment_of_area
Cantilevered fins (hanging out the back) are more susceptible to this because their support edge (root of fin) is so far away.

Given it will be flying on F motors I would expect no problems.
True, especially with 3 ply. You want the exterior grain direction to be the same as if you were using balsa or basswood. Furthermore, layout fins in the same direction in the same piece and area of your plywood sheet. Avoid cutting a set of fins from different plywood scraps. Plywood will warp over time, and you want any warping to be consistent and provide a bit of roll rather than pitch/yaw trim. Plywood fins should be flexible to absorb landing impact without breakage, but not so flexible that they flutter.
 
There are essentially three types of 1/8" plywood for hobby use: 3-ply plywood, 5-ply lite plywood, and 5 or 6-ply aircraft-grade plywood. As you move from left to right, the weight, stiffness, and surface hardness increases. 3-ply is very flexible and can actually be used to wrap curved surfaces.

Yup, that was the problem. It was lite plywood. I don't know how I didn't notice how flexible it was when I bought and cut it. Won't make that mistake again.
 
Are you still looking for a solution? I'm surprised no one has suggested a thin layer of epoxy on each side. Maybe there's a good reason. Personally, I'd go with a 1/32" wood lamination on each side, or maybe take the opportunity to dip a toe into the world of fiberglass.
 
I might be a bit late to the game but I'm dealing with something similar on a build. The Balsa included on my kit was really soft and flexible. I considered a number of suggestions from members here and tried coating the one fin in thin CA. After letting it sit over night the fin is rock solid. Might want to give it a shot.
 
I might be a bit late to the game but I'm dealing with something similar on a build. The Balsa included on my kit was really soft and flexible. I considered a number of suggestions from members here and tried coating the one fin in thin CA. After letting it sit over night the fin is rock solid. Might want to give it a shot.
Agree. Since the fins are already on the rocket, might be the easiest solution but could use up a lot of thin CA, even with a light coat.
 
Update: I used wood glue to attach a layer of 1/32 basswood on each side of each fin. It made it a lot better. Not as good as if I got the right plywood in the first place, but very close to it and good enough that I'm not worried about it.
 
Back
Top