Quantcast

Fixing Floppy Fins

The Rocketry Forum

Help Support The Rocketry Forum:

Gunstar

Well-Known Member
Joined
Mar 2, 2018
Messages
50
Reaction score
12
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?
 

Gunstar

Well-Known Member
Joined
Mar 2, 2018
Messages
50
Reaction score
12
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
 

neil_w

Chuffed as ninepence
TRF Supporter
Joined
Jul 14, 2015
Messages
9,784
Reaction score
2,894
Location
Northern NJ
:oops:

What kind of motors will you be flying it on?
 

crossfire

Lifetime Supporter
TRF Lifetime Supporter
Joined
Jan 16, 2011
Messages
4,270
Reaction score
427
Glue a matched fin on the same each side of the fins
 

neil_w

Chuffed as ninepence
TRF Supporter
Joined
Jul 14, 2015
Messages
9,784
Reaction score
2,894
Location
Northern NJ
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.
 

lakeroadster

Lonewolf.... No Club
TRF Supporter
Joined
Mar 3, 2018
Messages
2,159
Reaction score
965
Location
Central Colorado
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!
 

rklapp

NAR# 109557
TRF Supporter
Joined
May 8, 2020
Messages
778
Reaction score
507
Location
Oahu, Hawaii
Just to check, do the grains go along the leading edge or the root edge?
 

Gunstar

Well-Known Member
Joined
Mar 2, 2018
Messages
50
Reaction score
12
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.
 

H. Craig Miller

Well-Known Member
TRF Supporter
Joined
Sep 8, 2020
Messages
49
Reaction score
87
Location
Placer County, California
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.
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
4,299
Reaction score
1,499
Location
Melbourne Australia
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.

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.
 

Alan15578

Well-Known Member
Joined
Feb 11, 2018
Messages
372
Reaction score
94
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.

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.
 

Gunstar

Well-Known Member
Joined
Mar 2, 2018
Messages
50
Reaction score
12
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.
 

jqavins

Joseph Avins
TRF Supporter
Joined
Sep 29, 2011
Messages
3,984
Reaction score
1,467
Location
Howard, NY
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.
 

Back_at_it

Well-Known Member
Joined
Jul 16, 2019
Messages
189
Reaction score
78
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.
 

rklapp

NAR# 109557
TRF Supporter
Joined
May 8, 2020
Messages
778
Reaction score
507
Location
Oahu, Hawaii
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.
 

Gunstar

Well-Known Member
Joined
Mar 2, 2018
Messages
50
Reaction score
12
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.
 

Latest posts

Top