Fin Grain Orientation

[Bruiser]

I am working on a design now and am scratching my head a little. Here's the fin shape


The fin is thirteen inches long. The radius of the curve at the front is .75 inch. The width is 2.625 at the back. This a BT-80 rocket, 29mm powered. The fins will be 1/8th" thick. I haven't decided if I want to use balsa (lighter) or ply (tough but heavier).



If I were to use balsa and cut it to align with the "rake" of the fin, the very front edge radius would be end grain which is generally not good. It's just the 3/4" radius but 29mm...


Then I thought maybe I could make the fin two piece where the front inch or two would have the grain going away from the body. It's be more work with a butt-glue joint. I wonder if that is needed?

With ply fins OR is projecting 3.5 ounces of nose weight (with a G64). With bare balsa it shows I could lose an ounce. Not sure how to account for the papering.

What do you all think?

-Bob

 

[3stoogesrocketry]

Looks like a Saab RB-05a forward fin. I'd soak the balsa in thin epoxy after gluing it to the airframe and shaping it.

 

[SolarYellow]

Run the grain at 45 degrees to the airframe. Then you have end grain exposed on the full perimeter so it will soak up your CA hardening with maximum effectiveness. You will have some strength parallel to the fin to prevent flexing like the fins on the top and bottom of an eel, but will also have grain that will be strong in a direction to prevent the fin being split off.

I've been thinking about this quite a bit lately. I'm not buying that the ancient rule of thumb brought down from the mountain by Moses that the grain should run parallel to the leading edge is always the best. The only thing that's really obviously true is that you want to avoid a situation where any chunk of the fin can be broken off simply by the wood splitting along the grain. Apart from that, I can see arguments for any angle between parallel to the leading edge and parallel to the trailing edge, depending on circumstances. I'd be perfectly willing to consider putting the grain normal the the airframe on a trapezoidal fin, for example. Or maybe a built-up fin, with grain normal to the airframe, but a stronger tip (e.g., balsa fin with a bass tip) added on parallel to prevent the "eel" flexing.

 

[teepot]

I rarely use balsa. It doesn't hold up at all to our field surface. Even 1/8th ply is risking a broken fin. I would take the weight penalty and go with plywood. If you don't want to take the weight penalty try hard balsa. After I discovered hard balsa if I want the rocket as light as possible I use that instead of regular balsa. I get the hard balsa from BMS. I use the 1/4" stuff. Tuff as nails.

 

[Scott_650]

With ply fins OR is projecting 3.5 ounces of nose weight (with a G64). With bare balsa it shows I could lose an ounce. Not sure how to account for the papering.

What do you all think?

-Bob

The seemingly now inactive Daniel the RocketN00b did some interesting work on the weights of various balsa filling techniques - spoiler alert: papering is the heaviest

Here’s a snip from his final results and a link to the page -

- Papering with Avery Labels - 0.144g/square inch, 0.022g/square centimeter

- Papering with Glue and Copy Paper - 0.233g/square inch, 0.036g/square centimeter

http://rocketn00b.blogspot.com/2018/05/balsa-grain-fillers-and-added-weight.html

 

[Bruiser]

I was just playing around with the OR sim and when flipping thru the different views I saw that OR defaults to the grain going away from the body tube.


I was also thinking there are different types of ply. Back when I built RC aircraft there were two types of ply that we normally used. We called on "light ply" and the other "aircraft" ply. Light ply was made of a softer wood and had minimum layers. Aircraft ply was a harder wood and and had more layers. I've tried "googliing" for the weight of each type but didn't find anything on these types of ply. I wonder which type OR sims...

BTW, the rocket is an AIM-4D.


-Bob

 

[neil_w]

I think I would do one piece, somewhere between 30 and 45 degrees up from the BT.

And I *always* paper, so that’s a given.

 

[waltr]

We called on "light ply" and the other "aircraft" ply. Light ply was made of a softer wood and had minimum layers. Aircraft ply was a harder wood and and had more layers. I've tried "googliing" for the weight of each type but didn't find anything on these types of ply. I wonder which type OR sims...

BTW, the rocket is an AIM-4D.


-Bob

You can select what material the fins are made from in OR. It is in the component edit dialog.
With the selector the 'density' is noted.

 

[BABAR]

Concur with most of above, when leading edge is less than 45 degrees from the long exis, time to throw the “parallel with leading edge” rule out the window.

Also, no amount of paper makes up for wrong grain direction. I’ve tried.

In this case parallel to the dominant lateral edge is DEFINITELY wrong.

For one piece /single ply, I’m good with anything from perpendicular to 30 degrees from long axis (sweeping back) is okay. It is also the hardest to cut with a standard blade, tough to cut anyway because it will split easily until it is attached. If you go single ply, might be easier to paper the balsa BEFORE you cut. This may help it to “hold together” while you are cutting and placing it.

Something that may help place long pieces like this on the body tube are “cheaters” aka “balsa fillets”.

These are thin strips (1/16x1/16 chord length” works) that can easily be placed, particularly along “wing” or “chine” fins, along the ventral edge of the fin line marker. You’ll see it here post 2 pic 1 (hey, it’s waaay too big but it was a long time ago )


https://www.rocketryforum.com/threa...w-sr-73-raven-glider-and-new-gyskelion.38883/
Because it’s light it places easily with very little glue, dries fast .

Once glued in place, provides a great onboard alignment guide for the fin placement, an extra strong bond which can’t hurt. Doesn’t weigh much. Cosmetically often can hide it with a glue fillet.

 

[BABAR]

Making your own plywood with two pieces of 1/16 balsa at 90 degree angles works well. Tricky part is the pieces will curl away. May tape the edges and stick in parchment or waxed paper and stick in an old encyclopedia or other old book you don’t care about.

 

[Bruiser]

I wanted to try to keep this build simple. I really want to get one of my designs published in the Peak Of Flight newsletter. It's a bucket list thing.

Seems like it's gonna be way to complicated for that so I'll have to find something more than a 4FNC for the build but not as complicated as this.

So yeah, the ideas to have the grain going 30 to 45 degrees or so from the body and papering them seems to be the best bet for balsa "mid" fins. The forward fins could probably be papered balsa too but I think the aft fins probably need to be be bass or ply

Babar, I opened your build link. Not sure what you were telling me to look at but I did notice the grain on the front strakes. No shredding there? It's not too different than what I am asking about.


Anyway, Apogee hasn't responded to my emails asking if this is something they would be interested in. Well actually I offered a BT60 size for them. It be simpler 'cause I would skip the boat tail on that size. Maybe even the fin flanges but I guess it's a no-go.

I am going to cannibalize a Super Big Bertha kit for this build. I just got one in from AC Supply. The build will come soon.


Thanks for all the input. It's been really helpful.

-Bob

 

[bjphoenix]

I'm a structural engineer but not an aerospace engineer. I make my fins with balsa grain reasonably perpendicular to the body tube surface to give the most resistance to fins breaking off. I typically don't do fins that sweep back behind the back end of the body tube because those are most likely to break off on landing, but a lot of what I build are designs from other kits and sometimes they have swept fins. My most recent build was a scratch build of an Estes Fat Boy. I built it with balsa grain parallel to the rear edge of the fin, not the leading edge.

I also paper my fins for strength, to keep corners from breaking off and to help strength of fins with weird shapes. For BT-60 and less builds I just use printer paper, for anything larger than that as in the Fat Boy I use thin cardstock. I took a piece of 1/8" balsa that actually measured 0.132" thick. I weighed it and calculated density of 0.114 ounce per cubic inch. I papered a piece and weighed it, then calculated density of 0.156 ounce per cubic inch. I just put that density into open rocket.

 

[BABAR]

Babar, I opened your build link. Not sure what you were telling me to look at but I did notice the grain on the front strakes. No shredding there? It's not too different than what I am asking about.
View attachment 618189

First: no, didn’t shred.

second: wow, egg on my face with that one. Although to be fair, that was the setup for the chines on these Estes SR-71 which were however pretty heavily papered. And it was a decade ago, I was (and still am) learning.

http://www.spacemodeling.org/jimz/estes/est1942.pdf
I think the Apogee SR-72 DarkBird may have done the same thing, but I don’t have the rocket anymore. That was the inspiration for my upscaled and heavily modified SR-73 Raven.

https://www.apogeerockets.com/Rocket-Kits/Skill-Level-3-Model-Rocket-Kits/SR-72-Darkbird.
you can obviously get away with thin short no weight bearing segments, although I wouldn’t do it that way again. Even looking at the Estes build up, the wider rear ends of the wings were more more conventional (although even those were grain parallel to leading edge!)

@neil_w used about a 45 degree angle on his IRIS build for his looooooonnnng fins, which was a supercool scale missile build

https://www.rocketryforum.com/threa...-scale-iris-t-missile-1-3-8-ish.145053/page-2

Anyway, I think for loadbearing fin segments (meaning major fin segments which are providing significant stabilization AND/OR good chance the rocket is gonna land on them, which was not the case for the Sr-71 thru 73 chine segments.

Maybe 45 degrees is too conservative, but I’d try to keep it above 30 degrees off long axis.

somebody here must have built an AIM-9 or Phoenix, how did you deal with the grain? Unless of course you did plywood.

 

[Bruiser]

I was thinking the same thing last night when I couldn't sleep. That is, how did Estes do it on the Phoenix so I just Googled it.

I found several builds with pictures and the grain is orientated with the leading edge


Even the instructions


Hmmn...

Bit of trivia. I read somewhere that the Phoenix and the Maverick were both further developments of the AIM-4 Falcon. They certainly look like it.

-Bob

 

[bjphoenix]

I have a Phoenix that I built over 20 years ago, I believe it has the grain oriented just like that. The problem with the Phoenix is it's relatively heavy for balsa and it likely lands on one or two of the rear fins. Mine has broken a rear fin at least once, as much as anything it breaks the fin along the grain then breaks across the grain somewhere near the root. The last time I put that piece of fin back on and put a dab of fiberglass cloth over the joint on both sides. That's also the reason that I paper everything now. On my list is to replace the shock cord in that rocket so I can launch it again.

I've seen numerous rockets like the HiFlier XL with the stock fins put on differently so the leading edge became the root edge and the grain is parallel to the body tube. These work pretty well up to a point, I don't think it is all that much worse than the stock grain orientation. Just recently I saw an Estes Alpha with the fins put on wrong and the grain parallel to the body tube. Last year I found a lost Estes Nova that had the fins glued on in the wrong orientation, and one of the fins was missing.

 

[Orestes Mayo]

I am working on a design now and am scratching my head a little. Here's the fin shape
View attachment 618028

The fin is thirteen inches long. The radius of the curve at the front is .75 inch. The width is 2.625 at the back. This a BT-80 rocket, 29mm powered. The fins will be 1/8th" thick. I haven't decided if I want to use balsa (lighter) or ply (tough but heavier).


View attachment 618025
If I were to use balsa and cut it to align with the "rake" of the fin, the very front edge radius would be end grain which is generally not good. It's just the 3/4" radius but 29mm...

View attachment 618024
Then I thought maybe I could make the fin two piece where the front inch or two would have the grain going away from the body. It's be more work with a butt-glue joint. I wonder if that is needed?

With ply fins OR is projecting 3.5 ounces of nose weight (with a G64). With bare balsa it shows I could lose an ounce. Not sure how to account for the papering.

What do you all think?

-Bob

Have you consider to instead use shipping label which has adhesive. Then sand rounding the edges. When that's done use CA on edges and all over each surface. Sands easily to a smooth finish and I find it makes fins pretty resilient. That way you don't cut your fin.

 

[Bruiser]

I did think about papering balsa. I have done so in many builds. For this build I decided to go with some very light ply that I had. Well actually, the build I started is not a AIM-4D. It is an AIM-4G which has slightly different fins

https://www.rocketryforum.com/threads/aim-4-g-super-falcon-build.184424/

-Bob