Glassing beveled fins

So I have some plywood fins I want to put a few layers of glass on and I also want to bevel them. Do you typically glass first and then bevel or bevel first and then glass?

Thanks!

Bevel-Glass. Watch out for where the bevel on top and bottom reach the air fame when glassing. Totally do able with a little care.

I bevel then glass. After everything is set I normally coat the edges in a high temp epoxy to seal them and get a good slim edge.

So when you glass a beveled fin, do you go completely tip-to-tip, or do you stop the glass at the beginning of the bevel? I suspect that you run the glass clear to the edge, but it seems that the glass would blunt sharp leading and trailing edges.

BDB said:

So when you glass a beveled fin, do you go completely tip-to-tip, or do you stop the glass at the beginning of the bevel? I suspect that you run the glass clear to the edge, but it seems that the glass would blunt sharp leading and trailing edges.

Click to expand...

It sort of depends. Depends on how many plies you lay down (more than one) and if your plies are going to get larger or smaller. Meaning if the first ply is smaller than the next plies you lay, then really the top ply should fall at the edge of the bevel to create a "smoother" transition from ply(s) into the transition. This sort of lay up has a little more involvement with tow load transfers, but to be completely honest i'm not 100% sure of how much it effects the forces acted on the fin(s)

The other way would be to lay the largest ply first (assuming you are laying more than one) then to lay smaller subsequent plies. In this fashion the edge of each ply will be exposed all the way around covered by a smaller ply on top. For me I like this method more only because I can sand out a smoother transition from bevel, ply-ply. Doing so this way, for me, I would run the ply all the way to the edge and continue with the theme of sanding a transition.

Both have their advantages/disadvantages

The first description is probably the easiest, but you aren't able to sand any of the possible steps you may get from subsequent plies underneath the largest top ply. Why? You will end up burning through the tow in all of the ply to do so. If you aren't pulling vacuum you run the risk of little voids all the way each ply laying underneath the bigger ply above it. Just hope you stipple all the air pockets out, and none form after you are "done" (in my opinion if doing a contact layup without vacuum, you should check your lay up regularly for voids forming and stipple accordingly.) Under vacuum you shouldn't have any problem with voids, but will create more definite steps in each ply.

The second description for me, I can get a smoother transition. BUT you may want/need to seal the sanded edges with a quick finger swipe of resin just to be sure any possible fray edges don't lift.

Creating clean, EVEN, step backs/step ups in plies really helps in terms of looks and a smooth even transition. I.E. a lay up schedule.
Weave direction/Step Back
Ply 1 - 45 degree FULL COVERAGE
Ply 2 - 90 degree .25 "STEP BACK
Ply 3 - 45 degree .25" STEP BACK
Ply 4 - 90 degree . 25" STEP BACK
Ply 5 - 45 degree .25" STEP BACK

This lay up schedule would be an example of the second description. Reverse "STEP BACK" and "FULL COVERAGE" to "STEP UP" and "LAY FIN TEMPLATE PLY 1 inch FROM ALL EDGES", respectively.
My thinking comes from protoype aerospace. I may be completely off, but this is my thinking how to and why to do a lay up in this fashion.
Pull vacuum to reduce resin weight as needed. Pleat all fillets and outer edges of lay up to ensure a wrinkle free, tight bag.

Hope this helps.

dford said:

Hope this helps.

Click to expand...

Thanks. It looks like I have a lot to learn, but that is what I love about this stuff.

Yikes, with 5 plies it certainly will be strong but omigosh the work involved. Three fins, 15 layups! Four fins 20 layups!

With a 38mm minimum diameter cardboard LOC tubed project with 1/8" 5 ply plywood (I can't find 5ply 1/8" anymore) I did 1/3rd, 2/3rds and full span layups with 2 oz glass cloth. Twelve separate pieces on a 4 finned rocket.

What it means is one piece of cloth goes between two fins to 1/3rd the way up the fin span. Next one is rotated 45 degrees and 2/3rds the way up the span and the last is back to 90 degrees tip to tip. One can only do two fins at a time.

You go with 5 plies and it will be strong but......... A lotta work.

If shooting for stupid fast for stupid long, I like the post where a fellow laminated an aluminum strip in his leading edges. Held up very well. I don't care how many plies you use, you get close to Mach 3"ish" and your leading edges are going to
be eaten away due to aerodynamic heating. Makes me think that a lot of rockets that go that fast and are recovered are single use or require major rework for the next flight. Kurt

I should have been more specific.
I was giving an example of a lay up schedule. I agree five additional plies will be super strong. With that said if I were to do a five ply lay up, i'd lay them up an a table pre wet out before I laid the entire stack onto the actual part. It will make the lay up FAR easier.

With that said. (and i'm thinking about doing this) One could create a ply lay up pattern in such the plies would create a "pre air foiled" fin. Probably something like a 3-5 ply lay up on a flat non porous "tool" vacuum bagged to the tool. Once cured, flipped over and done again. Mirror image essentially on the back side. Sand out the steps, resin wipe and sand smooth x3.

...Yeah... I started a new job ... I have a lot of time to think about this stuff. Never know.

dford said:

It sort of depends. Depends on how many plies you lay down (more than one) and if your plies are going to get larger or smaller. Meaning if the first ply is smaller than the next plies you lay, then really the top ply should fall at the edge of the bevel to create a "smoother" transition from ply(s) into the transition. This sort of lay up has a little more involvement with tow load transfers, but to be completely honest i'm not 100% sure of how much it effects the forces acted on the fin(s)

The other way would be to lay the largest ply first (assuming you are laying more than one) then to lay smaller subsequent plies. In this fashion the edge of each ply will be exposed all the way around covered by a smaller ply on top. For me I like this method more only because I can sand out a smoother transition from bevel, ply-ply. Doing so this way, for me, I would run the ply all the way to the edge and continue with the theme of sanding a transition.

Both have their advantages/disadvantages

The first description is probably the easiest, but you aren't able to sand any of the possible steps you may get from subsequent plies underneath the largest top ply. Why? You will end up burning through the tow in all of the ply to do so. If you aren't pulling vacuum you run the risk of little voids all the way each ply laying underneath the bigger ply above it. Just hope you stipple all the air pockets out, and none form after you are "done" (in my opinion if doing a contact layup without vacuum, you should check your lay up regularly for voids forming and stipple accordingly.) Under vacuum you shouldn't have any problem with voids, but will create more definite steps in each ply.

The second description for me, I can get a smoother transition. BUT you may want/need to seal the sanded edges with a quick finger swipe of resin just to be sure any possible fray edges don't lift.

Creating clean, EVEN, step backs/step ups in plies really helps in terms of looks and a smooth even transition. I.E. a lay up schedule.
Weave direction/Step Back
Ply 1 - 45 degree FULL COVERAGE
Ply 2 - 90 degree .25 "STEP BACK
Ply 3 - 45 degree .25" STEP BACK
Ply 4 - 90 degree . 25" STEP BACK
Ply 5 - 45 degree .25" STEP BACK

This lay up schedule would be an example of the second description. Reverse "STEP BACK" and "FULL COVERAGE" to "STEP UP" and "LAY FIN TEMPLATE PLY 1 inch FROM ALL EDGES", respectively.
My thinking comes from protoype aerospace. I may be completely off, but this is my thinking how to and why to do a lay up in this fashion.
Pull vacuum to reduce resin weight as needed. Pleat all fillets and outer edges of lay up to ensure a wrinkle free, tight bag.

Hope this helps.

Click to expand...

Funny that you posted this today.....I just looked up the message that you sent me back in September of 2016 explaining it all to me from the Go Devil threads! I have a new house and a new shop, so this winter I'm planning on FINALLY putting this info to good use. Just today I was looking it all up to print it out and put it with my Go Devil project.

If you're glassing over the bevel, it will be a lot easier to keep the glass stuck down at the transition if you sand a faint radius into the bevel-flat transition. Glass doesn't like sharp corners. If you are not painting the layup, doing the smallest layer of tip to tip first will make it easier to get a nice finish.

My first attempt at glassing bevelled fins. Balsa and ply laminate, single layer of 200g glass on both sides. Vacuum bagged, trimmed and sanded

I just posted this question the other day. I beveled mine first, then glasses. I used a couple layers of bleeder cloth and I believe this helped keep the glass pushed up agains the wood as the bevels narrowed to the edge. Make sure you wet it out good enough. I agree with what was said earleir that setting out the cloth first would make it easier.

Perhaps a thread of mine might provide some ideas. I vacuum bag a set of bolt-on fins with reinforced leading edge, with pre-painted mylars.

Starting at page https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/page-2#post-435909 then picks up at https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/page-2#post-436268 from https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/

Gerald