High Powered Flis Praetor? Will the fins stay on?

[OKTurbo]

Picked out the two worst looking halves for the trial fin assembly. Mixed up epoxy with carbon and silica to thicken it. Glued the halves together and let it cure overnight. Trimmed it up after work today. Need some warmer weather before I can do the 2 part foam filler. Some more pics...you can see some of the bridging spots on the close up. Each one has been a little bit better as I get my technique down on the vacuum infusion.

 

[OKTurbo]

Been tough to do resin infusion with the cool weather. It was 44deg F in Oklahoma this morning. Progress is slow...each fin half take a whole day to complete. I will say that the membrane tube makes set up super easy...no resin catch pot on the vacuum pump side. Pictures are starting to all look the same.

 

[dhbarr]

This weather is great! I'll miss it when the summer gets here ( NE of Tulsa ).

 

[OKTurbo]

Yes! Today has been a beautiful day. Where are you at dhbarr? I’m in Pryor.

 

[OKTurbo]

I’m going to try something different. The molded halves aren’t turning out like I want. The finished fin is just too thick. I want it to be 1/4 inch thickness but I didn’t account for the carbon fiber thickness in my mold.

I’m infusing a flat sheet that I can cut the fins from. Use a carbon fiber skin on both sides of a sandwich core. See if this works better.

 

[Wallace]

It's never easy stepping back after all that effort is it. Sometimes Ya just gotta try something different if you're not happy with prior results. You really have me wanting to try resin infusion now. Where's all the mess? Didn't anyone tell you that's supposed to look like a disaster area. You're making it look too easy, but I just know it can't be....

 

[OKTurbo]

If you don’t make mistakes, you aren’t doing anything. This has been a learning experience for me. No big deal....here are some pics of the plain sheet.




Just popped it off. Finish is dull due to mold release PVA.




This is the toughest part. Getting the peel ply off is tough. Once you get it started it’s not too bad, but getting started is a chore.




After a wash in the tub.

The resin infusion isn’t too complicated. Actually fairly mess free since you’re not wetting out cloth. In my opinion, the MTI infusion tube and valve make it easy to get good results. The tube is a semipermeable membrane that lets air pass thru but not the resin. No catch can! The valve regulates the pressure on the resin inlet tube. This keeps even pressure on the bag as the resin flows in.

The tough part to get over is that all these are consumables that get thrown away. But...sure makes the finished product nice.



This is the white membrame tube that goes to the vacuum pump. Resin won’t pass thru it.



The black thing is the MTI valve on the resin supply.

 

[Wallace]

I Really, really like that...

 

[OKTurbo]

Going to see how this works out. Fin root is 3/16 ply. Outline is all basswood. Trying something different for the core.

 

[OKTurbo]

Okay...here’s the idea. Don’t have any 3/16 thick nomex honeycomb laying around, but I have lots of balsa. Balsa trellis core.

 

[Wallace]

Now run spokes through the middle and tension it. Can't remember who but thinking someone on Aus rocketry did just that, used titanium spokes for weight savings if I remember correctly...

 

[Wallace]

Oh, Btw; You have way too much patience...

 

[OKTurbo]

Oh, Btw; You have way too much patience...

Ha! I just like building things. Nobody will know it’s even in there. Using some skills learned building stick and tissue planes.

I’ve got the fin skins epoxied together. Letting it cure...

 

[OKTurbo]

This is what I’m going with. It’s heavier than I would have liked. Basically the same weight as one made from 1/4” aircraft ply....but it’s very stiff / rigid. Still need some final shaping in the leading and trailing edges.

 

[Wallace]

Reminds me of the B-17 I built when I was about 8, with the exception that yours look really nice. Was WAY out of my league, but I was motivated. It was "off" in every possible direction and had glue and dope blobs/fingerprints/hair and dirt all over it. To my utter amazement it actually flew really well unti l it got stuck in a tree and weathered into oblivion. Winding the rubber bands somewhat equally was a bit of a challenge though. I've looked into revisiting stick and tissue off and on over the years but have yet to commit. Thinking either a P-38 or an A-10 some day....

 

[Wallace]

I love the look of that carbon by the way. What cloth and where would I find it?

 

[OKTurbo]

I love the look of that carbon by the way. What cloth and where would I find it?

It's amazing some of those old Guillow's kits actually flew when built. The wood was very heavy. I remember working on a Fokker Triplane all winter long....sorta flew, but sure looked cool.

The checkerboard cloth is called Textreme. It's a spread tow fabric. Very thin. Instead of woven threads where the carbon 'tow' are twisted together...the individual tows are spread out flat. Holds less resin. I went with one layer of that and two layers of a triaxial tow cloth. The Textreme is very nice to work with since it has a binder that holds it all together when your cutting and handling it. Most carbon fiber cloth will just start fraying like crazy when you handle it.

This is where I bought it from.
https://compositeenvisions.com/text...w-carbon-fiber-12k-39-37-100cm-4-72oz-160gsm/

The triax cloth I got from these guys.
https://www.sollercomposites.com/Carbon-3K-Fabrics.html
They also have composite sleeves that are good for rocketry. I've used the carbon / Kevlar sleeves on some of the higher tech rubber freeflight planes for fuselage tubes.

 

[Wallace]

Composite Envisions used to carry a triaxial weave that had an absolute 3d effect/depth to it. Think it was about $95 a yard so I never committed. Now that it's gone....

 

[Wallace]

Found it. Looks like the price has gone up even though they don't have it. Looks to be strong in all directions...https://compositeenvisions.com/carbon-fiber-triaxial-spread-tow-12k-39-99cm-3-54oz-120gsm-t-700/

 

[Wallace]

And they're web address drives me nuts too, all I see is "composit teenvisions"

 

[Wallace]

Just noticed the Textreme uses 12K tow. How is it to work with? Obviously not too terribly stiff since it laid nicely in your mold?

 

[OKTurbo]

Just noticed the Textreme uses 12K tow. How is it to work with? Obviously not too terribly stiff since it laid nicely in your mold?

It’s fairly stiff. Good for flat parts. Doesn’t follow complex curves like a twill fabric. Awesome to work with on flat sheets. The molded parts did have some resin bridging at the edges because of it. Vacuum pulled it fairly tight, but not all the way in. I thought about doing a few layers of just it and rotating the bias between plays...lots of waste on the roll cutting at 45 degree though. Used the -60/0/+60 triax instead.

I used the Textreme 1017 btw....

 

[OKTurbo]

Progress for tonight.

 

[Wallace]

You have a jig for that?

 

[OKTurbo]

You have a jig for that?

No jig. I drew out the grid on some butcher paper. Cut the balsa with a stripper tool. Made a simple notching tool out of scrap balsa and a strip of sand paper. You can see the notching tool in the pic....blue tape on it to set thickness of the notch. Stack the wood strips and sand out the notches. Put it all together and thin CA to glue it up.

Lay the fin outlines on top. Mark and cut with razor blade. Old school on the inside...high tech carbon on the outside.

 

[Wallace]

I reiterate, way too much patience....

 

[Wallace]

I "should" currently be at about the same stage in making fins for my L3 project but I've fallen down the 3d printing rabbit hole and can't seem to find my way out. Like you, could'n't quite decide what would be the best way to go about it. Skeletonized w/skins, molded w/internal foam, wood frame w/foam, beveled foam core w/skins, etc. You've managed to come up with the one thing I'd not considered and I'm waiting to see how it turns out. May just have to appropriate some of your ideas if I ever get past the fng with a printer thing..

 

[OKTurbo]

Making another sheet for the fin skins. One sheet wasn’t enough for all four fins. I took pics of each step if anyone is interested in how an old okie engineer does resin infusion.

 

[OKTurbo]

Just popped off the new sheet. My best one yet. A couple of extra shots to show the scale of this thing. 4” LOC tube. Fat Boy for scale . I guess I have to figure out how I’m going to separate this for recovery....

 

[OKTurbo]

Finishing up the fins.