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Oct 18, 2021, 02:57 PM
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vonBird's Avatar
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Tube fabrication


I need to fabricate a cylinder, really half of one. It'll fit around a 4" propeller. The only mold I can use is a piece of 4" schedule 40 PVC. That'll make the ID 4.5". That's fine, I don't need super close tolerances.

I've looked at having the full design piece fabricated. It's just too expensive.

I have a good bit of fiberglass experience from when I renovated my sailboat. Pretty extensive experience, but I haven't done anything like this.

It's either fiberglass, Kevlar, carbon fiber, or some material I haven't considered. It needs to be at least as strong as PVC.

Any advice or suggestions is appreciated.
Thx!
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Oct 18, 2021, 04:30 PM
Pro Hoarder
turbonut's Avatar
Sounds like a channel wing...Fiberglass and maybe some carbon if needed..No kevlar..
Latest blog entry: In flight
Oct 18, 2021, 04:31 PM
B for Bruce
BMatthews's Avatar
If this is for a ducted prop or ducted fan you might want to consider that the tip losses, and thus drag, go down rapidly as the fit becomes tighter. So really you do want things to pretty well fitted in terms of diameter.

Similarly you want the inside to have an airfoil like shape with a venturi like entry.

So.... do you know anyone with a wood lathe? What I'm thinking is that you know the desired ID at the prop or impeller location. It would not be hard for someone to glue up some wood and turn an inside mold to suit what you're after. It should be something like the sketch below so it can slide off the small end and has the desirable venturi like inlet lip. Then you can seal and coat and polish up this mold and use it to lay up a nice smooth inner surface fiberglass duct. For the outer shape make up donut rings of blue or pink foam, carve to your liking and then epoxy glass the outside as well.
Oct 18, 2021, 07:08 PM
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vonBird's Avatar
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Quote:
Originally Posted by BMatthews
If this is for a ducted prop or ducted fan you might want to consider that the tip losses, and thus drag, go down rapidly as the fit becomes tighter. So really you do want things to pretty well fitted in terms of diameter.

Similarly you want the inside to have an airfoil like shape with a venturi like entry.

So.... do you know anyone with a wood lathe? What I'm thinking is that you know the desired ID at the prop or impeller location. It would not be hard for someone to glue up some wood and turn an inside mold to suit what you're after. It should be something like the sketch below so it can slide off the small end and has the desirable venturi like inlet lip. Then you can seal and coat and polish up this mold and use it to lay up a nice smooth inner surface fiberglass duct. For the outer shape make up donut rings of blue or pink foam, carve to your liking and then epoxy glass the outside as well.

Yes, I'll be shaping the ID similar to your image...but the prop won't be as close as it would be in a duct...and It'll be half a tube, cut down the length.]

...no, no lathe, just a bit more than basic tools.
Last edited by vonBird; Oct 19, 2021 at 05:55 AM.
Oct 19, 2021, 07:40 AM
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vonBird's Avatar
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Quote:
Originally Posted by turbonut
Sounds like a channel wing...Fiberglass and maybe some carbon if needed..No kevlar..

I still have lots of glass. I don't know if you can get small amounts of carbon...or what it costs. I also have West Marine epoxy, but it may not be any good. It's at least 6 years old.


AeroMarime told me to use furniture polish as a release. Anything better?
Oct 19, 2021, 08:23 AM
Mark LSF # 3792
West Marine claims their resin has an almost unlimited shelf life, if stored correctly. I still have part of a quart I purchased longer ago than I care to mention, certainly more than 6 years. It has always been stored in my house, it still seems to perform as desired. I did buy some new hardener about a year ago because the old 206 had turned a dark purple and the cap was getting stuck on the can. If you are worried just make a small batch to check it.
Oct 19, 2021, 08:31 AM
Registered User
AA5BY's Avatar
From my experience.. the resin shelf life is not a problem... but the hardener catalyst can go bad with time.
Oct 19, 2021, 08:38 AM
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vonBird's Avatar
Thread OP
A test batch is right. I've got 205 and 206. It's been stored inside, on a concrete slab, coolish environment.
Oct 19, 2021, 10:31 AM
Scott
Pylonracr's Avatar
You are into Composites 101 here, luckily you are making a fairly easy shape. With basic hand tools I would go the easy old fashioned way. Draw each end and cut from wood, glue blocks of foam in the middle. Rough sand to shape and lay a couple of layers of fiberglass cloth and epoxy. Finish shaping with Bondo and elbow grease. When you get your shape, brush a couple of coats of epoxy over it and sand smooth, being careful not to sand through the epoxy. Prime and sand smooth.

Release is a whole story of its own, (Kind of like asking what the best oil is), but the tried and true method is to wax with Partall #2 wax, 6 to 8 coats, and brush Partall #10 PVA on. The wax and PVA are available from any composite supplier in fairly small quantities and are inexpensive.

From there, resin and fabric to make your component. I would probably use a 2C2 layup, where there are 2 layers of fabric, a core material, and 2 more layers of fabric, where each fabric stack is 1 layer of 5.7 ounce carbon and 1 layer of 6 ounce glass on alternating bias. Your project is pretty small unless the tube is going to be quite long, so if you need some material let me know and I will send you some.

Scott
Oct 19, 2021, 11:05 AM
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vonBird's Avatar
Thread OP
Quote:
Originally Posted by Pylonracr View Post
You are into Composites 101 here, luckily you are making a fairly easy shape. With basic hand tools I would go the easy old fashioned way. Draw each end and cut from wood, glue blocks of foam in the middle. Rough sand to shape and lay a couple of layers of fiberglass cloth and epoxy. Finish shaping with Bondo and elbow grease. When you get your shape, brush a couple of coats of epoxy over it and sand smooth, being careful not to sand through the epoxy. Prime and sand smooth.

Release is a whole story of its own, (Kind of like asking what the best oil is), but the tried and true method is to wax with Partall #2 wax, 6 to 8 coats, and brush Partall #10 PVA on. The wax and PVA are available from any composite supplier in fairly small quantities and are inexpensive.

From there, resin and fabric to make your component. I would probably use a 2C2 layup, where there are 2 layers of fabric, a core material, and 2 more layers of fabric, where each fabric stack is 1 layer of 5.7 ounce carbon and 1 layer of 6 ounce glass on alternating bias. Your project is pretty small unless the tube is going to be quite long, so if you need some material let me know and I will send you some.

Scott

I wanna lay it onto 4" schedule 40 pvc pipe. That'll give me the 4.5" ID, but gluing foam onto the pipe might be a good way to form the shape. Carbon and glass layers? Interesting and surely cheaper then all carbon. I need to go 5/16s to 3/8" deep, but solid, no core material. Any ideas on where to order carbon-fiber?


Thanks
Last edited by vonBird; Oct 19, 2021 at 11:10 AM.
Oct 19, 2021, 12:04 PM
Registered User
MiteyF's Avatar
If all you're really wanting is half of a tube, you can always buy carbon tube and cut it in half

https://www.rockwestcomposites.com/s...n-fiber-tubing

Edit, just looked at prices... maybe not. Or email and see what they have?
Oct 19, 2021, 12:27 PM
Scott
Pylonracr's Avatar
Quote:
Originally Posted by vonBird
I wanna lay it onto 4" schedule 40 pvc pipe. That'll give me the 4.5" ID, but gluing foam onto the pipe might be a good way to form the shape. Carbon and glass layers? Interesting and surely cheaper then all carbon. I need to go 5/16s to 3/8" deep, but solid, no core material. Any ideas on where to order carbon-fiber?


Thanks
Go to US Composites and check their discounted fabric listings. You can usually get pretty good deals on end of roll material.

Scott
Oct 19, 2021, 06:01 PM
Registered User
vonBird's Avatar
Thread OP
Quote:
Originally Posted by Pylonracr
Go to US Composites and check their discounted fabric listings. You can usually get pretty good deals on end of roll material.

Scott

Yeah, their 5.7 discounted twill, a 40" yard is $24.50...1 yard should be plenty. I don't have a clue how many layers to get to 3/8" thick, but it couldn't be very many. I might experiment with some of the glass I have on hand, first.
Oct 19, 2021, 10:00 PM
Scott
Pylonracr's Avatar
Quote:
Originally Posted by vonBird
Yeah, their 5.7 discounted twill, a 40" yard is $24.50...1 yard should be plenty. I don't have a clue how many layers to get to 3/8" thick, but it couldn't be very many. I might experiment with some of the glass I have on hand, first.
UUUMMMM...
5.7 ounce fabric is about 0.010" when laminated, so figure 37 to 38 layers of fabric to achieve 3/8" thickness if vacuum bagged. Not only is this a LOT of fabric, it will be very heavy. Now you know why I recommended a core material with fabric on both sides. Not only will it be thinner, it will be lighter by orders of magnitude. If you do not vacuum bag you will end up with an extremely resin rich laminate that will not only be heavier but weaker and more prone to fracture due to unsupported resin between layers of fabric.

Scott
Oct 19, 2021, 10:59 PM
Registered User
vonBird's Avatar
Thread OP
Quote:
Originally Posted by Pylonracr
UUUMMMM...
5.7 ounce fabric is about 0.010" when laminated, so figure 37 to 38 layers of fabric to achieve 3/8" thickness if vacuum bagged. Not only is this a LOT of fabric, it will be very heavy. Now you know why I recommended a core material with fabric on both sides. Not only will it be thinner, it will be lighter by orders of magnitude. If you do not vacuum bag you will end up with an extremely resin rich laminate that will not only be heavier but weaker and more prone to fracture due to unsupported resin between layers of fabric.

Scott

Uumm is right. That's too many layers. All I need is something about as strong as schedule 40 PVC pipe, and a similar weight. I need a 4.5" ID tho, and thicker wall. Nobody stocks anything like that.

I'm going to experiment with the glass I have on hand and see what I get...then I'll know more.

If I could afford for somebody to CNC some T6, I'd consider that...haha, way expensive.
Last edited by vonBird; Oct 19, 2021 at 11:07 PM.


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