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Old Jun 16, 2001, 01:10 AM
Registered User
Burnsville, MN, USA
Joined Oct 2000
172 Posts
Fiberglass Duct is like Rubber! Help!

I thought I had finished a very nice fiberglass duct for a MiniFan 480, but I started to pre-fit it to the duct, I realized that it's more flexible than a gymnist! I could plug one end and suck on the other and the entire duct would collapse. Does the type of epoxy you use make a difference? I used Great Planes 45 minute finishing resin. Should I try some other brand? If so, which one would you guys recommend? Also, does carbon reinforcement help? Is it practical in the case of a MiniFan 480 on 9 cells and a Kontronik Fun400-36 motor?


Thanks,
- Razz
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Old Jun 16, 2001, 02:18 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally posted by razzor7:
I thought I had finished a very nice fiberglass duct for a MiniFan 480, but I started to pre-fit it to the duct, I realized that it's more flexible than a gymnist! I could plug one end and suck on the other and the entire duct would collapse. Does the type of epoxy you use make a difference? I used Great Planes 45 minute finishing resin. Should I try some other brand? If so, which one would you guys recommend?
How did you make the duct? Did you make up a sheet then roll and seam it or did you make it on some sort of mold?

What weight of cloth did you use?

Did you thin the resin with anything or did you mix it up and use it as is?

Dan
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Old Jun 16, 2001, 09:34 AM
Flying Welder Pilot
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Reno, Nevada
Joined Jan 2001
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It almost sounds like you used only one layer of cloth. You need two use at least two layers to get the rigidity. 1 1/2 ounce cloth minimum. The other thing would be not waiting long enough for the part to cure. Epoxy usually takes overnight.
I use my storm door window to make sheets of glass by waxing the glass and putting two layers of cloth some resin and squeegee the resin out to the edges. Let it cure and peel it off. Roll it up for some neat duct work.
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Old Jun 16, 2001, 11:27 AM
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mexico city, mexico
Joined Aug 2000
515 Posts
Better yet.

Make afoam plug for the duct, then cover it with papier-mache (news paper and diluted white glue). Then dissolve the foam with gasoline and voila ulta light and perfect fit.

rgs/eduardo http://www.inedesca.com
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Old Jun 16, 2001, 12:15 PM
EDF Head
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Stavanger, Norway
Joined Feb 2000
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What inedesca said - but wrap the foamplug with plastic food wrap or cover or with a balloon. Neither stick well to epoxy so removing it is easy.

PlaneCrazy is right on re. your problems I think. Try 3 layers of 2oz cloth or thereabouts and let it cure atleast 24hrs. The epoxy can take a day or two to fully polymerize.

I have just recently moulded some fuselages and after 24hrs they're firm enough to pull out of the mould but they do get stiffer then next 24hrs.

The epoxy I used had stated 24hrs curing time@20degC.
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Old Jun 16, 2001, 02:34 PM
EDF all the way!
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Gilbert, Az
Joined Jun 1999
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I just laid up some duct work for the single fan Rafale of EJF. I used hobby shop epoxy (Bob Smith). After cure very much like rubber. I check with our composite people at work (Boeing Aircraft) They stated the best is Saf-T-Poxy from Aviation Spruce and Supply. I was able to get some locally from Aeroloft Designs. Also check with Aerospace Composites. It needs to be a stiffer type of resin system. Good luck!
Bob
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Old Jun 16, 2001, 06:07 PM
EDF Head
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Stavanger, Norway
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I have used both some aero-grade stuff and WestSystems. Both cure nice and stiff but neither reach full stiffness untill 24-48hrs.

Have it cured nice and stiff by now? If not I suspect a faulty mix ratio. I dont know the GP Finishing resin so cannot comment on its suitfullness.
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Old Jun 16, 2001, 07:35 PM
EDF Head
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Provided the epoxy cures nice and stiff - yeah, carbon add a lot of rigidity.
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Old Jun 16, 2001, 08:22 PM
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Burnsville, MN, USA
Joined Oct 2000
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Thanks for all the helpful replies!

Dan: I made the duct by taking some waxpaper, and plugging it from both sides with my fans to help the paper retain its shape. I then proceded to use 6 oz. cloth just to get comfortable with the process, and no, I didn't thin it out at all.

Later on in the finished model, I'll probably use two layers of 1.5 oz. cloth, though.

Planecrazy: You are correct, I only used one layer. If I use two or more layers of lighter cloth, should I put both layers on simultaneously, or should wait for the epoxy to cure in the first layer, sand it, and then apply a second layer of 'glass and epoxy?

Haldor: I'll probably end up buying some West Systems epoxy. I don't want to play around and waste both fiberglass and my time with this Great Planes stuff. What I think it's good for, though (the Great Planes resin), is wing seam reinforcement, because wings always flex, and I'd bet you want flexibility in the epoxy if the surface is going to flex equally as much. And no, even up until now, it has not cured stiff; still very flexible. Resin like this could have other useful applications though

Bruff: Thanks for the info. I think this confirms that the type of epoxy can have an effect on the stiffness of the finished product.


Thanks again for the replies,
- Razz
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Old Jun 16, 2001, 08:33 PM
Flying Welder Pilot
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Reno, Nevada
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Lay up both layers at the same time. And try and pat up any excess epoxy with paper towels. The stiffness of the part is the action of two layers of glass working against each other.

West systems epoxy is excellent but as Haldor said you will have to wait at least 24 to 48 hours for a full cure.

I have used West System epoxy for composite carbon fiber landing gear and it is high quality stuff. Make sure you mix it by wieght 3 parts epoxy to 1 part hardener as per instructions. I use a gram scale with paper cups.

Good Luck
Gordon
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Old Jun 16, 2001, 08:34 PM
EDF Head
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Stavanger, Norway
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Just lay up the layers all at once - wet on wet.
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Old Jun 16, 2001, 11:14 PM
It wasn't me...
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Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally posted by razzor7:
I'll probably end up buying some West Systems epoxy. I don't want to play around and waste both fiberglass and my time with this Great Planes stuff.
I've used Bondo Marine Product marine epoxy resin (No. 3202) to mold parts. Dries overnight and cures to make pretty stiff parts. I buy it at my neighborhood Lowe's for $20.

No matter which epoxy you use, make sure that
you mix it thoroughly. (at least a minute or two) If you don't it will turn out rubbery.

Good luck.

Dan
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Old Jun 17, 2001, 07:25 PM
Scott Black, Montreal
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montreal quebec Canada
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Finishing resin does not have any strength. You have just discovered this the hard way (as I did). Safety Poxy, West, Proset or any of those products are Laminating epoxies used to make layed up parts rather than just providing a good finish. Any of them will work.

There is a considerable pressure drop across a duct and you have to ensure that it won't collapse. It has happened in numerous full scale aircraft too - like the P-80 prototype.

If you want to make a stiff layup, you could take CF tow and wrap it diagonally both ways so that the duct looks like an ugly leg in a sexy CF fishnet stocking (if you get my drift). That would stiffen it up a bunch and it would be easy to do. YOu could wet the CF out with more resin or just CA.

sb
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Old Jun 17, 2001, 07:47 PM
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Burnsville, MN, USA
Joined Oct 2000
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Thanks Scott. I'll have to get some carbon fiber too then, huh? Man, this is getting expensive. I'm almost over the $1000 mark!


Regards,
- Razz
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Old Jun 18, 2001, 10:25 AM
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Ed Waldrep's Avatar
Las Vegas, NV
Joined Dec 1996
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A couple of months ago I laid up inlet ducts for my scratchbuilt F9F Panther (still not finished building it! GRRRR). I used West Systems Epoxy and some realy thick cloth I bought from Aerospace Composite Products
, I beleive it was 10 oz cloth. I uses spray on mold release that I found at a local plastic supply store. I tried using the spray on Freekote brand that Aerospace Composite Products carries but it was horrible, it wouldn't wet out evenly on the plug, it just bunched up into little droplets on the surface. I've read about this habit of Freekote in Flying Models magazine also.
I just used the one layer, and as the resin cured I layed on some carbon fiber tow (Dave Brown brand) and put the whole setup in the bathroom and turned on a space heater to raise the tempurature. The next day I popped the parts off of the mold and holy moly I had some light parts that I made myself! That's a good feeling, I tell ya what. The carbon tow really stiffened the parts. I only put it on in two thick bands but maybe next time I'll do Scott's fishnet panty hose idea .
I used the heavy cloth because I didn't want to mess with more than one layer. I also have the 3/1 ratio pumps for the West Systems epoxy, but to be sure I added just a little bit more hardener than the pumps put out, and it seems to have made a difference. I'd use this epoxy before on a blue foam fuselage and it seemed to be kinda rubbery-probably just a bad mix on my part but I did use the pumps. Going by weight would probably be safest. I think a key proceedure is to get the part warm, I feel that makes the part cure better but I could be wrong, it's probably not necessary. Besides, this part was stiff because it was made of 10 oz cloth and not 3 oz cloth.
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