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Old Aug 05, 2012, 08:19 PM
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So, I started work on the intake ducting. This is the first time I make one, much less do fiberglass work... In the process, I learned a few things I haven't been told about or read...

1. Don't try to use one big 2 oz cloth to cover the whole duct or try to follow tight curves expecting no creases... impossible. Cut out smaller pieces and layer it up overlaping the pieces. In hind sight, perhaps more layers of thinner cloth might've been good too.

2. 2 oz cloth does not follow tight 90 curves like the lip on the duct. Should lay some finer cloth over the tight curves first.

The pics show the construction process of the plug, and shows the plug after it's been puttied, sanded, and finally PVA'd. I'll Post a pic of the glassed plug after it's finished curing and I'll show the final product.

Oh yes, I've decided that I'm going to split the duct to try to save it, I realised half way through glassing how much work is involved in making a duct plug, might as well avoid having to do it again if the duct sucks.
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Old Aug 06, 2012, 11:59 AM
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So it's been ~ 24 hrs. The tube is still a little bit sticky to the touch, but I'd say hard enough to cut and peel. I've trimmed off the glass to leave just the tube I want.

I'm a little concerned now, I PVA'd the foam really good, like 8 or 9 coats. In trimming, I found that in spots where I sanded the putty down to foam (only in a few minor places) the resin still stuck to the foam cause the PVA dried with a slightly rough surface... I'm going to try a warm water bath before pealing...

So these photos speak to my inexperience with fiberglass, there is a big air bubble at the fan rim, The 2 oz cloth is too thick. Having to do it again, I'd use more layers of thinner cloth. In any case, from the looks of it, the duct seems to have some strength to is, so it may be useable!

I'm gonna use a Dremel tool with a disc sander to slit the duct. How to I prevent from hurting the foam? It's an inevitability I think...
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Old Aug 06, 2012, 02:31 PM
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The duct is off the plug. The warm water bath was a must.

The air bubble at the rim led to a weakness there. There's a crack in the resin. The duct only weighs 100g and I find it weak. I think I'll reinforce it in a few places.
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Old Aug 06, 2012, 03:52 PM
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Yes, we try and learn! At least your making the effort to succeed in doing something. Roger Brecht(airsally) uses very heavy cloth.(5oz) and gets it from CST. It is very flexable. I save boxes of end pieces of .5-1 oz cloth. I use these in layers and wrap the plug across the width. I have ducts that withstand the power of a 1400W 70mm, with 5 layers of .75. I purchased some 20 oz cloth locally and separate the roving. After cutting the duct free, a wrap the roving in a cross-hatch using thin CA. the roving overlaps are about 2" apart on the top and bottom of the ducting.

Never had one implode yet. Next time your glassing remains sticky, douse it good with talcum powder and let it sit for an hr. Many epoxys will take atleast 72hrs for a full cure. And even after that it takes a week for full hardness. Talc is great stuff. I use it when silkspannng/laquer finishes. Forces the drying of the lacquer. Makes sanding any paint real easy, too.

Fuzz
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Last edited by St. Martin; Aug 06, 2012 at 05:42 PM.
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Old Aug 07, 2012, 06:52 AM
Life begins at transition
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I use one layer of 3/4oz glass, 3oz glass, then 12k carbon tow in a spiral each 2" on my F16 duct.
Run it up to over 2kw on a 70mm.

Hoop strength is the issue with ducts, but it takes effort to make lots of rings of carbon. One big spiral is much easier. All done at the same time, so both layers of glass and the carbon all cure together.
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Old Aug 07, 2012, 10:11 AM
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yeah, and in the "flattened" portion of the intake duct, there's very little resistance to squishing. The hoop strength provided by the cylindrical shaped part of the duct is much stronger.

On the real F-16, there's a structural member right at the inlet. I wonder what its purpose is.

I like your idea of a 3/4 oz cloth followed by a 3 oz cloth.
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Old Aug 07, 2012, 05:41 PM
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Mine was quite squishy in the flat portion too, but once the front of the intake it glued to the airframe, it doesn't collapse.

If you can bond the intake to the formers, they will keep if from collapsing, and you can get away with a lighter layup
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Old Aug 15, 2012, 10:11 AM
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I should note that I made my duct using polyester resin, which is what they gave me when I asked for "finishing resin". It has a strong fumes, I dislike the chemical smell and so don't like working with it. So I went back out and found some "laminating resin" which seems to be doing a great job on another project of mine. Hardly any smell at all.

Anyone know what the advantages or disadvantages are to these two resins?
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Old Aug 15, 2012, 10:48 AM
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poly resin is cheap.

Epoxy resin is good.



Poly is generally more brittle, the user can change the cure time by changing the catalyst ratio, and is (in my experiance) quite thick. Oh, and stinks to high hell!

Epoxy on the other hand is a bit more flexible, must be mixed accurately (I use a 0.01g scale), and varies in thickness from water to mud. No real smell apart from the hardner may have a tinge of ammonia to it.

I exclusively use epoxy, mainly for the smell, but now also for the mechanical properties.
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Old Sep 16, 2012, 12:56 PM
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Just a quick update to say this project is still alive. I got side tracked by another project.

I patched up the intake duct. I'm going to need to reinforce it somewhere else before it's ready. I'm pretty happy with it for first time using fiberglass.

Making the exhaust duct will be much easier, but I'll leave that to later. I'm going to work out the kinks of the airframe from the fan forward first. I think the design of aft of the fan can happen separately. I'll post pics after I've got the intake duct in frame.
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Last edited by arrowking; Sep 17, 2012 at 10:30 AM.
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Old Sep 23, 2012, 12:26 PM
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I've reinforced the fore bulkheads (#13 - #19) with a few layers of 2.5 oz fiberglass, the foam was too flimsy and likely wouldn't support a hard landing. I'm thinking or potentially getting all of the bulkheads redone by CNC, and using some 1/16" ply or 1/18" ply to reinforce a few of those fore bulkheads. Don't know yet.

I've been dreaming of finally installing the intake duct, which would mean gluing all of the bulkheads to the frame. No going back, at least with this frame. Check out the pics with the duct held in place by the fan and the string.
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Old Sep 25, 2012, 12:03 AM
ich bauen groß modell flugzeug
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Awesome!
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Old Sep 27, 2012, 10:38 PM
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Here's a few more pics. I'm using the intake duct to support the nose gear.
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Old Sep 28, 2012, 12:30 PM
ich bauen groß modell flugzeug
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Im a Falcon Driver too So this is Epic!
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Old Sep 28, 2012, 07:32 PM
ich bauen groß modell flugzeug
Keenan smith's Avatar
United Kingdom, London
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Arrowking could you send me the DWG's? i really wanna knock out one of these to have a ton of fun!

K
my email is 3DDFHobbies@gmail.com
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