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PROP-JET BINGE

Old 10-02-2010, 12:29 AM
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combatpigg
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Default PROP-JET BINGE

Still building those SWRs and working on easy ways to make FG fuselages with sacrificial foam plugs. This one is based on the A-7 Corsair, but it has been slimmed down in the overhead view. It will be closer to scale in the side view. Some compromises have been made to allow it to look somewhat like a jet, but I still want it to be fast enough to justify the noise it will be making.
The whole fuselage will be laid up in one shot, then cut up to seperate the cowl and the wing access.
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Old 10-02-2010, 07:06 AM
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smoknrv4
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Default RE: PROP-JET BINGE

Will be following along.
Old 10-02-2010, 08:50 AM
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Max_Power
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Default RE: PROP-JET BINGE

Great timing! I started a similar project recently. although like you I dont think I'll be able to make the fuse as fat as a real A-7 and get any speed out of it. I got my wing done its 28" span ...It looks a bit more like a F-18 wing as I don't have any sweep in it. I'm assuming on a bladder my Fora will run fine upside down? I was planning on having the cylinder hanging down into the intake duct and routing the exhaust dump or tuned pipe down the tube(I got the Fora .061 with the rear round exhaust so I could silence or pipe it possibly). I acyually set aside my morning to work on my Fuse a bit as its been on the back burner to bathroom construction for a few weeks so maybe more pic's soon. I'm definitely subscribing to this thread! Todd
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Old 10-02-2010, 10:43 AM
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combatpigg
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Default RE: PROP-JET BINGE

I've done 3 complete bathrooms and for each one drove to the store and back 100 times......
We found that the help at Home Depot was superior to Lowes...I don't know if it is because of universal hiring practices, but going through Home Depot cut down on SNAFUS, FUBARS, etc.
Old 10-02-2010, 11:12 AM
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Max_Power
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Default RE: PROP-JET BINGE

So where does one get a hold of the blue foam? At this point looks like I'll be doing a piece for the top/turtledeck. Do you just glass it and then melt the foam with acetone or similar? Looks like that stuff sands and shapes better than the pink stuff. Todd
Old 10-02-2010, 11:50 AM
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Default RE: PROP-JET BINGE

The blue foam is the same as the pink foam. Just different colorants added to identify the two big makers. Both work as well with sharp knived and fresh coarse sandpaper. And of course be sure you use epoxy laminating resin (look for that at the local boat building or specialty figerglass suppliers) instead of polyester resin. The polyester resin will melt the foam.

If you want just a light turtle deck a slick way to get it is to make your foam shape. Lay on a single layer of 2 oz cloth with resin. Blot away some of the escess with toilet paper and let cure. Once cured fill the weave pattern with a second coat of resin turned into a "sanding sealer" by using a fairly rich mix of microballoon filler. Thinning this coating with a little 99% isopropyl alky from the drug store will aid in making it a brushable mixture. You don't need to lay it on heavy. Just enough to fill the exposed weave.

Now a single layer of 2 oz cloth is pretty flexible and would not hold its shape well. So instead of removing all the foam by dissolving it chew it out with some cutting tools and then sand it until it's about an 1/8 inch thick of foam and fairly smooth. Now glass in a few one inch wide strips of the 2 oz cloth to the inside to form arch shaped "formers". Once cured I think you'll agree that this is a pretty nice way to make a strong and light deck. The "formers" will lock the shape in solidly to the point where if it seems too flexible while placing the former strips that you should tape the whole works back onto the fuselage using wax paper or plastic food wrap as a separator so that it is taped in place to the right shape and allowed to cure.

This is all a bit more work but it'll result in a part that is light and gives you a bunch of room inside for gear. But really at our small sizes if you don't need the room you may as well leave the foam in place. Or if you want to try doing a whole fuselage I THINK it would come out lighter to hollow out the foam and do an inner layer than it would to do two coats of cloth and a few spots in high stress areas where you use some patches as a third layer. In CP's case where he's doing the foam all in one part he doesn't really have a choice unless he bandsaws the finished fuselage into two pieces or cuts a sizeable hatch to allow the mechanical carving out.

If you want to carve out the foam a messy but VERY effective tool is to make up a "mace" for your Dremel. Take a 1.5 inch piece of 1/8 brass tubing and into the end shove 4 or 5 L shaped pieces of 1/32 music wire and fill the tube with solder with the wires going the whole length. Arange the wires so that they stick out at various angles so that the ends sort of describe a ball when rotated and grind the ends all to about the same length of about 3/16 to 1/4 long. This makes a super mean and totally clog free foam gutting tool. Just watch those fingers though ! ! ! It's not bad for hollowing out softer balsa blocks as well. Rough as blazes but from there you can finish it with finer stuff.
Old 10-02-2010, 04:41 PM
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Max_Power
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Default RE: PROP-JET BINGE

Thanks! Sounds like the Dremel mace should come with some sturdy safety glasses!!! Todd
Old 10-02-2010, 05:16 PM
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combatpigg
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Default RE: PROP-JET BINGE

If you pull up to a job site and offer a laborer a six pack for his troubles, you should be able to come away with some nice scraps of foam.

One of my favorite tools for hogging out the sacrificial foam is a hacksaw blade with 1 end ground to a point. Wood chisels are handy, too.
Old 10-03-2010, 12:20 AM
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Default RE: PROP-JET BINGE

Todd, I would never so much as think about even starting up a Dremel unless I have safety glasses on. The foam is light enough that the "mace" tool doesn't throw it hard enough to be dangerous but if one of the wires should fatigue and fly off..... well, the risk is up there and well worth wearing the glasses.
Old 10-03-2010, 04:57 PM
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Max_Power
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Default RE: PROP-JET BINGE

So how's this thing coming CP? I need to see how you're doing your tail!
Old 10-05-2010, 10:03 PM
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combatpigg
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Default RE: PROP-JET BINGE

Here's a little more work done on it. The wing is 1/16" top and bottom sheets seperated with 1/8" FG spar tubes and balsa ribs. The leading edge is 1/8" balsa square stock. I'm glassing the center section and wingtip area for some added stiffness.
The fuselage has a layer of 2 oz and a layer of 1 oz cloth applied with laminating epoxy. After the epoxy has cured for a few days it should be set well enough to do the surgery required to remove foam and make the cut outs for cowl, hatches, etc. .
It's going to be flimsy with only a 3 oz buildup of cloth.
This will be my weakest and largest layup attempted, so I imagine there will be a need for further reenforcing from within the shell later on.
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Old 10-05-2010, 11:13 PM
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Default RE: PROP-JET BINGE

Looking Sweet buddy! I think it's been over a year since I last fired up my 1/2a stuff but I'm glad to see some of you are still going strong and keeping this part of the hobby alive....
Old 10-06-2010, 12:20 AM
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combatpigg
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Default RE: PROP-JET BINGE

Last time we talked, we were comparing how much snow we got!
This forum has picked up some new members who like to build, so it is always nice to see that going on.
Old 10-06-2010, 11:26 PM
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combatpigg
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Default RE: PROP-JET BINGE

All I got done today was cutting the hatches out and gutting out the foam. There is still some more cleanup work left to do where fragments of tape remain inside the shell.
I spent more time gutting out the foam, than I did designing, shaping and glassing the part itself.
Gutting out the foam on a big part like this is no fun, thankfully the ShopVac was there to help keep the mess to a minimum.
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Old 10-07-2010, 01:25 AM
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Rendegade
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Default RE: PROP-JET BINGE

what was your layup for that?
Old 10-07-2010, 08:56 AM
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Default RE: PROP-JET BINGE

It was done in 2 sessions.
The first layer was 2.2 oz cloth and West Systems epoxy
Then I sanded down the high spots and seam overlaps.
Final layer of 1 oz cloth and West Systems.

I like the way it turned out generally. There will be a little bit of body work to even out minor divots but it wont amount to a 1/2 teaspoon of Bondo.






















































Old 10-07-2010, 12:36 PM
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Default RE: PROP-JET BINGE

i wonder if there is a solvent that will eat the foam and not the epoxy?

gas sure eats EPS pretty quick
Old 10-07-2010, 04:08 PM
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Default RE: PROP-JET BINGE

Very nice!
Old 10-07-2010, 07:40 PM
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Default RE: PROP-JET BINGE

I've seen some gut jobs done with gasoline, lacquer thinner, etc. and they left a gummy residue. I've got other gluing operations to carry out inside the shell, so I feel safer about cleaning at least 99% of it with elbow grease and do a final swabbing with acetone or MEK.
Old 10-07-2010, 08:42 PM
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Default RE: PROP-JET BINGE

Yer liken that West Systems arent' you.
Old 10-07-2010, 09:00 PM
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Default RE: PROP-JET BINGE


ORIGINAL: cncswiss1

i wonder if there is a solvent that will eat the foam and not the epoxy?

gas sure eats EPS pretty quick
Once the epoxy is cured it is quite solvent resistant, that's not a problem. But the solvent resistance improves after a few days, or more quickly with some heat to speed up the last few percent of curing. I would think there would be less chance of a gummy inner layer after a few days, how long do yours cure CP? You'll also get that if the epoxy mix is off ratio, and unless you mix by weight chances are there is a shift left or right of a perfect ratio (which within reason is not a big deal) which will contribute to that.
Old 10-07-2010, 09:45 PM
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Default RE: PROP-JET BINGE


ORIGINAL: vicman

Yer liken that West Systems arent' you.
It's a good product, but it'll give the same poor results if you try to laminate FG veil cloth [5/8-3/4 oz] over raw foam. The resin soaks into the foam and the cloth is incompletely saturated in spots. The solution has been to first cover the foam with clear packing tape...use the lightest weight tape you can find.

MJD, I notice a pretty full cure within a week...the part is about as tough as it's gonna get. The gummy layer I've seen is from the foam that still clings to the FG skin after the solvent treatment.
Old 10-08-2010, 08:12 AM
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Default RE: PROP-JET BINGE

Yup, in a week it would be as good as it is going to get. So your goo is just foam snot, makes sense. I would expect that if the part were cured overnight, i.e. structurally sound enough to remove the foam but with a few percent of cure left to go, that any ketone use might result in a surface film of tacky epoxy residue. I say that simply because the solubility goes from completely soluble in uncured state to insoluble when fully cured, so while those last few percent of cure are going on, the part will be sound but there is still the potential for a bit of surface attack from solvents.
Old 10-08-2010, 10:36 AM
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Default RE: PROP-JET BINGE

Hi CP- nice thread! Epoxy is great for this kind of work but a little expensive. I have made more lost foam cowls than I can remember so now just make plugs for molds in case more parts are needed. You can use polyester resin in lieu of epoxy but an extra step is involved. To me, the extra step pays for itself because the polyester resin is a lot less expensive. After you carve your foam just 'seal' it with Elmer's wood glue or similar. A couple of coats should do just fine. Let it dry. The polyester resin doesn't penetrate the dried glue seal so your foam won't melt! Then apply your fiberglass matt (for plugs) or cloth (for parts) using the polyester resin just like you did for the epoxy. I always make my foam plugs about 1/4" smaller than needed to account for build up of the resin and fiberglass.
Old 10-08-2010, 04:51 PM
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Default RE: PROP-JET BINGE

I'm not sure what weight my cloth is...Is the weight per sq. Ft? Yard? I want to weigh my cloth to determine if it is sufficient. Todd

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