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help with f-18 plug

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Old 07-24-2003, 02:33 PM
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corey-RCU
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Default help with f-18 plug

Hi all, I am building a plug for a small f-18 which will be my first time doing a fiberglass fuse. I know you have to do the mold in two halves but I don't know if I need to do it in top/bottom halves or left/right halves. The thing that is causing me problems is the wing/fuse fairing... it slants down a bit so I would think if you did the left/right halves the layup would get trapped in the mold and top/bottom halves I think you would need a very curved skert to follow the lines of the fairing. Anyone know how to go about this?
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Old 07-24-2003, 05:21 PM
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Default help with f-18 plug

Email me and I can help you out. Very simple.

Russ
Oakdale Aircraft
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Old 07-24-2003, 05:35 PM
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Default help with f-18 plug

I am curious as to Russ's solution, can you share?

Can you post pictures?

Do you think one way is a three or more piece mold?
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Old 07-24-2003, 05:56 PM
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Default help with f-18 plug

My solution isn't anything special. It's simple b/c it's been done by so many people.

You can do a left and right half and have the wing/fairing nub very short. There might be a small undercut when molding, but it should be able to pop out no problem. Iíve done larger molds with some pretty good size undercuts and the parts popped out no problem.

If you donít want to do this, do a top and bottom. Use a Sharpe and mark the parting line on the mold. The parting line does not have to be straight. It can be as curved and wiggly as you want. You then need to attach a parting board to that line. See www.ctsales.com for a tutorial on how to do this. Wax and PVA the mold. Then lay down a surface coat of resin. Use something to thicken the resin up just a little (like Microballons). Make sure you push the mixture into all the corners (including the corners where the mold intersects the parting board). Let this begin to cure until it is tacky. Then lay a piece of light oz cloth over it. Let fully cure (couple days). Then start building up the layers until it is pretty darn thick (3/16Ē-1/4Ē). Let cure a couple days. Then remove the parting board. Wax the other side, including the lip of the glass that was created from the parting board and repeat. Then let the whole thing set a week or so. Then pop the molds off.

Itís really not that hard. Thereís a lot of other guys on here that have done this as well. Thereís also a wealth of information as to how to do it. Research a little and youíll find it. Email me if you have any questions, suggestions, or corrections.

Post some picts so we can see your progress!

Good luck.

Russ
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Old 07-24-2003, 05:57 PM
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Default help with f-18 plug

BTW, what are the specs for your aircraft? What kind of powerplant?

Russ
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Old 07-24-2003, 07:48 PM
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Default help with f-18 plug

Thanks for the info, it is going to be for the Toki .18 unit and the size is 35.5 span by 50 length. another thing I noticed with doing the right/left mold is at the bottom of the fuse near the tail cone and where the front fuse meets the belly the mold will curve in towards these grooves which I think would lock in the fuse in the mold... do you know what I mean and is this a concern? Thanks again for the help.
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Old 07-24-2003, 08:07 PM
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Default help with f-18 plug

If you could email me a sketch, CAD model or pict I could better help you out. Also, I'm interested in what you're doing. Email me and let's talk about some possible opportunities, if you're interested.

Thanks

Russ
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Old 07-24-2003, 11:30 PM
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Default F/A 18

Do a top and bottom mold for the F/A 18 your seem should run
from the nose to the lex and follow the lex to the tail straight.
You will have no problems pulling parts from this.
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Old 07-28-2003, 02:31 PM
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Default help with f-18 plug

Is it better to fill in between the formers with foam of sheet the thing with balsa?
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Old 07-28-2003, 10:19 PM
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Default help with f-18 plug

I've done both. Personally, I think you can get a better surface if you use the foam. Make sure you use a high density foam though. It's easy to cut to get the shape close to the formers, then sanding it exact. When you're done, do two layers of a 3oz fiberglass cloth and sand that surface.

If you use the wood, I found that if you don't sand it perfectly, you'll see the edges of the planks in it. Even if you glass over the wood, it's hard to get the edges out.

Just my opinion though.

Russ
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Old 07-29-2003, 02:15 AM
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Mike James
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Default Top/Bottom molds

One detail you may want to investigate, if you're doing top/bottom molds, instead of left/right molds....

The shape of most of these jet fighters is roughly cylindrical, but I've noticed when doing CAD models of these things, that relative to the center "waterline" of the plane, the noses tend to slope downward from roughly the cockpit area to the tip of the nose.

What this means is that, to avoid getting your plug (and later, parts) trapped in the mold, the parting board must slope downward in the front. I recently did a large Long EZ, which has the same trait.

Here's an image of the molds, where you can see the slope. Basically, you want the parting board to stay on the centerline, (or let's say the widest part of the structure) and that's why mine had to be cut, then angled down at the nose.
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Old 07-29-2003, 02:20 PM
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Default help with f-18 plug

Thanks guys, I am doing the top/bottom mold and I'll make the parting board curved to match center line. I also got some pink foam yesterday and glued the pieces in place[what kind of glue do you guys use for this?] after some thought I figured it would give a better finish as well. Today I will sand the foam back so it should start to look like something but I do not have a digital camera yet so I can't post pics, I do have a couple of people who take pictures for me but they are both on vacation right now until next week. I will post pics as soon as I can though. Do you think it would be better to do an all composite jet or have the wings and fins foam/balsa? Also retracts or no retracts?
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Old 07-29-2003, 02:34 PM
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Mike James
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Default Glue for foam blocks

Sticking foam blocks together:

I use epoxy for this job, mixed with microballoons, but I'm careful to only glue the "centers" of the blocks, so that when I sand the blocks to shape, I'm not sanding any epoxy seams.

PS on the parting board "curve"...
I didn't actually "curve" mine, but instead I scored it, propped it all up (on the plug) at the right angle, and then used some hardwood blocks to bridge the scored area, so it would retain the shape.

Can't wait to see your results. Good luck.
(and hey! Buy a digital camera, with some extra memory... It's worth it.)
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Old 07-30-2003, 02:26 PM
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Default help with f-18 plug

Thanks, the plug is going well and I have the sanding very close, it actually looks like an F-18 now and is getting me excited! What would a guy do if he got to excited and sanded a little to much in one spot Do you fill it up before you glas it and what kind of filler do you use...dry wall filler?
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Old 07-30-2003, 02:35 PM
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Default Fillers on foam

Any soft filler will work, as long as it doesn't melt foam, (!) but I've had better luck by just continuing on with the sanding until the shape is nearly complete, then adding filler just before glassing. In fact, if there's a fairly major ding or crevice, I'll fill that with epoxy and microballoons, right before the first layer of glass. That way, the different density of the filler doesn't hold you up during the basic "get the shape right" stage.

Once you get it glassed, there's still plenty of sanding to do anyway, so don't worry about small dings at this point.

... Can't wait to see this baby.
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Old 07-30-2003, 02:51 PM
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Default help with f-18 plug

Thanks Mike, I can't wait either! I hope it comes out light enough for good performance.
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Old 07-30-2003, 10:22 PM
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Default help with f-18 plug

I've learned the hard way to make the job as easy as you can get it. I use drywall filler, it sands SOOOOO easy, a lot easier than epoxy, bondo, or what ever you may put on after you've glassed it. I've learned to get it right before you glass it ( I hate sanding).
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Old 07-31-2003, 02:12 PM
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Default help with f-18 plug

Not ready for this step yet but once you put on the two layers of 3oz cloth with epoxy what comes next, what are the steps for that smooth finish and what will I need?
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Old 07-31-2003, 02:21 PM
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Default Plug finish

If it were me, I'd put at least two layers of heavier cloth on first, like 6 oz. cloth, then add some lighter cloth, to avoid having a lot of weave to fill and to strengthen the plug.

With a foam plug in particular, you want to give it strength on the outside, not just for general durability, but so that you won't run the risk of going through the cloth during the sanding phase. (done it... That's how I know)
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Old 07-31-2003, 02:32 PM
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Default help with f-18 plug

O.K. I'll put two 6oz and a lighter layer on top, once that is done do you even need to fill the weave or just sand and what kind of filler is good for this step? Also what grade of paper should I use?
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Old 07-31-2003, 09:27 PM
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Default Plugs

Since (within reason) weight isn't an issue with plugs, I generally start by sanding the existing surface, before applying any fillers. (You need to scuff up the surface anyway to get a good bond.)

Since you'll have lightweight cloth on top, there won't be much weave to fill, and it's a judgement call when you look at it, whether to use a filler mixed with epoxy, or just go straight to primer. I like the nice-sanding lacquer-based primers best.

Automotive primers are great, but you can use something like "Pactra Prep" early in the game. At the end, just prior to making plugs, is where I'd spend the money on the best lacquer-based primer I could afford... something that dries as hard as possible. ("Duratec" for example)

If possible, I try and keep the plug surface a uniform hardness, so it sands evenly. Microballoons and epoxy is a good choice, but not too much fun to sand.

I use 80 grit for tough areas, then 180 and 220 wet, and finally 600 grit wet.
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Old 08-01-2003, 02:17 PM
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Default help with f-18 plug

Perfect! Another thing I thought of is when I put cloth on the hatch area on the plug and on my hatch, the hatch will not fit right anymore which is built to an exact fit as is right now... do I not glass in some areas or am I missing somethig? Sorry for all the questions.
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Old 08-02-2003, 12:03 AM
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Default Hatches

Well, it's best (for durability) to glass everything, but not absolutely necessary.

As long as you have smooth and flat surfaces where your hatches mate, you'll be ok. Just remember to be generous with the wax and PVA at the end, when you make your molds.
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Old 08-02-2003, 01:38 AM
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Default help with f-18 plug

Corey, can not help with your molding questions as I'm still learning as well and it seems Mike is giving some great answers.

Just wanted to say good luck on the project and tell you the Toki .18 was dropped off in Fargo, ND at Kens shop as per Len's shipping instructions this morning.

Just be sure to balance the fan and you will be in good shape

Thanks,
Mark
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Old 08-02-2003, 12:28 PM
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Default Fill ther weave

I usually fill the weave rather than try and sand it away. You can use your resin and cabosil scraped into the weave with a credit card or just use Spot/glazing putty scraped on in a thin layer or if the weave is not too deep just use a good primer/filler sprayed on like FeatherFill.

My $.02

Ted
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