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Old Jun 17, 2011, 11:09 AM
Yeeah buddy!
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Northern Colorado
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Wow! This is turning out really well! Awesome job!!
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Old Jun 18, 2011, 07:30 AM
Now in TN!
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Chapel Hill, TN USA
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I see that you have your 335 started. Looking great. Nice work Pat.

J
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Old Jun 18, 2011, 01:38 PM
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Originally Posted by J Morgan View Post
I see that you have your 335 started. Looking great. Nice work Pat.

J
I appreciate that John. It was your Do-335 build that inspired mine. (Shaping the tail is a huge PIA!)

Pat
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Old Jun 19, 2011, 07:11 AM
Now in TN!
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Chapel Hill, TN USA
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Shaping that tail WAS a PITA! It was the reason I had to make a second plug.

J
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Old Jun 20, 2011, 11:10 AM
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Joining the wings.

I traced the wing root onto cardboard and used the template to cut the center section from a block of foam. The dihedral brace ends were joined with a scarf joint and two 1/32" plywood plates. The wings were propped up to the correct angles and allowed to dry. I remembered to run some strings to pull the servo leads through later and glued in the middle part of center section with a 1/8" hard balsa piece to bridge the ends of the main spar. Then I glued in the rest of the center section. After sanding the center section to match the wings I'll give everything a coat of WBPU. A little filler here and there and it'll be ready to cover with kraft paper and WBPU.
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Old Jun 21, 2011, 10:06 AM
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I covered the wing with 30lb kraft paper (lightest you can get, similar to newsprint) and WBPU. After filling any large imperfections, I gave the whole wing a coat of WBPU, let it dry and sanded with 220 grit paper. The weight before covering was 251g.

To cover with kraft paper I first cut it roughly to size then brush water over the paper to saturate it. I want it throughly wetted but not dripping. Then I brush WBPU on the inside surface of the paper and the wing. I covered each wing half with a single piece of paper overlapped at the TE. The center got two layers of paper. After the paper is applied and smoothed out, I brush on another coat of WBPU. After ten minutes or so I wipe it down with dry paper towels to try to even out the wetness.

At the aileron cutouts I trim the wet paper and fold the edges over so they overlap. At the tips I fold and roll up the excess paper over the edge and clip it in place with small binder clips. The paper will conforms to the tip surprisingly well. When the paper is trimmed off there will be about a 1/4" wide gap where the paper is not well adhered. I'll trim that off and cover the gap with a strip of fiberglass cloth.

The paper shrinks as it dries stiffening the wing. It's really important to let the paper dry slowly and evenly; I hang the wing from the tip and let it dry overnight in still air. Even after 12 hours it's still a little damp. Anytime I've tried to speed up the drying with fans, I got a warped wing.

Final weight was 333g. Over the next day or two it will lose another 5 to 10g of moisture. I like this method. Besides being economical, it's the lightest way I've found to build full airfoil wing other than a film covered open frame. Sheeting with 1/16" balsa and covering with 1/2 oz. glass cloth would have added 50 to 75g. If I'd have used a solid foam core with balsa sheeting the wing would have been about 100 to 125g. heavier.
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Old Jun 21, 2011, 11:25 AM
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Spring Hill, Tn.
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Sweet build

I like how you did the fuse and got the skins formed to your mold. What did you use for a heat source in your hot box? Also I really love the wing construction. I've been cutting and sheeting cores for years but was never happy with the weight, this is just to cool and finishing with paper keeps the cost down. You made my day!!!

Dave
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Old Jun 21, 2011, 01:25 PM
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Quote:
Originally Posted by diggerrcdave View Post
I like how you did the fuse and got the skins formed to your mold. What did you use for a heat source in your hot box? Also I really love the wing construction. I've been cutting and sheeting cores for years but was never happy with the weight, this is just to cool and finishing with paper keeps the cost down. You made my day!!!

Dave
Dave,

The heat source for the oven is from a space heater. I now use a small ceramic type. You have to bypass the over-temp switch and the heater's thermostat - everything but the element and fan. I still use the heater's housing, it can easily stand the heat. I use a thermocouple plugged into my multimeter to monitor the temperature. I'm the thermostat! Switching the heater on and off to maintain the temperature at 210F. In one of the threads I cite in post 2 there's a drawing of the oven and a detailed description.

I'm with you on foam cores. It's great when you can knock out a wing in an afternoon. And they are tough! But suitable balsa isn't always readily available and it's not cheap. Whatever you cover the balsa with, film or fiberglass also can be spendy. And of course, there's the weight.

I've tried covering foam cores with paper and it can be done but the expanded foam beads telegraph through the paper unless you fill and sand. I've had bad luck with warping too. You still need a substantial spar because the paper and foam have little compressive strength.

For me, the laser method is a best compromise between weight, strength, time, and cost.

Pat
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Old Jun 21, 2011, 04:46 PM
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heater

Thanks for the info Pat.
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Old Jun 23, 2011, 10:27 AM
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Back to the fuselage. I've decided to go with the scoop as is. It's not the right shape but I'll fix that on the next one.

I marked the centerlines, wing and stab cutouts. I did it before I covered the fuselage so I wouldn't sand them off later. I cut a couple scraps of plywood to the height of the front and rear motor centerlines and clamped the fuselage above the table. When I had everything lined up, I secured the fuselage to the stand offs with some hot glue. I marked the lines using another piece of plywood with holes drilled at the right heights to hold the marking pen.

I was all set to cover with 3/4oz glass and epoxy when I discovered I didn't have enough cloth. I had been planning to try covering a fuselage with paper sometime and it looked like the time had come. Then I discovered I didn't have enough 20lb kraft paper! I did have a roll of 60lb kraft paper (similar to a grocery bag in weight) so used that. I made a guess as to the largest pieces I could use and still conform to the curves and cut the long side pieces and applied those wet, with WBPU. Then I filled in with smaller pieces. For that I used some 20lb. paper which will conform to curves better.

I didn't even try to cover the canopy and scoop with paper. I'll do those with glass cloth and epoxy. I'll have to re mark my lines now that they're covered with paper.
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Old Jun 24, 2011, 11:44 AM
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I was going to fiberglass the canopy and scoop but decided to use "Liquid Sheeting 2" instead. LS2 is a 2 part polyurethane you brush onto to foam to form a strong, hard coating. I bought some this winter to experiment with. It's interesting stuff. It takes several coats to build a layer thick enough to add significant strength to the foam. You can apply multiple coats over the course of several hours (wait 30 minutes to an hour between coats). The environmental conditions are important: humidity over 50% and 70F or above. It cures with moisture and if it's too dry and too cold it takes days to set up.

3 to 4 coats is roughly comparable in weight and strength to 3/4oz glass cloth and epoxy. It takes practice to apply it correctly. Too much and it runs, too little and it won't level out. Also you have to apply it and leave it, with minimal brushing or it won't self level. When done correctly, it leaves a smooth, hard finish.

It's perfect for complex shapes that would be hard to cover with cloth like the scoop and canopy. I masked off the paper covered areas and sanded the WBPU coated foam to 220 grit. The surface needs to be "primer ready" smooth. The LS2 is hard to sand but if the surface is smooth the LS2 will be too. I gave it 3 coats, which earlier testing showed is about the minimum to give significant protection to foam.

I was surprised how well it worked. The three coats are almost the same thickness as the paper. I did a little block sanding to blend the paper and the LS2 and that looks good so far. The paper is still damp so it'll be a day or two until I can sand it out and prime it to see how well it blended.
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Old Jun 25, 2011, 05:44 PM
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I sanded down the fuselage and blended the paper overlaps and LS2 into the paper. I was pleased how well the paper-LS2 joints sanded out. I shot some primer to check the joint to see if it needed filler and it looked good.

The stabilizer and rudders are hot wire cut cores sheeted with 1/32" balsa (NACA 0012 airfoil). Since I had the LS2 out, I gave them two coats. It didn't fill the grain completely but close. If I would have covered them with 1/2 oz. cloth and epoxy they should have come out to around 75g. With two coats of LS2 I they came out to 66g so that was nice. The surface seems tough, better than 1/2 oz. glass over bare foam in fact.

I printed out the stabilizer cross sections on label stock and stuck them down using the centerlines I drew as a guide and cut them out. Then I dry fit the stabilizer. It fit well the first try! I used the template I had traced of the wing to do the wing cutout. (airfoil is NACA 2412 with 1 degree positive incidence)
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Old Jun 25, 2011, 09:53 PM
Yeeah buddy!
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Northern Colorado
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This build is just awesome! Great job again Pat!!
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Old Jun 26, 2011, 06:53 PM
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Working on the wing saddle.

At the front of the cutout I glued in a 1" thick piece of foam to strengthen that area. I used the section of the fuselage cut away to trace the curve on the foam. I sanded it to fit and glued it in. (in the photo it's taped in place, just noticed it was crooked!) I faced the front and rear of the cutout with 1/16" balsa.

The sides were lined with two layers of 1/16" balsa to give more meat to the wing saddle. They also support the 1/8" plywood mounts for the wing bolts.

I bolted the wing on to see how it looks - gonna be sweet!
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Last edited by pmullen503; Jun 27, 2011 at 11:52 AM.
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Old Jun 28, 2011, 12:21 PM
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Pat,

DO335 is lookg great! You are moving along at a fast clip.

Keep it up -- Stacker
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