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Sep 15, 2018, 11:26 PM
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Build Log

Scratchbuilt, Scale, Dollar Tree Foam, 53" Piper J3 Cub, with Plans!!


This is the beginning of a build log I will be doing for a Piper J3 cub. I've always been an admirer of the cub, and hopefully this build will do justice to this timeless aircraft. I've started this build by modeling my design in Solidworks. I have an educational subscription to Solidworks through my EAA membership, which is a great perk.

To start my design I'm using the attached, publicly available, drawing views of the J3. These were found by means of a quick google search.
After modeling up my design, I put together some flat pattern plans for all of the required pieces. These plans can be found attached. The primary building material for this build will be dollar tree foamboard (with the paper removed). I will use some balsa and bass wood as necessary to reinforce critical joints. The balsa and bass wood I use is sourced from Hobby Lobby.

Hopefully everything goes together as anticipated! I do expect that there may be areas I decide to revise or improve as the build progresses, but for the most part I think the plans will be an adequate map to completion.
Last edited by Plane15; Oct 07, 2018 at 02:55 PM. Reason: Added Photo of Completed Build
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Sep 15, 2018, 11:46 PM
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Plans Printed, First Parts Cut


Here are some shots of the first page of the plans printed out. You can tile them in print settings to 8.5x11, then afterh they are printed they can be spliced back together with packing tape. After the pages were back together, I proceeded to cut out the part templates. From there, the templates were traced onto either Dollar Tree foam, balsa, or bass wood. On one set of the pdf plans the parts have labels on them denoting the material of the part shown. DTF obviously stands for dollar tree foam.

It might be worth noting that I first remove the paper from the dollar tree foam before I cut parts from it. To remove the paper, I soak the foamboard either with a hose (it could be done in a shower too probably). Soaking the paper breaks the bond of the glue on the paper. After it has been soaked, I let it dry, being careful to let both sides dry evenly to avoid warping. After it is dry, the paper can be peeled up and removed from the foam with ease.
Sep 16, 2018, 09:30 AM
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Lawn Dart Flyer's Avatar
i'll be watching
Sep 16, 2018, 09:57 AM
Jim
Quote:
Originally Posted by Plane15
Here are some shots of the first page of the plans printed out. You can tile them in print settings to 8.5x11, then afterh they are printed they can be spliced back together with packing tape. After the pages were back together, I proceeded to cut out the part templates. From there, the templates were traced onto either Dollar Tree foam, balsa, or bass wood. On one set of the pdf plans the parts have labels on them denoting the material of the part shown. DTF obviously stands for dollar tree foam.

It might be worth noting that I first remove the paper from the dollar tree foam before I cut parts from it. To remove the paper, I soak the foamboard either with a hose (it could be done in a shower too probably). Soaking the paper breaks the bond of the glue on the paper. After it has been soaked, I let it dry, being careful to let both sides dry evenly to avoid warping. After it is dry, the paper can be peeled up and removed from the foam with ease.
That's gonna be wicked cool but too much fiddly cutting for me! My hats' off to you if you do all that by hand!!
Sep 16, 2018, 03:36 PM
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More Parts Cut, Assembly Started


Got around to cutting some more parts out, and starting the assembly. The parts fit together with tabs and slots, which creates a good friction fit. I use a thin application of 5 min epoxy to all of the joints to set the final bond. The brand that works best for me is the RBC 5 min epoxy, sometimes branded as "hammerhead" epoxy. It works very well on the Dollar Tree Foam, and creates the cleanest, strongest bond I've seen. It's only downside is that it is literally a 5 min epoxy, at about 2-3 minutes it has already pretty much set, and by 5 minutes you can barely put a mark in it with a fingernail, so you need to work fast and be prepared with any clamps, etc. that you might need.

It might be noted that the tabs only extend into the slots about 3/4 of the thickness of the foam. This was done intentionally, as these shallow gaps were filled in with DAP fast'n final lightweight spackling. After the spackling dries, it can be sanded down for a nice smooth finish across the foam.

For the side-fuselage skins notice that very slight bevel grooves were cut on the inside. These grooves help the skin bend to the shape of the formers, and give crisp definition to the final shape.

Balsa stringers were used to help give shape to the rounded portion of the fuselage just aft of the firewall. These stringers were covered with some thin foam from a styrofoam plate.

The firewall is built from some 4ish mm plywood. There are holes in the plywood which house neodymium magnets. These will mate up with magnets on the removable cowl. The aluminum motor mount is screwed down to a spacer block to get the right longitudinal placement off the motor. The spacer block of wood is attached to the firewall with wood glue and a screw from the backside of the firewall.

The aluminum motor mount is made for a hextronik DT750 outrunner.
Sep 17, 2018, 11:37 PM
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Windows Complete


Finished up the windows. I taped the window template with packing tape to a piece of cardboard, and it made a good working surface. I used elmers wood glue to bond the balsa window pieces. The wood glue didn't stick to the packing tape at all, making it very easy to peel up the glued parts.
The outside of the windows is covered in a thin sheet of balsa to give nice rounded corners and a finished look. Some clear plastic cut out of a salad box was used to make the windows. The windows are glued (from the inside) to the radiused corners of the outer skin. I went ahead and painted the window frames yellow and did a dry fit.
Sep 18, 2018, 11:44 AM
Jim
Very impressive building skills sir! It looks awesome!
Sep 18, 2018, 09:08 PM
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Thanks for the kind words Jim.

With the windows done, I turned my attention to the cowl. The very forward most point of the cowl is made with 3 laminated pieces of DTF. The templates for these pieces can be seen in the plans. From there, balsa stringers run aft to the former that will mate up to the firewall. This former has holes in it for the magnets that line up with the magnets in the firewall. While gluing in the stringers, I found it helpful to locate the laminated nose piece relative to the former by using a temporary fixture. I've attached a pdf with templates for the fixture pieces. Once all of the stringers were glued in, I used styrofoam plate as a skin to cover the cowl. The styrofoam plates are quite thin, and they can be shaped easily to the curves of the cowl. When determining the shape of the skin pieces, I find it helpful to start by honing in the shape using a piece of paper, and then once the paper fits well, I transfer it to the foam. With all of the skin pieces glued on, I added some of the DAP lightweight spackling to all of the seams. Once the spackling dries, and is sanded down, the seams become almost invisible.
Sep 18, 2018, 09:13 PM
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Cowl Sanded, Painted, and Polyurethaned


With the spackling dry, I went ahead and sanded down the cowl and added two coats of paint. Note that I sanded before and after each coat. After two coats the coverage looked good, so I went ahead and added the final finish with a coat of water based polyurethane. It makes the yellow really look nice!
Sep 19, 2018, 05:53 AM
Warbirds Lover
Dreamcatcher's Avatar
hi there,

I will take a chair and follow this build :-)
Great job so far !
Sep 19, 2018, 01:57 PM
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elewon's Avatar
looking good !!
Last edited by elewon; Sep 19, 2018 at 03:36 PM.
Sep 19, 2018, 10:40 PM
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Vertical Stab and Rudder


Here are some shots of the vertical stabilizer and rudder. In the plans, these pieces were originally designed to simply be made from dollar tree foam, but I thought that they might benefit from some extra strength, so I went ahead and wrapped the exposed perimeter of the parts in balsa. The seam between the balsa and the DTF was then filled with lightweight spackling and sanded smooth. For the rudder, I reinforced the bottom with two pieces of bass wood. Each piece has a slot so that when the two pieces are sandwiched together it leaves a cavity. This cavity is for the tailwheel wire so that the tailwheel will actuate with the rudder. I also went ahead and glued in the hinges in the vertical stab. The hinges are just rectangles cut out of a floppy disk, and then they are glued into slots. I finished up by adding a coat of paint, and then connecting the control horn. The horizontal stab and elevator should be up tomorrow.
Sep 20, 2018, 10:10 PM
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Horizontal Stabilizer and Elevator


Finished up the horizontal stabilizer and elevator. The process was the same as that used for the vertical stab.
Sep 21, 2018, 01:22 PM
Retired CAD guy
birdofplay's Avatar
VERY NICE CAD WORK.
A great effort.

It was easy to transfer to MPF foam and scale it up a little to fit inside MPF dimensions.

Do you "hang" with any of the Aero Tow guys over at Union Gap ? ( Gene, George ... )
Sep 21, 2018, 10:34 PM
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BirdofPlay,

I'm glad the PDFs scaled well for the MPF foam! I don't know the guys at aero tow. Although I do sometimes fly in Union Gap, so who knows, I might run into them.

Best Regards,
Isaac


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