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Old Apr 12, 2011, 11:28 PM
Trapped in California
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United States, CA, Arcadia
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Build Log
Build Log - Simple Twin-Boom Pusher

This will be my first build log with RCGroups. I want to share a very successful design I have that is very cheap, quick, easy, and fun, based on my "So" series.

Here are some details:
Main Construction Materials: Dollar-Tree foam board (black, 2 pieces 20x30"), 2" wide clear packing tape, hot glue, 5mm carbon tube (spar)

Wingspan: 30"
Airfoil: 10% KFm3
Target RTF Weight: 16-18 oz
Power: Turnigy C2409-1600, Turnigy 3S1800 20C
Controls: throttle, elevator, aileron (single)

So let's get going!

First, the wing. Cut from foam board (I'll just call it FB from now on) a piece that is 30" long by 9". By the way, have you ever found a perfectly straight sheet? It seems they all have a little warp to them, but the warps mostly disappear when I make wings. You will also need a strip of foam board measuring 30" by 1.75". This strip is the intermediary piece of the KFm3 airfoil. The 6" and 3" pieces will be folded, so don't cut all the way through.
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Old Apr 12, 2011, 11:58 PM
Trapped in California
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On the halfway-through cut, bend the FB all the way over. Using your favorite knife, cut a deep bevel on both sides. Be careful to not cut the final paper sheeting, this sheeting will serve as a sort of hinge. "It takes a steady hand"! This allows the FB to be bent into the wing. The bevels will be on the inside of the wing, and this fold forms the leading edge.
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Old Apr 13, 2011, 12:01 AM
Trapped in California
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By the way, this is what I'm drinking during construction. You can't beat a good Hefeweizen!
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Old Apr 13, 2011, 06:47 PM
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Lay the wing flat and apply tape along the outside of the bevel cut that was just made. This tape will strengthen the wing's leading edge.

Draw some marks at each wing tip, 1-1/2 inches from the trailing edge. These marks serve to position the intermediary strip of FB which forms the KFm3 airfoil.

I like to paint the edges of the FB when I use black FB. For this plane I chose a lime green. Why? Doesn't black and lime green look good together? For me it's easier to paint the FB edges before the wing is assembled.

Next, hot glue the intermediary piece of FB onto the main wing using the marks made previously. When I use hot glue the temperature is 170F - 180F (dual-temp glue gun on the low setting).

A quick note about applying the clear packing tape - make it as smooth as possible with few wrinkles. Something I do is press down the tape well with a film covering iron set at 240F - 250F.
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Old Apr 14, 2011, 12:55 PM
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The wing is almost done! Glue in a carbon tube (I used a 5mm OD tube from Hobby King, they're 750 mm in length which is almost the span of this wing). Then fold over the leading edge and glue well.
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Old Apr 15, 2011, 09:59 AM
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More photos of the wing....
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Old Apr 15, 2011, 10:16 AM
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Next is the aileron. Cut the trailing edge of the wing panel 8 inches from one wing tip. This will be a hinge cut, so don't cut all the way through. The aileron measures 8 by 1.5 inches. Fold the aileron back and do the bevel cut, then apply tape to both top and bottom of the hinge.

On the first cut, widen it to about 1/16 inch to prevent binding of the aileron movement.

So why only one aileron? Because if there were two, it would be extra weight, cost, time, and effort for something that isn't needed. It doesn't matter what side of the wing the aileron is on, but on this model I put it on the left side.

The wing is now complete!
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Old Apr 15, 2011, 12:53 PM
jclassic1
New Houghton, Derbyshire, UK
Joined May 2006
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Really love the "So" series of planes.Great you are doing a build log and I will be following it with great interest.I only use 6mm depron for all my builds but should be ok with that.Will you be doing plans or does this plane not need them.Is the Kfm3 better than the Kfm2 for this build.I like to keep all my foamies around 12 ozs if I can so may use a smaller lipo such as a 1300 mah.
I have been looking at the "So" family for a while but never got around to building one,perhaps now is the time.

Thanks Foamflyer for renewing my interest.

John
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Old Apr 15, 2011, 01:31 PM
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You're very welcome, jclassic1. Of course the thickness of material for this plane really doesn't matter. You could hot-wire it out of solid foam too, as I often do, but I've found foamboard to be easier to work with for some things.

The only plans that will be available are the drawings contained in this build log. It doesn't need anything else. As for the difference between the KFm2 and KFm3, try it, there's nothing to lose.
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Old Apr 15, 2011, 01:46 PM
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Fuselage Construction

OK, the wing is done, here we go on the fuselage (don't worry, it's easy!)

You can start by cutting the parts. You'll need two fuselage sides, a long strip that goes between them, and a plywood firewall. See the drawing for dimensions. The long strip, given these fuselage dimensions, needs to be about 36 inches in length to go all the way around the top and bottom of the fuselage.

You might ask why the fuselage is so long? It doesn't need to be, but it flies great as-is. If you go with a shorter fuselage you'll just have to adjust the battery location for the correct center of gravity (CG), no big deal.

I started hot-gluing the strip at the back of one fuselage side, then work your way all around the nose and up to about where the leading edge of the wing would be. The FB is easy to bend and shape into curves, no cutting required. Try to keep the strip 90-degrees to the fuselage side.

Then attach the other fuselage side.
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Old Apr 15, 2011, 01:54 PM
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The firewall was cut from 1/8-inch birch plywood, and measures 1 inch high by 1-3/8 inch wide. Glue it to the rear of the fuselage, and ensure it's at a 90-degree angle to the fuselage (the plane will fly better if it is). I like to add hot-glue fillets on the inside for added strength.
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Old Apr 15, 2011, 02:00 PM
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Can you believe the fuselage is almost done?

The next step is to apply clear packing tape to the bottom of the fuselage. This will help protect it during landings. I like to center the tape across the width of the fuselage and wrap the sides over. Again, use the film covering iron to seal it down good (the heat will also slightly shrink the tape). Have the tape extend onto the nose too.

Also note I've already painted the FB edges lime green!

I also added a paper canopy with regular wood glue and painted it blue.

That's it! Fuselage done!
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Old Apr 15, 2011, 06:18 PM
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Let's make the booms.

Cut two pieces of FB that measure 15 inches by 4.5 inches. Make two cuts (not all the way through) at 1.5 inch intervals. Bevel all the length-wise edges. Fold the pieces into triangles (with the bevels on the inside) and glue. If it doesn't easily make a triangle, make the bevels a little deeper. Then cover them entirely in clear packing tape.

The booms really need to be covered in tape. If not, you'll see stress cracks form in them near the trailing edge of the wing.

On this model I then cut the front of the booms at an angle, and left the rears square. And of course don't forget the lime green paint!
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Old Apr 15, 2011, 06:21 PM
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A few more photos, and the tail booms are done!
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Old Apr 15, 2011, 06:25 PM
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By the way, you don't need a lot of space to build this plane. I'm building mine on a folding table 5 feet long and 2-1/2 feet wide. I'm not even using all of it due to other stuff on the table.
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