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Old Dec 02, 2011, 12:32 PM
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United States, WI, Fond du Lac
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Funbuild 5 Beechcraft D-18

I may be jumping the gun but here's my entry for Fun Build 5: 1/11 scale Beechcraft D-18. I found some excellent 3 views so other than scaling up the 3 views and drawing in some structure there was very little to do to get a workable plan.

I plan to do the fuselage in heat formed FFF and wings using the "laser method" like I did for my Do-335 and my Lockheed Vega series.

Wingspan: 52"
Motors: Turnigy 2209-28
Battery 3S 2200 mAh
Weight: 50 oz
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Old Dec 02, 2011, 04:08 PM
What's 3D?
trumps's Avatar
Australia, WA, Ellenbrook
Joined Feb 2008
3,316 Posts
most excellent choice mate, the best looking twin ever!
can't wait to see this one's birth

Cheers
Craig
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Old Dec 02, 2011, 07:56 PM
It's a Great Day to Fly
LenBFP's Avatar
Melbourne Beach, Florida
Joined Jan 2002
3,863 Posts
Great looking twin. Subscribed.

len
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Old Dec 03, 2011, 10:44 AM
Dog is my co-pilot.
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United States, GA, Brooks
Joined Mar 2005
877 Posts
Gotta follow this one.
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Old Dec 03, 2011, 12:09 PM
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I got started on the D-18 last night. I started with the tail surfaces. The tricky bit was getting the rudder linkage buried in the stabilizer. I wanted the linkage to be easily adjustable and low profile enough to cover with scale looking covers.

I started by cutting the stab outline out of FFF. Then I glued down 1/16 light balsa around the perimeter. In the center I added a more balsa to form a track for the Sullivan flex push rod (#507). The slots in that piece make pockets for the hinges.

The balsa strengthens the otherwise fragile edges of the foam, gives me a handy center line reference when shaping the airfoil, automatically centers the hinges and stiffens up the whole stabilizer.

Another layer of FFF was glued down and then sanded to an airfoil shape. The rudders were done the same way except I used 1/32" balsa for the center layer.

The stab was traced onto the rudders and the foam cut away down to the balsa center layer. The rudders were glued on. This provides a nice strong connection between the stab and vertical stabilizer. In my experience, twin rudders are always catching on something and take a beating.

The hinge surfaces were faced with balsa. The elevator got a 1/4" balsa LE. The rudders were just sanded round. After everything was finish sanded I gave it two coats of Liquid Sheeting II (aka Styrospray 1000). This is a two part polyurethane coating for foam that cures to a nice hard shell. It's roughly comparable in strength to an equivalent weight of epoxy and glass. It's great for something like this that would be a pain to fiberglass. WOWplanes does entire aircraft with it.

A 5g. servo was mounted and connected to the push rod. I still need to fabricate the fixed covers for the rudder linkage out of pop can aluminum or maybe I'll vacuform something. You can see the linkage cover in the full size D-18 in the photo.

It came out pretty good. A little heavier than I would like (82g.) but the linkage moves freely and is adjustable. The rudder to stab connection is very solid after the LSII coating. A little filling and some fillets and it will be ready for paint.
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Old Dec 03, 2011, 03:05 PM
Lookin' up at the centerline
taildragger1589's Avatar
Lawrenceville, Georgia
Joined Feb 2005
2,225 Posts
That is just outstanding!

Nick
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Old Dec 05, 2011, 04:21 PM
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United States, WI, Fond du Lac
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Wings

I got started on the wing. I’m using the so-called “laser method”. It offers certain advantages over built up and foam cored wing construction. Like hot wire cut foam cores, you don’t have to draw and cut ribs, you just need root and tip templates. On larger wings there is a significant weight savings over a solid foam core. (Though at this size the weight savings are small). Where I live, anything over 2” thick EPS is not readily available. So being able to make the ribs out of cheap, easily available foam board is also a plus. Another big advantage for the scratch builder is that you can drop in ribs wherever you want. (Software like Profili will draw all the intermediate ribs for you as long as they are evenly spaced.). Adding spars is easy because you cut the slots all at once with a hot wire. I find it easier cut holes for wiring and such with this type of construction than to rout channels in solid foam cores. The building boards apply clamping pressure only where the ribs are when skinning. Dihedral and washout are built into the templates.

Start by drawing the wing outline and rib locations on the building boards. I’m using ĺ” melamine coated shelving. Then glue down the rib blanks. (I used hot melt glue) In this case, I did the ribs from the nacelle to root first. I’m adding two 1/8” lite ply dihedral braces to carry the landing gear loads so I cut those slots first. The motor/LG mounts will be a pair of 1/8” lite ply plates sandwiched between (3) ĺ” ribs. The lite ply and foam ribs were clamped together to ensure the correct spacing while the ribs were glued down.

Once the dihedral brace slots were cut, I glued down the rest of the ribs and cut the main spar slot. I added a block of foam for the aileron servo too. Before adding the top building board, I used two straight edges clamped to the root and tip to make a hot wire cut to make sure all of the tops of the ribs were straight and level. Then the top board was glued to the ribs (Gorilla Glue in this case – too many ribs to glue at once for hot melt). At this point, you have the ribs (spar and brace slots cut) glued to the top and bottom boards.

Next, add the root and tip templates and make the first hot wire cut.
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Old Dec 05, 2011, 04:51 PM
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After the top cut is made, lift off the top building board and glue down the top skin. I pre-form the DT foam skin into a curve by rolling it with a piece of plastic pipe on a couch cushion. The harder you push and the smaller the pipe, the tighter the curve. Apply glue to the ribs and place the skin on the ribs. I use white Gorilla Glue. If you apply just the minimum needed to attach the skin you can hot wire through it with no problems.

Replace the top board and weight it down to clamp the skin in place while the glue sets.

With the soft DT foam, pre-forming is needed. Otherwise, the weight needed to get the skin clamped to the ribs will dent the foam. (Not a problem with balsa skins.) The building board only applies clamping force to the ribs so the skin doesn't sag between ribs.

I used Dollar Tree foam and Iíll cover it with kraft paper and WBPU. Balsa also works great with this method and makes a really strong wing. The foam and paper will weight about 25% less than 1/16Ē balsa and a light glass cloth/epoxy finish. Iím also going to splice in a section of LE to handle the area between root and nacelle and that will be easier with foam so thatís what I used.

Once the glue is dry remove the top building board and trim off the excess skin at the root and tip. The excess at the LE and TE will be trimmed off when the cut is made for the bottom of the wing.

Add the templates for the bottom of the wing. Before replacing the top board apply several pieces of double-sided tape to the rib cut offs stuck to the top building board. Carefully replace the top building board using the spar slot cuts to make sure itís properly lined up. The idea is to temporarily attach the wing to the top board to hold it in place when the bottom is cut off.

Make the cut for the bottom surface.
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Old Dec 05, 2011, 05:34 PM
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Now it's time to add internal structure and the holes for the wiring.

1/8" lite ply for the dihedral braces was slid into placed and marked. The braces are cut out and glued into place.

The main spar is a sandwich of 3mm bamboo top and bottom with 1/8" balsa between. It's made by dropping a 3mm round bamboo stick into the spar slot and sliding 1/8" balsa into the slot after it and marking. The balsa spar was cut (subtracting 3mm for the top bamboo stick) and the bottom bamboo stick, balsa and top bamboo piece glued into place. I've used this type of spar on larger planes and it's more than adequate.

I cut the holes for the wiring with a sharpened brass tube. Unlike with a solid foam core, you can see exactly where the wiring holes are going. On longer wings I've clamped templates to the ribs and cut the wiring channels with the hot wire.

The bottom skin is then glued on in the same fashion as the top. When the glue was set I set up templates to trim the LE and TE. Since the flap and aileron edges are a straight, continuous line, I made a 45 degree cut at the TE and capped it with 1/8" balsa. I'll make ailerons and flaps from balsa sheet rather than trying to use the foam cut offs. The LE gets a 1/4" balsa cap. It's possible to do the LE in foam but the balsa cap adds needed dent resistance.

The D-18 wing should really be done in 5 sections, the center and root to nacelle and nacelle to tip for both wings. But when you look at the full size airplane, there is no obvious change in shape for the back half of the airfoil. That and making the skin continuous should make a stronger wing, lead me to attempt to just splice in a new airfoil section to match the plan view.

I glued on a 1/8" balsa rib extension at the root and a brace at the LE. I cut away the top and bottom skins to make room for the new airfoil section. The I pre-formed more DT foam into the new airfoil section. You must cover the outside with packing tape before forming the foam to keep it from cracking. Once the replacement piece was formed I glued it into place. It came out better than I had hoped. The outboard part of the splice will be hidden by the nacelle and the rest blends nicely.
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Old Dec 16, 2011, 12:04 PM
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Here's a shot of how the wing panels are joined. I used the building boards to build a wing holding jig. The boards themselves are lined up and set with the proper spacing between the wing halves and hot glued to the bench. The root and tips were traced onto pieces of foam (the dihedral angle is built into the tip supports) and the foam pieces cut and glued to the building board. Now I can drop the wing halves into the jig to trim the braces. I cut one side and used it to mark the other. After a few minutes of trimming, the two halves fit nicely. The braces will be joined with thin plywood plates or maybe just wrapped with glass cloth and epoxy. The center section will be cut from solid foam and fitted around the braces.

This may seem like an odd way to do things but it will allow me to fabricate and install all the wiring and test it in place before gluing everything up. Installing the wiring this way will be easier than trying to do it with the wing assembled.
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Old Dec 16, 2011, 10:13 PM
The "pro" in procrastination
Steve85's Avatar
Canada, ON, Kingston
Joined Mar 2004
2,660 Posts
Pat,

These are great techniques, thanks for posting them in so much detail! I'll be trying the laser technique on my Lancaster, and your rudder setup looks like a great place to start for my rudders too. Please keep posting your techniques!

Steve
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Old Dec 18, 2011, 03:20 PM
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Originally Posted by Steve85 View Post
Pat,

These are great techniques, thanks for posting them in so much detail! I'll be trying the laser technique on my Lancaster, and your rudder setup looks like a great place to start for my rudders too. Please keep posting your techniques!

Steve
Thanks Steve,

Here's how I did the wing tips. Wing tips seem to take a lot of abuse so I added a strip of bamboo around the edge. The bamboo comes from a bamboo blind I found in the home center clearance bin a while back. I now have a lifetime supply of 3mm round sticks.

I made a form from some scrap 1/4" plywood. I printed out the wing tip about 10% undersized to account for spingback. I taped one end of the bamboo dowels to the form and slowly bent them around the form while applying steam from a tea kettle (wear heavy leather gloves!) After the bamboo sticks were bend around the form I taped down the other ends and let them dry over night. When released from the form they will spring back some but the shape is now fixed.

The I cut the wing tips from foam and glued them in place along with a 1/16" balsa tip rib. After rough shaping them, I cut a shallow groove in the foam and about an inch along the leading edge balsa and glued the bamboo in place. When the glue was dry, I did the final shaping of the foam. A little filler here and there and the wing halves will be ready to cover.
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Old Dec 18, 2011, 05:34 PM
Lookin' up at the centerline
taildragger1589's Avatar
Lawrenceville, Georgia
Joined Feb 2005
2,225 Posts
Just to throw in my two cents...
If you don't like to bend bamboo with steam, it can be easily shaped by bending it over a soldering iron.
I've made more than a few proctor kits using this method.

Nick
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Old Dec 18, 2011, 09:58 PM
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Quote:
Originally Posted by taildragger1589 View Post
Just to throw in my two cents...
If you don't like to bend bamboo with steam, it can be easily shaped by bending it over a soldering iron.
I've made more than a few proctor kits using this method.

Nick
True, especially for free hand work! A heat gun works too. I've done a lot of steam bending over the years; building canoes and furniture and find it enjoyable.

Pat
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Old Dec 19, 2011, 09:25 AM
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I covered the wings with 30lb kraft paper. I first gave the wings a coat of WBPU (I used Carver Tripp "Safe and Simple") and let it dry to raise the grain of the balsa and give a harder surface to the foam. Then I sanded with 240 grit paper.

I cut out the paper and misted it with water to wet it. Not soaking wet, just enough to get it to relax and stretch out. Then I gave the side of the paper that will be against the wing and the wing itself a coat of WBPU and applied the paper. The paper is overlapped at the TE. I added an extra layer of paper where I spliced in the area between the nacelle and the root to toughen it up because the LE is just foam in that area. When everything was smooth, I gave the wing a final light coat of WBPU and hung the wings from the dihedral braces to dry vertically in still air. It's important to let the wings dry evenly on both sides to prevent warps.

For the tips I used small binder clips to clamp the paper down while it dried. I'll cut away the excess when it's dry and cover it with a strip of bias cut fiberglass cloth and WBPU. I've cut slits in the paper and overlapped it in the past, much the same way you would do a heat shrink covering, but I still got wrinkles and bumps and ended up covering the edge with a strip of fiberglass cloth anyway. I think I get a better result doing the tips this way. You can see in the last picture how nicely the paper molds to the edge.

The weight of the two wing halves is 238g. My "budget" is 300g for the completed bare wing so I've got about 60g to spend on the center section, flaps, and ailerons. Because the wet paper shrinks as it dries, the wing really stiffens up. I dropped it back into the wing holding jig to check for warps and it's good.
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