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Old Sep 07, 2014, 05:39 PM
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United States, MI, Grand Ledge
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How do you layout fuslage skins?

When building up the skin of a fuselage, what technique do you use for generating a skin pattern? Cross section not box.
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Old Sep 08, 2014, 10:06 AM
AndyKunz's Avatar
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Are you asking how to design formers?

Andy
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Old Sep 08, 2014, 11:22 AM
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For sheeting I start with tracing or parchment paper. You can see through it to get locations on the fuse formers. Once I have a close fit I go to medium density cardboard (like gift box cardboard) and adjust the fitment to be very close. Bend the cardboard to the fuse and make any final adjustments to the pattern. Cut the balsa give it a light shot of windex on the outside surface and form to the fuse. On scratchbuilt planes I keep the cardboard patterns in case I build the model again.

Here is an example on a Sukhoi I'm building now in this balsa section.







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Old Sep 08, 2014, 11:23 AM
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Hi, Andy. No, I already have formers and the longerons attaching them together, the skeleton, if you will. Now I want to bend sheet metal (really wood) that will cover that frame. Because of curvatures, and the sheet metal starting out flat, I will have to piece together and bend to create the shape I want. Think of all those rivets on a metal P51, P38, whatever.

Now, working in wood isn't nearly as flexible as metal, not much stretching or other forms of bending can we do, never mind the complexity of grain.

So I wonder, since I now face this challenge, how do others tackle skinning a fuselage. No planking allowed, unless you can convince me that layering up a bunch of planks will weigh equal to or less that a thin skin. Don't forget the glue!

A step by step procedure would be way helpful. I am very dense at times.
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Old Sep 08, 2014, 11:38 AM
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Dpetsel,

Thank you. So you use two passes with paper. The light one gets you close, the heavy one for durablity? Have you found that the cardboard and the balsa share about the same characteristics for bendability? In other words if I can get the cardboard to fit properly, will the balsa or ply fit ok as well? I see also copious use of notes on the template. I'm beginning to be a true believer in that and photos. Best thing that ever happened was the digital camera. You are skinning with what thickness on the balsa? Heaviest curve goes cross grain? Do you do anything with a plywood underskinning, reinforcement? If so you use same template for ply and balsa? Or is the ply in a physically different location within the former? What types of glues do you use for these procedures?

Lots of questions, sorry, but this is the outside of my plane. I want it right and I want it light. Maybe I can come back with more questions?
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Old Sep 08, 2014, 11:40 AM
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As with my pics above I mention Windex. Step 1. Take a piece of your sheeting balsa and spray windex on the outside surface. Step 2. Bend that piece of sheeting in your hands gently to get the starting bends that will conform to your fuse. Step 3. Fit to you fuse lightly and continue to make bends (gently) to conform to any additional bends. Step 4. I usually push on my sheeting lightly to make sure I am contacting all of my formers uniformly. Step 5. Once I am satisfied with my contact and fitment I use very light clamps to hold it in place for the finished contours.

If you were careful with the Windex and just got it on the outside of the sheeting it should only take about 10 to 15 minutes to dry.

Glue it in place and start the template for the next piece of sheeting.

You can also use velcro straps wrapped around your sheeting (on the formers)to help shape your sheeting.
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Old Sep 08, 2014, 12:05 PM
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What happens when the bend is too much, either compound or simple? I imagine that the pattern would need to be redefined by adding in stringers where the worst bowing occurs, and the separating the original pattern into 2 or more so that the curvatures aren't so pronounced? Also seems to me that locating the line of maximum curvature might be a challenge. Will this problem show up while working with the parchment paper? Be nice if it did.
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Old Sep 08, 2014, 12:44 PM
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Quote:
Originally Posted by sdad View Post
No planking allowed, unless you can convince me that layering up a bunch of planks will weigh equal to or less that a thin skin. Don't forget the glue!
I find planking to be the best solution for compound curves. It's fast, easy, and best of all, super light! (Even including the near-zero weight of Titebond).

I have this secret weapon called sandpaper, and then follow it with another secret weapon, a flexible sanding block. It gets rid of all the "panel lines" you get from sectioned sheets much more nicely.

The few places that have sectioned sheets, I use cereal box cardboard to make templates. That would be for places like wing roots, as shown above.

Andy
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Old Sep 08, 2014, 07:56 PM
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I sheet everything.
There are more compound curves in my Seabee than a waterslide.
I used 1/16 sheeting on this build. I use velcro straps to form the sheeting on the formers. If there are recess curves I use the velcro straps with styrofoam shims to form the reverse curve. To glue the finished sheeting on the fuse I will use Sigbond or Titebond. Setup on the Sigbond is very fast so be sure you have all of the registration correct before you lay in your sheet. For sheeting on wings I will usually use Medium CA on all formers stringers and spars and set the sheeting onto it. I will lay out all of the wing sheets and CA all of the butt edges before I trim the sheeting. I then sand all of the sheets together for uniformity of thickness and to clean up the butt edges before I apply the sheet.

Don't use CA if you are not comfortable with the process. Use Titebond as it will give you more time to align the sheeting (or remove it if it breaks or wasn't cut correctly)


Here are some crazy compound curves on a 40" fuse.












Here is the Seabee painted.
Only sanding sealer and high build primer used to fill grain.


You can use 1/2 oz fiberglass and resin.
You can use 1/2 oz glass and the West System Epoxy
You can use 1/2 oz glass and Minwax polyurethane
You can use standard shrink covering Ultracote/Monokote







I used Medium CA on this wing.
2 sheets 6"x 48" butt glued and sanded. I needed 1" extra in the corner so I spliced it in with a butt joint.

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Old Sep 08, 2014, 08:12 PM
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With full disclosure..........
If I have a super tight bend I have used planking. I will try sheeting first. If I can't get the balsa to make the bend without breaking I will plank it. It has to be a pretty small radius though.
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Old Sep 08, 2014, 08:33 PM
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Dpetsel, Before coming to this thread, I spent some time looking over your SeaBee build. First thing that came to my mind was the complex curve on the rear fuse. You pulled it off very well! Cudos.

On the SeaBee at least, I see that formers and stringers in place prior to any sheeting. No need to add in additional stringers along the way? Any multi layer work?
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Old Sep 08, 2014, 08:51 PM
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AndyKunz,

I have spent some time doing fine scale boat modeling. A very common technique is to do a 2 layer hull. An interior layer done with a thicker strip will give the hull shape and rigidity, and a thin layer of a more exotic wood on the outside. Boat hulls have compound curves that give a guy nightmares, convex into a concave. Yikes. As you said, sanding these small strips into a smooth curve is quite easy. But I am using hardwoods, also.

So back to the plane, what thickness of strip wood do you typically use? Would 3/32 be adequate on an electric 40 sized ship? I am somewhat concerned that the strip wood wouldn't offer up the strength and rigidity that a semi monocoque will.
How about this? What if I double layered the fuse like my boats, but use a strip of 1/32 ply for the bottom layer, 1/16 balsa strip on top? The ply will certainly pick up the buckling loads that my pattern plane will create, and yet give me the formability of balsa.
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Old Sep 08, 2014, 09:06 PM
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Quote:
Originally Posted by sdad View Post
Dpetsel, Before coming to this thread, I spent some time looking over your SeaBee build. First thing that came to my mind was the complex curve on the rear fuse. You pulled it off very well! Cudos.

On the SeaBee at least, I see that formers and stringers in place prior to any sheeting. No need to add in additional stringers along the way? Any multi layer work?
No additional stringers.

1/16 balsa would be fine on a fuse for strength but impossible to carry around without damaging it. 3/32 soft balsa will work fine on most larger fuselages and gives the strength to handle the model.


It might be easier to give you info if we knew the .40 size model you are building?
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Old Sep 09, 2014, 11:30 AM
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Quote:
Originally Posted by sdad View Post
I have spent some time doing fine scale boat modeling. A very common technique is to do a 2 layer hull. An interior layer done with a thicker strip will give the hull shape and rigidity, and a thin layer of a more exotic wood on the outside. Boat hulls have compound curves that give a guy nightmares, convex into a concave. Yikes. As you said, sanding these small strips into a smooth curve is quite easy. But I am using hardwoods, also.
That's exactly what I'm doing on my Baby Bootlegger. 1/16" mahogany over 3/16" balsa.

Quote:
So back to the plane, what thickness of strip wood do you typically use? Would 3/32 be adequate on an electric 40 sized ship? I am somewhat concerned that the strip wood wouldn't offer up the strength and rigidity that a semi monocoque will.
I just finished a 40-sized plane with a planked chin and top. The top separates into front (hatch) and rear (fixed to fuse). I did it all with 1/8 x 3/18" strips, then separated the hatch with a knife. Both parts are amazingly strong for their weights.

BTW, the flat fuse sides are 1/8 medium balsa. A 1/4" balsa stringer is at the edge where the top and chin meet to them.





I'll be publishing the plans and construction soon.

Andy
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Old Sep 09, 2014, 08:14 PM
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Nice looking build Andy.
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