thenated0g
Very Strong User
On my big 78" starfire i had the same problem. I decided to bag them separately, 4 cores, and than fiberglass tape the seams top and bottom.
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Obelix Build Log
- Thread starter Mucksmear
- Start date
Doc J
Very Strong User
I developed a nice bag method millions of years ago, well, about 1970.
This works well for straight TE panels, and can with a little jiggery also do slight curves.
OK: I used to use melamine (formica?) leaves to bag the wings in order to provide a really nice finish and an arrow-straight TE.
If you cut a couple of "leaves" very slightly larger than the wing to be bagged (Not so easy on multiple tapers I have to say) then put them close together and close up the TE side with scotch tape or similar, you can then use release wax on the leaves which are now an "open book" and paint them before bagging. The idea is that you can "soft mould" the cores to a great finish using the melaimine leaves.
To bag, just add your fabric to the leaves, not the cores however you want it, and then slip the dry core(s) inside. Then bung the whole assembly into the vac bag and proceed as normal. There is a small amount of "squish" that will allow you to push the core(s) towards the TE, if needed and rearrange them nicely in the "book" leaves, and in good position on the foam shucks before switching on the vacuum.
Works pretty well and you get a nice straight wing with a sharp TE (sometimes too sharp) and a coloured hard finsh.
TE is easy to finish with a sanding block, but the LE takes a little more time.
Old skool...but sometimes good skool
Doc.
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Interesting Doc, your melamine leaves method sounds similar the method of using pre-painted mylar sheets in the vacuum bagged layup? Only with melamine u ensure a very straight TE. The melamine is flexible enough to curve against the airfoil even towards the narrower wingtips? I’ve tried neither method so I’m just guessing here.
Doc J
Very Strong User
Hi Muckster,
Point 1. Yes you are on the button. Mylar sheets work great for balsa or really hard foam structures but not so well for softer stuff which crushes.
Tip: Its best not to reduce further than 13PsiG per sq In, I.E. less than one atmophere, when using all but the hardest white foam. If you do a quick sum and add up the wing area x 13 (less than one At) you will soon find that even a small wing panel can have a pretty large amount of pressure.
Example for a 60" x 12" power plane wing panel:
60" x 12" = 720 in/2 x 13 = 9360 lb (It looks even worse in metric) which is over 4 tons on each side.
That would be hard to achieve using lumps of lead, extra kids, wives, or other heavy objects from around the workshop.
Point 2. Second on the button: The TE produced by this method is very straight and sometimes a bit sharp, and the entire surface if the wing is normally really straight too. Of course the shinier the malamine (or other) is, then that surface will be reproduced.
Point 3. Yes, Melamine is OK for small wingtips - I used to use it for tailplanes too.
If you are a serial builder, like Nate, you can of course use the leaves again, or if not then cut them down for smaller panels.
Cheers,
Doc.
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Elliot, is that 2 or 3" pink foam I see in the background? You probably told me, but where do I get some of that?
We finally have the bugs worked out of our CNC and I NEED some of that. Actually, I just need a source for it.
2" thick.
I found it at the Home Depot Pro in Colma:
91 Colma Blvd
Colma, CA 94014
Important note: At the time, I was unable to find un-scored sheets. This foam is pre-scored to aid installation between wall studs meaning a single 4'x8' sheet will actually end up as 8 panels like so:
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| | | | |
| | | | |
------------ <-- (EDIT: this one may/may not exist)
| | | | |
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Yielding
4 panels @ 48"x16"x3"
and
4 panels @ 48"x8"x3"
EDIT: if the mid span score does not exist, then it’s actually
2 panels @ 96”x8”x3”
And
2 Panels @ 96”x16”x3”
While not ideal, I figured most sport and scale double taper wings would fit within these sizes. Added bonus for me was this made it easy for me to cut into pieces that fit inside my car - i don't have a truck.
Owens Corning FOAMULAR NGX F-250 2 in. x 4 ft. x 8 ft. SSE R-10 XPS Rigid Foam Board Insulation 52DDNGX - The Home Depot
Owens Corning FOAMULAR Scored Squared Edge Insulation Sheathing is perfect for use on wood, steel framing or concrete walls.
www.homedepot.com
I found it at the Home Depot Pro in Colma:
91 Colma Blvd
Colma, CA 94014
Important note: At the time, I was unable to find un-scored sheets. This foam is pre-scored to aid installation between wall studs meaning a single 4'x8' sheet will actually end up as 8 panels like so:
------------
| | | | |
| | | | |
| | | | |
------------ <-- (EDIT: this one may/may not exist)
| | | | |
| | | | |
| | | | |
------------
Yielding
4 panels @ 48"x16"x3"
and
4 panels @ 48"x8"x3"
EDIT: if the mid span score does not exist, then it’s actually
2 panels @ 96”x8”x3”
And
2 Panels @ 96”x16”x3”
While not ideal, I figured most sport and scale double taper wings would fit within these sizes. Added bonus for me was this made it easy for me to cut into pieces that fit inside my car - i don't have a truck.
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Indeed - I was surprised to see how much weight I could/should be using. The DIY vacuum bag system I used on my Coyote build is a diagram based aquarium air pump that probably only pulls 3-5 in Hg or only about 1.5 psi. That would be 363 pounds on this Obelix’s center panel alone. Using cinder blocks and magazines I might be at only half of that.
Doc J
Very Strong User
Oh crap! This is unfortunate. On this build, I did not tape the skins down along the edges prior to placing the top core bed and stacking the weights. This combined with the overly thin nature of the veneer allowed some chord wise slack to get trapped as I added the weights. I lucked out with the bottom skin and the other two panel top skins. I will try to surgically remove the offending section and re-skin.
Side note: these skins are just over 1/32” thick poplar. I would have prefer 1/16” and possibly the lower density of a different species but this what I found at the time.
Side note: these skins are just over 1/32” thick poplar. I would have prefer 1/16” and possibly the lower density of a different species but this what I found at the time.
Sliced the offending sections and managed to remove 2 pieces with minimal disturbance to the cores.
A reapplication of epoxy and some tape to hold the pieces in place and just taught enough to avoid trapping any wavy slack prior to stacking on the weights.
Just removed the weights and it looks good enough to move on to the next steps.
A reapplication of epoxy and some tape to hold the pieces in place and just taught enough to avoid trapping any wavy slack prior to stacking on the weights.
Just removed the weights and it looks good enough to move on to the next steps.
Onwards! I marked the cut lines for the elevator and ailerons and used a metal straight edge and a single edge razor blade to just slice through the poplar veneer avoiding a deep cut into the foam. Once both top and bottom surfaces are cut it’s a simple task to carefully slice through the foam using the cut edges of the veneer as a guide. If I’m careful enough, there is minimal sanding to true up the exposed face.
The elevator and aileron cutouts get a 1/8” sub-TE. The long piece of tape running parallel to the sub-TE reduces the stain/mess of glue squeeze-out.
A small sanding block 1/2 masked off with tape easily sands down the excess sub-TE material until it is flush with the wing surface with no danger of over sanding
The elevator and aileron skins will be trimmed back to provide the required bevel prior to receiving their respective 1/8” LE strips.
The elevator and aileron cutouts get a 1/8” sub-TE. The long piece of tape running parallel to the sub-TE reduces the stain/mess of glue squeeze-out.
A small sanding block 1/2 masked off with tape easily sands down the excess sub-TE material until it is flush with the wing surface with no danger of over sanding
The elevator and aileron skins will be trimmed back to provide the required bevel prior to receiving their respective 1/8” LE strips.
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Hard balsa side ribs epoxied to the exposed sides of the center panel above and behind the outer panel roots. The ends of the control surfaces and their cutouts are capped with 1/16” balsa. At this point all exposed foam has been clad in balsa except for the wingtips, LE and fuselage pod cutout areas:
Rough cut and initial shaping of the block balsa wingtips:
When sanding down blocks of balsa on the belt sander, things often get out of square from inadvertent unequal pressure across the block. Last thing I want to do is sand beyond my target surface. Here’s an idea - stick a bubble level on the material to get live feedback on the sanding angle! -
Rough cut and initial shaping of the block balsa wingtips:
When sanding down blocks of balsa on the belt sander, things often get out of square from inadvertent unequal pressure across the block. Last thing I want to do is sand beyond my target surface. Here’s an idea - stick a bubble level on the material to get live feedback on the sanding angle! -
1/2” basswood LE made from laminated 1/4” stock. I often stack these 1/4” thick scraps to create load spreaders for the limited number of clamps I have:
The outer panel LEs taper in height towards the tips:
The center section LE extends outwards and overlaps the outer panels with a nice 5” splice shown here taped up for a test fit:
The outer panel LEs taper in height towards the tips:
The center section LE extends outwards and overlaps the outer panels with a nice 5” splice shown here taped up for a test fit:
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thenated0g
Very Strong User
I almost hope you clearcoat that thing. Looking good.
Hawaii Sloper
Very Strong User
Really enjoying this build.
The strip of tape on a sanding block trick also works on a plane with a minor tweak. I added a couple strips of tape to one edge of the plane just ahead of the blade and then a couple of longer strips that extend over the blade. The number of initial strips of tape needed will depend on how far your blade sticks out of the plane. The tape prevents the plane from gouging the wing skin, but needs to be inspected more frequently than a tape/sanding block tool because that blade can easily pierce through the tape and then gouge the surface you are trying to protect.
With the roughly shaped LE now attached to the wing, I planed it down to just shy of the final profile.
The final shaping to blend the LE smoothly into the wing is done with an 80 grit sanding block with the strip of tape to avoid sanding the veneer.
A straight edge with a light shining from above and behind casts a shadow that outlines the profile. It’s really easy to spot high and low spots and discontinuities in the LE transition.
With the roughly shaped LE now attached to the wing, I planed it down to just shy of the final profile.
The final shaping to blend the LE smoothly into the wing is done with an 80 grit sanding block with the strip of tape to avoid sanding the veneer.
A straight edge with a light shining from above and behind casts a shadow that outlines the profile. It’s really easy to spot high and low spots and discontinuities in the LE transition.
Faced the fuselage cutout area with 1/32” ply.
The fuselage pod is a basic box structure. I like to add triangle stock for rounding although I’m this case I will need to resolve how the rounded corners interact with the top and bottom of the wing - probably a big epoxy/cabosil fillet.
The fuselage pod is a basic box structure. I like to add triangle stock for rounding although I’m this case I will need to resolve how the rounded corners interact with the top and bottom of the wing - probably a big epoxy/cabosil fillet.
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Progressive chamfers measuring left/right and top/bottom to maintain some semblance of symmetry. 80 grit makes fairly quick work of it. The hard part is outlining my intended chamfer lines along the curved tapering nose in a symmetrical manner.
I have found that key to sanding smooth rounds is using a single light source (or the Sun) and rotating the part so I’m always sanding along the terminator.
I have found that key to sanding smooth rounds is using a single light source (or the Sun) and rotating the part so I’m always sanding along the terminator.
