stunthanger.com
Building Tips and technical articles. => Building techniques => Topic started by: Ron Varnas on March 04, 2006, 04:34:34 PM
-
Hi, I'd like to know the best method for shimming the "inside" cores of Foam wings? A lot of the guy's that have made foam cores can relate to this.. meaning
the core centres on most foam wings arrive with "play" or "slack" they rattle inside the wing , not a tight fit , this is because of the thickness dia. of the wire
cutter eg: if the wire is 1/16" the final gap around your inside core would be 1/8"
Theres noway you could compress expoxied wing skins onto cores that have this
gap inside........otherwise the true airfoil your after would come out like a "prune"
My idea is to shim that gap between the inner cores and wing with thin strips
of mylar before the skins go on , to actually fill the space left by the wire cutter.
I'd appreciate anyones methods or tips on this ???
-
I would think that masking tape would work.
-
Why not core ?
-
>>Why not core?
How can you replace core section thats been melted away in the cutting
process? even if the cutting wire was 1/32 in dia, you'd have a nice 1/16 gap
all around ........correct?
-
I don't know what happened. The question was why not core them after they are sheeted?
-
To core wings after sheeting is probably the best solution, but if your tooled
up to do this , most likey you'd make your own cores anyhow.
My original question was on already "cored out " wings as you'd get them
from the supplier.
I think Chris is on the right track with the tape.
-
Shim the gap with 1/16th balsa. Or whatever thickness you need to keep the wing from being crunched by the outer shell. The balsa you use will not be so damaged you cant use it on your next airplane. I think you will use alot of tape to take up the gap. You should only need to shim one side. Use strip about 1/4wide at the sharper bends of the leading edge.
use a bit of tape to hold the wood to the core while inserting into the wing. should be easy.
Greg
-
Strange, but I have never had the need to shim the cored out pieces before. (??)
No problem with the wing crushing or otherwise. What am I doing wrong?
-
If the wing is tapered you can just push the cores peices a little further into their holes. They'll take up the space accurately enough and if you don't have enough length to trim off the unsupported 1/2" or so of centre section you can trim up a slice of foam to temporarily fill that gap.
-
I think a number of the methods described all have merit, but unless someone
can really convince me , that the foam airfoil magically comes back to it's proper
shape after it's had a very decent load placed all over the wing panel in the
skinning process, then I'll shim it with what ever it requires ::)
-
Ron,
Interesting topic and replies...
The gap is more likely more like a saw kerf than an actual image of the cutter wire. Saws with teeth cut the material to at least the full width of the offset of the teeth, often a hair more.
A hot wire cutter cuts by melting the foam, and that melt width is wider than the wire. It depends on the heat and rate of motion.
For the relatively few foam wings I've cut, I've used NiChrome wire of less than 0.01" diameter, or occasionally clean, new braided flyiing line of about 0.012" diameter. Both work. I've never used 0.031" or 0.0625" as cutting wire.
\BEST\LOU
-
I think a number of the methods described all have merit, but unless someone
can really convince me , that the foam airfoil magically comes back to it's proper
shape after it's had a very decent load placed all over the wing panel in the
skinning process, then I'll shim it with what ever it requires ::)
squishing the foam is a pounds per square inch phenomenon. It really doesn't take much force to keep the sheeting down snugly on the foam while the glue dries. 50 lbs of weight on a 750 sq.in. wing is only 10 lb. sq.ft. Standard 1lb. white foam normally is spec'd to take a minimum of 10 psi(144 lb/sq.ft.) without permanently deforming. By comparison, running a thumbnail over the foam can easily generate over 10 psi and mark the foam.
That said, shimming the foam cores by adding masking tape to them won't hurt anything, as long as you don't go overboard and deform the outer surface of the wing by bowing it out.
-
Good answer Phil C, so how many pounds in weight would you put on say
a 30" wing panel for the sheeting process? ???
-
I don't sheet wings. I have methods I like better. Somebody else mentioned 50 lb. on a 750 sq.in wing. 20 lb/sqft should be more than enough. Personally, I'd use something like Elmer's Ultimate polyurethane glue because it foams up and you need less. Push the cores in until they just fill the gap, as Ash says. Use some tape on the root edges to make sure the foam doesn't separate from the sheet. Lay the wings out on a table as they will fit on the plane, left and right, and then clamp with concrete blocks. Make sure the blocks "balance" on the wing so the load isn't concentrated for or aft and twisting the wing. Once the blocks are on take a close look to make sure everything settled down smoothly and there aren't any curves in the LE or TE.
Once the glue is dry, inspect the skin carefully for bubbles or unglued areas. Tap it lightly with a pencil or something. Unglued areas will sound echo with a different note. Try gently flexing and bending the assembly and listen for creaks or cracks. If it cracks the same way every time you bend it something is moving and not glued down well. bubbles up to 2x3 in. or so can be glued down by injecting a few drops of foam safe CA and holding until its dry. If there are more than a few bubbles, if they are large, and if they are near the center and high point you didn't get a good bond. Make another wing.
-
>Strange, but I have never had the need to shim the cored out pieces before. (??)
No problem with the wing crushing or otherwise. What am I doing wrong?
Me too. I'm not going to stay up all night worrying about it though.
Actually, I usually used the "polymerizing aliphatic glue" method given me by a "Gasp" great RC builder. Tom Dixon had an article on this at one time in stunt news. It works! and no weighting down anything. Leave the cores in, be sure wing and skins are clean, spread the glue with a playing card on skin and wing, let it dry for an hour or two, carefully align the skins and lightly go over the skins with a clothing iron at the right(pretty hot-I use wool setting). Instant permanent bonding if you do it right.
Remove the cores, install LE, TE, trim it all up and get ready to install controls and get the two halves together by your favorite method.
Bonus: the aliphatic glue is very light after it is dry.
Jim
-
Use constructon paper the heavy kind WORKS GREAT
-
Actually, I never have considered the questions here.
The "I-beams" left in the panels when they are triple cored take quite a bit to compress much. Since we use about 50 or so lbs of weight spread out over the panels, I have just not seen any deformation of the panels.
Guess I really am the "blind hog". :)
-
Where do you get your Cores from Bill ???
-
Where do you get your Cores from Bill ???
Hi Ron,
My foam wing panels come from Randy Smith/Aero Products.
-
As a matter of interest are these Jim Pearson cores? :)
-
As a matter of interest are these Jim Pearson cores? :)
Actually, Ron, I have never asked Randy aboiut who is cutting his cores. Might be Ronnie Farmer. (???)