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Carbon Tubes for Spars

Old 02-04-2006, 04:00 PM
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jwreffner
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Default Carbon Tubes for Spars

I'm building the Haffke 1/4 scale Gee Bee Z. The spruce spars listed in the plans just aren't going to cut it (for a variety of reasons). So I'm thinking about going with carbon tubes for the fore and aft spars. Does anyone have any experience using them for spars and what is the perferred method of bonding them to the ribs?

Thanks,
Jay
Old 02-05-2006, 02:32 AM
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BMatthews
 
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Default RE: Carbon Tubes for Spars

Tubes for spars are not a great option. If you look at how the forces are loaded in a bending spar all the loads are at the extreme top and extreme bottom. This means only about 1/4 or less of the tube is providing the proper spar function. In addition the curved fore and aft portions are not in a position or of a proper shape to provide good support (as in shear webing) to the small portions of the tube that is actually doing the work.

I don't know what your reasons for not using the plans are but I would suggest you reconsider. The best spars are of I beam like cross section or a box cross section with the top and bottom elements of proper materials and sized to withstand the comppresive and tesile loads that will occur. Henry followed this pattern in all of his designs that I'm familiar with and it works.
Old 02-05-2006, 10:14 PM
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jwreffner
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Default RE: Carbon Tubes for Spars

No he did not use the I-Beam construction technique with the Gee Bee Z. It is a solid spruce stick running through the middle of the ribs. I'm not too found of cutting out an internal hole in all of the ribs to accomodate a spruce spar. Plus 1" x 3/8" spruce spars are impossible to find.

Any thoughts then?

Thanks,
Jay
Old 02-06-2006, 12:56 AM
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Default RE: Carbon Tubes for Spars

Ah, I see. That changes things then. I hate cutting out holes of that sort in the center of the ribs and feeding them over long sticks too.

If he did this to keep the spars buried below the covering so it would keep a smooth look then you'll need to consider some options.

One would be to use a pair of smaller carbon tubes located just below the upper edge and just above the lower edge of the ribs with the outer skins of the tubes more or less in the same spot as the top and bottom of the spar. Then shear web the tubes together with vertical grain 1/8 balsa in between the ribs. This at least keeps the rib holes to round ones that would be easy to do. With tubes of about 3/16 to 1/4 diameter you'll see them working as top and bottom caps in an adequite if not ideal manner. Not a perfect solution but if you were to test it a built up like this would be likely a little stronger and stiffer than the spruce and probably a little lighter. It's a bit more work to fit and glue in all the shear webs mind you. Maybe an hour per wing for this one additional task?

The other method that I'd suggest is also a bit more work. Cut spar slots for 3/8 x 1/8 flat laid good quality spruce top and bottom spars but cut them about 3/16 to 1/4 deeper than you need or as required to position the spruce spars at the same location as the top and bottom of the original planned spar. But if you can keep them further apart with ease then do so. The deeper the spacing the more strength and stiffness the spar has. Here again you need to web the spars. 1/8 vertical grain fitted between is ideal but in a pinch 1/16 for and aft glued to the faces will work but use a flexible glue like aliphatic resin and clamp all the joints to ensure a TIGHT glue line. In this case to fill in the deep spar notches you need to glue in little fillers and sand them to match the rib curve. TA-DA! Buried spars without needing to feed them through the ribs. The use of the 3/8 wide flat spruce is all you need. In the case of the solid 3/8 x 1 spar the upper and lower 1/8 inch of the spruce was carrying about 90% or more of the spar loads anyway. The big bit in the middle just keeps the upper and lower parts from collapsing together.

You like either of these?

Ideally if you want to use carbon the top and bottom elements should be rectangular flat sections of carbon strip rod of something like .050 x 3/16 inch with a vertical grain 3/16 webbing between them. Now that would be a spar that would hold up and be super stiff and far lighter than the spruce spar it replaces. The carbon flat rectangular section could beinserted through 3/16 holes drilled in the ribs and not glued to the ribs until you glue it to the precision table saw cut sections of vertical shear web material. Glue in the webbing with slow cure epoxy use some method of small rubber bands to wrap around the whole spar and lock them together at the top as clamps. A short length of 1/8 sq could be used as a toggle to do this. Feed the one end loop through the second end loop and put a 1/2 inch length of balsa through the first end loop so it toggles the second loop much like those toggle buttons on some jackets. After it all cures lock the spar to the ribs with a little dollop of epoxy mixed heavily with microballons to form a nice putty like consistency. This produces a great gap filler that is not too heavy.

Actually of the three I like the last one best. It's a far better and lighter spar than either the spruce or the big section carbon tube. It leaves the ribs with far less damage by limiting the intrusion to just two 3/16 holes. Although it took a lot to write it there's also the advantage that you don't need to fit and glue all the rib fillers and sand them down like with the spruce method. And finally even with the use of all the epoxy the spar and glue will be far lighter than the spruce spar and glue needed to hold it into place. If the orginal spruce spar was intended as a mount point for some hardware then just glue 1/16 plywood onto the webbing where required. Or if it's for a through spar fitting then perhaps 1/32 on each side. If it's bolts that will put crush loads on the balsa webbing then drill out that area of the webbing with a 3/8 drill or brass tube hole saw, insert a short 3/16 long section of good fitting hardwood dowel and then cap with 1/32 ply pads on either side. Now that's a strong point that is light but will completely resist any local crushing loads within reason.

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