TWF Techniques: Sparless Wing Joiner Tube Installation (Part 1 of 4)
Posted by J.S. Decker on Dec 30, 2007, 15:01
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My goals with my sparless wing joiner tube installation technique are 1) to minimize time spent preparing the cores for vacuum bagging the skins on, 2) result in a sufficiently robust structure, and 3) be simple to execute. Using the technique described below, I can have an Ultron, JARTeron, or Rotor prep’d for bagging with a few hours of work, if I do not make any mistakes that require additional time to fixed. I have yet to have a wing in-flight failure, and for those less than optimal landings, this joiner system performs better than a conventional plywood rib and sub-rib system. And, the technique does not require any special jigs or other tools not found in the typical scratch builder’s shop. Here is the step-by-step procedure for joiner tube installation that I use...
1. Mark the reference line(s) on the cores. I like to visualize what I am about to do before getting lost in the details associated with preparing the cores. I find that marking the reference lines on the cores first helps me with this visualization. I mark three things, first is a reference line on the right and left wing core that is used to align the right and left wings to each other. The idea is to have a line on each side of the wing that you can sight down or lay a straight edge one to align the wings relative to each other. The second line that I mark is the centerline of the wing joiner tubes. And, the last line that I mark is the high point of the airfoil, which helps me align the root ribs when gluing them to the cores.
Frequently, two or all three of these three lines will be coincident. Such an example is my JARTeron wings, where the wing alignment reference, joiner tube centerline, and airfoil high point are the same.
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| Poster board template used to block foam, locate reference lines, and cut laminate. |
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| The hole in the template on the reference is used to mark the cores. I later connect the marks with a line using a straight edge. |
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| Wing half lined up with the reference line. |
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| The joiner tube reference line is in front of the wing alignment reference line for this set of Xica wings. |
2. Mark the root rib angle. The angle of the root rib is dependent upon the shape of the fuselage, which defines how the wing fits to fuselage and the thickness of the root rib needed. My general rule of thumb is that if the fuselage tapers more than an 1/8 of an inch from the wing LE to TE, I adjust the angle of the root rib to match the taper of the fuselage. If the taper is less, then I simply trim the root rib to match the fuselage shape after vacuum bagging.
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| Some planes are require that the root rib be installed at angle to match the taper of the fuse, like the Xica wing half on the right, while the ORCA wing half on the left is installed perpendicularly to the reference line. |
3. Make a root rib sandwich. Balsa wood is easier to sand than plywood. The bond between end grain balsa and composite material is comparable to the bond between plywood and composite material. So, I use a thick root rib that is a balsa-plywood-balsa sandwich. The idea is to have more composite skin to root rib contact area to transfer loads, while making the root rib easier to sand to the foam core. In my opinion, this beats slapping a 1/8 or ¼-inch piece of plywood on the root, because the larger contact area of the wider root rib helps reduce damage to the LE and TE of the wing root associated with wings moving back and forth on those less than perfect landings. It is also easier to sand the balsa-ply-balsa rib to the match the cores.
Typically, I use a root rib that has 3/16-inch balsa attached to both sides of the 1/8-inch 5-ply plywood. The resulting root rib thickness is ½-inch, a nice round number that is easy to work with. If my fuselage tapers a lot or has a funky curved surface around the wing pivot point, I add additional balsa thickness to the fuselage side of the root rib, since that is where material will be removed with the wing root is trimmed to match the fuselage profile after vacuum bagging.
The balsa is edge glued with Ambroid (works great if you have not tried it) or Titebond (aliphatic resin) to make a sheet of balsa root chord width or wider. The balsa is attached to the ply wood with spray adhesive, e.g., 3M77, and tossed into a vacuum bag for a half an hour or longer. I typically, make slabs of root rib sandwich that are big enough to build at least a half-dozen planes.
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| Balsa-ply-balsa sandwich root rib. The grain of the balsa is oriented so that the end grain touches the laminate skin. |
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| Balsa-ply-balsa root rib stock in a vacuum bag after gluing the balsa to both sides of the plywood with 3M77 spray adhesive. |
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