Created 25 Oct 00 MD Edited 17 May 01 MD Sparcap/Web/Rib Assembly ------------------------ Bevel the ends of each web segment to exactly match the "sweep" of the ribs. Likewise for the joiner ply webs. All the webs in one panel have the same angle, so it makes sense to make little mid-panel and tip-panel angle sanding jigs by double-sticking small wood blocks down on the table. There must be NO gaps or voids between the webs and ribs. Test fit the web/rib stack-up before gluing. 0.014" bagging Mylar is an ideal material for the necessary 0.015" (close enough) shim under the bottom sparcap. Cut it into strips the same size as the sparcaps. Use the slowest epoxy you can find for the assembly. Laminating resin is OK, since it will be thickened. The small tip panels are built first, which will allow you to judge whether you have enough pot life for the bigger mid and center panels. Thicken the epoxy with Cabosil until it gets difficult to apply. It should hold its shape like Vaseline. The best applicator is a disposable epoxy brush (with the rolled metal handle). Trim the bristles to half their normal length to make the brush stiffer for the thick epoxy. A passable applicator is a square balsa stick the same width as the spar (1/2, 3/8, or 1/4) trimmed at a bevel. - - - Addition: 17 May 01 MD Before doing the midpanel web/rib/spar assembly, glue the ply/rod joiner units to the mid-panel bottom sparcap, aligning them carefully over the plan. This is the bond which includes the mat or glass in the bond line. The main assembly can then proceed as soon as this bond sets up enough to handle. This pre-bonding of the joiner rod units in the correct alignment eliminates the possibility of them squirming out of position when the main assembly is weighted down. Checking their alignment after weighting is difficult since the view of the plan is obstructed by the clamping straightedges and weights. The same pre-gluing technique can also be used for the ply/tube units of the center panel, but here the possibility of their squirming is not an issue if the completed midpanels are used to hold the ply/tube units in position. - - - Apply an excess of epoxy to the whole sparcap, and start laying down web, rib, web, rib... Butter the ends of each web segment before you put it down. Wipe off epoxy which oozes out from all the web/rib bonds just after clamping down everything (or just before clamping if the access will be limited). Do NOT wipe off the epoxy which oozes out from the web/sparcap bonds. These are better cleaned off afterwards. IMO, the best glue for all the other wood/wood joints is Titebond or any other equivalent yellow wood glue. CA is OK but is heavier and more brittle. CA is not suitable for attaching the top LE sheeting. Use slow epoxy with lots of microballoons for gluing in the balsa triangle fillets to the front spar side, and for gluing the top filler strip to the top spar side. This fills the divots between the Kevlar tow wraps without adding much weight. The bottom filler strip does not carry loads, and can be glued onto the spar with Ambroid for minimum added weight. Any voids which develop in the bond line between tows don't matter. Weight down the strip firmly until dry. Kevlar wrapping --------------- Transfer all the 60 feet of Kevlar tow to a piece of large dowel or piece of broomstick. This is most easily done by chucking the dowel in a lathe or drill press, and running at low speed. You cannot apply good tension on the Kevlar if it's wound on something flimsy. Have a helper hold the Kevlar dowel while you wind the Kevlar onto the spar with both hands. Or anchor the dowel somehow if no helper is available. Unwind the kevlar from the dowel the same way as you wound it on. Don't pull it off the end of the dowel, since this will put twists into it. Twisted Kevlar tow will wind onto the spar as a nasty small rope rather than a nice flat ribbon. Over each panel-end rib bay (those with ply webs), aggressively round off the sparcaps and ply webs and thin out the sparcaps as you approach the end ribs. The sparcaps ideally want to taper over the joiner bond, from their normal cross section at the next-to-end rib, down to zero at the end rib itself (don't go to this extreme, though). The taper evens out the shear stresses and actually gives a stronger spar/rod and spar/tube connection. Thinning the sparcap at the end rib also allows adding more Kevlar wraps right at the end to prevent bursting. If the sparcaps are not thinned, the thicker Kevlar can protrude out of the airfoil contour on the bottom. The glass/Kevlar wrap over the joiner bays is the most difficult to wet out. After mixing the epoxy, apply it at the joiner bay first, when its viscosity is the lowest. Lightly heating of the spar at that bay before epoxy is applied greatly enhances wicking (this is recommended with medium-viscosity epoxies like West 105/206). The Kevlar on the rest of the spar wets out very easily without heating. Put masking tape on the carbon rod and mask off the brass tube opening before wetting out the spar. Blotting off extra epoxy is much easier if a bit of heat is applied.