The aluminium tube and spruce strip (3mm thick on this wing)are measured against the core and 2 x slots cut and sanded to shape,. This glider will have a shear web to about 1.3 mtrs out from the root , I use a sharp new blade and a straight edge for the shear web slot , the foam sliver is lifted out with a thin flat screwdriver. The tube slot is made with a rounded sanding attachment on my Dremel tool. It is important that slots are cleaned down to the existing carbon spar and the shear web fits with a little play, the shear web (spruce ) is trail fitted and marked so that it can be sanded to a taper and flush with the pre-sanded spar groove.The aluminium tube is sanded with some rough sandpaper for extra grip and the open end filled with some tissue or a foam plug to keep epoxy out.
The aim of the shear web is to ceate an I beam , it must hold the spars apart (under compression)This is the most common failure on long glider wings, the top surface/spar fails. It must also keep the top and bottom skins together (under tension). The spruce is strong and resists flexing in the verticle moment , but could split along its grain. So a carbon braid (with the strands running at 45 degrees) Is pulled tight over the spruce and onto the aluminium tube. I tack this in place with cynano glue. Another way is premade sheets/strips of glass and end grain balsa, light and strong but a little more time consumming, or even light ply-wood.
The next steps all take place as one exercise so it is important to have the next set of carbon tows already precut for the bottom spar.
The carbon braid is wet out with epoxy resin (I mix about 50gr at a time)and set aside , I then paint resin with a small brush against the exposed carbon top spar inside the slot. At this time there would still be a fair amount of epoxy left in the pot, to this I add glass flocks(cotton flox) until I achieve a runny mix (like syrup) A bead is layed into the slot with a large syringe as well as plenty in the area for the joiner tube, which is wedged in possition to create some di-hedral.
The shear web and tube is now pressed into possition, some mix "goop" should ooze out
then you know it is well bonded.
Any excess can be scraped off, thickened with more "flox" and a thicker mix used to fill hollows , especially around the joiner tube area.
Moving right along while everything is still wet , get the carbon tows wet out and layed into the pre-sanded groove. Using a plastic phone card you can flatten and spread the epoxy through these, until they are all nicely wetout and flush.
Dont panic if the tows are below the groove edges , this is easy to fill later, you dont want to sand off part of the carbon if it's proud.
Place the core on some paper ,on a flat surface, wet side up and weight it down to prevent a bow in the wing,
I do both wings at the same time and needed to mix extra epoxy in 50gr batches , mixing more on a hot day is looking for trouble with an exo-thermic runnaway, where
the epoxy curing accelerates generating so much heat it can catch fire(been there done that).
Now pour a long cold one and start thinking about the the next step , the mylar preparation and glass composite wing skins. (part 3)
Cheers
Mike
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