Why do these things, you ask? Most of the reasoning has to do with the gross weight increase. When asked about the required changes to support a Corvair engine, Roger responded to increase the shear web thickness to 1/4" and change the cap depth to 1". I checked the design calculations for a normal category, 1200 lb gross weight, with 1.5 safety factor and felt that the spar cap doublers were needed to handle the moment due to aileron deflection. This airfoil appears to have a significant pitching moment change as the flaperon is deflected. The upper doubler is even thicker than the lower doubler to account for the additional compression stress due to the lift strut. The additional intercostals control shear web buckling. As time goes on I hope to post all my calculations in support of these changes, many of which are based on EAA spreadsheets.
The Hoerner wingtip simply seemed like the best looking tip for the labor involved. The look of the cub style bows doesn't appeal to me. I don't feel like investing in the fiberglass droop tips either. Although the efficiency improvements from Hoerner tips are debatable, this chosen design should give me an effective 30' wingspan at the very least.
My first task was to rip the 1/4" birch plywood into strips the depth of the spar with the top edge angled about 10 degrees to match the airfoil shape. I then scarfed them together into one 15' long piece. The bottom of the strip was placed against the aluminum angle fence and clamped tightly as the epoxy set, ensuring a straight spar.
2 hr
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( 2.4 / 23 )I built the big table. I chose to make a 16' x 4' table in two sections. It consists of a ladder frame of steel studs with 1/2" MDF faces on each side. I used an additional pair of steel studs to tie the two table sections together. The whole table is torsionally and flexurally very stiff - I estimate that a section warps by less than a 1/16" when picked up by a corner. I used inexpensive steel folding sawhorses and shims to level the table in our garage. Along one edge, I have affixed 1"x1"x1/18" aluminum angle that acts as a straight edge.
Although it probably wasn't needed, I welded the frame together. This did make assembly a little easier, and also allowed me to practice my acetylene welding techniques before moving on to the rudder pedals or landing gear.
4 hr
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( 3 / 10 )Scarfed up another side panel from 1/16 inch birch ply for the starboard root rib.
1/2 hr
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( 3 / 10 )I created a nose rib template out of some leftover flooring laminate stock. I use this with a vaneer trim bit in the router to create the noseribs with a fair level of duplicity. After cutting out a few nose ribs from 1/4" birch ply, I glued one in place on the root rib. I also glued up the root rib for the starboard wing. Pictured at left is the full complement of ribs for the port wing.
2 hr
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( 3 / 10 )Glued the port side root rib to the scarfed plywood.
1/2 Hour
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( 3 / 10 )I spent some time today working on making nice scarf joints. The ply that I had ordered wasn't long enough to cover a rib in one piece. I had my best results using the router jig shown in the first pic at the left. It is simply two steel rails attached to a board, spaced about 3.75 inches apart. One rail is shimmed up 0.25 inches to establish the 15:1 taper. Simply slide the plywood under the higher rail, then glide the router over it. I sacrificed a cedar shingle to hold the thin plywood flat. You can see the results in the second picture.
The last two images demonstrate what happens when you aren't careful using the sanding technique. I sanded one taper too long, and so even though one side looks pretty OK, the other side has exposed the laminating resin of the plywood. Scrap.
2 hr
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( 3 / 15 )Glued up the root rib for the port side wing. As this rib will have birch ply covering one side, I left the bottom gussets out.
1 hr
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( 3 / 15 )That's the last of the ribs requiring aileron hardpoints.
1 hr
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( 3 / 15 )1 hr
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( 3 / 20 )1 hr
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( 3 / 25 )
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