Wing 8. Reinforcement Plates

Photo shows router setup to mill butt end of spar for plywood reinforcement

The 1/16th inch plywood reinforcement plates must be flush with the surface of the spar, so the spar is milled down 1/16 inch on both sides.  I used an ordinary router.  First I set up a stop made of two 1x2's clamped to the spar.  The base of the router contacts the 1x2 stop and makes a nice even square end to the milled down portion of the spar.  I used a 3/4 inch diameter flat bottom router bit set to 1/16 depth.   It took several adjust and try cycles before I got the depth just right.

Cutting the recess is a matter of moving the router left and right across the surface of the spar.  Keep the router level by keeping the router base flat on the uncut portion of the spar.  Go slow, no hurry.  Chew gum, listen to music, stay loose.  You want the cut portion of the spar to be as smooth as possible.

After milling with the router, dress the cut with sandpaper until the router marks disappear.

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The Wing 7 Rib installation

Photo: Ribs being installed, note yellow string lines

Ribs are nailed to the wood spars with tiny cement coated aircraft nails. 

Install the rib closest to the butt end of the wing first.  Then install the full rib closest to the tip next.  Installing these two ribs first allows you to tie a couple masons strings between the butt and tip to use for aligning the remaining ribs.  The ribs are a snug fit fore and aft but are able to move vertically, so the string lines are needed to ensure accurate vertical alignment.

Aircraft nails are so small that it's difficult to hold them.  If you attempt to hold them with your fingers you end up smashing the fingers with the hammer everytime.  There are two ways around this problem: (1) you can hold the tiny nail with a needle nose pliers while you tap it with a tack hammer, or (2) my favotite method is to place the little nail head on the magnetic end of the tack hammer, place the nail point where you want it and then tap the tack hammer with a big ball peen hammer...the nail sets into the wood about half way with that one blow.  Finish setting the little nails the last 1/4 inch with an ordinary nail set. 

Be sure the ribs just touch the string line and use a square to ensure the ribs are vertical and perpendicular to the spars.

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Wing 6 compression struts

Compression struts tie the front and rear spars together.  They are steel tubes with a flat plate (flange) welded to each end.  Bolts through the wood spars and the flanges attach the compression struts to both spars.  Large diameter wood washers are used under the bolt heads.

The wing begins to aquire rigidity with the addition of compression struts, like building a lightweight box.

Go to Wing 7: Rib Installation

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Varnish

Photo: using a Q-tip to varnish the inside of a hole.

Ahh, varnish.  Being a sailor and boatbuilder I love the look and the smell of varnish.  I love what it does to the wood  A good varnish job gives your wood a beautiful honey glow, and it's functional too, varnish preserves the wood and protects it from damage.

Half of the art of varnishing is sanding.  Start with a 120 or 150 grit sandpaper on the raw wood.  Don't use an electric sander or a sanding block.  Use your hand.  Tear the sheet of sandpaper into 4ths and then fold each 1/4 sheet in half and then half again until you have a approximate 2in by 2in pad of sandpaper.  Sand with one side of your sandpaper pad until it loses effectiveness, then turn it over and sand with the other side until that side is worn out...then unfold and repeat until all four sides are used. 

Sanding is a two handed job: the right hand moving the sandpaper over the surface while the left hand comes along behind feeling the surface for smoothness and missed spots.  Use a very, very light touch.  Do not attempt to remove wood.  The idea is to only smooth the surface and you should be kicking up just very little fine dust.

After sanding the raw wood, remove the dust with a tack cloth and then apply the first coat of varnish.  Thin the first coat with about 25% mineral spirits so it will be readily absorbed into the wood.  I use a traditional spar varnish for the first two coats.  Traditional varnish takes a long time to dry, all the while soaking into the wood.  And spar varnish gives your wood a beautiful color, a nice warm glow. The second coat can be full strength spar varnish, and the third coat should be an epoxy varnish.  Using the spar-spar-epoxy combination gives excellent protection to the wood and a beautiful finish.

Use a good varnish brush..(they are expensive, don't cheap out on your brush).  A good brush is very soft and has a really decadent feel to it.  If the hardware store guy won't let you take it out of the package and feel it before buying, just walk away. 

After the first coat is dry (overnight).  Sand, very lightly again, with a 220 sandpaper.  Remove dust with a tack cloth.  Brush on the second coat of varnish.  Let dry.  Sand with 320 sandpaper.  Tack cloth.  Then brush on the third and final coat.  Resist the temptation to add more coats of varnish.  Three coats is just right:  protects the spar, looks very nice, but not too heavy.

Don't forget to varnish inside all those holes you drilled and be sure the ends of the spar get a good coating too.

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Wing 5, Ribs

Transfer the positions of the ribs from the old spar to the new spar.

The ribs are nailed to the spar with tiny aircraft nails and weather you removed the ribs rough or easy they will leave distinctive marks on the old spar where they were nailed on.  Start by laying the new spar and the old spar side by side on sawhorses.  Check that they are perfectly aligned by using a square on the butt end.  Then slide the square down the spar to the first rib position, align it with the nail holes and draw a line.  One line is not enough, because when you go to install a rib the line will be covered, so draw two more lines one on each side of the nail hole line.  The two extra lines should be about the same distance apart as the width of the rib.

Once all the rib positions have been marked, flip both spars over and mark the rib positions on the other side.

Go to Wing 6: Compression Struts

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Wing 4, Trim wingtip to length

Photo shows two spars, the one on the left has been trimmed to length and tapered.

Before the newspar/oldspar sandwich is unclamped (after drilling holes), trace around the wingtip end of the old spar with a pencil. Unclamp the two spars and set the old spar aside. Then using a carpenters circular saw carefully and slowly cut the taper in the end of the new spar,.  You can make a precison cut with a circular saw by holding the guard open with your little finger and then watching the blade as it moves through the wood.  The blade should remain on the waste side of the line and the cut should just barely touch the pencil line.

After cutting with the saw, dress the cuts smooth with a small block plane.  Remember to plane in the direction of the grain, if you try to plane against the grain the plane will catch and rip out little chunks of wood.  It's also possible to do the same thing with sanding block and some course sandpaper.  It takes longer but you're unlikely to damage the wood. 

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Wing 3, Drilling holes in new spars

Photo of drill press set up to drill holes in spars.

I mounted my benchtop dirll press on the rolling cart that normally supports my sandblaster.  The old spar was clamped on top of the new spar blank with the butt ends perfectly aligned.  Then it was a simple matter to drill through the old spar holes into and through the new spar.  The drill press assures that the holes are perfectly straight and true and perpendicular to the face of the spar.

One little trick: the oldspar/newspar sandwich should NOT be clamped to the drill press table.  If the sandwich is loosely set on the drill  press, it is allowed to "float" as the drill bit enters the old spar hole and any misalignment will be eliminated as the spar sandwich is able to move to align itself with the drill.  Another tip is to use a scrap piece of wood under the spar sandwich so the drill bit does not burst through the wood as it exits.  That way you get a nice clean hole on both sides of the spar.

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