Tuesday, February 14, 2017

More Wing Fittings

I needed to make some more wing fittings.  Rich Wilbur was making faster progress on his wings than I was, so I sold him all my wing fittings.  I figured I'd have time to get new ones made before I needed them.  I also need to make fittings for each of us to comply with the spar AD.

There is only one Airworthiness Directive (AD) for the WACO NINE.  The inboard ends of the upper wing spars were prone to splitting.  The Feds believed it was due to poor quality spruce used in the spars.  I happen to agree.  The original spars I have are made from very poor quality spruce.  A repair was developed which clamps the spar ends with steel plates on the top and on the bottom of each spar.  The plates are held in place with four 1/4" bolts.  There are also 2 sets of smaller 3/4" wide straps bolted to each side of the spars.  I think these are intended to assure the wing attach bolts, which run vertically through the spar and the clamping plates, can't pull out of the ends of the spars.
  All these steel pieces are made from 0.090" thick steel.  I don't know anyone with a shear which can cut 0.090" 4130 steel and Aircraft Spruce doesn't sell 3/4" wide strips 0.090" thick.  WACO used 1025 steel bar stock which was rolled to 3/4" wide.  With a good blade in the band saw I can cut about 6" per minute so it goes a lot faster than you would think.

I find that for long straight cuts I do better if I draw 2 cut lines 1/16" apart.  With just one cut line I tend to wander too far from the line.  With 2 lines you can see when you are getting too close to either line.

The 0.090" steel is too thick to punch with my Whitney punch so I drill the holes with center drills so the holes don't wander as much.  All the holes are center punched using templates so they match within a couple thousandths of an inch.  Once the holes are drilled they are reamed to 17/64" to provide clearance for paint and variations in bolt diameter.

The wing strut attach fittings are made from 3/16" thick steel.  I find it helps to cut as much as possible before separating the parts so it's easier to hold them while cutting.

I also grind as much as I can before separating the parts.  It's easy to get small parts too hot to hold while grinding the edges.

I lay out parts with a felt marker so I know everything fits well before punching or scribing the steel.  I drill all the holes to match the template before scribing the cut lines around the templates.  You can see that on the strut fittings above I had one which I somehow mis-drilled a hole.  Because it hasn't been scribed the steel can still be used for some other part.

I only scribe the cut lines, never on the part surfaces which could cause a crack.  Using templates avoids the need to lay out holes, etc. on the part.

These are the top and bottom plates for the spar AD.  They get a piece of 3/16" plywood riveted on before installing.

There are lots of holes.  The 1/8" holes are not drilled with a center drill.  I find the pilot end too fragile to use on anything harder than aluminum.

There is over 9 pounds of steel in the 76 fittings used in the wings.  I need to bend and weld some of these, then paint everything.

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