October 27, 2011
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False Spar Cove
The next logical step on my wing was the false spar/aileron cove. I first test fitted a piece of metal cove and realized it would not fit well so I decided to make a wood cove by laminating two pieces of 1/16″ ply to match the end profile of the ribs. I made a jig using particle board to laminate the cove. Laminating was very easy using epoxy resin to bond the surfaces together. When removed from the jig there was no spring back and the cove fit perfectly. I was concerned about how the cove would react under the pressure of the covering material so I made a test box representing the wing aft of the rear spar between two ribs. My first covering showed a lot of bowing on the cove so I removed the cover and added a top edge of 1/16″ ply to the cove and re-covered the test box. The results were much better. I still had some scalloping of the edge so I decided to make reinforcements for the cove similar to the ones used by Piper. I used the Piper drawing to fabricate a similar part out of wood and installed after the cove was glued in place. I followed that with a edge blocking for the final top ply edge. The cove came out very nice. I still have to add the bottom edge but will wait until the wing is flipped over when working on the fuel tank bay.
Below is a picture of the trial fit of the metal false spar/cove. Not a very good fit at all.
Next, the jig and fabrication of the coves. This step was surprisingly easy.
Here is a test fit of the finished cove.
Next is the test box I used to see how the cove reacted under pressure from covering without any top edge. It did not turn out well with out the top edge.
The second time around I added a rig across the middle so I can practice rib stitching after I cover the box a second time.
I decided to make a wood reinforcement as per the Piper drawings to help with distortion of the cove. I installed the pieces between each rib bay.
Lastly I glued on a blocking edge of spruce along the top edge of the cove followed by the 1/16″ top ply edge. All in all I am very pleased with the outcome of the cove. I tested the wood cove against the metal to compare weight and the wood was lighter by about 20%. Cost for the wood cove for both wings was about $40 compared to about $300 for metal.
Next step is installing the cables and pulleys; a major head ache for sure!
