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Date:  2-17-2008
Number of Hours:  1.97
Manual Reference:  6W7A
Brief Description:  Drill & cleco stiffeners to rear ribs 6-9

1. There has been considerable discussion by plans builders on the Zenith Forum regarding bowing of the rear ribs after the lightening holes are flanged. Some builders removed the bowing by carefully rolling the edges of the flanges. Others suggested adding a stiffener to the ribs to remove the bow.

2. Since I hadve 8 186-mm stiffeners that I couldn't use for ribs 1-4 because I had trimmed the corners at 30°, I decided to rivet these stiffeners to L & R ribs 6-9 to correct the bowing.

3. Before proceeding I calculated the approximate added weight. The 8 stiffeners weigh 88g after drilling pilot holes, or 11-g/ea. 20 A4 rivets weigh 19-g, but the mandrels weigh 14-g. So the installed rivets weigh ~0.25-g/ea, and 40 rivets weigh 10-g. Then, the 8 stiffeners, installed, will add ~98-g or ~3.5 oz.

4. Draw rivet lines 10-mm from the edge of each stiffener, and mark 5 evenly spaced holes.

5. Pilot drill the stiffeners, 3/32" . See Fig. 1.

6. Draw a rivet line midway between the 1st two lightening holes and perpendicular to the bottom flange of each rib. Mark the 1st hole, 36-mm from the bottom flange. That will center the stiffener between the flanges.

7. Drill & cleco each rib & stiffener 3/32" & #30.

8. Deburr the holes, scour the joint line with ScotchBrite, and wash with laquer solvent.

9. Prime the joint lines with Cortec 703 primer. See Fig. 2.
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Pilot drilled stiffeners

Pilot drilled stiffeners

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Primed stiffeners & ribs

Primed stiffeners & ribs

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