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Section 29, Mid Fuse Side Structure

asw20c

Well Known Member
Hi All,
I'm going to add this to the 'Gotcha' spreadsheet but thought I would post here first. On pages 27-04 and 05 there are notes on the figures that specifically tell you NOT to dimple the flanges on the firewall. That is because on page 29-08 the flanges, along with the side skin are used to match drill the hinge that will eventually be used to fasten the cowling to the fuselage. The issue is that there does not seem to be a step in the instructions, or a note anywhere on the figures (that I can find) that tells you to dimple the flanges after you are done with the match-drilling. The reason they have to be dimpled is because the hinge is countersunk, the shim is dimpled, and the skin is dimpled in that location. There are 4 layers; from inboard to out, it is the hinge, shim, firewall flange, and side skin. Therefore since the flange is sandwiched between the hinge/shim and the skin, to fit properly the flange must be dimpled too. If you, like me, follow the plans religiously, but don't read ahead much, this will bite you on the rear because you will get to the step where you begin riveting the side skins to the fuselage assembly, but then once you drive that first rivet the skin is never coming off again and will be in the way of dimpling the flanges which you previously left un-dimpled.
I called Van's to discuss this with them and they agreed it made sense to dimple the firewall flange after the match-drilling was complete, and that there appears to be a (significant) oversight in the plans since there is no step telling you to dimple.
 
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