November 12, 2019

Continuing with Wing Skins

Last week I emailed back and forth with TAF regarding the hole misalignment between the wing lower skins and the main spar caps. I got word back from Mike Blyth at TAF that the hole misalignment is ok, and that they see the same on their factory builds – they simply elongate the holes. Our mutual finding was that the pitch of the holes on my skins is ever so slightly less than the pitch of the holes on the spar caps – we’re talking hundredths of millimeters.  For example, on the wing root end of the spar cap, I measured the holes at 1.99999cm pitch, while on the skin they were 1.99788cm, a difference of 0.0211 mm. It doesn’t sound like a lot, but that error x62 holes turns into a misalignment of 1.29mm by the end of the skin, more than a quarter width of a rivet, which is why they stop fitting along the rivet row.

Now, I need to decide how to address this error, and then elongate holes on the skins to make it all fit nicely.  Option (a) – I could start at one end of the spar, fitting in as many rivets as possible until they no longer fit, like I did in this picture:

… or option (b), I could try to start from the middle, get things lined up as best as possible, then elongate holes at either end of the skin.  Option (a) removes more material from the skin at one end, while option (b) spreads the removal over more holes.  Decisions, decisions…

I guess this is both the benefit and cost of pre-drilled parts – when they’re lined up well, it greatly reduces the need to match-drill.  But then there’s the problem of misalignment like I’m seeing, when parts are drilled at different times on different machines.

Before I bought the Sling 4, I took a 2-day class building a rudder with parts that were NOT pre-drilled, and it was pretty time consuming, and required more tools–like a strap duplicator.  So, overall I’m happy that I went with a pre-drilled kit.

 

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