Flap twist

cnpeters · Post by **cnpeters** » Sat Aug 04, 2007 10:00 pm

I noticed 3/16-4/16" of twist

in my left RV-9A flap that is almost finished (TE to be riveted still). Is this considered acceptable or should I try and fix it? If I do a fix, the plan would be to drill out the lower skin to spar rivets, flatten the flap, and reset them. I am not too keen on this as I may enlarge the holes a little from drilling out. Some are already oversized a little from the countersinking. Plus it is a PITA to reach in and buck - would have to also drill out a bunch of expensive MK-319-BS rivets in the lower skin to ribs to gain access. Or I can just order all new parts - aaargh! Thoughts?
Thanks.
(I posted this to VAF also - didn't know if anyone here has run into this)

Post by **Spike** » Sat Aug 04, 2007 10:21 pm

Uggg, the dreaded twist. I fought this with my elevators and now that I am working on the ailerons I am paranoid about it. First I think you could probably leave it and suffer no real ill effects. We are probably in the domain of what you can live with. Remember, as Vans says, your building a tractor.

Did you build the work surface that Van's mentioned in the instructions?
Is the flap ok with respect to your work surface?
Did you set the end rivets yet?

If the thing is good with respect to the work surface (my elevator problem was) it will be hard to fix without redoing your work surface. There are a bunch of rivets to drill out. Keep in mind that if you drill them out perfectly, and then reassemble, you could very well have a warped part again as they may set the way they did originally.

cnpeters · Post by **cnpeters** » Sat Aug 04, 2007 10:49 pm

Yeah - the table is flat (jointed boards holding top, checked for twist with winding sticks). The flap has twist relative to this table. The only deviation I made was the plan called for weighting down the upside down top skin (already attached to spar and ribs) while you reached in and bucked the lower skin to spar rivets. Well, I wasn't sure how to weigh down the skin against the table when I had to reach along them and under the bottom skin (which is on top as the flap is upside down) to get to the spar. Perhaps flat bags of sand?? I DID cleco the TE to the board in existing holes, and held the rest of the upside down flap against the table - thought it was all flat.
Anyway, I will go ahead and rivet the TE tomorrow, but I doubt it will do one bit for the twist - this is likely locked in from riveting the skin to spar. And yeah, for the next flap, I'm going medieval with jigs, clamps, lasers... . But maybe it should have a symmetric twist, though

.

Spike wrote: Did you build the work surface that Van's mentioned in the instructions?
Is the flap ok with respect to your work surface?
Did you set the end rivets yet?

cnpeters · Post by **cnpeters** » Sun Aug 05, 2007 6:31 pm

Well, I set the trailing edge rivets after weighting the whole mess down with 4 2x4's,, 8 bricks, and my two bins of clecoes. After riveting the TE, it actually brought more than half the twist out, so I am left with just under 1/8". Not bad for an 8 foot flap. As a bonus, the TE is dead straight. Feel much better - just was surprised the TE riveting would do any straightening, even with the extra weights. Time for a drink.... .