When you are building something with .020" or .016" thin skins, things can pucker up between clecos and then holes won't line up as you rivet the sheet down. Or you end up with wrinkles between the holes. I guess it wouldn't be needed so much on thicker skins.
I like to divide an unriveted section in 1/2 with my next rivet. Then cut those two halves each in half, etc...... Playing the mental game with myself to see how perfect I can end up at the end...... Playing with the math to see if my final run can be every other hole......Thicker skins can do it too. Not chasing the skin can help greatly, by that I mean don't rivet in sequence. Pin the sheet in a few places along its length with rivets, then fill the gaps.
Billski, you literally took the words out of my mouth. I had contacted a close friend who works the assembly line at Bell Helicopter, and he said the EXACT same thing, almost to a T!On long straight runs, every 4th is probably fine. On highly curved areas, I can see every hole. Here is why:
With flat stock wrapped around a curve, you get secondary curves. The outside of the curve gets longer around the curve and shorter across the straight line. The inside of the curve gets shorter around the curve and longer across the curve. The piece wrapped around the outside of a curve ends up sort of saddle shaped, with the edges lifting. You can see this where skin panels abut on a wing or turtledeck. This is plate theory...
To keep all the holes aligned, some folks put Clecos in every hole as they follow around the curve trying to keep it all laid down. In addition to getting the holes lined up around the curve, this will usually reduce wrinkling and puckering that comes with the tendency of the skin to become saddle shaped. Sometimes you can get the straight lines across the curve misbehaving - Clecos in every hole can help that lie down and make sure the holes are all aligned too.
True, but that is not what we are talking about. Wrapping a skin around a nose rib, the skin goes further as it goes around than the rib does, and is trying the whole time to straighten out. If we skip holes with the Clecos as you go around, the holes in rib and skin will end up match drilled to a skin that is at larger radii than it should be. Then when we do pull it down with rivets, holes no longer match OR the holes will match but the skin will not be pulled down everywhere. So, you Cleco everywhere around a curve.But cleco's won't fix poor match drilling and deburring.
Those are flat surfaces. There is just about zero positional error going with a Cleco every three or four holes on flat panels, unless the flat stock took a set when rolled or was otherwise curled. Then you have the same problem as putting flat stock around a curve.This is what I would consider normal:
Now that looks like gratuitous Cleco-ing. As if building an airplane did not take long enough already.Then you have this:
Maybe you should read post 6 (mine) again. A simple curve does not really exist. RV wing leading edges, fuselage lower chines, and turtle decks are designed as simple curves, but during the elastic bending of the sheet around the form, it unavoidably is a little saddle shaped. Even if the shape is rolled or hit on a brake to plastically deform it and hold a curvature, the plastically deformed part will be slightly saddle shaped, and that combined with the remaining elastic deformation to lay it down will still result in raised edges on the panels.There are not a ton of compound curves on E/ABs, they are kinda designed out of the projects to make the buildable, or are supplied/purchased formed.
Not buying 'better cleco ever 3/8ths an inch so the skin doesn't warp/wrinkle/oilcan. No one is slapping a flat sheet over wildly curving and complex forms and hoping it stays in place with cleco's. They are forming it to a large extent. In A&P school you have to do all sorts of rivets (flush, countersunk etc) and once you get the first few set the skin does not move the remaining cleco's are really there to just keep the sheet against the substructure. Once you establish the inital alignment, you really can't walk a cut/formed skin wildly off kilter.... to to the point where a cleco in every hole vs every 3-4th hole would make a difference.Now that looks like gratuitous Cleco-ing. As if building an airplane did not take long enough already.
Those are nice flat panels going onto a fairly uniform substructure. Try the mid-side skins on an RV-8 fuselage, where the sides curve slighlty to meet longerons with compound bends/twists. You have to start by clecoing every match hole, then final drill those match-holes before you drill into that virgin top longeron. And, you've gotta drill the skin onto the longeron, clecoing every hole as you go.But cleco's won't fix poor match drilling and deburring.
This is what I would consider normal:
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Then you have this:
There are not a ton of compound curves on E/ABs, they are kinda designed out of the projects to make the buildable, or are supplied/purchased formed.
If you need to use them, then use them. But I showed the ridiculousness, or maybe just inexperience or lack of understanding that I see all over the E/AB world. Making a battery box? Better add 300 cleco's to turn a 30 minute project into a 2 day affair etc.Those are nice flat panels going onto a fairly uniform substructure. Try the mid-side skins on an RV-8 fuselage, where the sides curve slighlty to meet longerons with compound bends/twists. You have to start by clecoing every match hole, then final drill those match-holes before you drill into that virgin top longeron. And, you've gotta drill the skin onto the longeron, clecoing every hole as you go.
Then, after you take it all apart and debur, prime and back rivet miscellaneous substructure to those side skins, you've gotta line it all up and futz with cleco's until you can put a rivet into every hole without cheating and augering out a few holes at each end.
That's the only way you're gonna get those skins on tight with good looking rivets.
Maybe once you get it all lined up, you can pull clecos out of every other hole. But, not before. Even Van's says repeatedly, "use as many clecos as it takes to get everything to fit correctly, even if it takes a cleco in every hole."