wingtip attach bulges

[lostpilot28]

I've got a slight problem that's bugging me...I'm hoping someone can enlighten me on how to fix it. My fiberglass wingtips (older -7 style batwing tips) don't seem to fit very well on the wings.

Basically, when I tighten the screws down (I installed nutplates on the tips and use #6 screws to attach to the wings) the aluminum of the wing gets pulled down to the wingtip slightly. The result is a slight bulging of the aluminum between the screw holes. I've seen other -7's and the wingtips seem to fit perfectly flush. Any ideas?

[Bob Axsom]

I have two sets of wingtips and tip tanks that I attach with screws and I do not have the problem you describe. I have to suspect the hole preparation. I use #8 flathead stainless steel screws, dimpled wing skins, countersunk tips and platenuts for the interface. The problem could be in skin and tip hole mating interface configuration such as an incorrect dimple die or countersink for #6 screws or no attempt to match the configuration such as no dimples or no countersinks or one but not the other or some such incompatibility. There could also be an imperfect hole alignment problem which can be relieved in some cases by replacing fixed platenuts with floating platenuts.

Bob Axsom

[Bob Brown]

Sonny, I know you've probably already tried this, but sometimes there's an order that the screws have to be tightend in that pull the part into proper fit and eliminate the bulging in-between screws. Just like the order you're supposed to drill these holes in when you fit up the tip, sometimes you need to tighten the screws from the front (leading edge) side and work your way toward the back.

[az_gila]

...shrank, or distorted, a little with age, and then did not fit flush with the skin overhang. Tightening the attach screws could then cause pillowing. It sounds like this might be the effect you are having.

Some builders found that a slightly oversize foam rib installed in the end of the tip could push out the shape a little and make the rib push gently up on the skin overhang.

It would be a pretty easy test to see if this might work in your case.

[lostpilot28]

Thanks guys. Bob (Axsom)...I hope it's not a hole alignment issue, but I guess anything is possible. I would think that I would have to have misaligned all the holes in the same direction to get the bulging effect. Any ideas on how one would identify if it were a hole alignment issue and how to rectify it?

Bob (Brown)...yep, I tried that! I was formerly tightening from the back to the front, but I tried from the center out and then from the front to the back. Same result.

Gil, I may try the foam rib...I would hate to paint my airplane with the bulges and have it turn out like garbage. Maybe I'll try to get a picture of the bulging and post it online soon. I also have a couple of flat spots on the tips, so maybe the rib will correct that, too.

[nucleus]

I have those bulges too, so if you figure out to make them go away, let me know!

[Bob Axsom]

If you can get all of the screws in the alignment is probably OK. Try installing the tip with clecoes and see if the bulges appear. Another thought, maybe you are over tightening those tiny little #6 screws _ I use #8. I can envision the skin being pulled down into a series of holes below the tip skin line.

Bob Axsom

[bobmarkert]

Bob Private message sent

[Bob Axsom]

1 - Screw location on my airplane is not uniform. I have 48 screws in each tip and the spacing is smaller at the forward part of the upper side of the wing there the airfoil is more curved and larger where the airfoil is closer to a straight line.

2 - I install all of the screws down to where they are 1 or 2 turns loose then I tighten them in sequence starting with the rear screw on the bottom and progress forward around the wing ending with the last screw on top of the wing at the rear.

Bob Axsom