I am scratch building a little sport plane of RCM plans and I can't figure this out. The plans call for a wing bolt filler block to be positioned at the trailing edge. I understand its function. However, the plan does not specify what material I should use. The actual wing bolt block in the fuselage will be plywood. Will 3/16" balsa sheet suffice for the filler block, or does it also need to be plywood.

Thanks

Is the block going inside the wing or on the surface? Inside it can be balsa, just harden the holes with thin CA. If it is on the surface of the wing, thin ply would probably be best. You just want to insure that the bolts won't pull through the wing.

Thanks for the help. The blocks are going inside the wings. So should I take the balsa, drill the hole, then just saturate around the hole with thin CA?

I always use a basswood or even a maple block in the fuselage so the threads don't eventually wear out. I drip in CA and then freshen the threads with a thread tap. Hardwood, but glued in with carpenter's glue and not epoxy so the wing will pop off in a ground strike. As opposed to tearing out a huge hunk of fuselage or snapping off the wing at the roots.

The reenforcement in the wing is less critical as long as the surface has a good stiff facing so the covering and underlying wood doesn't twist and pucker. Balsa is fine. To don't want threads in the wing section. Only the fuselage blocks. That snugs the wing in.

EDIT: missed the post that says the block goes inside the wing. Oh well, the following is a good suggestion for models that are heavy enough to stress the wing/fuselage joint.


The filler goes between the bolt block in the fuselage and the wing, right. So the filler block will simply bear against a section of the wing, right? It's to help the wing. It spreads the compression area out that the wing feels when the wing bolt tightens the wing against the fuselage sides. Fuselage sides are usually pretty narrow.

I've been considering placing a scrap balsa filler in the rear of my Tiger60 where the bolt block is in the attached picture. The wing shows compression lines where those narrow fuselage sides press against the top center of the wing. Then just the sides won't be where all the force is on the wing.

Not really sure what you are describing. Like da rock said, if you have a recessed wing mounting block, and you are trying to fill the area between that and the top of the wing saddle, then use a balsa block, and sand it to the shape of the wing saddle. You will have to drill through it and through the ply mounting block for your bolts.
If you are talking about the area between the bottom sheeting and the top sheeting of the wing, at the TE where the bolts go through, then that too can be balsa filler, sanded to shape to keep the sheeting from collapsing when you tighten the bolts. in addition, you may want to use some thin ply on the top sheeting of the wing to further distribute the pressure of the wing bolts.

He is referring to a filler block INSIDE the TE of the wing

Like Charlie said, Balsa is fine. No need to reinforce the holes with CA.

However, you should also add a thin (1/6"?) plywood plate to the outside of the wing to help prevent the bolt heads from pulling through.

To further help with overtightend bolts, put the balsa filler end grain to the bolt head. Balsa has a lot higher compression strength in this direction at no increase in total weight. The thin ply is still a good suggestion as the sheeting is cross grain and it would be easy to pull through the sheeting by overtighenting the bolts.

A note to keep in mind related to the nylon bolts. They will stretch and stretch as you are tightening them. It is very easy to over tighten them, pre-stressing them so just an added amount of stress will cause them to fail. A rule of thumb I use is to finger tighten them, then using the wrench, go 1/2 turn more. This is enough friction to prevent the bolt from working out, but still leaving the bolt with all is tension strength avaiable.

Don