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Best method of "pinning" firewall or wing block through the fuse sides?

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Best method of "pinning" firewall or wing block through the fuse sides?

Old 12-09-2003, 03:18 AM
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Ernie Misner
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Default Best method of "pinning" firewall or wing block through the fuse sides?

Greetings,

After hearing some stories of the wing mounting block ripping out of Ultra Stick 40's, I thought it might be best to pin the wing block to the fuselage sides. (I already ran extra epoxy around the block)

My question is what to use for the "pins". Drilling too large of a hole for dowels would cause a weakening of the block and be counterproductive I think. I was wondering about drilling only a 1/16" hole on each side and using short pieces of music wire for the pins.

Has anyone tried this? Thanks for any input!

Ernie
Old 12-09-2003, 09:26 AM
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Ed_Moorman
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

I have used cut down toothpicks and also button head screws. Never had a failure.
Old 12-09-2003, 10:01 AM
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MinnFlyer
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

I'm not familliar with the set up on this plane, but if the wing hold-down blocks are two seperate blocks (as opposed to a single block that runs the entire width of the Fuse), pinning them from the sides will really add VERY little extra strength.

you would be much better off by epoxying triangle stock under them.
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Old 12-09-2003, 03:45 PM
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Ernie Misner
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

Excellent answers guys, thanks loads!

The block on the US 40 is one large piece which is good. It is extremely difficult to get under it to do much work though, and I have already fouled up the triangular brace idea by forming fillets under it with epoxy....

..... so back to a couple of pins through the fuse sides for security, how about a couple of servo screws???

Ernie
Old 12-09-2003, 06:25 PM
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

This is a .40 size aircraft? Forget the pins. They are not necessary. There is very little force at the trailing edge of the wing in flight. A piece of triangle like MinnFlyer said is more than adequate.
Old 12-10-2003, 01:57 PM
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Steve Collins
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

Drill the appropriate size holes and use round toothpicks. Keep the holes small enough that the toothpick has to be tapped lightly to get it all the way in. Use some medium CA in the hole.
Old 12-15-2003, 10:33 PM
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

Hey Ernie:

You can still put in the triangle stock. Just use some big stuff like 1/2". Trim down the point of the triangle that goes against the fillet until it will fit flush. Then epoxie it in place. Should do the trick for you.

Mark Shuman
Old 12-16-2003, 02:34 AM
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Ernie Misner
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

Thanks Mark and to the others who contributed. It's too cold and windy to fly here in the NW, so I'll probably have triangle stock AND pins in there by next spring!

Ernie
Old 12-16-2003, 11:47 AM
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MinnFlyer
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

phread took the words right outta my mouth
Old 12-16-2003, 12:29 PM
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

Toothpicks work ok, but I prefer 1/8" dowels. Make sure you measure this "supposed" 1/8" dowel with a caliper before using your 1/8" drill bit. They are not always perfectly sized. I put a little tape on the drill bit to gauge the depth of my hole to about 3/4" (depending on thickness of the fuze side)I slightly round the end, and tap in with a hammer with medium CA.

This works best with ply fuze sides, but I think it does still help with balsa sides. Its easy to do, and adds virtually no weight.

Dav
Old 12-17-2003, 03:59 PM
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

.......<<Toothpicks work ok, but I prefer 1/8" dowels...........

Thanks Dave. This does sound logical and very sturdy. A question though.... what about my idea of using servo screws as pins. Wouldn't this work excellent also?

Ernie
Old 12-18-2003, 02:00 PM
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

Servo screws would work fine, but I don't know if I would want the heads sticking out on the outside of the fuse. If they fit under a cowl, they would be fine.

Most of the kits available today, really do not need to have the firewalls pinned. I like to do it for a little extra piece of mind.

Dave
Old 12-18-2003, 02:06 PM
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Ed Kincer
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

I've had good luck using BBQ Skewers you buy at the food store. They are about 1/8!" diameter and seem "tougher" than dowels and tooth picks. Cheap too!
Old 12-20-2003, 03:52 AM
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

I think pinning is over-kill. I've had wrecks that have popped the glue joint, and made the repair fairly easy. Adding strength is one thing, but if you hit hard enough, the joint is going to separate regardless. I have had some really messy repair work on things that were pinned. The pins tend to stay in place while the wood that was drilled through rips away. It transfers the shock of the blow from the joint to the attached wood which is usually balsa. Kind of like thinking that you can save you home from earthquake damage if you run a nail through the foam board insulation into the stud.

If your US 40 is like my US 60 in design, I would use a piece of 1/4 triangle stock on the top of the hold down block up against the rear bulkhead of the wing saddle/radio compartment. There should be plenty of room between the wing saddle and the wing to put that in. If you have already put an epoxy fillet there.... mission accomplished. That should work too.

There are some situations where you would really want those blocks to break free, because god knows those nylon bolts never seem to sheer when you want them to. Like when you cart wheel a landing. It the wing pops off at the blocks, repair could be light. If they don't, you are probably looking at repairing not only the wing, but the tail feathers, and the engine mount.
Old 12-20-2003, 04:24 AM
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

Capgains - you make a good point about the nylon bolts shearing. That is a problem of using the wrong size hardware and not installing it correctly. For a .40 size model, #10 nylon bolts are more than adequate to hold the wing on. Somehow 1/4" bolts became the standard, but the same size is used for .90 size planes? If that's the size needed for a .90, why on earth use that size for a .40?

Also, the gap between the blocks and the wing should be zero. Any gap allows the bolt to flex instead of shear.

I fully agree with your thoughts about pinning as well. Unnecessary and will not prevent damage but actually cause more.

Build to fly - not to crash.
Old 12-20-2003, 12:23 PM
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Default RE: Best method of "pinning" firewall or wing block through the fuse sides?

I'm with you that the blocks should be flush against the wing... but in the arf world, it was not to be. It's probably easier to jig the wing block in on the assembly line. In this case there is what looks to be a 1/2 inch gap between the block and the wing saddle top. There is a plywood doubler inside to add strength, but that doesn't fix the flexing problem you mention.

I have heard of guys taking 1/4 nylon bolts and drlling a 1/16 hole down the center of them to weaken them so they will shear easier. Not sure of the science on this so I have nver done it myself.

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