Longeron Question

[WingsOnWheels]

Ok, the manual calls for the longerons to be flat (square) to the jig at F604 and F605, this makes perfect sense. However, it states that the inside of the longeron should be shimmed (as required) at F606, F607, and F608 to match the contour of the bulkhead sides.

So if I'm understanding this correct, the longeron is twisted at the firewall to flat through F604 - F605, then twisted slightly at F606 - F608 and then is flat again at F610-612?

That doesn't make a whole lot of sense to me, so maybe I am misunderstanding. If the longeron is twisted at those bulkheads, what is going to maintain that twist once the fuselage is removed from the jig? I know the shim will probably only be 1/16" or so, but it just seems odd.

 

[Stevea]

It has been a long time since I bent longerons, but I don't recall using any shims aft of F604. The only twist is from the firewall to F604. Aft of F604 the longeron should lay flat against the jig cross piece (and be parallel to the bulkhead flange).

 

[noelf]

I agree with Steve...

...I do not recall having to shim any of the longeron aft of F-606.

As you lay in the bulkheads, you will be able to determine if the bulkheads have a slight arc at the longeron-bulkhead joint. If they do, then a slight shim may be needed to align the longeron "flange" parallel to the bulkhead flange.

If a shim is required, the longeron will keep its shimmed position in the jig while you are drilling and riveting the fuselage bottom skins on. Once all bottom skins are riveted, you will be "flip'en the canoe" and the longeron will keep its slight twist.

 

[WingsOnWheels]

Thanks for the help. I was concerned that the anti-twist of the .125 longeron would be stronger the the pro-twist of the thin bulkhead. I guess once everything is together, it is not an issue.

Does anyone remember what thickness of shim they used?

 

[noelf]

What thickness ??...

...the easy answer is "whatever it takes".

Really though, it will not be much. Take a look at Drawing #24. This is the dimension drawing for the bulkheads (and it might only show up in the preview plans set from Van's). From this drawing, you can see where the longeron will lay in the "slight" radius of each bulkhead. It will not be much.

As each bulkhead is positioned and the flanges formed for the tail cone angle taper), you can lay a straight edge on the bulkhead flange and determine if the longeron flange is parallel or needs any shim "help" to become parallel to the bulkhead flange.