F-303 Center Section Build-up

[Ironflight]

I am studying up to get ready to assemble the wing center section on the -3B with Louise, and wanted anyone who's done this recently to check my thinking. The instructions show what is essentially a full-bulkhead jig in drawing SK-17, but the instructions (dated ca. 2000) talk about just building what amounts to false spars to use as spacers for assembling the front and rear parts of the bulkhead. I don't see much advantage to the SK-17 fixture, and am assuming that the "false spar" method is a later way of doing it...and sufficient?

Thoughts?

Paul

[danbenua]

Hi Paul,
I went with the false spar, which also doubles as a mount point for short legs to support the fuselage after it comes off the jig.

That said, I was very careful to align the F-303A/B side bulkheads before drilling them to the F-303C/D bottom bulkheads. I layed out a rectangle on a sheet of plywood and clamped everything down to get everything square and with the correct dimensions across the open top of the U.

Another tip is to delay drilling the F-303E side plates and F-303F bottom plate until the other parts are primed and riveted together. Then you can bolt the forward and aft sub-assemblies to your wing spar roots to get perfect alignment between them. I installed a few "tack" rivet holes to secure the side and bottom plates. If you are really thinking ahead you can lay out the "tack" rivets so they won't interfere with your final skin rivet patterns.

Finally, when drilling the false spar for through-bolts use a drill press to make the holes accurately perpendicular to the bulkheads. You want to keep everything in alignment while the skins are drilled. All this is to avoid the disaster of having your spar bolt holes mis-aligned with the bulkhead holes at final assembly.

Cheers,
- Dan Benua

[rph142]

I used the false spar and it worked fine. If you dig around in your parts bin you will find the U shaped spacer pieces that go between the two bulkheads. I recommend grabbing them and inserting them between the bulkheads to ensure proper spacing near your main longeron. If you drill the bulkheads even a little too close together in the longeron as I did, then when you go to insert them 6 months from now, you will find they cause your skin to bow between them a micro inch. Nobody but you will notice the bow, but it will still be very frustrating. Good luck.

[Ironflight]

Great feedback guys - thanks!

[randylervold]

Paul, I too used the false spar method though I had a pal machine some aluminum blocks that were .05" larger than the spar just so they's slide in.

If you're building your fuse structure now be careful with the front-to-rear spar distance, you're best off to have your wings done first or wait for your QB wings to arrive for the exact measurement.

Yes, I still monitor this forum, I'm not so off into Cub-land that I've forgotten about y'all! BTW, I had a pal doing a ferry job for me and he left his RV-8 here in Yakima. I'm still on his insurance and he left me the key so I took it up for about a half hour... forgot how well they fly!!!

[Ironflight]

I figured you were still lurking Randy - hard to break old habits! We've already got the QB wings sitting mostly finished (no need to rivet on those skins until we have to...), so yup - we can make sure we get that spar distance right.

Louise laid out all the firewall stiffeners today - nicely done, all square. We'll get that riveted together and in the jig in a few days, and be on the way.

Paul

[longranger]

Here's Sam Buchanan's alternative to the false spar I intend to plagarize when I get to that point. I've already joined the front and rear halves with the tie plates, using the spars themselves as a jig, but something like this (or a false spar) will still be requred to prevent floor ribs from tweaking the assembly.

[REHughes]

Originally Posted by randylervold

If you're building your fuse structure now be careful with the front-to-rear spar distance, you're best off to have your wings done first or wait for your QB wings to arrive for the exact measurement.

Paul,
Let me emphasize and amplify Randy's concept a bit. To prevent spacing and alignment problems when mating the wings, the best procedure is:
1. Complete your wings.
2. Fabricate an exact template of the relationship of the main spar (rear face and upper and lower borders) to the rear spar for both right and left wings. Two pieces of poster board cut to match each spar, then joined together in their correct relationship and transferred as a pattern to a piece of 1/4" plywood works well.
3. Use these spar spacing jigs when you are laying out the exact fuselage position of the center section bulkhead (complete with false spars) and the rear spar carrythrough. This should prevent any surprises when you are mating the wings. Make sure you are using the right wing jig on the right fuselage side...with the fuselage inverted it is easy to get them reversed, not that I ever would of course.

The last airplane build that I followed ran into difficulty because the tilt of the center section bulkhead in the fuselage jig was a tiny bit off, and the rear spar attachments would not match up at the correct incidence angle without forcing them into place. Tough decisions at that point.

[Tony Spicer]

Originally Posted by REHughes

Make sure you are using the right wing jig on the right fuselage side...with the fuselage inverted it is easy to get them reversed, not that I ever would of course.
.

And to use the same line, not that I ever would of course. Have a look here:

http://picasaweb.google.com/tonyboyt...44212086129506

Tony

[longranger]

Originally Posted by REHughes

...Make sure you are using the right wing jig on the right fuselage side...with the fuselage inverted it is easy to get them reversed, not that I ever would of course...

I know these are airplanes, not boats, but would it help to label things port/starboard instead of left/right, just to make you think about it?