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How to "capture" shape for a custom rib?

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dcarr

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I need to fabricate a vertical "rib"/mini-bulkhead that fits at an intermediate point between two fuselage stations on my metal airplane. Forming the rib should be pretty straightforward, but I'm not sure how best to capture the skin curvature at that location and transfer it to my forming block.

Are there any neat tricks for doing this?

Would something like this hold it's shape well enough?
http://www.amazon.com/Quint-Measuring-Systems-FC36-Flexible/dp/B000WS5IZ2/

Thanks,
David
 

oriol

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I do not know about flexible curve rulers but I think that replicating the rib with cardboard/plywood can do just as well or even better.



Oriol
 

cheapracer

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Sheet of plastic tight on the surface, can of spray foam over it and later cut through it to get your section shape.
 

dcarr

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Thanks for all of the input so far. The spray foam is very clever.

As a quick clarification: The fuselage is already constructed, the cross section is continuously changing, and there are no existing formers/ribs at this station. The challenge is to essentially take an accurate cross section of an existing structure.

Davdi
 

BJC

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Put packaging tape on the inside.

Cut a piece of 3/4" plywood to fit with a clearance of approximately 1/4" all around.

Make a large hole in it to allow access to the back side from the front.

Shim it in place and mark some reference lines.

Apply Bondo to the gap on one side of the fuselage. Ensure that there are no gaps in the Bondo.

Let cure.

Pop loose.

Put back in place.

Bondo the opposite side.

Let cure.

Remove.

Sand the Bondo flush with the plywood on both sides.

You now have the shape of the front and back of the bulkhead flange within the accuracy of the tape thickness, which can be compensated for by shifting the bulkhead aft slightly, assuming a tapered fuselage.


BJC
 

TFF

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Unless you have access to a indoor LIDAR ir one of the CAD drawing arms, you are stuck with the trial and error of cardboard, foam, paper templates until satisfied.
 

don january

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This may be way out ther but can you place a mirror at the front station and shine a bright light at mirror and capture a shadow in the area where your new rib will be and transfer the shadow line onto wood ect.??35 ford004.jpg
 

stevel

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There's always the method that boat guys have been using for ages. Try a search on "tick stick method" for several good explanations and a couple of videos. It would work from either the inside or the outside. I have used it many times, and found it to be quick and easy.
 

dcarr

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So many great ideas here!

I think I'll start with a variation on the bondo method---there's some around here somewhere and it seems like a straightforward (if stinky) place to start.

It'll be a little while before I finalize the rib location, but I'll report back on the results.

David
 

Eduardo Fadul

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São Carlos, SP - Brazil
The easy (and hardest) way could be with a "jig" a plumb an a ruler.

Some time ago we have to obtain the points of the airfoil that no-one knows which was it was, so we made a jig with wood a couple of rulers and one plumb. We take several measures until the point when we "feel kind of confident" about the final result. You can take as many measures as you wish. The good part with this method is that actual you can make and calculate a new rib at will.

ADvice: Take much more measures at the leading edge than in the rest of the shape ;)

regla.jpg

Regards.
 
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