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Author Topic: Wood sizes for a scaled-down model  (Read 192 times)
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GeoffinIN
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« on: December 11, 2021, 12:24:12 PM »

I downloaded Dave Rees's Citabria plan, reduced to 60% and am unsure how far I should go in reducing the wood size, if any.  Since Rees was famous for feather-light designs, I think I'd be loco to scale the wood down anything like 60%.  The original was a coconut scale (36"span) model; mine will be 22".  I don't enter contests, so going super-light isn't an issue.  Anybody have a suggestion?

Geoff the lousy builder
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GM
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« Reply #1 on: December 11, 2021, 02:49:33 PM »

There is a lot of wood in the fuselage so I would go for 1/16th square minimum but no more than 3/32 square. Split ribs for the wing from 1/16th sheet. Tailplane and fin from 3/32 square. Sizes can be adapted also depending on the density and stiffness of the wood at hand.

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GeoffinIN
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« Reply #2 on: December 11, 2021, 02:56:12 PM »

Thanks, GM.  Since I have some good a-grain 1/20" wood I can use that.  Also have quite a lot of light metric-dimensioned wood.  Should I be safe in using 1mm thick spars, or will 1/20" be a safer bet?

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« Reply #3 on: December 11, 2021, 03:10:53 PM »

1/20 would only be ok for indoors. Best go for light and stiff 2.00mm for fuselage longerons and spacers. 1.00mm to 1.5mm for the wing spars.

Just my guess.
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TimWescott
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« Reply #4 on: December 11, 2021, 05:45:00 PM »

1/20" should be good for the wing ribs.  I don't have the plans in front of me, but if the fuselage is a box-and-former design with stringers, then 1/20" would work for the formers.  I would use 1.5mm or 1/16" square (same thing, essentially) for the stringers, if it has such.

Something I haven't seen or tried, but want to someday, is stringers that are deeper than they are wide -- so on that model, perhaps 2mm x 1mm, with the 1mm side showing on the outside.  This ought to be stronger than 1.5mm square, and a hair lighter -- but there's a lot of folks out there with more experience than me, and I always see square stringers, so...

And just a note -- every time I'm done with a plane and I calculate the mass of the wood vs. the mass of the finished plane, it's usually something like 50% or 25%.  If you want to build light, use less and lighter wood, for sure, but before you knock yourself out with small sections and contest-grade wood, get in the habit of using the absolute least amount of glue you can get away with.  On a model that size, depending on my mood I'd use cellulose (Duco, Sigment, Ambroid), CA or aliphatic -- but no matter what, I'd be applying the glue with a pin or a piece of music wire with a 1/32" diameter loop in the end of it, to insure that I used the minimum amount of glue necessary to stick things together.
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lincoln
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« Reply #5 on: December 11, 2021, 06:49:26 PM »

I'm amazed Mr. Rees was able to keep such a lightly built model together. Most of those wood sizes wouldn't be out of place in a 22 inch model down outdoors, that is, not scaled down.

You could look at some other models that care known to be good flyers and see what sizes they use. Maybe look at a design by Mike Nassise. Or maybe some old Comet designs, though not all are good. Their Severesky P-35 holds up pretty well, except where you grab it. It does, of course, have more stringers. None of these have Rees style wings, though. There's a Rees design for a Mr. Mulligan on Outerzone. Apparently he was thinking about mass launch and fantasizing about it becoming legal for that event. I dunno, maybe it IS legal by now. In any case, you'd find his sizes for a more robust, 24 inch model in that plan.
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GeoffinIN
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« Reply #6 on: December 11, 2021, 08:50:36 PM »

I'm not an inexperienced builder; just not a very good one!.  I'd normally use 1/16 or 1.5mm stringers and spars, but, as mentioned above, Rees built his designs with the absolute minimum of wood.  I'm only wondering how little wood I can get away with and have it as robust as most models.  The model in question is in the NPA plans gallery.  https://www.hippocketaeronautics.com/hpa_plans/details.php?image_id=4639&mode=lightbox
You can check some of his stuff out here:  https://www.youtube.com/watch?v=OAni2MkwxXU

Building something even close to his stuff is NOT easy!
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