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Quarter Sawn Balsa

How important is it?

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Levanter18/04/2018 13:19:43
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706 forum posts
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I am looking at two plans that have caught my eye. Both refer to quarter sawn balsa for the ribs.

​Looking up the qualities of quarter sawn it seems to be stiffer across the grain and recommended for ribs but not much else. Certainly my local model shop does not make any distinction and although it does have some quite nice stuff I have never seen quarter sawn and although a have a few random pieces in stock I have never singled it out for any particular use although I think it has an interesting look.

​I can understand it being used for sheet surfaces such as tail planes and fins but many designs show tips or inserts at 90 degrees to stop any warping.

​Most of the ribs I make are under 30mm in section and anyway are further supported by top cap strips which are curved, spars, sheer webs and the front and rear strips.

​How important is it to use quarter sawn where it is specified and where does it become really important.

​Levanter



Peter Miller18/04/2018 14:01:31
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Just use a harder grade of balsa.

1/4 sawn has a sort of speckled appearance

I just use medium hard sheet. As you say the cap strips etc help, It was also more inportant in the days when dope and tissue or nylon coverings were used. I have seen control line stunter (Mercury Cobra) with "S" shaped ribs due to an over enthusiastic application of dope.

Martyn K18/04/2018 15:06:41
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1/4 grain has - as Peter noted - a speckled 'Mackerel Sky' appearance. The benefit is that its stiffer across the grain than normal straight grained wood and if decent quality you can go down a size in wood using light weight material

Mainly used for components that you want to keep warp free, like fins, rudder and wing ribs.

Decent wood is as rare as hens teeth. I have still got some that is 30 yo - keeping it for a special occasion. Probably use it to line my coffin laugh

Martyn

Martyn K18/04/2018 15:45:24
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4541 forum posts
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A photo of what Quarter grain wood looks like..

p6230006.jpg

Both of the lower sheets (1/32 balsa) is quarter grain although the top half (in the photo) is cut towards the edge of the quarter area. The bottom part is beautiful and demonstrates exactly what you need to look for. The very long cross grain shows it was at 90 degrees to the growth rings in the tree. Quarter grain is not ideal for sheeting wings though as it is rather stiff and does not bend easily to the profile of the wing section.

Martyn

Nigel R18/04/2018 16:15:09
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"How important is it to use quarter sawn where it is specified and where does it become really important"

It's a bit more resistant to compression and warping, and more brittle when it comes to bending a sheet crossgrain (so not great for sheeting, near useless for spars).

If you don't have any, you could

1) Use a harder grade as Pete says and suck up the extra weight.

or,

2) redesign structure a little.

e.g.

replace the 1/4 sheet tailplane, with 1/8 structure sheeted with 1/16

laminate two thinner pieces together as a ply

Balsa cabin will sell you quarter grain but by gosh do you pay for it. I'm not convinced it is worth the extra.

I would choose option 2 myself.

Tom Sharp 219/04/2018 01:40:47
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2831 forum posts
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Different cuts could make a huge difference when Balsa was harvested from the wild.

But I don't think there is much difference in the modern plantation grown Balsa.

Don Fry19/04/2018 09:18:31
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2347 forum posts
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I would be tempted to use 3 or 6 mm depron as a direct replacement. It's got good qualities for ribs, and is as cheap as chips. No need to alter the plan, except where the rib face is visible, i.e. Where the aileron slot leaves a bit of the last full length rib. Face the rib with 1.5 mm balsa at this point.

Only downside I can see is you are now having to work with a view to not getting inappropriate glues and solvents on the foam.

Levanter19/04/2018 11:29:41
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706 forum posts
346 photos

Good idea Don
​I having been thinking about getting a bit of depron in stock to play around with so I think I will get on with it.

Levanter

Don Fry19/04/2018 15:29:09
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2347 forum posts
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I bought some 3mm the other week from a local DIY store, France so no use to you, 2 sheets, 1225x700 mm, about £6. Lot of ribs.

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