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Apr 06, 2004, 10:20 AM
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no more kits for me

I build sailplanes and small electrics. All of the kits I have bought have been a dissapointment in the quality of the wood. So I have been building from scratch.
I have a problem with spars. I cannot get spruce at my hobby shop and I am too scheap to pay shipping and handling of $7.00 on an order of less that $10. So I have available lots of oak (left over wood flooring. I also have poplar, birch, beech and of course pine. I have ripped down oak for foamie spars and some electrics but sailplanes have far more stress on their wings. Can anyone suggest which one might be best (strength/weight ratio) for a bird of Time. I will probably use carbon on the spars, the rest will be built per plan.

Also I plan to cover the wings with monocoat and the fuse with silkspan and polycrylic either colored with food coloring or spray painted after covering. How many coats of poly do you suggest...or is the even a bad thing to do. I have been told that its lighter than fiberglass and just as scratch resistant.

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Apr 06, 2004, 11:43 AM
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chlee's Avatar

Compressive strength parallel to grain is the value of interest. You can also divide the compressive strengths by specific gravities if you want to compare strength/weight ratios for different woods.

Also see this discussion of carbon-wood sparcaps:

- Chung
Last edited by chlee; Apr 06, 2004 at 11:51 AM.
Apr 06, 2004, 01:25 PM
Registered User
Use birch spar caps on the root wing panels and rock hard balsa on the tip panels. Select a piece of birch that has little or no grain runout to rip your spar stock from. If the grain is angled to the length of the spar it will weaken the spar. If you have a local hobby shop you can go there and select the very hardest balsa they have in stock. If they don't have very hard balsa in a usable size then use their bass wood for the tip wing panels. If you don't have any of the above, perhaps you could find a piece of straight grained douglas fir or bald cypress to rip for the tip panel spars.
Last edited by Ollie; Apr 06, 2004 at 01:27 PM.
Apr 06, 2004, 02:04 PM
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Thanks guys. I suspected that the grain should go with the length.
In fact I cut a spar out of oak this am both ways and answered my own question.
Apr 06, 2004, 11:37 PM
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John Boren's Avatar
BASS Wood works great for spars as well.

John Boren
Apr 07, 2004, 12:30 AM
capncrunch's Avatar
Ollie -

how would you join the spar caps spanwise so that there's less weakness at the joints? I'm imagining a simple butt-joint here. I suppose with a polyhedral wing there's not much getting around making joints, but is there a way to build a straight spar cap of two or more pieces without butt joints? Is the best way to make a straight spar cap just to use one piece of wood that tapers, or could you make a spruce/balsa spar cap that's stronger and lighter?

Apr 07, 2004, 06:40 AM
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vintage1's Avatar
Thdere is aq wonderful book out there called 'understanding wood', by a greast american woodworker. Hoadley> omething like that. I used to have it but gave it to the carpenters who built my house as a thank you.

It is a brilliant mix of the natural properties of wood, and how to uitilise it, and how to machine it carve it treat it and in fact do what we do with it,.

Ther are alos tables of all teh woods we commonly use in construction with propeties.

I went into the LHS last week, and fingered a spruce spar The bending stiffness was about 4 times as great at one end as the other. The grain ran all over the place.

Nuff said.

Ther are better woods out there in this sort of dimension. Lime (bass) is good, poplar is underated, birch, ash, yew - plenty to choose from. Even willow, and bamboo.
Apr 07, 2004, 09:17 AM
Registered User
One of the very best woods for spars is ramin. It is a rare tropical hard wood that is close grained, usually very straight grained, machines well and glues well. Ramin is 80% stronger in compression than the best, old growth sitka spruce and only 60% as dense as spruce. That gives Ramin a 12.5% better strength to weight ratio than the best sitka spruce. Here in the USA, ramin is no longer available as lumber. It is readily available in large diameter dowels that can be ripped into spar stock. The ramin dowels are competitively priced with maple and other hard wood dowels
Apr 07, 2004, 10:28 AM
Registered User
Wow this is great I am learning a lot here! Thanks.
I read that bambo is stringer than steel by weight/strength but where would you find flat bamboo?
Apr 07, 2004, 10:44 AM
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Robbydog's Avatar
Originally posted by Ollie
One of the very best woods for spars is ramin. It is a rare tropical hard wood that is close grained
Quick! Lets chop the ramin down before it disappears!


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