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Info: Balsa Grading and Characteristics

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    Posted: 07/13/2006 at 10:58pm

Most information I found was from Model Airplane sites.  I thought I would post this info since someone in another thread said it would be interesting to know.  But I can't seem to find that thread anymore.  I also posted the links to the sites  for reference.

Enjoy.    

- SEAN

Interesting Facts About Balsa Wood - http://skyhooks.ca/balsa.htm

Model airplanes are no different than any other type of flying machine, large or small - THE LIGHTER IT IS BUILT, THE BETTER IT WILL FLY! With that in mind, it is easy to understand why balsa wood has been the standard material for model airplane construction since it first became readily available in the U.S. in the late 1920's. Its outstanding strength-to-weight ratio enables hobbyists to construct durable models that fly in totally realistic manner. Balsa also absorbs shock and vibration well and can be easily cut, shaped, and glued with simple hand tools.

WHERE DOES BALSA WOOD COME FROM? Balsa trees grow naturally in the humid rain forests of Central and South America. Its natural range extends south from Guatemala, through Central America, to the north and west coast of South America as far as Bolivia. However, the small country of Ecuador, on the western coast of South America, is the primary source of model aircraft grade balsa in the world. Balsa needs a warm climate with plenty of rainfall and good drainage. For that reason, the best stands of balsa usually appear on the high ground between tropical rivers. Ecuador has the ideal geography and climate for growing balsa trees. The scientific name for balsa wood is ochroma lagopus. The word balsa itself is Spanish meaning raft, in reference to its excellent flotation qualities. In Ecuador it is known as Boya, meaning buoy.

HOW DOES BALSA WOOD GROW? There is no such thing as entire forests of balsa trees. They grow singly or in very small, widely scattered groups in the jungle. For hundreds of years, balsa was actually considered a weed tree. They reproduce by growing hundreds of long seed pods, which eventually open up and, with the help of the wind, scatter thousands of new seeds over a large area of the jungle. Each seed is airborne on its own small wisp of down, similar to the way dandelion seeds spread. The seeds eventually fall to the ground and are covered by the litter of the jungle. There they lay and accumulate until one day there is an opening in the jungle canopy large enough for the sun's rays to strike the jungle floor and start the seeds growing. Wherever there was an opening, made either by a farmer or by another tree dying, balsa will spring up as thick as grass. A farmer is often hard put to keep his food plot clear of balsa. As the new balsa trees grow, the strongest will become predominate and the weaker trees will die. By the time they are mature, there may be only one or two balsa trees to an acre of jungle.

HOW LONG DOES IT TAKE A BALSA TREE TO GROW? Balsa trees grow very rapidly (like all pesky trees). Six months after germination, the tree is about 1-1/2 inches in diameter and 10 - 12 feet tall! In 6 to 10 years, the tree is ready for cutting, having reached a height of 60 to 90 feet tall and a diameter of 12 to 45 inches. If left to continue growing, the new wood being grown on the outside layers becomes very hard and the tree begins to rot in the center. Unharvested, a balsa tree may grow to a diameter of 6 feet or more, but very little usable lumber can be obtained from a tree of this size. The balsa leaf is similar in shape to a grape leaf, only a lot bigger. When the tree is young, these leaves measure as much as 4 feet across. They become progressively smaller as the tree grows older, until they are about 8 - 10 inches across. Balsa is one of the few trees in the jungle which has a simple leaf shape. This fact alone makes the balsa tree stand out in the jungle.

THE PERFECT NURSE! Nature evidently designed the balsa tree to be a "nurse tree" which would protect the slower-growing species of trees from the scorching jungle sun during their critical early years. For instance, in an area of the jungle that has been ravaged by a tropical storm or other natural disaster, the balsa trees will quickly sprout and beginning to shoot up to impressive heights in a very short time.Their fast growth, and the extra large leaves they have in their early years, provide shade to the young seedlings of the slower-growing forest giants. By the time the seedlings are established enough to take care of themselves, the balsa tree is beginning to die. Undoubtably, the balsa tree's rapid growth, fast spreading crown of first very large and gradually smaller leaves, and it's relatively short life span were intended to make it the "perfect nurse" in the jungle ecosystem.

HOW ARE BALSA TREES HARVESTED? While nature intended the balsa tree to be a short lived nursemaid, mankind eventually discovered that it was an extremely useful resource. The real start of the balsa business was during World War I, when the allies were in need of a plentiful substitute for cork. The only draw back to using balsa was, and still is, the back breaking work that is necessary to get it out of the jungle. Because of the way the individual balsa trees are scattered throughout the jungles, it has never been possible to use mass production logging procedures and equipment. The best way to log balsa trees is to go back to the methods of Paul Bunyan - chop them down with an axe, haul them to the nearest river by ox team, tie them together into rafts, and then float the raft of balsa logs down the river to the saw mill. The logging team usually consists of two native Ecquadorians, each armed with a broad Spanish axe, a machete, and a long pole sharpened like a chisel on one end for removing the bark from the downed trees. Because of the hilly terrain, an ox team may only be able to drag two logs to the river per day. At the saw mill, the balsa is first rough cut into large boards, then carefully kiln dried, and finally packed into bales for shipment to the U.S. via ocean freighter.

Final cutting and finishing of our model aircraft balsa is done right here at the SIG factory. As a result of the balsa tree's fast growth cycle, both the quality and lightness of the lumber obtained from a balsa tree can vary enormously depending upon the tree's age at the time of cutting.

WHY IS BALSA WOOD SO LIGHT? The secret to balsa wood's lightness can only be seen with a microscope. The cells are big and very thinned walled, so that the ratio of solid matter to open space is as small as possible. Most woods have gobs of heavy, plastic-like cement, called lignin, holding the cells together. In balsa, lignin is at a minimum. Only about 40% of the volume of a piece of balsa is solid substance. To give a balsa tree the strength it needs to stand in the jungle, nature pumps each balsa cell full of water until they become rigid - like a car tire full of air. Green balsa wood typically contains five times as much water by weight as it has actual wood substance, compared to most hardwoods which contain very little water in relation to wood substance. Green balsa wood must therefore be carefully kiln dried to remove most of the water before it can be sold. Kiln drying is a tedious two week process that carefully removes the excess water until the moisture content is only 6%. Kiln drying also kills any bacteria, fungi, and insects that may have been in the raw balsa wood.

HOW LIGHT IS KILN DRIED BALSA WOOD? Finished balsa wood, like you find in model airplane kits, varies widely in weight. Balsa is occasionally found weighing as little as 4 lbs. per cu. ft. On the other hand, you can also find balsa which will weigh 24 lbs. or more per cu. ft.

However, the general run of commercial balsa for model airplanes will weigh between 6 lbs. to 18 lbs. per cu. ft. Eight to twelve lb. balsa is considered medium or average weight, and is the most plentiful. Six pounds or less is considered "contest grade", which is very rare and sometimes even impossible to obtain.

IS BALSA THE LIGHTEST WOOD IN THE WORLD? No! Most people are surprised to hear that botanically, balsa wood is only about the third or fourth lightest wood in the world. However, all the woods which are lighter than balsa are terribly weak and unsuitable for any practical use. The very lightest varieties don't really resemble wood at all, as we commonly think of it, but are more like a tree-like vegetable that grows in rings, similar in texture to an onion. It is not until balsa is reached that there is any sign of real strength combined with lightness. In fact, balsa wood is often considered the strongest wood for its weight in the world. Pound for pound it is stronger in some respects than pine, hickory, or even oak .

 

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STRENGTH OF BALSA WOOD COMPARED TO OTHER WOODS

Species  Weight Lbs./Cu. Ft.  Stiffness Strength  Bending Strength Compression Strength
 Balsa 8 72 70 75
 Balsa  10 100 100 100
 Balsa  14 156 161 149
Spruce 28 230 260 289
Yellow Pine  28 222 277 288
Douglas Fir 30 241 291 341
Hickory 50  379 638 514
Oak   48 295 430 366
Basswood 26 261 288 288
Black Walnut 37 301 506 512

 


NOTE ABOUT CHART: The strength of balsa varies in direct relation to its density or weight - the heavier the wood the stronger it is. The above chart was designed with 10 lb./cu. ft. balsa as the median. In other words, balsa at IO Ibs./cu. ft. has been tested given a value of 100. The other woods were then tested in the same way and given a figure that is numerically in proportion. By comparing the relative strength figures in the chart, it will be seen that balsa is as strong or stronger, pound for pound, than most of the species shown.

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GRADES - http://www.pldaniels.com/flying/balsa/balsa-properties.html

http://www.pldaniels.com/flying/balsa/balsa-grain.png

'A-grain' or also known as a Tangent cut. This cut of balsa has a long grain, smooth creamy apperance and is very flexible but lacks stiffness. This cut is called a tangent cut because the sheets of balsa are sliced off at tangents to the growth rings.
Ideal for: sheet covering on curved fuselages. Wing leading edges (D-boxing etc). forming tubes (motor holders) and flexible spars. 

http://www.pldaniels.com/flying/balsa/tn-a-grain-balsa.jpg

 'C-grain' also known as a Quarter grain. This cut of balsa has shorter grains, mottled in appearance (typically) it is quite stiff but subsequently brittle and prone to splitting along the length of the sheet. The sheets of balsa are cut at a perpendicular angle to the growth rings. Of all the cuts this is regarded as the most asthetically beautiful.
Ideal for: Sheet balsa wings and tails, flat fuselage sides, wing ribs, fuselage formers, and wing trailing edges. 

http://www.pldaniels.com/flying/balsa/tn-c-grain-balsa.jpg

 'B-grain' also know as Random or Mixed grain cut. These sheets vary a lot in properties, often a single sheet will vary from A to C grain across its width. If possible, avoid buying sheets that change grain type across the surface, ie they appear to be C grain on one side and A on the other edge these will cause warping issues.
You will find that most LHS stock tends to be in the 'B-grain' catagory simply because from a given tree there is only going to be a limited number of quality A and C selections. This cut of wood is useful as a general purpose selection, if you don't have specifically a Tangent or Quarter grain stock selection to use for the job at hand, then the random cut will suffice. 

http://www.pldaniels.com/flying/balsa/tn-b-grain-balsa.jpg

Density:

Balsa, like all woods can vary in density, or "mass per volume". The denser a piece of balsa, the more it will weigh. The relationship between weight and strength is not always consistent so don't assume that heavier = stronger.

Density Usage
Metric Imperial


100kg/m.cu 6lb/ft.cu

Contest grade balsa, hard to obtain reliably. Used a lot for indoor planes where lightweight is more important than outright strength

 
120~200kg/m.cu 8~12lb/ft.cu

Common grade balsa, widely available, cheap. This density of balsa is what most model kits are made of.


380kg/m.cu+ 24lb/ft.cu

Don't use for models, if you need extra hardness in wood use something like spruce or birch. Leave this balsa for building houses

Sean Ramones - USA 91709
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ttman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ttman Quote  Post ReplyReply Direct Link To This Post Posted: 07/13/2006 at 11:06pm
sweet dude ill check this all out tmrw its going to help with my home made blades
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SDRamones Quote  Post ReplyReply Direct Link To This Post Posted: 07/13/2006 at 11:11pm
Cool . . . that would be great.  If someone used the info to make decent Balsa blades.  Good luck
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aeoliah Quote  Post ReplyReply Direct Link To This Post Posted: 07/14/2006 at 12:25am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote haggisv Quote  Post ReplyReply Direct Link To This Post Posted: 07/14/2006 at 2:03am

Thanks for that excellent info SEAN!

Yes I was me who asked, and said it should be useful information. I can't remember which thread it was in either!

I asked because the Balsa used in Friendship blades seems very different to that used in solid Balsa Blades (like some Dr N blades), and I was wondring why...

So I guess from reading your info, it could be just a different grade of balsa, no doubt a cheaper one, but this may not make it worse... just different.

Thanks again!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote SDRamones Quote  Post ReplyReply Direct Link To This Post Posted: 07/15/2006 at 10:04am

No problem guys.  Glad to share.  I was wondering the same thing especially when there are a lot of Balsa blades coming out. 

If anyone else find different information, please post.  It is very interesting.  I'll add it to the Original post, as well, so that it will be easier to read together in one piece.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote haggisv Quote  Post ReplyReply Direct Link To This Post Posted: 07/15/2006 at 6:25pm

I heard a comment from an older player recently that sort of makes sense... he said the balse layer in these Friendship blades is probably not pure balse, it's pressed together with grass and other fibres...

Possible I suppose, although I think the ITTF does insist on natural wood, not  particle board and stuff like that.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote chin_penholder Quote  Post ReplyReply Direct Link To This Post Posted: 07/15/2006 at 8:59pm
niiiice info man!
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