Balsa Wonders

[Chris Fretz]

So how does one determine if a piece of balas wood is nice weight or too heavy? What is ment by 4lb wood and so on? What kind of wood is in a Brodak kit? Can you build a Brodak kit light?

I bid on a box of modeling stuff an it had a old Tower Hobbies box of wood that says  Grade AAA. What does that mean? Some of it feels heavy but what do I know.

Just wondering 
Chris

[Brett Buck]

So how does one determine if a piece of balas wood is nice weight or too heavy? What is ment by 4lb wood and so on? What kind of wood is in a Brodak kit? Can you build a Brodak kit light?

I bid on a box of modeling stuff an it had a old Tower Hobbies box of wood that says  Grade AAA. What does that mean? Some of it feels heavy but what do I know.

   Balsa has essentially two characteristics - density and grain. The density is as it sounds, usually expressed in lb/cubic foot. Under 6 lbs is usually considered "contest grade", and very light, 6-10 is medium and above 10 is hard. This is a general characteristic of the tree it was cut from, and how it was dried.
   
   Grain is the direction of the with respect to the "flat" side, and depends on from where in the tree it was cut. "A" grain has the grain running across from one flat side to the other, will easily bend across in line with the grain. "C" grain has the grain running nearly parallel to the flat surface, and is very stiff to bend across the grain, and if you try, it splits. "B" grain is in-between.

   People spend a lot of time worrying about the density, and sometimes don't pay any attention to the grain. You can build the same weight airplane with almost any wood, because the heavier it is, the stronger it is, and in most places you can just make the part out of thinner stock and get the same weight and the same strength of the completed part. Getting the right grain for the part is sometimes a lot more important. You would want C grain for fuse sides and solid flaps, and you want A for sheeting curved surfaces and spars. You almost always want light wood for blocks and medium for sheets.

    Ultimately, it's nice to have the best 3.5-lb of the perfect grain for everything, but it doesn't make all that much difference. How the material is used and the cleanliness of the workmanship is far more important.

      Brett

[FLOYD CARTER]

Everyone seems to like the term "Grade AAA".  It doesn't always signify quality.

Balsa is graded in lbs. per cu ft.  If you want to go to the trouble, calculate the volume of a piece, in cu. ft. and weigh it.  You can calculate the density that way.  More experienced modelers can estimate density just by handling a piece of wood.

There's much more to it when one considers the cut: This can determine the stiffness of a piece.

Dense balsa has many uses, when strength is required.

Floyd

[Chris Fretz]

So since the ribs in my current airplane bow when you squeeze the thickness of the wing, that means the grain is a A and should be a C?

Most of the wood in this box feels heavy an very stiff.

Boy there sure is a lot to building a excellent airplane, I don't know how you experts do it. I'd love to see you guys do this stuff in person.

[Chris Fretz]

   Balsa has essentially two characteristics - density and grain. The density is as it sounds, usually expressed in lb/cubic foot. Under 6 lbs is usually considered "contest grade", and very light, 6-10 is medium and above 10 is hard. This is a general characteristic of the tree it was cut from, and how it was dried.
   
   Grain is the direction of the with respect to the "flat" side, and depends on from where in the tree it was cut. "A" grain has the grain running across from one flat side to the other, will easily bend across in line with the grain. "C" grain has the grain running nearly parallel to the flat surface, and is very stiff to bend across the grain, and if you try, it splits. "B" grain is in-between.

   People spend a lot of time worrying about the density, and sometimes don't pay any attention to the grain. You can build the same weight airplane with almost any wood, because the heavier it is, the stronger it is, and in most places you can just make the part out of thinner stock and get the same weight and the same strength of the completed part. Getting the right grain for the part is sometimes a lot more important. You would want C grain for fuse sides and solid flaps, and you want A for sheeting curved surfaces and spars. You almost always want light wood for blocks and medium for sheets    Ultimately, it's nice to have the best 3.5-lb of the perfect grain for everything, but it doesn't make all that much difference. How the material is used and the cleanliness of the workmanship is far more important.

      Brett

Ohhhh by the way, Hi Brett! It's been a while since you have told me about the world of control line.  You know I was hoping it was just as simple as, get contest balsa an you are good to go. Can "you" tell the grain of the wood just by looking at it?

Chris

[Brett Buck]

Ohhhh by the way, Hi Brett! It's been a while since you have told me about the world of control line.  You know I was hoping it was just as simple as, get contest balsa an you are good to go. Can "you" tell the grain of the wood just by looking at it?

   Usually, yes. I will take some pictures unless someone comes up with it faster. Of course, if you have a clean cut on the end grain, you can look at it directly.

     Brett

[Tim Wescott]

"A" grain means that the cut is along a line from the bark to the center of the log.  If you could see the growth rings they'd be going across the thickness (not the width) of the sheet.

"C" grain means that the cut is at right angles to the cut of "A" grain.  You can't get perfect "C" grain because trees are round (well, you can't get perfect anything).  If you could see the growth rings they'd be going across the width of the sheet.  Because they'd be longer, they'd add stiffness to the wood.  "C" grain wood has a speckled pattern to it that "A" grain doesn't have -- it's obvious if you know what you're looking for.

People used to specify "B" grain -- that really means that the cut is somewhere in between perfectly "A" and perfectly "C" grain.  I'd use this sort of mixed-grain wood for ribs, and save the "C" grain for making hand launch glider wings.  Once you've seen good "C" grain wood you'll kind of be able to tell "B" grain.

When in doubt, bend the sheet across the width (not length).  Take weight into account, but if it bends readily without cracking, it's "A" grain or close to it.  If it's stiff (and if it breaks when you try too hard) it's "C" grain.

You also want to look a the sheets to make sure that the density and grain is consistent on the sheet -- trees are living things, and therefore not perfect.  There's plenty of balsa out there that doesn't have straight grain, or that has grain running diagonally across the sheet.  There's balsa out there that varies in density within a sheet -- it's not at all uncommon to have considerable variation from one edge to the other.  I've had balsa that's hard as rock (probably 14-pound or more) on one edge, and light as a feather on the other.  I've also gotten balsa that varies from "A" grain to "C" grain along the width of the sheet.

The bottom line is to look at what you have and think about where you're going to use it.

[Tim Wescott]

Density is pounds per cubic foot.  You can weigh the sheet and calculate the density.  When I get balsa in the shop, the first thing I do is weigh every sheet and write the density on the end so that I can pick out suitable balsa quickly.

I'm sure that there are calculators out there, but if you stick to 3" x 36" wood it's really easy: weigh the wood, then divide the weight of the wood in ounces by the thickness in inches.  The answer is the density in pounds/cubic foot.  (and remember that to divide by 1/4 you multiply by 4, divide by 1/8 means multiply by 8, etc.).

One could make a chart.  Or make a calculator card out of clear plastic; that would be more fun.

[Norm Furutani]

Sig has a nice online balsa brouchure. Maybe someone can figure out a direct link. Here's how to get there http://www.sigmfg.com/cgi-bin/dpsmart.exe/IndexBuidlingMaterialsEquipmentF.html?E+Sig. On the Sig top menu bar, select Product support,> Building info >About Balsa

60+ years of breathing balsa dust, working this computer, not so much!

NormF

[Ara Dedekian]

      Try this, it makes calculating the density quick and easy.

      Ara

[Ara Dedekian]

         To get the above program to work, go to the National Control Line Racing Association site, nclra.org and click on 'Helpful Programs'.

         Ara

[Mike Haverly]

Or just click on this.  http://www.nclra.org/Programs/BalsaDensity.php

[Brett Buck]

Sig has a nice online balsa brouchure. Maybe someone can figure out a direct link. Here's how to get there http://www.sigmfg.com/cgi-bin/dpsmart.exe/IndexBuidlingMaterialsEquipmentF.html?E+Sig. On the Sig top menu bar, select Product support,> Building info >About Balsa

60+ years of breathing balsa dust, working this computer, not so much!

NormF

  Try this:

http://www.sigmfg.com/cgi-bin/dpsmart.exe/InformBalsaJV5.html?L+Sig+vyzo8349


    Brett

[tom creasey]

      Try this, it makes calculating the density quick and easy.

      Ara

     Thank you for the post....got it!!! Web site bookmarked

[Chris Fretz]

Far out fellas!  That's is some good info! Now all I have to do is remember it all, or remember to come back to this post as a reminder. I like that simple calculator, I'll have to give it a try later!

Chris

[Chris Fretz]

When you guys order balsa from National Balsa or wherever can you specify you want A grain or C grain? Or do you order a specific lbs and get what you get in grain?

[bill bischoff]

"A" grain means that the cut is along a line from the bark to the center of the log.  If you could see the growth rings they'd be going across the thickness (not the width) of the sheet.

"C" grain means that the cut is at right angles to the cut of "A" grain.  You can't get perfect "C" grain because trees are round (well, you can't get perfect anything).  If you could see the growth rings they'd be going across the width of the sheet.



Isn't this backwards?

[Jim Hoffman]

Here are some spread sheets showing the density of balsa

One spread sheet is in grams, the other in oz.

Both have nice photos of A, B, and C grain

The various tabs cover the standard lengths and widths generally available

Hope this is helpful
Jim Hoffman

[Steve Helmick]

"A" grain means that the cut is along a line from the bark to the center of the log.  If you could see the growth rings they'd be going across the thickness (not the width) of the sheet.

"C" grain means that the cut is at right angles to the cut of "A" grain.  You can't get perfect "C" grain because trees are round (well, you can't get perfect anything).  If you could see the growth rings they'd be going across the width of the sheet.



Isn't this backwards?

I always thought SIG had this diagram swapped backwards between A & C. Maybe Mike Pratt will lay down the law? FWIW, the pictures are correct. That pretty speckled balsa is C grain fer sure.   Steve