Airfield Models - Construction Materials

About Balsa Wood

July 10, 2009



Home
About
What's New
History
Models Gallery
Model Building Safety
Articles
Mail & FAQ
Site Map
Site Feedback
Contact
Register
Add to Favorites
Tell a Friend
Comments
Design and Build Contest
Items For Sale
Search Airfield Models

Back to Construction Materials

 



 

Airfield Models (http://www.airfieldmodels.com/)Balsa Wood

Balsa Wood is the traditional construction material used in flying model aircraft.  Balsa comes in several shapes and sizes and you will need some of all of them if you scratch-build.

If you build from kits, you will still need a piece here and there.  Sheet balsa is the most basic.  Useful sizes are from 1/32" to 3/8" with some sticks, tri-stock and blocks to round things out.  If you can saw your own sticks and triangles you will save A lot of money.

Generally speaking, you will save money buying bulk or larger sizes.  For example, if you buy 4" wide by 48" long balsa sheets, you will generally pay less per cubic inch than if you buy 3" x 36" wood.  Sometimes it is just the opposite because the size may be difficult to obtain for various reasons.

Also see

 
 

Wood Selection

If you scratch build, it is important for you to be able to select your wood properly.  You should also be able to determine what kind of balsa to use in various situations.

For example, wing spars should be strong and straight-grained.  They tend to be heavier wood, but just because a piece of wood is heavy does not mean it will make good wing spars.

Grain Pattern

The grain-pattern and density are the determining factors for selecting a piece of wood.  I suggest you use an accurate postal scale or beam-balance to weigh your wood.

Balsa Wood

Balsa Wood

Two typical Balsa Wood samples.  Balsa has a variety of grain patterns.  In some cases it is very important to select the correct grain balsa.  Example are long, load-bearing members such as spars and longerons.

Another example is planks such as used for the tail surfaces of many sport models.  Selecting the incorrect grain will almost guarantee warping.

In other cases, the grain isn't so important such as using filler blocks that are primarily cosmetic.  In these cases weight is the most important issue.  Another consideration is consistency of the block so that it carves and sands uniformly.

Click the images to see them at higher resolution.  Both of these pieces were sanded with very fine glass finishing paper.  The image at the bottom in particular has very deep grain that no amount of sanding will remove.  Further sanding may remove grain that is open now, but will open other grain.

This is what makes balsa so difficult to finish and why some type of covering such as tissue, silk, silkspan, fiberglass or other will make finishing faster and give better results.

If your goal is to create a grain-free finish then you should always apply a covering first.  If you use sanding sealer or primer to fill the grain then you probably won't hide the grain completely anyway and you'll add a lot of weight to the model.

Using a covering over bare balsa will result in a lighter finish.

Balsa is classified as having one of three grain patterns:

  • A-Grain

    A-Grain wood has long, straight grain lines and is best used for sticks.  However, it warps easily and should not be used in large, unsupported planks such as tail surfaces.

    A-grain tends to bend more easily than B or C-grain and can be used for turtledeck sheeting, leading edge sheeting (when there are plenty of supporting ribs) or small radius curves that will cause the sheet to support itself.

  • B-Grain

    B-Grain wood has mid-length grain that is not as long as A-grain.  It is stiffer than A-grain, yet does not crack as easily as C-grain.  Therefore it is a good choice for flat sheets such as fuselage sides, leading edge and trailing edge sheeting, ribs and formers and other general uses.

    It is usually safe to pick B-grain wood when you do not know the correct wood to choose.  However, you should always use A-grain for long, load-bearing members such as fuselage longerons and wing spars.

  • C-Grain

    C-Grain wood has a mottled appearance due to its very short grain pattern.  Proper use of this wood will result in models that are light and strong.  However, C-Grain is the hardest wood to judge and can vary dramatically in strength with little indication of being weak or strong.

    For example, C-grain can appear to be strong because it is stiff across the grain.  The same piece of wood can also be pulpy and weak in spite of how strong it feels when you try to flex it.

 
 

Wood Defects

  • Poor grain pattern (veers wildly all over the sheet)

    Generally useless wood.

  • Non-uniform density

    If the density is consistent along the grain, but varies across the width of the sheet, then I will use it if I can strip a piece off that is all the same density.  I do not use the full sheet at one time because it will not sand evenly.  For example, I wouldn't use it for wing sheeting.

  • Hopelessly twisted like a pretzel

    I do not buy any wood that is severely warped.  A little warpage is normal and OK.

  • Knots

    Balsa knots are tiny and nothing to worry about.

  • Insect damage

    Looks like 1/32" holes drilled at angles through the wood.  The mills do a good job of weeding badly insect damaged wood out.  I have never seen a piece that I would discard due to this kind of damage.

 
 

Mill Defects

  • Joined wood

    Some mills glue blocks of balsa together before sawing it.  There is an obvious line down the sheet of wood where it has been joined.  While there is no fundamental reason not to buy this wood, sometimes the sheet has two different densities or grains of wood.

    For example, half the sheet may be soft and flexible and the other half may be heavy and stiff.  If the wood seems to be the same across the sheet, then it is ok to buy.

  • Sticks sawed or stored in such a manner to have taken on a permanent warp

    These sticks should be avoided because they will create unwanted stress in your model after they are braced into the correct shape and glued into the structure.  Often these sticks were stored standing up or they bowed after being cut from a sheet.

    Sometimes wood looks like it just did not go through the saw right and has a long 'S' curve shape.

  • Tapered cuts

    You would be surprised if you knew how many sheets of balsa tapered in thickness across the width of the sheet.  I have seen wood that varied by more than 1/32" in thickness.  While that may not seem like much, think of what will happen if you join these together to sheet a wing.  You will have to sand a lot of wood away to get the sheet even and it will end up being too thin.

A taper milled across the sheet is a defect.

Tapers across thickness (end grain shown)

Similar to this is wood that tapers towards the edges from the middle.  I do not have an explanation for how this happens, but I have seen it happen on occasion.

A taper from the middle to the edges is also a defect.

Tapers from middle to edges (end grain shown)

The above images are more extreme than the actual problem, but they get the idea across.

 
 

Working with Balsa

  • Before building a new kit, ensure it has been in your shop at least a week to climatize the wood.All wood needs to be acclimatized to your shop before you use it.  In other words, when you buy a sheet of wood (or a new kit) from the local hobby shop, you should let the wood sit in your shop for several days before using it.

    Wood shrinks and swells significantly.  If you glue a piece of wood that you just brought from another climate (such as from your air-conditioned home into your non-climate-controlled shop) to another piece of wood that has been in your shop then you are asking for warps.

  • Because Balsa is soft, there is a general conception that it is easy to work with.  That is both true and false.

    Balsa is easy to cut and to shape, but it can be difficult to smooth.  Balsa end grain tends to fuzz especially on compound curves such as wing tips.  There are a couple ways of dealing with this.

    First, shape the part as you would normally.  Then finish sand using finer and finer papers until you reach about 800 grit (wet or dry used dry).  If the fuzzing still occurs and it will cause problems, then rub some model airplane cement in the grain to harden it and make it easier to sand off the fuzz.  You can also brush on a coat or two of model airplane dope.

  • Sheet balsa always needs to be trued if you need it to have a straight edge (wing sheeting, for example).  I have had to cut as much as 1/2" off on edge to get it straight because of the amount of curve it had.  Do not ever assume that the edge of a sheet of wood is straight when you buy it.  It is not.

  • Sometimes after you slice a piece off a sheet it will bow.  At this point you can do one of two things.  Normally what I do is go ahead and use the sheet and remove the bow with pins, tape or some type of clamp.

    The other option is to slice the sheet a little wider than you want at first because there is no way to tell in advance if a sheet will bow after it is cut. By cutting it oversize, the sheet will probably bow as it would if you cut the sheet to size, but there is still wood left to trim or sand.

  • If the edge has to be perfectly straight then I weight a sheet down with my heavy straight edge with just a quarter inch or so off the edge of my bench.  Then I use a long sanding block to true the edge.

  • When sanding a thin sheet of balsa on one of the faces, do not sand back and forth.  Sometimes the block will grab the wood on "back" stroke and crumple the wood.  Use "away" sanding strokes on thin wood (1/32").

  • I love my Microlux Table Saw.  There are other small table saws on the market.  This is the only one I have owned, so I can not compare it to other makes.   The best thing about it is that when I need matched sticks, such as wing spars, I can cut them from a sheet of wood instead of standing in a hobby shop trying to match sticks through scientific bouncing, bending and guessing weights.  The bonus is I save a lot of money on the cost of stick wood.   Before I cut the sheet into sticks, I use a colored marker on the end of the sheet so I know which sticks go together.

  • Do not buy just the lightest wood you can.  And do not buy heavy wood that bends easily.  The whole point of heavy wood is to have stronger, stiffer wood.  Heavy, flexible wood is generally useless.

  • When using balsa sticks, make sure the grain does not suddenly veer off the edge of the stick.  If it does, it will crack there if subjected to a load.  You could use this stick for a leading edge that gets capped, sheeted or otherwise reinforced, but do not use it for spars or longerons.  The same thing applies to any piece of wood you would use for a spar.

  • Every time you need a part, go to your scrap box first.  Find the smallest piece of wood that is big enough to do the job.  Be sure to hang on to small cut-offs of special shapes such as triangle stock.  I save anything that is at least 1/2" long and put them in a baggy so they are easy to find.

 
 

Balsa Wood Sources

Your local hobby shops should carry the most common sizes of balsa sheets, sticks, triangles and trailing edge stock.  Hobby shops tend to be more expensive than bulk suppliers.

If you build from kits then it is probably best to buy just what you need from a hobby shop rather than by mail or internet.  The reason being that you can hand select the wood so that it is most appropriate for the application.  Balsa is also available in some art and craft stores, but this is the most expensive source I've found as well as having the smallest variety.

I buy my wood in bulk because I scratch build most of my model aircraft.  Buying in bulk means getting a variety of weights of wood so I always have what I need.  I also order contest balsa in bulk when it is available.

I order my wood from Balsa USA and Lonestar Models.  I have also ordered from Sig Manufacturing in the past but their wood is more expensive.  I also think Sig wood has a nicer finish.  Balsa USA wood tends to be a little on the rough side, but I sand everything anyway so it's not a problem.  I'd rather save the money.

The other major source of balsa wood that I know of is Superior Balsa.  I have never ordered from this company so I can't recommend it.  Others tell me they have been pleased with the wood they received from Superior.

 
 

Previous
Next

Adhesives used for Model-Building
Plywood and Hardwood for Model-Builders

Comments about this article

 
 

Back to Construction Materials
Airfield Models Home

 
 

Copyright 2002 Paul K. Johnson