Balsa Wood For Freeflight Models
Let us start by first rejecting all that you have read about balsa wood and structure. About every other year one of the magazines prints that classic article containing a cross-section of a log and fortune cookie advice about the effect of cleaving the log along this plane or that. I do not think that “thermals will always fawn on wings that are quarter-sawn;” or is it “balsa with freckles costs many sheckles”? In any event, unless you live in Ecuador, forget that stuff.FAC News #89 Glue Guru – Fuselage Structure
Understanding Balsa Grain
As any newcomer to stick and tissue freeflight models quickly learns there’s ton’s of, often conflicting, information about just every aspect of the hobby. In all honesty, the best way to learn what works best for your models is to BUILD, FLY and HAVE FUN! It’s your hobby – learn what works best for your type of models, construction methods and flying style.
Sheets of balsa wood are usually classified into three different Grain Types, the following information is copied verbatim from an aforementioned “classic article” by SIG Manufacturing:
- A-Grain sheet balsa has long fibers that show up as long grain lines. It is very flexible across the sheet and bends around curves easily. Also warps easily. Sometimes called “tangent cut.”
Do: Use for sheet covering rounded fuselages and wing leading edges, planking fuselages, forming tubes, strong flexible spars, HL glider fuselages.
Don’t: Use for sheet balsa wings or tail surfaces, flat fuselage sides, ribs or formers.
- B-Grain sheet balsa has some of the qualities of both type A and type C. Grain lines are shorter than type A, and it feels stiffer across the sheet. It is a general purpose sheet and can be used for many jobs. Sometimes called “random cut.”
Do: Use for flat fuselage sides, trailing edges, wing ribs, formers, planking gradual curves, wing leading edge sheeting.
Don’t: Use where type A or type C will do a significantly better job.
- C-Grain sheet balsa has beautiful mottled appearance. It is very stiff across the sheet and splits easily. But when used properly, it helps to build the lightest, strongest models. Most warp resistant type. Sometimes called “quarter grain.”
Do: Use for sheet galsa wings and tails, flat fuselage sides, wing ribs, formers, trailing edges. Best type for HL glider wings and tails.
Don’t: Use for curved planking, rounded fuselages, round tubes, HL glider fuselages or wing spars.
And there it is Balsa Butchers, you’re now experts! Haw-w-w-w! Not quite. Fledgling peelots with little working experience with balsa, or any other wood for that matter, may have a tough time wrapping their heads around what this grain stuff is all about. If this is the case, purchase a few sheets of balsa wood with grain resembling the various photos above. Cut the sheets into 6″ to 8″ long strips and investigate the differences.
Bend the sheets with and against the grain. Twist the sheets and try to roll them into tubes. Experiment by breaking the sheet with, against and diagonally across the grain. Keep notes and photos of your findings. Experience will be your best teacher.
When reading model plans you’re certain to come across lingo such as, “Use 8lb A grain balsa”. You already know about balsa grain, but what’s that “8lb” number all about. Here’s a snippet explaining balsa density according to William F. McCombs:
Balsa comes in various weights or “densities”. A slower growing period in the tree’s life (less rain or colder weather) results in denser wood, and because of this part of a sheet of balsa may be denser than the rest. Denser wood is also heavier, harder, stiffer and stronger. Balsa is usually classified by modelers into 5 types of density, density being express as pounds per cubic foot or #/cu.ft., as in Table 9-1.McCombs – Making Scale Model Airplanes Fly
A simple way of determining the density of a standard sheet (3″ x 36″) of balsa, and hence of strips of pieces to be cut from this, is to weigh the sheet in ounces and then divide this weight by the sheet’s thickness in inches (this give the average density; part may be more and part less). For example, suppose a sheet of balsa 3″ x 36″ and 1/16″ thick weighs .55 oz. Its density is .55 / 1/16 [or .0625] = 8.8 #/cu.ft. …McCombs – Making Scale Model Airplanes Fly
A cheap Jewelers Scale can be purchased online for less than ten smackers and is well worth the investment. Remember to weigh each sheet before hacking it up. It’s smart to write the original sheet size and density on each balsa sheet with a marker for future reference. Here’s a calculator to help you determine the approximate density of most any size rectangular sheet of balsa.
Balsa Wind Checking
When Balsa trees are growing, especially the fast growing ones, they are subject to wind bending which actually compresses the grain in some sections. For all practical purposes the compressed wood is broken; it just hasn’t fallen apart yet.
A fast way to identify a wind-checked sheet is to hold it up to a florescent light, at an angle, to simulate a reflection and wind-checks show up as a spider web. This wood is good for tips or nonstructural fill in. Strips are harder to identify but may be felt by gently pulling the stick through your fingers putting a slight bend on it. The wind-checked strips will feel like they have facets and will give slightly…
Another feature to look at is uneven density across the sheet width and length. Hold the sheet up to a fluorescent light like an X-Ray film. The dense areas will show up darker.
If you accidentally used wind checked wood, a light coat of thin CA, on both sides, will make a very good repair.Wind Checked Balsa – Gene Wallock – From The Winding Stooge, Newsletter of the Nebraska Free Flighters, Hank Sperzel, Editor
Flying Aces Magazine Wise Crack-Up
Balsa Wood Summary
There’s lots to know and understand about balsa wood. However, you don’t need to know it all to build successfully flying models. Experience and experimentation will improve model performance. This takes time and will be an ongoing lesson.
Don’t be discouraged. Buy various pieces of balsa wood and put ’em through the ringer. Play and have fun, this is what the Flying Aces Club is all about! Again, be sure to document the lessons you learn – this information will go a long way as you grow with your hobby.
A word of caution regarding balsa wood, be weary of spongy sheets of balsa wood. If the sheet is squishy or spongy pass it by. This super-soft balsa is not good for very many things. Yes, it’s usually a light sheet of balsa but it has no strength in any direction and will fail extremely quickly.
For those of you Daddy Warbucks out there with expendable cash and the inclination to get a head start you can purchase wood to your specifications from online stores such as Specialized Balsa Wood. They allow you to select balsa sheet size, density, grain, etc. Be prepared to pay for their expertise and time. Once you’ve built a few flyin’ crates you may find investing in the good stuff worthwhile for building more serious models.
Alan Orthof’s “Snowbird” Stick Job
There’s no better way to gain experience working with balsa wood than to build a few balsa flying stick jobs. Here’s a simple design by the famous Alan Orthof that is a simple build. It can be easily modified to use a plastic prop with a nose bushing. Be patient and understand the plan and instructions before you start.
Shape all the parts and airfoils with a sanding block or a quality emery board. Use a glue similar to Testors Green Tube #3505. It applies easy, joins metal to wood and “sets” in a few short minutes. Download the plan here and get busy!
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