# BALSA wood aircraft?

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#### captarmour

##### Well-Known Member
Hello All,

The fact that Balsa trees grow so fast, and therefor a renewable resource, can entire aircraft be built from BALSA?

#### plncraze

##### Well-Known Member
HBA Supporter
The Sorrell Guppy used quarter inch balsa wing ribs. There were options for other construction methods.

#### sachaknoop

##### Well-Known Member
Balsa when used as a sandwhich in boats works very well especially because its water resistance. It surprised me, but I heard this from a composite ship builder. When the outer layer lets through water the balsa does not soak. The grain of the balsa used is vertical which helps stopping the water migrate into the sandwhich. With the grain in this direction compressibility/weight performance is the best you can get, although the endproduct will be a little heavier. Most foams are lighter then Balsa.
Balsa can be a very durable solution in some cases.

Sacha

#### captarmour

##### Well-Known Member
A marine surveyor told me that if it is epoxied it would come part as the fibers would separate from each other, that is why in composite hulls the balsa is on end.

however a boat builder said it absorbs the epoxy better than most woods this penetration makes it very strong.

as they say doctors differ, patients die...

wow 1/4 inch balsa ribs! It must be a lot stronger than I thought!

#### Himat

##### Well-Known Member
wow 1/4 inch balsa ribs! It must be a lot stronger than I thought!
I did once read that balsa wood is classified as a "hardwood". If that is right, balsa is light with a low densinity, but with the "structure" more like a heavy and dense hardwood.

#### TFF

Balsa is a hardwood and Spruce is a softwood. Funny huh? Has to do with cell structure; probably got named when someone was messing with oak 400 years ago. It is one of the strongest per weight woods around; just not one of the strongest woods. I use about 50 lbs a year on my models, so I wish it was cheap here as it is there. Per board foot Balsa is equal if not more expensive than spruce in the US. A 1/4"X3"X36" piece of balsa is about $4-5. #### Jay Kempf ##### Curmudgeon in Training (CIT) Lifetime Supporter Balsa is a hardwood and Spruce is a softwood. Funny huh? Has to do with cell structure; probably got named when someone was messing with oak 400 years ago. It is one of the strongest per weight woods around; just not one of the strongest woods. I use about 50 lbs a year on my models, so I wish it was cheap here as it is there. Per board foot Balsa is equal if not more expensive than spruce in the US. A 1/4"X3"X36" piece of balsa is about$4-5.
I used to buy balsa by the board feet from Aircraft Spruce. They used to sell spruce and balsa odd lots. Still have a lot of balsa 2x4 stock and bundles of spruce lots. That and a narrow kerf blade on a table saw and a band saw and you can pretty much build anything. I think I still have a pretty large rack of lite ply and a lot of glow motors as well. Of course I was in the business back then. Still have lots of carbon fiber, kevlar, S and e glass, gallons of epoxies beyond their shelf life, micro balloons (cubic feet), and also tongue depressors and lots of Dixie cups. Yes I have a large shop. Now we are building foam/electric kits which is an interesting seque when looking at hybrid technology for full size stuff. Electrics are amazing.

#### TFF

##### Well-Known Member
Someone at my club is importing balsa; about one container a year. I also got to see the B USA wherehouse with 20,000 sqft of balsa 20 ft high, and it was only half full. The electrics are amazing, especially the small ones, but I am an engine guy.

#### davidb

##### Well-Known Member
A marine surveyor told me that if it is epoxied it would come part as the fibers would separate from each other, that is why in composite hulls the balsa is on end.

however a boat builder said it absorbs the epoxy better than most woods this penetration makes it very strong.

as they say doctors differ, patients die...

wow 1/4 inch balsa ribs! It must be a lot stronger than I thought!
When corrected for specific gravity (density) most woods are about the same strength. Balsa is about 1/3rd the weight/ 1/3rd the strength of Sitka spruce. However, the shear strength of balsa parallel to the grain (even when corrected for density) is significantly less than Sitka spruce. A glued joint can't be stronger than the surrounding wood. So, it won't just be an issue of up-sizing the stock to account for the lower density balsa. You'll also have to enlarge the glue joint surface area--larger gussets? The fact that epoxy soaks deeper into balsa won't make the joint stronger...the joint will just fail where the penetration stopped.

Other than for ribs and core material, it seems balsa will be problematic to design around. It does vary widely in density/strength--not sure how you would deal with that issue. With it's low compression strength, bolted joints (spar?) will be an issue too.

I'm not an engineer but it does seem like a low aspect ratio delta wing design is a good direction if you have to use balsa. Let us know what you come up with for structure design.

#### captarmour

##### Well-Known Member
Any thoughts on white cedar? I have a few trees I could probably harvest.

#### WonderousMountain

##### Well-Known Member
Does white cedar have problem with sap seperating the boards like an oiled seem? Cedar is very good wood to use, the important thing when selecting aircraft wood is that the grain runs quite strait, the density and rings aren't pethy, or split too easily, and it must be dried properly. If you're not in a hurry, to build, cedar balsa sandwhich ought to work very nicely. There are a number of good epoxies, which have to be used, because regular wood glue will absorb moisture where you live, even water-resistent won't hold up over time.

Cedar has been used in many planes, and the differences between species aren't too great. Redwood appealed to me when it was available locally, but yellow poplar was the best wood locally afordable in Tennessee/N Carolina. Could you test some small pieces of lumber from your area?
That would give you the most reliable answer, better to let the wood speak for itself.

#### Jay Kempf

##### Curmudgeon in Training (CIT)
Someone at my club is importing balsa; about one container a year. I also got to see the B USA wherehouse with 20,000 sqft of balsa 20 ft high, and it was only half full. The electrics are amazing, especially the small ones, but I am an engine guy.
Yeah, I have a pretty healthy supply of Super Tiger high end stuff too. Got rear exit pattern motors. Large capacity stuff, small capacity stuff.... Boxes and boxes.

#### TFF

##### Well-Known Member
The good Tigers. I like the old pattern have a Summit 1 and a Vertigo II kit. Kind of funny that the thread is about balsa.

#### davidb

##### Well-Known Member
Any thoughts on white cedar? I have a few trees I could probably harvest.
Take a look at http://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf Chapter 5 has charts of the mechanical properties of various woods. You'll see that Atlantic white cedar is quite a bit lighter/weaker than Sitka spruce. The FPL Wood Handbook and ANC-18 are good sources to study before you start designing/building a wood aircraft.

How big are your cedar trees? Harvesting and milling your own aircraft grade lumber is no small undertaking. Are you really planning to design your own aircraft? I'd guess it'd be easier to design around white cedar rather than balsa.

#### captarmour

##### Well-Known Member
A couple are about 2 foot diameter most are much smaller. Must admit I have a thing for balsa, not sure why, maybe because its so light.

Really I'm attempting a low ar flying wing type Wing In Ground effect. My models are showing good promise. The object of the exercise is to fly without CAA being involved. I realize I have to sacrifice some efficiency for stability. I am experimenting with inverted airfoils. Really flat with windward bevel LE and delta shaped so that AC is at the center of area to minimize aft cp movement in GE.