Selecting sheet balsa

[R/Cflyboy]

I wanted to get started on a new project today so I headed to my LHS to get some wood. My local HS is the old fashion kind, they stocks lots of wood, Balsa, Bass, and Spruce. I've gotten wood there before, just picked what I thought was good wood for my purposes from the regular stock. They carry both contest and select balsa. This time I took a hand held balance postage scale and a chart from SIG showing the various weights of 2",3", and 4" by 36" wood for 6lb,8lb,10lb,12lb, etc. wood. I was surprised to find wood that weighed the same as contest wood in the select grade wood. Also, how the wood varied in stiffness and hardness in these lighter weights. I found very stiff wood that was on the low side of weight for contest grade wood along with hard and soft wood of contest grade.

My question to the members who do a lot of scratch and building from plans is does the weight of the wood make a defference in the strength? should I be using heaver wood for high load areas and lighter wood for low loads?

when I got home I checked some of the wood I had in my reserve stock and found that it was pretty uniform but not as light as I selected using my scale. Also, by selecting out the lighter grade of wood from the select grade I saved about 50 cents per sheet or more depending on thickhess and width.

Whats the answer?

[fhhuber506771]

selecting balsa takes experience...

there are variations in colot and grain patterns that you can learn to see.. and you can eventually learn to select by feel and a slight "flex test" to see how springy the wood is..

Lots and lots of working with the wood... thats how you leearn to select the best pieces for the application. Note... sometimes you WANT the heavy hard stuff...

[BMatthews]

You've learned a bucket load from just that one shopping trip. You're right on all counts. Harder wood has more fiberous content as well as the cell walls having more lignum in them so the wood is both heavier and stronger. Lighter wood can suffer from inadequite fiberous content and should ge used for lesser tasks.

I run into this a lot with my free flight models. I'll select my wood first for weight and then for parts, especiallly small section strips, I'll cut a small sample of a sheet and break it. If it breaks off clean with little or no fiber splinters then it's used for stuff like ribs or area fills. If it shows signs of light fiber content then it's fine for ribs and lightly loaded stuff. Wood with high fiber content is prized for very light wing spars, fuselage longerons and similar higher stressed duties. The medium to heavy wood generally does not need to be break tested but I do it anyway to avoid unpleasent surprises. Higher loaded areas like larger model spars and big rubber model main fuselage longerons get the hard and strong stuff.

Then you find the different grains. The stuff that has a checkered appearance is C grain and you'll find that it's more stiff if you try to flex it cross grain. There's more radaial rays so it's stiffer to flex. The other A and B grain differences are more subtle and can basically be ignored.

Contest wood is selected by an extra inspection process and that's why they charge more. But as you've found the generic wood often has contest grade stock as well. But you're saving the extra inspection price.

[R/Cflyboy]

Thanks for the info., this really helps. I've been building and flying mostly woody sailplanes for the last few years. Prior to that I built rubber contest planes. Lately, I've gotten into big electric scale where weight savings is more critical. I guess what you're saying is there is a place for all weights of wood i.e.. Heavy strong wood for spars and load baring structures and lighter wood for ribs, infills, fill in stringers. I watch for the different grained wood as each grain pattern has it's own particular use.

Am I correct in thinking that in all cases lighter is better? That is the general drift that I get from what I read here in the various threads But, it seems to me that long load baring structures would benefit more from harder and therefore heavier wood...a trade-off or compromise of weight to strength for the job. Am I on or have I missed the point? Also, does light hard wood have the same strenght as heavy hard wood? How can it if heavy wood has more fiber and lignin and thus the weight.

Gary

[BMatthews]

Lighter is always righter.... up until the parts fail... :D

It's a tradeoff as you say. Spars and other key load bearing elements require harder and stronger wood. But if you pick the more fibrous stock you can often get away with a medium weight grade as we often overbuild from a purely airload standpoint. And then there's the idea that if the overall model is lighter than you don't need the high strength in the first place. As a contest rubber modeler I'd say you're well aware of that idea even if you only made a few of that style of model from kits and similar.

Electric adds it's own quirks. The flight gear is such a heavy and contentrated load that the model must be able to deal with it. The real key there is to rely on some form of composite center structure even if it's only composite in terms of a built up balsa core that the rest of the model attaches to so that the loading is spread out to more of the outlying structure. But carbon or kevlar in various forms can be used to great effect in supporting localized point loads and spreading these loads out to more of the airframe.

I'm also a big believer in putting more of my structure out near the surface where it can do more good assuming decent support to prevent buckling. For example I would rather build up a strip box than use a center beam keel. Wood at the core of a structure does little good for the weight.

[R/Cflyboy]

Same subject different question: is there a rule of thumb as to the size (thickness) of shear webs? I'm replacing 1/16 webs that are glued on each side of a 3/8th spruce spar cap with 3/16th shear webs located in the middle of the spruce spar cap. I know the 3/16th will be stronger and less prone to failure but, how do you size them? I used the TLAR (That Looks About Right) method.

[Sparky Paul]

3/16" between the ribs all the way out is serious overkill.
The most load is at the root.
For 2 rib bays either side, 3/16" should be fine.
Then 1/8" for a couple,...3/32..;.. then down to 1/16" for the rest.
Mass out in the wings is to avoided, if it's not needed.
On balsa selection, SIG has the best mediums to soft.. almost never any really hard balsa.
Midwest is mostly medium to hard, and about twice the SIG price, locally.