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May 30, 2013, 01:02 PM
Jim in the Desert
Thread OP
Discussion

What's the difference between "good" wood and not good wood


I'm getting interested in building my own planes. I've only flown foam planes until I started getting trained on our SIG LT-40. What a plane. I'd like to buy wood/covered planes but they seem a lot scarier to crash than foam planes.

But it occurs to me if I build it, I can fix it. But now I read about how kits from decades ago had great balsa, spruce, and ply, and new stuff might have "punk" wood or just cheap bad wood.

I could not tell the finest wood from the worst. You experienced guys, how is good wood different? How it looks? Weighs? Feels?

And why does it matter?

Thanks
Jim
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May 30, 2013, 01:59 PM
Registered User
Here's an excellent article on selecting Balsa;

http://airfieldmodels.com/informatio...ding/balsa.htm

and one for ply and other wood.

http://airfieldmodels.com/informatio...ng/plywood.htm

Selecting wood that is appropriate for the purpose will, inevitably, result in a lighter, stronger, straighter model than just throwing any wood into it. For example, what we look for in a sheet of balsa for a fuselage side, is different than we want for skinning a wing (not just thickness) and different again from what we want for carving and shaping a cowling or fairing.

A lot of wooden kits tend, for cheapness, not to have "selected" wood in them. Meaning that for the manufacturer, if the ribs are to be cut from 1/16" sheet, then thats the only factor considered, so whatever sheets arrive from the supplier are just chucked into the machine, so you might find you end up with one soft fuselage side, and one rock hard one, meaning that a warp is highly likely in the finished model.

I'm not sure the quality of wood in kits has got that much worse, but the quality of the average "balsa stand" in a LHS certainly has. I haven't seen any really really good wood in a hobby shop for a long time. The saving grace is that there are a number of online merchants where you can select the grade, grain and density of the wood you want.

Hope that helps some

Matt
Last edited by MCarlton; May 30, 2013 at 02:04 PM.
May 31, 2013, 01:31 AM
Registered User
One thing I learned early on is that when selecting sticks you bounce them on a hard floor. For spars and longerons you should hear a "ting" sound and the stick should bounce quite a way back up. If you hear any kind of dull thud or the stick doesn't bounce back nicely then there is something wrong with the wood. Since spars and longerons are pretty much the most important parts of your structure, just getting these pieces right will go a long way toward ensuring your plane is flight worthy. And that test is for any wood - balsa, spruce, basswood. Drop it from about 18" or so.

Oh yeah, inspect the stick before you drop it. A lot of the sticks will be too soft, won't be straight or have bad grain so they never even make it to the drop test.
May 31, 2013, 02:16 AM
Never stop being a kid.
BrundleFlyBy's Avatar
Quote:
Originally Posted by CafeenMan
One thing I learned early on is that when selecting sticks you bounce them on a hard floor.
So you're the guy who mushrooms the ends of all the balsa sticks in my LHS!

No doubt the same guy who bruises all the avocados at the grocery store.

Swine!

May 31, 2013, 02:29 AM
Registered User
Well, if it mushroomed then I did you a favor - you know not to buy that piece because if it were worth having I would have bought it.

Yeah, and I love avocados.
May 31, 2013, 06:33 AM
Jim in the Desert
Thread OP
Thanks so much Matt. Look forward to reading these this weekend.

Jim
May 31, 2013, 03:43 PM
Built For Comfort
Tepid Pilot's Avatar
Argh! Must resist...

But seriously. The airfieldmodels.com link is absolutely marvelous. Bookmarked! Selecting balsa is an art worth studying for sure. Never heard of bouncing it til now, I prefer to heft and flex it.

TP
May 31, 2013, 05:18 PM
Registered User
IMHO, this is a bit more informative source of Balsa information:

http://www.go-cl.se/balsa.html


Please check out the end grain chart, showing how the A/B/C grains are sawn from a log. This is woodshop 101 stuff and has direct application to nearly ALL structual uses and stability factors of wood, not just balsa, used to its best avantage. Balsa even has some advantages that most other woods don't usually have. Its how the annular rings of the tree are oriented to the finished surfaces and edges of the individual board/plank/sheet.

The familiar grain grading is a direct result/function of the sawing process. If you apply the axiom that "vertical grain" appearing on the front/rear face of a spar is the strongest orientation. C grain fills the vertical grain requirement most excellently, although A grain usually gets the spar nod because of its percieved relative cost (modelers tend to save "rarer" C grain for ribs and formers and don't think of it as the best choice for Spars, which it definitely is, at least from proper grain orientation practice).

Also note the direct contradiction regarding general strength characteristics and the hardness factor, as indicated by airfieldmodels.com. Not huge deal, but interesting view from another slant..
Last edited by packardpursuit; May 31, 2013 at 05:26 PM.
Jun 01, 2013, 09:32 AM
Registered User
Packard - what is the contradiction you mentioned regarding strength and hardness?
Jun 01, 2013, 09:47 AM
Built For Comfort
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Quote:
Originally Posted by packardpursuit
IMHO, this is a bit more informative source of Balsa information:

http://www.go-cl.se/balsa.html
Yeah. I have a hard copy of that floatin' around in my piles. As I recall it came in one of my Sig kits, or possibly in a catalog.

TP
Jun 01, 2013, 03:05 PM
Registered User
This from airfieldmodels.com under "Wood Selection"

"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."

According to the Sig info, weight has a direct bearing on balsa strength.

I guess I jumped to wrong specific conclusion. airfied is, IMHO, correct.

mea culpa
Jun 01, 2013, 04:24 PM
Registered User
Quote:
weight has a direct bearing on balsa strength
I'd say that put the other way around, that might be correct;

Stronger balsa tends to be heavier, but heavier balsa isn't always stronger.

A spar made of plaster of paris would be extremely heavy, but would not be strong enough for the task

Straight, narrow grain is more important IMHO, there is a limit to how "strong" any piece of balsa can be, and rather than trying to find stronger/heavier balsa, substitute a stronger material of a smaller dimension.

There does tend to be an association with weight=strength in some quarters, and there's a vicious cycle involved. The heavier a model is, the stronger it needs to be, so if you beef up the structure in order to gain strength, you may actually end up making it weaker in terms of strength to weight ratio.

If, say, a wing on a 5lb model can cope with a load of 50lb (10G) and a stronger heavier wing on the same model can cope with a 75lb load, but the model now weighs 10lb, the maximum G loading goes down (ie 10lb x 7.5G = 75lb) so it is MORE likely that the "stronger" model will fold a wing
Last edited by MCarlton; Jun 01, 2013 at 04:33 PM.
Jun 01, 2013, 05:21 PM
Registered User
" The strength of balsa varies in direct relation to its density or weight - the heavier the wood the stronger it is."

From SIG site linked above.

I used to fly FF A-1 Towline glider. Frequently the designs of the 70-80's featured wire wing joiners and a mono-spar made up of 1/16x1/4 (flat) spruce caps, one over the other, with shear webbing between them and each rib bay. I generally substituted hard balsa with never a spar failure, However, I frequently had the wires bend or have the tubes fail completely in gusty tow conditions. Balsa, in my experience is more than adequate for most modeling uses.

This does not mean that I always use Balsa. Spruce,Hemlock and Pine make excellent modeling materials, as you suggested. This can be for "scale effect", strength, not to mention availability and comparative (to Balsa) cost.
Jun 02, 2013, 08:47 AM
Registered User
"I guess I jumped to wrong specific conclusion. airfied is, IMHO, correct."
"mea culpa"

No worries. I was asking an honest question because I don't want to present bad information. The ONLY reference I had for balsa when I was growing up was the Sig catalog which I loved to browse through. I read through the Sig site a few years ago and their information is the same now as it was then. The only point I was trying to make is to help people avoid making the mistake of just because it's heavy it's strong. Usually true but you still need to know how to read wood grain to be sure.


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