Hi all, I am building a big (89" wingspan) airplane and inadvertantly glued a few pieces together incorrectly (and quite permanently) so need to get some new lumber for the main spar assembly. It was originally supposed to be basswood but all I can find locally is spruce in the sizes I need. Is there a big difference in strength/flexibility of the two that I should go to the trouble of mail ordering the right stuff or is this a fair substitution? Thanks.
Jack
Spruce is what they use in "real" airplanes. Go for it.
Red S.
My guess is that spruce is stronger than basswood.
Just a guess, though... and I thought spruce was what was normally used anyway.
Good flying, desmobob
Robert/Red, I found an interesting chart comparing various woods. Here is the URL:
Jack
On Mon, 21 Nov 2005 19:32:36 -0500, "Red Scholefield" wrote in :
I agree with Red and Robert.
Do let us know if your flight testing produces different results than we expect. :o)
Marty
Basswood = hardwood Spruce = softwood
Spruce is the aviation "standard" but... I wouldn't substitute without putting a lot of thought into this. You may need to use a larger spar.
Most models are severely overdesigned and built. I suspect that very little true engineering is done in their design given the variability of the materials. More likely seat of the pants "that looks about right" is the technique most often used. I have substituted balsa spars for spruce in a couple of plane with dire predictions from the gallery. They were plenty strong.
First you should understand the very good reference information that was given.
The modulus of rupture is higher for Spruce, both compression and shear parallel to the grain are higher for spruce.
Spruce while it weighs a bit more than basswood (about 20%) it is better suited for use in spars.
Why do you think it is the choice in full size aircraft if it is inferior to basswood (which is lighter)?
Red S.
Gee, Balsa is stronger than I thought, about half as strong as basswood or spruce, and less than half the weight. Sounds like making spars of double thickness of balsa would make sense.
Can somebody tell us how tall a basswood tree grows? What's the maximum length of plank we could get out of it without having a lot of grain deviation? Real airplanes need long pieces of wood. If a basswood tree offers usable sticks only 12 or 15 feet long, that's why we don't see it in real airplanes. "Usable" means, according to aircraft construction requirements, at least 8 grains to the inch, no more than 1:15 slope to the grain, no knots, pitch pockets, shake, cracks, splits, compression failures, spiral grain and so on, and the wood's moisture content should be around 15%. As I understand it, basswood is more brittle ("brash") than spruce, so that it fails more suddenly. Great for carving, though. Spruce grows in tall trees with the branches all at the top, leaving a knot-free trunk for most of the lower length. There are several types of spruce, too, with Sitka being the wood of choice.
Dan
The OP said "in the sizes I need", meaning he's going to use the same size as the basswood pieces.
I didn't think your advice that "Spruce is what they use in "real" airplanes. Go for it." was very compelling.
Spruce may, or may not, be "the" choice. Seen many spruce propellers?
I haven't said it is inferior.
As Sport Pilot noted, balsa has the best strength/weight of them all. Why isn't balsa used in full size aircraft?
Common woods used in aircraft
lbs/cubicfoot | bend/elastic limit | max. crush | shear strength pine 27 | 6000 | 5300 | 640 spruce 27 | 6200 | 5000 | 750 bass 26 | 5600 | 4500 | 720 douglasfir 34 | 8000 | 7000 | 810
in the 1936 aircraft manual this came from, it says that with similar grain, spruce, fir and pine are good for beams and other structural members, but basswood is marked for best used as the inner core of plywood,or for blocking, but rarely for structural members. More of a Birch replacement than Spruce.
** mike **I have never seen a balsa spar fail. The Great Planes kits use them almost exclusively including the Kaos and P-51 kits. I believe that most kits are greatly over-engineered because actual engineering would be too much trouble for the gains.
Spruce is specified (Government publication AC43.13) for most wooden airplane structural members. Some highly-loaded parts may be ash, oak, mahogany or birch. Propellers are the most highly loaded part of an airplane, having to resist thrust, bending, torsional, and centrifugal loads, and besides being too light to act as a flywheel, it's too flexible and has too little tensile strength. Most props are of hardwoods such as maple, birch and so on.
It is, often as a core of a sandwich construction. Sometimes has fiberglass facing. It's not used for structural members, since those members would have to be too large and don't bolt up well. Too little crush resistance.
Dan
"Paul McIntosh" wrote in news:ekRgf.665$4v.116@fed1read03:
I'll mostly agree with that, but if by "over-engineered" you mean that they're built stronger and heavier than necessary, I'd take a more nuanced view. There are certainly a lot of planes that have known weaknesses (not typically the spars) where they really should have been built stronger.
There *are* a lot of poorly-designed spars out there. When I'm designing a wing (hey, it happens), I actually do the math. I make a number of simplifying assumptions - if anyone's interested, I can post 'em - but, yeah, a lot of the designs just don't make sense. And for small planes, solid balsa spars have been underrated. The weight of the shear webbing and glue will often exceed the weight saved by having a more efficient I- beam or c-channel cross-section.
The ~300 sq in House Of Balsa planes have an interesting combination of solid balsa spars with dowel leading edges. I still haven't decided about them. They're not particularly light because of the heavy dowel, and they're not very pretty, but they build quickly, you can't ding the leading edge, and I've done some pretty violent flying on those wings without incident.
One also has to ask if they're over engineered for normal flight, yet not engineered enough for those times when the runway suddenly jumps up a couple of feet on landing.
AC 43.13 is advisory. It doesn't specify that Spruce be used for anything! But it does list some permissible substitutes for repair work:
Douglas Fir - stronger than spruce, but more difficult to work Noble Fir - slightly stronger except in shear Western Hemlock - stronger, but less uniform in texture Northern Pine - less strength, excellent working qualities White Cedar - stronger, easy to work with handtools, difficult to glue Yellow Poplar - slightly less strength, excellent working qualities
And this info is for repairmen, nothing says the manufacturer had to choose spruce in the first place.
Propellers are the most highly loaded part of an
Bass is generally straighter and more warp free. I think its from Lime (Linden) trees which grow to over 100ft...
I have had several balsa spars fail..but then I build down to a weight for ultra light electrics.
Balsa varies HUGELY in strength and stiffness...sprice and bass are far more predictable.
I prefer bass when I an get it because the grain is better, its more free of defects, and it tends to warp less.
If you want a really good spar try laminating thin ply and soft balsa or depron.
| I have never seen a balsa spar fail.
Obviously you don't fly gliders off a winch very often :)
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.