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.
Robert/Red, I found an interesting chart comparing various woods. Here is the URL:
It shows weight and several different strength measurements (all relative, not absolute). From this I gather that Basswood is a bit stronger but only by about 8-10% over Spruce and is also heavier by about the same factor. So given availability I would stick with the Basswood and understand why it was chosen by the manufacturer/designer. However for this small a difference I'm not concerned for my application. For the main spars I was able to glue a piece to thicken what I already had and get what I needed. I now have a composite spar consisting of 3/8"x1/4" thick basswood (two 1/8" thick pieces) and one 3/8" x 1/8" of spruce. All bonded securely together to form a very substantial main spar that I have great confidence in. If this ever breaks, I'm sure it will be at the end of a very ugly sequence of events, not the other way around. Thanks for your input.
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.
lists both spruce and basswood. While basswood has a higher ultimate tensile strength both spruce and basswood take the same amount of energy to break -- meaning that if you slowly load the wing the basswood may hold on longer, but if you thump the spar hard they'll break about the same time.
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.
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.
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.
"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.
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.