Spar Stock

[dejh22]

I have a chance to buy some douglas fur. He is telling me that he has 12 boards total, rough sawn, 2x6x28, clear douglas fur. I have not looked at it yet. Just need some pointers for what to look for. The price is good enough that if out of all of it, I only could get 4 good spars I would still be doing good. I have notes from doing searches on here on size, vert grain,slope etc.

[Pearson]

Re: Spar Stock

AC43.13 has info on what to look for when identifying wood suitable for aircraft use. But even with that, if you don't have any experience in this area I would suggest finding someone who does to look at the wood for you. Aren't most spars made from spruce?

[Ragwing nut]

Re: Spar Stock

Fir needs to be oversized if I remember correctly. That could be an issue when it comes to the rib installation.

[astjp2]

Re: Spar Stock

Undersize, its 15% heavier and 15% stronger, but one of the old time manuals on wood says to not undersize it, just use it as is. Tim

Fir needs to be oversized if I remember correctly. That could be an issue when it comes to the rib installation.

[Hank Jarrett]

Re: Spar Stock

Tim has it. Doug Fir is stronger and heavier (the division will show you that it is a little more "heavy" than "strong"). If you re-design (DON'T) the spar you could make it slightly smaller (lighter) for the same strength, but not quite as light as the spruce one. The weight gain would only be a few %. If you make a dimensionally identical spar from Fir it will be heavier AND stronger. That is what you want, deal with the weight, it isn't that much more weight, even for a spar.

Now for the bad news. When a Sitka spruce tree is felled a crane is normally used to hold the trunk up while it is cut. If a trunk is literally "felled", it can fall over something on the ground that will bend the wood and cause compression fractures at the contact point. This isn't a huge problem if the wood will be used for a house, but for musical instruments, fine furniture or ESPECIALLY key structural components like airplane spars it is. The compression fractures are VERY hard to detect, usually with a microscope and ofter requiring cutting the wood (which makes it pretty useless for a spar) and that is why it is cost effective to use the crane and certify the method of harvesting the tree rather than doing the inspection. There are other ways to detect the flaws, but they are even MORE expensive.

Bottom line is, the NORMAL inspections are pretty well beyond most of us. You can learn them, but it isn't all that easy. The processing for those rough boards into good lumber for a spar is going to be difficult at best. A good mill works shop could do it, but there are risks that they may not handle the wood properly either. Next is the moisture content has to be within specs and the grain run-out and ring count must be good enough for a spar. You won't know until you mill it. The waste is HUGE in making spar blanks. That is why they are so expensive.

All this said, I bought a beautiful piece of Doug Fir for my 45. I know how to inspect it and have access to the inspection tools. The wood will be used to make new stringers and trim wood. I would NOT use it for structural parts. I got my Doug Fir from a specialty wood source and it was originally a piece of stock for a racing sailboat mast. It was also crane harvested. Sometimes you can get a really good deal if someone at the mill screws up and cuts a little too short.

Hank

[dejh22]

Re: Spar Stock

Hank, you might of talked me out of it. I was thinking if a few of them looked good enough ( straight grain, slope, no defects ) I would pick them up. The cost would be less then $80.00. I do know there would be extra cost in doing the mill work. I would have no way of knowing how they were harvested or moisture content.

Tim has it. Doug Fir is stronger and heavier (the division will show you that it is a little more "heavy" than "strong"). If you re-design (DON'T) the spar you could make it slightly smaller (lighter) for the same strength, but not quite as light as the spruce one. The weight gain would only be a few %. If you make a dimensionally identical spar from Fir it will be heavier AND stronger. That is what you want, deal with the weight, it isn't that much more weight, even for a spar.

Now for the bad news. When a Sitka spruce tree is felled a crane is normally used to hold the trunk up while it is cut. If a trunk is literally "felled", it can fall over something on the ground that will bend the wood and cause compression fractures at the contact point. This isn't a huge problem if the wood will be used for a house, but for musical instruments, fine furniture or ESPECIALLY key structural components like airplane spars it is. The compression fractures are VERY hard to detect, usually with a microscope and ofter requiring cutting the wood (which makes it pretty useless for a spar) and that is why it is cost effective to use the crane and certify the method of harvesting the tree rather than doing the inspection. There are other ways to detect the flaws, but they are even MORE expensive.

Bottom line is, the NORMAL inspections are pretty well beyond most of us. You can learn them, but it isn't all that easy. The processing for those rough boards into good lumber for a spar is going to be difficult at best. A good mill works shop could do it, but there are risks that they may not handle the wood properly either. Next is the moisture content has to be within specs and the grain run-out and ring count must be good enough for a spar. You won't know until you mill it. The waste is HUGE in making spar blanks. That is why they are so expensive.

All this said, I bought a beautiful piece of Doug Fir for my 45. I know how to inspect it and have access to the inspection tools. The wood will be used to make new stringers and trim wood. I would NOT use it for structural parts. I got my Doug Fir from a specialty wood source and it was originally a piece of stock for a racing sailboat mast. It was also crane harvested. Sometimes you can get a really good deal if someone at the mill screws up and cuts a little too short.

Hank

[Ragwing nut]

Re: Spar Stock

I did not remeber which way it was. I would definitely not make it smaller. Hank has a good point, but all wood should be inspected regardless of where it comes from before using as spar stock.

[astjp2]

Re: Spar Stock

BUY IT, you will get never it again for that cheap. If you use it for trim and stringers, its still cheap. I paid 156.00 for 2 each 14' pieces of dug fir for my stringers and it would be great for that....tim

Hank, you might of talked me out of it. I was thinking if a few of them looked good enough ( straight grain, slope, no defects ) I would pick them up. The cost would be less then $80.00. I do know there would be extra cost in doing the mill work. I would have no way of knowing how they were harvested or moisture content.

[Hank Jarrett]

Re: Spar Stock

$80 for that much nice Doug Fir is probably a good price anyway. It makes MUCH better stringers than Spruce because it is stiffer wood. It won't get that "wavy line" you see on the sides of some planes. Spruce is still about as good for teh weight as you can get for blocks, but the difference between it and fir is almost impossible to measure.

Additionally I have patterns for some of the most comfortable sling rockers you ever sat in that I plan to use my left over wood for (someday, when I get the time, you guys know how that goes)

Hank

Doug Fir make REALLY nice model plane spars, but them you don't die if one of those fails. I WOULD use a Fir spar like you are looking at IF I actually loaded the finishes spar to VNE Stall bending load before use. If it didn't break there were no compression faults, but the test is not simple (FUN! but not simple)

[dejh22]

Re: Spar Stock

I have the aluminum stringers. So I really would not have any use for them. He is asking 20.00 per board. If I knew I could use them I would buy them. My thinking was to use them for spars. But without having someone who would know if they would pass inspection I'm going to play it safe.
I just got off the phone with an IA that told me the best thing to do is just buy spars from Aircraft Spruce and not have to worry about anything. Plus he is coming out next week to look over my spars to see if they are still good. I had an IA look at them already and he told me they were ok. Then two weeks later called me to say he was thinking about something he saw and wanted to look at them again. He thought he saw a crack that looks to me like a laminate line.He never did come back out. Then a week later he called and said it was just to far for him to travel (he is in his 80's) and I would need to find someone else. This was last year and I finally found someone that will look at my project.So I guess I will wait and see what he says. Maybe I won't even need to replace the spars.

[lawheelock]

Re: Spar Stock

I bought spruce spar stock a couple of years ago for my rear spar on my BC12D. I also found that the broken F19 rear spar I had was identical. I used my old spar as a pattern for milling the butt end on a joiner-planer and sawing the taper on the tip end. It came out nice, but still have to re-assemble the wing. I drove to Wickes and hauled the spar in a 6" sewer pipe that I spliced and hauled on top of my Minivan.

Larry Wheelock TF671