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spruce vs basswood for spars

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spruce vs basswood for spars

Old 03-09-2006, 08:59 PM
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woodys3b
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Default spruce vs basswood for spars

Tomorrow I start building a Spirit 100. The spars are 1/8 X 3/8 basswood. Back in the day, most kits came with spruce for the spars which makes sense as I understand that spruce has the highest strength per weight of any wood. Old Howard Hughes was no idiot! Anyway, is it worth the effort of replacing the basswood with spruce? Also, the plans call for wrapping the spars with fishing line (4-6 lb test) in the area around the wing joiner. Any recommendations for other string that may be better?

Thanks
Andy
Old 03-09-2006, 09:16 PM
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Default RE: spruce vs basswood for spars

I don't think you could go wrong either way. As for which has the best strength/weight ratio I could not say for certain. Both generally are both light and strong. Still wood has a tendancy to to vary in its density.

Whichever you chose, weigh them. That will be the deciding factor. Get a pile of each, all being the proper size and weigh them.

Robert
Old 03-09-2006, 10:54 PM
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uliner
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Default RE: spruce vs basswood for spars

If you can get spruce, replace it. Spruce is much stronger in bending than bass. In my experience, bass tends to be brittle and short grained. Easy to carve, but not good spar material.
Old 03-10-2006, 12:02 AM
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Default RE: spruce vs basswood for spars

Old Howard Hughes was no idiot!
Don't know if he was an idiot....
But he always got upset when people called it the "spruce goose", since it was actually made mostly of birch plywood.

From http://www.sprucegoose.org/ :
The Hughes Flying Boat represents one of man’s greatest attempts to conquer the skies as the largest airplane ever constructed. It flew only one time on November 2, 1947. Conceived as a personnel and materiel carrier, the single hull prototype was designed to fly Trans-Atlantic to avoid World War II German submarines that were sinking Allied ships in large numbers. Completed in 1947 after the end of the War, the wooden winged giant is nearly six times bigger than any aircraft of its time. The press insisted on calling the Hughes Flying Boat the “Spruce Goose,” a name that its billionaire builder Howard Hughes despised. Most of the huge plane is actually made of birch, with only small amounts of maple, poplar, balsa, and, yes, spruce. Birch was chosen because testing proved it light, strong, and resistant to splitting, dry rot and deterioration.
But back to the topic:
Yes, spruce is much better for spars, I would replace the basswood.
Old 03-10-2006, 02:17 AM
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Default RE: spruce vs basswood for spars

With a top and bottom cap plus webbing spar system it's not the bending strength that you're worried about. Instead it's the tensile strength on the bottom cap and the compressive strength on the top cap.

As for the fishing line there's lots of far better alternatives. You want something that won't stretch. If you can buy a short length of kevlar cloth from a fiberglassing outlet and unravel a long strand of that then this would be about as good as you can get. Wrap it tightly and then bond it in place with a coating of glue of some form. Even glass fiber would be good but you'll probably have to slightly round the corners and use a light weight cloth so the threads are very fine to resist snapping at the sharp corners. It IS glass after all. If you can find some carbon cloth that would work great as well.

Some shops provide tow fibers. This just means that it's a long bundle of non woven fibers that you can use for stuff like this.

Anyhow, without looking up a chart of tensile and compressive yield points for the woods I'm pretty sure we have seen in the past where good sitka spruce is better in both regards. So if you can find some nice tight and straight grained spruce I think you would be better off.

Or cap the bass with some of that 1/2 x .007 carbon fiber unidirectional strip material.
Old 03-10-2006, 10:07 AM
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drela
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Default RE: spruce vs basswood for spars

Yes, spruce is much better for spars, I would replace the basswood.
The difference is nowhere near as big as you imply. There are the numbers from a wood properties table, in an old 1940's book on aircraft structures:

Wood strength modulus density
Spruce 5000 psi 1.30 Msi 25 lb/ft^3
Bassw. 4500 psi 1.25 Msi 25 lb/ft^3

So spruce is about 10% better. But I haven't seen quality spruce in a long time. Basswood is very consistent and easy to get.
Old 03-10-2006, 01:17 PM
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Default RE: spruce vs basswood for spars

I stand corrected.
I would have thought there is a bigger difference between the two.
Maybe it was just the basswood which I have had so far, it seemed to be a lot softer and not nearly as strong than spruce.

Anyway, here's some more info on the subject , which I found on the net :

In continuation of the discussion re virtues of basswood, many years ago I used a lot of basswood with excellent results, buying it from a local lumber yard in huge planks, then sawing and sanding it to any size I desired. I found it easier to work than spruce.

My wood design reference book calls basswood a hardwood, while spruce is, of course, a softwood. Basswood density is listed as 26 pounds per cubic foot, while both Sitka and White spruce are slightly heavier, at 28 pounds per cubic foot, all at 12% moisture content by weight.

Modulus of rupture is a useful indicator of solid beam bending strength. It is slightly higher than ultimate tensile strength, due to reduction of elastic modulus near the ultimate strength.

Basswood has a modulus of rupture of 8700 psi (pounds per square inch), while Sitka and White spruce chime in at 10,200 and 9800 psi, respectively. Thus, basswood appears to have a strength to weight ratio about 92% as high as Sitka spruce's.

Basswood's tensile strength across the grain is nearly the same as spruce's, at 350 psi versus 370 psi, so it should have about the same resistance to splitting as spruce. Basswood's shear strength parallel to the grain is a bit lower than spruce's, at 990 psi versus 1150 psi. Basswood's modulus of elasticity is given as 1460 psi, versus Sitka spruce's 1570 psi, so it is the same as spruce in relation to its weight.

Sitka spruce has been very popular among full-size aircraft builders for many decades. The reason may be its truly outstanding attribute, impact strength, at 56% higher than basswood's. Basswood finishes in a distant second place in crash damage resistance.

Basswood also takes a back seat to spruce in crushing strength across the grain, at 4730 versus 5610 psi. Sitka spruce appears to have the highest strength to weight ratio of any wood, higher even than that of hickory, the strongest wood listed.

I have conducted quite a few tests for modulus of rupture of balsa, and have found that balsa weighing at least 12 pounds per cubic foot to have a strength to weight ratio nearly the same as spruce's. Six pound stock had only about 1/4 the strength of 12 pound stock, so it has about half the strength to weight ratio, and soft balsa's impact strength is vastly inferior to that of the harder stuff.

Wood is a wonderful material for airplanes. It has a strength to weight ratio virtually the same as the WW2 vintage aircraft aluminum alloy 2024; much higher fatigue resistance and impact strength in relation to its weight; and it is easier to join. The success of the all-wood DeHavilland Mosquito bomber testifies to this.
Old 03-12-2006, 08:10 PM
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Default RE: spruce vs basswood for spars

For wrapping spars, you can buy 100 feet of kevlar tow for $5 including shipping. Here is the link:

http://www.rcgroups.com/forums/showthread.php?t=429236

Full disclosure: I'm Littleflyer's dad!
Old 02-01-2010, 12:26 AM
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Default RE: spruce vs basswood for spars

from what I know, Spruce has an advantage in strength vs basswood, however it's not like a night and day difference. Also, if you are using ca the Basswood will have stronger joints due to better glue absortion so overall it might be close to a wash. I think China (for some reason) is buying up a huge amount of spruce so the price has gone up in recent years. In my experience, Spruce is preferable but Basswood is certainly acceptable. Someone earlier mentioned impact resistance...in a bad crash even basswood spars would still likely be one of the last things to break. In other words, if you can easily get some spruce I'd replace the basswood, but don't go to the ends of the earth trying to find the stuff. Hope this helps
Old 02-01-2010, 09:11 AM
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Default RE: spruce vs basswood for spars

I also just bought a Spirit 100 kit. I intend to replace the basswood in the kit so I ordered the "bargan bundle" of spruce from Aircraft Spruce, because I do a lot of scratch and plan building as well as mill my own sheets and sticks. There is a lot of short, unusable pieces in the bundle but I did get 4 - 5' 1x1s and 4 - 30 to 41" 1x4s so all in all there's quite a bit of spar material in the batch. Can anybody think of a use for 2' spruce? At around $35 a bf it's ashame to waste it because this stuff doesn't grow on trees !!!!!

How does Douglas fir figure into the mix? I know of at least one glider kit that uses it for spars. It is very heavy, a 36" 1/4 X 1/2 spar weighed 1.5oz. I have a bunch of D-fir I got from a remodel job, beautiful wood. I milled a 48" 1/4 x 1/4 and bent in around over 180 degrees before it snapped. the break was about 4" long so it is fairly straight grained. It still seems too heavy for spars unless it has some magical property to make up for the extra weight.

Rick
Old 02-02-2010, 12:22 AM
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Default RE: spruce vs basswood for spars

What would you think about using Long Leaf Pine? I have some small strips of heartpine leftover from a job and was thinking of it for a 154" wing. It is heavier no doubt, but about as stable as it gets.
Old 02-02-2010, 01:40 AM
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Default RE: spruce vs basswood for spars

Poplar has very close properties of spruce in tensile, bending and "shore hardness".

This is what an engineer has looked up for me.

So off to Lowe's to buy some poplar boards and will have to do some ripping on the O'l table saw.

Gunny
Old 02-02-2010, 01:59 AM
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Default RE: spruce vs basswood for spars

When you guys are looking up spruce be sure you're comparing apples to apples. Typical house building spruce is a far cry from aircraft grade Sitka spruce that has a high ring density. Having worked with poplar on some wood working projects I would never think of using it for wing spars. It may have the tensile strength for the lower spar but the wood is far too soft and punky to have a good compresive strength for the upper spar. Basswood would be far, far better in this regard and a high growth ring count sitka spruce would be way better than that. The numbers the institutes give make bass look almost as good as spruce. And for some grades and types of spruce this is very much the case. But show me a plank of Sitka that has 16 or more growth rings per inch and very little runout or wiggles in the rings and I'll show you a plank that will produce first rate spruce spars that make basswood look like plasticene by comparison. Wood is such a variable product for many species that with some of them it's near impossible to trust a lot of the more general specs from various wood industries. Look at balsa. It runs from less than 4 lbs/cu ft to more than 16 lbs/cu ft. I've seen it where it's literally got the consistency and strength of a foam packing peanut to where I'd be happy to build a house out of it. How does one learn about balsa from reading a single tensile and compression strength reading in some list? Simple, they can't.
Old 02-02-2010, 09:59 AM
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Default RE: spruce vs basswood for spars

Ditto what Mr Mathews said. Spruce used in building is actually lodge pole pine, at least in this area. It NO WAY compares to aircraft grade spruce. If I couldn't find spruce I would only use basswood as a replacement. Since the local box stores don't carry basswood check with a local cabinet supplier, or maybe a cabinet shop will sell you a board or two.

The pic is a 1x6x19" piece of AC grade spruce, note the uniform tight ring banding. This piece weighs 1lb, 10.8 oz and came from Aircraft Spruce & Specialty in their bargin bundle deal. I ended up with 12 of these shorter pieces so I recommend buying a single spar of whatever lenght you use most. Much less waste.

Rick
Old 02-02-2010, 10:01 AM
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Default RE: spruce vs basswood for spars

Oops, here's the pic of the spruce.

Rick

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Old 02-02-2010, 11:24 AM
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Default RE: spruce vs basswood for spars

thats a nice slab of spruce, pity the crud they sell us here as spruce isnt that quality. I had a nice supplyof local beech, which worked quite well, but its hard to find now too. While quality spruce is the go to timber for spars/stringers, it isnt available reliably to many builders. As has been said before in this thread, the variations in timber can be quite a lot, I am a cabinmakerjoiner by trade and also a qualified shipwright, and I am disgusted by what merchants call "select grade" timber, when I had my last workshop I used to travel up to 1800 miles just to handpick my timber for upcomin jobs. For my models I use the best I have at hand, and if I feel a bit dubious about the strength I will add some c/f or kevlar tape.
Old 02-03-2010, 08:31 AM
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Default RE: spruce vs basswood for spars

I plan to make a set of 149" wings for my sailplane, and needed wood for the wing spars. The wood that I purchased for this purpose is aircraft quality spruce from the Aircraft Spruce & Specialty Company, and I was impressed with the quality of their products. Wonderful quality wood. It should be perfect for this application. You can generally order thicknesses and widths for a reasonable price. Since this is spruce that is used to construct full scale airplanes, for modeling purposes the spruce trim panels are what I mostly looked at. You will be hard pressed to find better quality. You can visit their website at www.aircraftspruce.com.

I've also got a Spirit 100 under construction, and plan to use the included basswood. It's lighter than spruce, and this since plane ends up very heavy when it's finished compared to other 100" sailplanes that I've built, any place where I can reduce weight is probably a good choice. For this plane, the basswood should work fine.

juggler
Old 02-05-2010, 04:11 AM
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Default RE: spruce vs basswood for spars

Well, Very interesting.

Glad I did'nt go to Lowe's yet.

Should have done my own homework.

Thanks

Gunny
Old 02-05-2010, 06:34 PM
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Default RE: spruce vs basswood for spars

I have an old Balsa USA Nomad kit - the spar material I believe is western cedar. Should I use it or trash it???
Old 02-05-2010, 07:41 PM
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Default RE: spruce vs basswood for spars

If the cedar is fine and tight grained I'd use it, I have used cedar many times in the past, never had a problem. You can always add some c/f or kevlar tape to strengthen and stiffen them if you like.
Old 02-05-2010, 11:51 PM
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Default RE: spruce vs basswood for spars

Use it.....mine is over 20 years old and still flying...
Old 02-06-2010, 08:38 AM
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Default RE: spruce vs basswood for spars

And you won't have to worry about termites getting into your spar !!!

Rick
Old 02-06-2010, 11:09 AM
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Default RE: spruce vs basswood for spars

If you're looking for GOOD Spruce, then you need to check this place out. This is their main job. supplying aircraft grade Spruce for full scale aircraft. For modeling stuff look in the "Cap strip" section. Lots of sizes that are perfect for modeling. If you have a good cutting tablesaw you can buy a large board and pretty much have Spruce for life. Most hobby shops I've hit in the last few years don't have near the quality of days gone by, and longer lengths are very hard to find.

Another plus is that they have shops on both sides of the country. Corona, CA and Peachtree City, GA. I found them when looking for spruce for the fuselage framework on the Zogling SG-38 I'm fixing to start soon. If only the Pilot figure had been so easy to source out!

http://www.aircraftspruce.com/menus/wp/spruce.html

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