Does it has to be spruce?

[ferincr]

What is that makes spruce the choice for all the spars?
Is it the straight grain, or the high content of resin on it?
Flexibility? all of the above?
Just wondering if other kind of pine can be used to replace spruce with.
I can not get spruce, and I'm trying to find something to use instead.
There is a large selection of woods here but none of the ones you may find in the USA, most of them are tropical woods and I can get some cypress and pine too.
There are some hardwoods quite flexible but they are heavy too.
I'm not thinking of building any aerobatic planes (at least not for the moment).
Any sugestion? I know is hard to give advice if you don't know the woods I can get, but may be a general guide on what to look for.
Thanks,
Fernando

[SoCalGliderFlyr]

Spruce had the tight straight grain necessary for strength. At least 15 growth rings or lines per inch. Could use fir if you can find a decent section. This gets down to about 10 lines per inch in some places.

Depending on the design hard balsa with proper shear webbing will be adequate.

[ferincr]

Thanks SoCalGliderFlyr,
I'll see what can I get with those characteristics. It's hard to find woods with tight grow rings here (if any), but there is some cedar (not like the ones you get there, the wood is similar but it is not a conifer, this one has real leaves) has a very similar grain as balsa does but it's harder, and quite light, I'll experiment cuting some spars of it to see how it looks. Definitely stroger than just balsa.

[ejett]

Can you get CF?

Can you get basswood?

If you can get a tight grained piece of pine without any knots, you could probably use that.

I always wondered what kind of a spar bamboo would make. Socal, what kind of compressive strength does bamboo have?

Ollie likes ramin, but it is on the 'endangered' list I think.

If you can get some CF, that would be the best alternative.

EJ

[ferincr]

I thought of bamboo too, but you'll need to splice it because you have a knot every couple of feet or less

[SoCalGliderFlyr]

That's the other wood I was trying to recall; ramin. Might be an equivalent from South America. It's an Asian wood (IIRC).

I'll have to look up bamboo. Stripped thin it makes great TE's on small planes.

[Mirage1]

I salvaged 300 5 foot long Douglas fir 2 x 6's from our plant warehouse. They had been there about 25 years. No nail holes and some fantastic close growth rings. I sawed a couple up so the growth rings/lines were vertical and ran them through my planer into 3/8 x 1/8 spars for my Mirage and BOT . They were very light and have performed very well.

[ejett]

The reason I mentioned bamboo is that I have owned a couple of split bamboo flyrods over my lifetime. I was always impressed by how strong it was. It may be that its modulus of elasticity is too low (making it too flexible).

It has probably been tried, but I guess spruce has a better combination of properties for model aircraft use.

EJ

[SoCalGliderFlyr]

I've found some impressive fir 2 x 2's down at Home Depot. At least four foot of good grain in some sections. Guess I'll have to think about using it when I run out of 36 years of spruce.

[BMatthews]

Local woods are fine if you can find some that have good tensile and compresive strength and are not too heavy. If you have a local wooden small craft boat building industry or craft shops then look into the woods they use for bent planks. That wood will probably be reasonably light but flexible and strong. It would be a good starting point at least.

[Ollie]

It is a complex question. What "best"?


Vert. grain balsa 40%, plus and neg 45 degree grain 100% webs.


Most wood spars in untapered cross section are 2/3 for the ride to throw out to much weight. Design spar is more important than type of wood. Spruce can be 5610psi compression, 13,000psi tension. Ramin can be 10,000psi compresssion. Carbon can be 50,000psi (poor) or 275,000psi (best).

Wood vs, carbon ~ equal $/strength. ~20x carbon strength/weight vs 1x wood strength/weight.

[Spekd'out]

Here is an example of spar design in wood which I built to show the extent that can be taken in design.The spar caps tapered in depth thickest at root of course,and had a scarf joint at 40% span changing to hard balsa( Strength closely proportional to weight). Shear webs were ply both sides at the root,45deg grain, one side switches to balsa,vertical grain, then both, then one side only and finally last few bays no shear web at all.

[schrederman]

When I went to A&P school, Sitka Spruce was considered to be the strongest wood per weight in the world, if it was the close, straight-grained stuff that would make the grade. That's why it was used for aircraft construction. I have heard lately that the Ramin mentioned above has now got that distinction, but it's not readily available. Douglas Fir is acceptable, just as strong, but not as light. I've used it, and the few grams of weight gain isn't noticable...

Jack Womack

[SoCalGliderFlyr]

There's 400 million acres of dead Sitka on the northern coast -- bark beatle or moth. Can't cut it because of "environmental" concerns. Going to make a great fire!

[John Gallagher]

Douglas Fir can be substituted for spruce. It's a close second in strength but it's a denser material so that the size would have to be scaled down.

[Phat2Wheel]

Look at Ollie post above and use carbon fibre for spar caps. I am sure you can web order from ACP composite for their tapered carbon fibre laminates. Here is my current project (http://rc-glider.blogspot.com). I use vertical grain balsa with tapered carbon laminates, thick at root and thin out at the tip.

Phat
rc-glider.blogspot.com (http://rc-glider.blogspot.com)

[Ollie]

Use plus, neg. 45 deg. grain ply balsa 2.5 x webs strength, not vert grain balsa.

[markdrela]

As Ollie says, +/-45 balsa "plywood" has about 2.5x greater shear strength than vertical-grain balsa. However, to realize this strength, the web must be continuous, with no ribs going through it. If the ribs DO go through the web, then one might as well use a vertical grain web.

[fprintf]

I am having difficulty visualizing how to make a single continuous web of +/-45 balsa plywood since the widest sheets of the stuff I can find are 4" wide. Even cut on a 45 degree angle, that makes the uninterrupted length about 5 3/4". Is the trick to overlap those 45 degree pieces so the joints do not run over one another? Or does it not matter since the joints are running in opposite directions?

[ejett]

My thinking is that the best approach would be to lay them out so that the butt joints between individual sheet segments do not lay on top of each other. OTOH, even if they do the resulting one piece shear web lamination will be stronger than the typical end grain balsa webbing, which in itself has proven to be strong enough when done with no gaps.

The question becomes do you want to go to the extra trouble to get the additional strength that most likely is not necessary in practice.

The assembled one piece shear webbing will have to be precision cut on a bandsaw or table saw. I guess it would be possible to assemble with precision cut pieces working against a straight edge. If you do this, be sure and post some info on the process you used and the end result.

EJ

[markdrela]

The question becomes do you want to go to the extra trouble to get the additional strength that most likely is not necessary in practice.

Yep. I can think of only one situation where a +/-45 balsa web makes sense:
All-wood wing with spruce sparcaps and a continuous web across the span. Two-piece ribs butt against the spar in front and back.

Two situations where a +/-45 balsa web does not make sense:
* One-piece ribs interrupt the web, like on most woodies. The ribs will fail in shear before the stronger +/-45 grain balsa web does. No advantage.
* Bias glass wraps the entire spar, like in a composite wing. The balsa web will then see mainly a vertical compression load, so vertical grain is best.

[Mirage1]

OK, Lets say I cut a series of + -45 balsa shear webs glue them end for end and I am using two 3/8 wide x 1/8 spruce spars,and lets say 3/8 thick balsa for the center panel I also have the technology to to taper this long shear web and use thinner materials for the outer panels of my wing. I butt and glue the two piece ribs up. What kind of strength will I be getting? Is it equivalent to the exotic composites.

[markdrela]

OK, Lets say I cut a series of + -45 balsa shear webs glue them end for end and I am using two 3/8 wide x 1/8 spruce spars,and lets say 3/8 thick balsa for the center panel I also have the technology to to taper this long shear web and use thinner materials for the outer panels of my wing. I butt and glue the two piece ribs up. What kind of strength will I be getting? Is it equivalent to the exotic composites.
No. Not even close. The only advantage of a +/-45 balsa web is that it can be made only 40% as wide as a vertical-grain web, for the same strength. So for example, a 1/4" wide vertical web can be safely replaced by a 1/8" wide web consisting of two 1/16" layers at +/-45 degrees. Whether this weight saving is worth the extra work is another matter. And this narrower web must not be interrupted by ribs, else it won't have the expected strength.

[Mirage1]

Thank you for the info. I am a R&D guy at Gulfstream Aerospace where we build mostly aluminum business jets but are increasingly going to composites for control surfaces etc. Actually I should go over to the composite building and see what they are throwing away in the scrap bin. I am a woodworker at heart though and love to experiment. I am going to build a few experimental + - 45 balsa/spruce spars of different thicknesses, maybe 4 feet long and put them to a load test. They won't match CF etc but inquiring minds want to know. : ) I will post some pictures of the build process and the results if anyone is interested.

[BMatthews]

Yes, please Mirage! It's so seldom any of us actually cut and try stuff. Not many want to go to the trouble of breaking what they build even if it is in the pursuit of knowledge.

[Mirage1]

I picked up some 3/16 balsa x 6" wide and some 1/8 x 2" wide and started by sawing them up on the mitre saw. I made a flip up stop so I could slide the balsa over to preset width and then flip up the stop so the balsa wouldn't bind when I chopped. The wind from the blade blows it out of the way. The 1/8 balsa required more pieces but the 2" was all they had in stock.

[Mirage1]

After cutting the webs, I thought ejett had a good idea so I built a glue up jig out of 3/4 " MDF. I will cover it with parchment paper to keep the glue contained and end glue the webs and then laminate two layers with the grains going opposite directions and staggered so the end joints don't fall on top of each other. I will push them up aaginst the backstop and clamp them.
Pictures of that tomorrow.

[ejett]

You gotta love those R&D guys. Go, Mirage1, go!

EJ

[Mirage1]

After the glue jig was dry I end glued the webs laid out a strip, rolled out a layer of glue on the other strip and clamped them up.

[Mirage1]

I then went about to make some spars. Found a 2 x 6 piece of douglas fir with 40 growth rings per inch. I ripped it up so the growth rings were going vertically and then planed them down to 1/8 of a inch.

[fprintf]

Ever seen the Woody Woodpecker cartoons (or it is Bugs Bunny) where they have a broomstick making factory that takes full size trees and turns them into broomsticks? That is what I think of when seeing the pictures of a table saw making spar sized sticks of wood. (although I know what you are doing is not wasteful, it just is what sprung to mind!) :) :D

[ferincr]

[QUOTE]
That is what I think of when seeing the pictures of a table saw making spar sized sticks of wood. (although I know what you are doing is not wasteful, it just is what sprung to mind!)

If you know where I can get a spruce tree that has 3/8 x 1/8 braches I can just snap off let me know, I'd love to have one in the garden. (he, he... )

[SoCalGliderFlyr]

With a 200 tooth panel cutting blade you loose about 1/16".

[fly1milehi]

Im thinking that the spruce stock in the bin at the LHS was most likely cut the same way!!!
You could claim to cut them with a bandsaw and save maybe another 1/32" in the saw kerf but you will promptly throw it away again when you run the stock through the planer to remove the uneven surface.

There is even some loss in laser cutting... "do you know how hard it is to finance a frickin laser?" Dr. Evil!

[fprintf]

Ok, just to be clear about my post. I do not think that cutting the spars that way is/was wasteful. Just that it reminded me of the cartoons (saw size versus eventual product, lots of sawdust).

[Roy Simpson]

With a 200 tooth panel cutting blade you loose about 1/16".

Panel cutting, what’s that? :D

I cut Sitka spruce with a hollow ground saw blade (Cobalt steel, mist cooled) setup, 15,000 RPM.

We used .031” (~ 1/32”) kerf (kerf is the width of the saw cut) blades in a slitting saw setup.

If you have to cut a few pieces, any standard table saw blade works great.

When you slit 5,000+ pieces a shift, then saw dust can make a real difference.

Should anyone need a source of those blades, PM me.

Roy

[SoCalGliderFlyr]

Panel cutting, what’s that? :D

I cut Sitka spruce with a hollow ground saw blade (Cobalt steel, mist cooled) setup, 15,000 RPM.

We used .031” (~ 1/32”) kerf (kerf is the width of the saw cut) blades in a slitting saw setup.

If you have to cut a few pieces, any standard table saw blade works great.

When you slit 5,000+ pieces a shift, then saw dust can make a real difference.

Should anyone need a source of those blades, PM me.

Roy


Used to cut. Sold your business now you are selling your tooling. I think that 5,000 pieces in an 8 hour shift using a mist cooled 15,000 rpm blade is exactly what the home hobbiest had in mind.

NOT!

The 200 tooth blade is at Sears for $12.