flybyjohn

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Just thought I would get some ideas of what some are using for mainspars for scratch built airplanes. I have looked at many kits and some use balsa, basswood, and spruce. I will be able to cut spars on a band saw out of wood from the local lumber store however I would be looking at birch, maple, mahagany, and other similar hardwoods. Are these woods used commonly for spars or do I need to break down and order some large chunks of basswood or spruce.

Thanks for any comments,
John

Laird SS

You might try Aircraft Spruce & Specialty for various sizes of spruce especially for aircraft (good quality, not mediocre). Here's a link to them [link=http://www.aircraftspruce.com/catalog/wppages/sprucesparstock.php]Aircraft Spruce & Specialty[/link]

In lieu of spruce you might try Douglas fir. It would be cheaper, but not quite as good.

flybyjohn

My Feedback: (2)

Thanks Laird SS, I did come across some very tight grain Hem-Fir at the lumber yard also that had a very straight grain. I just did not know what the elasticity was like in the hem fir.
Is a very stiff wing more desirable then a wing that can flex some without breaking or the other way around.
Thanks

longdan

I think a wing with some 'elasticity' in it is a good thing. Kind of like the suspension in a car. For models, I'm thinking more along the lines of crash resistance than flyability.
I know that for full size planes, it's critical to have a certain amount of springiness and flex in the wings, but this has little to do with how well the wings will survive a cartwheel landing.
The wingtips on the new airbus A380 flex upwards by several meters on takeoff and landing.

TFF

Fir is an alternate for spruce in real planes and is used in spars in aerobatic planes a lot because it is stronger but it does not glue as well as spruce. So there are tradeoffs. White pine is an alternate too. I use balsa, spruce, and bass; just depends on the plane.

CoosBayLumber

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A few years ago, I got ahold of some "Piano Quality" pine strips from a Northern California lumber mill. Not too often do they locate such timber, but ehen they do it is not wasted, a buyer is informed and they wood sold within less than one week. They cut up the majority of the tree already and shipped out the wood to some firm. The stuff was laying around awaiting an OK to ignite and burn up. Several pieces went in to the auto trunk then. Was too thin for them to cut, and too thin for them to market. Cost me a 1/3 tank of gasoline to get the free stuff. Much of the free stuff was heavily tapered, and I only got mabye fifteen 5 foot long strips of this extra strength and light material.

Wm.

dreadnaut

My Feedback: (5)

This depends on the application. An aerobatic airplane will not do some manuvers, like snaps and such, very well if the wing flexes too much. On wings with very thin sections, like sailplanes, a little flex is usualy a reasonable tradeoff to get lighter weight.

dicknadine

what size model are you planning on building ? sure would help in the wood selection - Balsa - Spruce - Pine - Etc. dick

tucker1865

My Feedback: (6)

I would not use any of he construction grade lumber, Hem-fir or Douglas Fir, since they have not been kiln dried. As it dries it can cause you problems. I would stick to the more stable woods, such as spruce.

Tony Hallo

Why reinvent the wheel? Just duplicate what is used in other planes of similar size and design.

aerowoof

what wood to use depends on the finished size,weight and application of the airplane.by using bass or spruce you can use smaller sizes than balsawood,also using shear webs will make a wing extremely strong for very little weight increase.

dhal22

with carbon fiber, composites, etc being used everywhere today i say reinvent the wheel. planes are lighter and stronger than ever with some of this stuff.

CrateCruncher

My Feedback: (1)

For those who want to know more about the structural properties of wood read [link=http://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf]THIS![/link]

For those who want to maximize the strength to weight ratio of their spars start [link=http://en.wikipedia.org/wiki/Second_moment_of_area]HERE![/link]

dreadnaut

My Feedback: (5)

Translation;

For those that don't have the math to figure all that out, you can get a pretty good approximation in this application by using the parrallel axis formula.

I=Ix+Ad^2

A is the area of the flange (what modelers call the ''spar''). d is the distance from the center of the web to the center of the flange. You can get equal values of I for both thin and thick wing sections by varying the area of the flange. For a thick wing, like a 3d fun fly, use a smaller flange. for a thin wing, like a sail plane, use wider flanges. If you are reasonably good with Excel, you can set up a spread sheet that will let you explore your options.

Bax

My Feedback: (11)

For many glow models, up to .61-.90 size, you can use different sizes of balsa for spars, with good shear webbing. You can build wings up to aboug 8-feet span without resorting to hardwood. You'll have ply in a few places, but that's about it.

For the larger models, 1/2" x 1/4" makes a good spar cap. Double it on the inboard 1/3-1/2 of the wing. Use 1/16" sheet webbing on front and backside of the spars, or 1/8" webbing glued to the inside of the spars (making an "I" ). Both will be very strong. If you have 3/32"-1/8" sheeting in the center, you won't even need a spar joiner. 6-ounce fiberglass cloth that goes 3"-4" to either side of the center joint will provide tremendous strength, as long as you've glued well the rest of the wing.

Don't want to wrap the center section with fiberglass cloth? Then use a 1/8" ply joiner on the front and back of the spars, and insides of the leading and trailing edges. Make it go at least 6" to either side of the center joint. Epoxy it well.

If you must use a hard wood, then good-quality pine, bass, spruce, fir, etc, will all do well. You can probably make the spars slightly smaller, but you should still use the webbing.

I've built a 60" span model for .15-.25 power that used 1/4" x 1/4" balsa spars with 1/16" webbing on one side. It was plenty strong for anything you could make the airplane do.

I've also built an 11-foot glider that had 1/8" x 1/4" spruce spars with shear webbing for the full span. They were doubled on the inboard 1/3 of the wing, and had a set of rear spars for the inboard 1/2 of the wing. Plenty strong!

All of the open-structure .60-size models I've built had balsa spars, some with webbing, some without, but all did well.

It's all dependent upon the model and the materials at hand.

flybyjohn

My Feedback: (2)

Thanks for all the replies. The model I am trying to build is very similar to the towerhobbies p51 fun fly or profile plane. It is a very short wing with a thick airfoil and fairly light, I think about 3.5 lbs. This is why they probably are using balsa. All the kits I have build have used basswood or similar for mainspars and so using balsa seemed a bit too soft. I figured that the balsa would crush under compression if the wing was flexed too much. I had a wing made with a popalar spar that crushed under loading ( during flex testing , not flying) but I was loading with a pretty good weight.

One of the reasons I wanted to cut my own spars is that for a 44 or 48 inch wing, I would like to use one full length spar so no splicing or center joiner is needed. It is hard to find straight wood in those lengths if they have them at all. Is this reasonable or am I thinking outside the box too much?

thanks again.

Bax

My Feedback: (11)

Nope. If you want a one-piece wing with zero dihedral, then cutting your own spars is most certainly one way to go. If you have the material, why not? You eliminate a center joint that you don't have to add structure to for reinforcement. Just web the wing all the way through. The only reason ply is used in center joints is because the loads have to be carried across the joint for both sides of the wing to support. A joint is a weak spot, and a dihedral joint with its bend is doubly-weak, so it gets reinforced with plywood, carbon, or fiberglass.