Strength of Balsa Spars

[Jackson T]

One of my school subjects requires me to conduct a scientific research project on a topic of my choosing. Obviously I chose a plane-related topic . My research objective is to find a way to mathematically predict the strength of balsa spars of the upper/lower spar cap + shear web design. I am also making sample spars to test the formulas I end up with. I can't find any similar work online, has anyone come across any projects/articles related to this or done something similar themselves?

 

[speedbirdted]

RCM tested this subject I believe in the late 70s or early 80s. They tried a lot of different configurations, the two spar with shear web design you describe being one of them. It had a lot of heavy math involved as the purpose was to find the spar design with the highest strength-to-weight. I think the winning configuration was using a slightly thicker upper spar and shear webs made of 1/64 balsa and thin carbon mat sandwiched together (which was pretty much alien-level technology for the time) I'll have to take a dive into my stack of vintage magazines to find it, but if I do I'll definitely let you have a look.

In any case if this yields any useful data please post it as I've often thought about this kind of thing myself...

 

[Ketchup]

I would definitely like to see this. I know that the project is focused on balsa but hopefully your findings can be used to make better wings with other materials too.

 

[Jackson T]

I will most definitely post my findings, I'm sure (if I actually find something) that it would be useful for other modelers. @speedbirdted let me know if you find that article, sounds like a good read!

 

[cyclone3350]

Dittos here on all the responses. Great subject. As U can see, U have a lot of folks that will be interested in what U come up with.

 

[Merv]

I do all my testing in the air, I use bamboo bbq spars. In the center section, I use 2 bbq’s on top & 2 on the bottom. On the outer wing section, one on top & one on the bottom.

I have broken a single center section. So far I have not been able to break the double center.
Don’t worry, I’ll keep trying. I just need a bit more speed for my square circles.

Here are some formulas that may help you calculate the answer at the work bench.
https://mechanicalc.com/reference/beam-analysis

 

[Jackson T]

I do my testing in the air, I use bamboo bbq spars. In the center section, I use 2 bbq’s on top & 2 on the bottom. On the outer wing section, one on top & one on the bottom.

I have broken a single center section. So far I have not been able to break the double center.
Don’t worry, I’ll keep trying. I just need a bit more speed for my square circles.

Sounds like an interesting idea, do you have a photo of your spars?

 

[Piotrsko]

Hmm bamboo is supposedly stronger & lighter than spruce for torsion ( they bend in compression) and them 4 ft bamboo skewers in the grocery store now look really tempting. A piece of steel standoff for center connector........

How to get finance director's buy in?

 

[Monte.C]

Hmm bamboo is supposedly stronger & lighter than spruce for torsion ( they bend in compression) and them 4 ft bamboo skewers in the grocery store now look really tempting. A piece of steel standoff for center connector........

How to get finance director's buy in?

The way they use bamboo scaffolding in Southeast Asia tells ME something. Bamboo has got to be the Wonder Wood.

 

[Phin G]

The way they use bamboo scaffolding in Southeast Asia tells ME something. Bamboo has got to be the Wonder Wood.
View attachment 203604

Why aren’t my bamboo poles that tall

 

[Merv]

Sounds like an interesting idea, do you have a photo of your spars?

I’m packing for FF, here is a quick picture of the approximate layout of my spars on a swept wing. The spars on the bottom are in the same location. All the spars are imbedded into the foam. I dig out a channel and use gorilla glue to secure the bamboo. I use the standard FF foam board spar as a separator, the idea is to form an I beam, or shear web.

The second picture is a strength test of an old wing, it's holding about 28 pounds. For a 30 oz plane, that would be 14-15 G.

 

[Jackson T]

I’m packing for FF, here is a quick picture of the approximate layout of my spars on a swept wing. The spars on the bottom are in the same location. All the spars are imbedded into the foam. I dig out a channel and use gorilla glue to secure the bamboo. I use the standard FF foam board spar as a separator, the idea is to form an I beam, or shear web.

The second picture is a strength test of an old wing, it's holding about 28 pounds. For a 30 oz plane, that would be 14-15 G.

Nice! So you use them for balsa construction too?

 

[Merv]

Nice! So you use them for balsa construction too?

Yes I would, but I’ve not built a balsa plane in a very long time.

So a few years ago, I found a curtain made from bamboo in the trash. The slats were about 3/4 inch wide, I cut them in 1/3rds & use those strips for spars on the outer wing section & bbqs for the inner section.

Taking off for Ohio, hope to see everyone @ flite fest

 

[telnar1236]

I've used some smaller bamboo skewers for balsa construction, but they are pretty heavy compared to balsa. I could see some use for them as structural elements in larger planes, but apart from locking elements or as main spars for the wings of very light planes they tend to be outperformed by balsa. A good shape tends to be more important than good materials. That said, I also make kites and I always use bamboo. It's a great material when you need a thinner spar.

Another great option is the larger coffee stirs. They are pretty light and strong, but they can be harder to find.

 

[telnar1236]

One of my school subjects requires me to conduct a scientific research project on a topic of my choosing. Obviously I chose a plane-related topic . My research objective is to find a way to mathematically predict the strength of balsa spars of the upper/lower spar cap + shear web design. I am also making sample spars to test the formulas I end up with. I can't find any similar work online, has anyone come across any projects/articles related to this or done something similar themselves?

Here are a few good starting points. The Wikipedia articles cover some basic theory and the MIT paper focuses on balsa wood's material properties. Depending on where you are in your studies, you may have already seen some or all of this.

Bending - Wikipedia
Shear and moment diagram - Wikipedia
https://dspace.mit.edu › Mech. balsa_draftall04.pdf

The two most likely causes of failure are the bending stress (compression and tension at the outermost surface of the spar cap) and the shear stress (the upwards force pushing on the wing and the counteracting downwards force the fuselage produces on the spar). I have never seen a wing fail in shear since the bending stresses tend to be so much larger, but it is technically possible. Unfortunately for more complex geometries, you need either very complex calculus or FEA to successfully model them, but for simpler geometries like an I-beam, you can do it on paper more easily.

 

[telnar1236]

I think cali bamboo is the best option for this... because cali bamboo is very light and stronger than the others...

What are you talking about?

1) I did not mention Cali Bamboo in my original post and have never heard of them. You added that word when you quoted me.

2) From what I can see, Cali Bamboo makes flooring and is thus useless for rc airplanes.

 

[OliverW]

What are you talking about?

1) I did not mention Cali Bamboo in my original post and have never heard of them. You added that word when you quoted me.

2) From what I can see, Cali Bamboo makes flooring and is thus useless for rc airplanes.

Ah yes, the bots of the forums are trying to get you to buy stuff lol

 

[Jackson T]

I've completed the tests, all 9 sample spars are broken! The assignment is due in 3 days, so I'll focus on getting it finalised and submitted, then I'll work on posting the findings here.

Here are a few good starting points. The Wikipedia articles cover some basic theory and the MIT paper focuses on balsa wood's material properties. Depending on where you are in your studies, you may have already seen some or all of this.

Bending - Wikipedia
Shear and moment diagram - Wikipedia
https://dspace.mit.edu › Mech. balsa_draftall04.pdf

The two most likely causes of failure are the bending stress (compression and tension at the outermost surface of the spar cap) and the shear stress (the upwards force pushing on the wing and the counteracting downwards force the fuselage produces on the spar). I have never seen a wing fail in shear since the bending stresses tend to be so much larger, but it is technically possible. Unfortunately for more complex geometries, you need either very complex calculus or FEA to successfully model them, but for simpler geometries like an I-beam, you can do it on paper more easily.

I somehow was not notified of this post and only just saw it, dunno what happened there. Thanks @telnar1236, that's just the sort of stuff I'm looking for! I've seen the second wiki page and the MIT paper, but didn't read through the MIT one very far last time. I'll do a more thorough read this time. I guess for a wing to fail in shear the material's shear strength would have to be way out of proportion to the tensile and compressive strength compared to most materials. Another potential mode of failure for my spars is horizontal shear in the webs. I did not see any shear web failure, but one spar had the shear web pull away from the spar cap, taking some of the spar cap with it (see photo). This could have caused the failure, or could have happened as the spar failed from other means. I will further analyse my photos and will most likely include this topic in my discussion section.

 

[Jackson T]

The assignment was submitted this morning! I am so glad to be finished. It was a very interesting project, but a lot of work. A formula was derived, but I'll post that with an explanation later. By using the formula to graph bending strength against different dimensions, the following two discoveries were made.

1. Strength does not increase proportionally with spar cap thickness. A graph of this is shown below for a spar 12mm wide and kept at a constant total height of 18mm, simulating the thickness of a wing. This is because a bending moment is a force x distance, and the added balsa wood on the inside is at a smaller distance than the rest of the balsa spar caps.

2. Bending strength increases greater than proportional to the total spar height. This means when you double the total spar height, the bending increases by a factor slightly greater than 2. This is caused by a similar reason to the spar cap thickness disproportionality.

 

[cyclone3350]

The assignment was submitted this morning! I am so glad to be finished. It was a very interesting project, but a lot of work. A formula was derived, but I'll post that with an explanation later. By using the formula to graph bending strength against different dimensions, the following two discoveries were made.

1. Strength does not increase proportionally with spar cap thickness. A graph of this is shown below for a spar 12mm wide and kept at a constant total height of 18mm, simulating the thickness of a wing. This is because a bending moment is a force x distance, and the added balsa wood on the inside is at a smaller distance than the rest of the balsa spar caps.
View attachment 206645

2. Bending strength increases greater than proportional to the total spar height. This means when you double the total spar height, the bending increases by a factor slightly greater than 2. This is caused by a similar reason to the spar cap thickness disproportionality.

I will be interested in seeing the details along with pics of your 9 samples and how each one performed.