The lightest balsa wood I have is 3.8Lb per cu. ft. The heaviest is 17 Lb per cu. ft. The low density balsa also absorbs more adhesive and dope than the high density balsa. Structure design is important. I used to build 1/2A pwer model wings with ~ 60 degree geodetic rib angles and the finished wing weighed 1.75 ozs. The new wing I built of the same area with very close to 45 degree rib angles, was noticeably stiffer in torsion yet weighed 1.25 ozs. finished. Barnes Wallis designed the WW II Wellington bomber with geodetic fuselage construction which was able to withstand significant damage yet not suffer structural failure that downed other aircrraft designs.
Very cool! Yeah, that is quite a range of density. The lower density stuff definitely absorbs glue much faster than the higher density wood. It behaves very much like a sponge. That is one reason I recommend when gluing joints (with CA at least), that you have to hold them longer for higher densities, and especially when using basswood. 5-6 sec is fine for normal balsa, but you might need to press the joints together for 15-20 sec for basswood or very high density balsa - similar if you have a glue film on the wood from a previous attempt
I used to use balsa cement but found over time that it continued drying out and eventaully pulled away from the wood. Not only that but wings etc. would break readily at the joints in a "heavy landing". I switched to slightly thinned PVA wood glue which never shrinks away from the wood and gives a slight degree of flexibility to the structure so withstands shock much better. CA I use mainly for fixturing and on field repairs. Downside is the joints are brittle. Can force cure using Zip KIcker on hardwoods but as CA cures with moisture on the joint surfaces, damping them with water accelerates cure time. Just breathing on the CA joint can speed up cure times. Best to assemble the joint dry then apply the CA. Watch for the vapour when curing fast on light weight woods as the vapour stings the eyes badly. Get cured CA off fingers etc. with nitromethane.
What would be interesting would be an equal mass comparison with different thicknesses and different densities in a square cross section. The low density thick square has less mechanical disadvantage against it than the high density thin square, when laying horizontal and supporting a beam in the middle, even if the high density material is stronger.
The suggestions you made at end of what kind of balsa to use, does it apply to towers too? I’m currently wanting you buy balsa online for first time and not sure what density to buy. Your benchmark Div C tower build uses such light balsa that I thought they would be under “extra light” density. Please help. Thank you!
Hi! I would say for the Div C tower build, the legs are on the light end of "light" density range, and the cross members are on the very light end of "light" or heavy end of "extra light". The best way to deal with this is to try and get as much material as you can and experiment with different sizes and densities but trying to match the total layer weights if you can. Good luck!
I first wanted to do a pure tension and compression test vs density, but it's really hard to build a test rig to isolate that. Compression might be possible, but creating something that can hold the pieces tight enough for a tension test usually destroys the ends by crushing them. I have found that the tension and compression strength correlates pretty well with these results from my general experience. Balsa is about 70% stronger in tension than compression, but the relative performance vs density holds pretty true.
@@balsaengineering6686 would it be possible to glue balsa sticks to two wood blocks and apply tension force to the bottom block for tension test? And is Hobby Lobby a good place to buy balsa sheets & DIY cut or some online stores for consistent density/strength? Thanks.
@@annylee4208 Hi Anny. We actually did that exact type of thing for testing the bass tension part of the boomilevers. You can see the test rig in this video here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-NrDhMu_wQt8.html It wouldn't be very practical to do that at a large scale. It's really important to be able to apply a constant tension force. You'd probably need an engineering tension testing tool to automate this with any consistency. The bending test I do here is a pretty good general test that correlates decently to tension strength. I'm sure Hobby Lobby has fine balsa wood, it's just a matter of if you can find exactly what you're looking for. I like to use Specialized Balsa for my balsa sheets, specifically, I like to get 4" wide by 36" long sheets which is a nice balance of best price and easy to strip (as opposed to the 48" long pieces). specializedbalsa.com/products/balsa_sheets.php
Hi, I ordered 1/4 x 1/2 balsa wood and it weighed ~4g. This was going to be used as my legs but I am concerned that it’s too heavy for a good Efficiency score for a bridge. I am also going to be cutting the 1/4 x 1/2 balsa slightly. Is there anything that I can do to improve the weight.
That does seem a bit heavy. I would recommend in the future to buy your wood in sheets and cut it to desired size and then you can have a bigger range of densities to choose from. For this wood, 1/2" wide is pretty thick, you can probably cut at least 50% off and maybe even in half depending on the density. Good luck!
@@balsaengineering6686 Thanks for helpful information. I have a balsa stick weighing ~12g with dimensions 1/4 x 1/2 x 36. I am planning to cut it into 1/4 x ~2/3 x 21 pieces. What would you expect the weight of those pieces to be?
Hi Ravidath, some fairly simple math can predict what your sticks will weigh (on average). We just need to compute the area ratio of your stick to the entire piece. We can use area instead of volume because the 1/4" thickness is the same for both. First, put everything in the same units, I'll use mm. Your original stick is 1/2"x36" or 12.7mm x 914.4mm = 11612.88 sq mm. Your desired stick is ~2/3 of 1/2" x 21cm or 8.47mm x 210mm = 1778.7 sq mm. The ratio is 1778.7/11612.88 = 0.153, which means each of your new sticks should be about 0.153 * 12g or 1.84g each. That seems fairly reasonable to start with. You probably won't get 4 sticks exactly the same though due to balsa being so non-uniform, but they should be close to that. Good luck!
Hi! Just a quick question! Would it be at all possible for you to share the deflection of the piece of balsa wood? I am working on deriving a set of Pareto-optimality equations and am interested in these values.
Hi! I didn't record the deflection for the pieces. That would have been very challenging to accurately measure and would have most likely had to be done visually with a camera precisely lined up to the pieces. Typically, they do bend "a lot" before breaking, but that depends on the grain type as well. Most of these pieces were A-grain which would bend quite a bit. C-grain would not bend as much.
Thanks so much! This is kind of an odd question, but is the 1/16 bass piece shown at the end of the video 2ft long or as long as the span of the bridge (about 47-48cm)?
That is showing the cut down piece. The 1/16 x 1/16 24" mass for what I've been using with these builds is around 0.6-0.65g or so. I am currently experimenting with even smaller tension members, 1/32x1/16 and 1/32x1/32. Both are worthy of checking in your own builds.
For the grain of the balsa, based on the diagram, I would think that A-grain balsa would be best right? I have some sticks that are light for my legs, but they tend to be c-grain sticks, I'm thinking that they will break quite easily due to the grain pattern.
@@balsaengineering6686 Yeah, the reason I asked was just because I've been noticing the inconsistencies in some of the wood I have. I discovered that some parts of the wood are really squishy, meaning that they aren't very dense, and then some parts of the wood are much denser (these observations coming from one 30cmx10mm (3/32") leg piece). Is there a way where I could contact you directly with a couple of questions, because I feel like I might be revealing too much about my future designs when I comment.
Hi Aiden, yeah, unfortunately, that is not super uncommon to have big density variations even in a smaller piece. When that happens, basically, don't use it. Some sheets are much better than others, even from great sources like Specialized Balsa. Feel free to reach out to me at mklinger@comcast.net if you'd like to share specific questions you don't want public.
Hi Balsa Engineering, I have another question. So if I was to buy from Specialized Balsa, would the legs you have used in the Div C build video (the 2098 efficiency one, specifically the 1/16"x10mm and the 3/32"x10mm) be within the range of light or medium light balsa? I have noticed that a lot of wood has been wasted from our end due to the fact that it has not met the standards we want (in terms of mass). I have made a few bridges at high efficiencies (I don't want to note them numerically), but I'm running of wood that's around the same mass, so would it be worth it to just go to light balsa (this is after doing the math) since I don't wanna waste money going for wood that might be too heavy since wood takes a while to arrive at the school I do S.O. in.
I'm at the stage where I'm trying to make a bridge to hopefully land on the podium at the state tournament in mid-March (in NY, which is really competitive at the moment).
Hi Aiden, doing the calculations on the legs I used for the 2098 build, the 3/32"x10mm was roughly 7.7 lbs/ft^3 and the 1/16"x10mm was ~9.3 lbs/ft^3, so yes, according to Specialized Balsa's range, that would be "Light Balsa" which they have as 6-10 lbs/ft^3. Good luck!! Let me know how it goes for you!
@@sibaiyan It's hard to know without seeing a picture of your build, but in general, yes, those things are very much related to each other. Feel free to reach out to me at mklinger at comcast dot net if you'd like to share a picture and I'd be more than happy to provide a more detailed response
