This major assignment this semester in Survey of Engineering Academy was the balsa wood bridge which corresponded to what we were learning in the physics portion of the class. I place any wood I am steaming on top of the basin, I usually have at least one or two pieces that are longer then the basin and some that are shorter, but the shorter ones lay on the longer pieces so that nothing is actually touching the water at this time, for the goal was to steam and not soak the wood. Most woodworking joints were in fact designed to avoid the application of glue to an end-grain surface. You can start by checking out this website because they have a great selection of Balsa wood sticks, sheets, and blocks. This results in a splotchy finish where the wood putty stands out rather than concealing the defect.
Saucer guy, what I meant about inflexibility is that when you double glue well fitted uprights to longerons with a cellulose based glue like thinned Duco, Ambroid, or Sig, allow to dry adequately, then try to glue the crosspieces between the fuselage side, the sides bend easily and form nice curves. Use the drawing you’ve created, and lay each strip of wood where it needs to be on the drawing. If the parts are not pre-cut then the outline of each one will be printed on to sheet wood and it’s your task to accurately cut them out with a sharp modelling knife.
However, balsa is easy to dent and can’t be used for any large or weight-bearing projects. A common reason for using plywood instead of plain wood is because plywood is more stable and because it is less prone to change (shrink, twist or warp). Miquel hasn`t designed the tower but Marc got the balsa wood cut up and they are accurate. Many competitions require you to have your tower weighed, and excessive glue adds a considerable amount of additional mass to a tower.
The best balsa wood tower designs require the sparing use of glue and other liquid adhesives. Take all your broken peices, split them down the middle to be thinner peices, then glue them back together in that overlapping style on a flat surface, they should be about as strong as new. Elmer’s Wood Glue is inexpensive glue that is quite effective for holding balsa wood together.
For example, if you do not do this and simply start sanding the joint, glue blobs can break loose and leave deep gouges in the skin. Particle board (sometimes called chipboard) is a material manufactured from wood particles (for example, wood chips, sawmill shavings, or even saw dust) and a synthetic resin or other suitable binder, which is pressed and extruded.
We could use only the materials provided, six square inches of balsa wood and 2/3 oz. of wood glue. There are 3 main kinds of joints: Lap joint, end/butt joint, and notched joint. While lumber from a harvested tree is no longer alive, these tissues still absorb and expel water causing swelling and shrinkage of the wood in kind with change in humidity. My only trick for stuff that is already put together is to use carpenters glue (alaphatic resin stuff light tight bond or the gorilla wood glue) take an empty glue container and mix 30-40% glue with super hot water. The gusset creats a lap joint, which is strong in both tension and compression.
I destroyed a couple of ARF’s and a kit build recently, I intentionally tore them apart and was alarmed as to how many of the bonds between the joints just fell away, they were CA’ed together, and in fact, only 60% of the elaborate lazer cut componentry was actually held together by glue, the rest was simply floating around. The thinned glue will penetrate deeply into the balsa, extending the bond between the pieces rather than creating a light surface bond. Getting started: Start by researching designs and plans for the most stable and lightweight bridge possible. You probably have been told what the winning bridge last year was able to withstand before collapsing.
The arch is hard to make and I thought by bending the wood it might weaken it. By doing an easier design I would be able to spend more time on craftsmanship and the details of the bridge. If you need to joint edges of balsa wood together, then add small overlapping pieces of balsa wood over opposing sides of the joint to strengthen it. The dado can also be used in regular lumber as long as the dado runs along the grain – otherwise, any glue used to secure the joint would have to adhere to the endgrain. From experience i know that the best way to build a bridge with balsa wood is not to build high.
You generally want slightly denser wood for areas of compression, depending upon the amount of compressive force. This glue is excellent for those times when your balsa project breaks and you don’t have hours for the glue to dry. Again I may be impatient, but it works for me. I then apply glue to the wood, and wait 4-5 hours for it to set (that is the amount of time my glue takes to set). If you can’t level the joints when the skin is flat on your workbench then you’ll never get them level after the skin is glued to the model.
Make sure it is hot and work it into the wood then press it under some weight and allow it to dry for a few days so that all of the water evaporates. Note that some joints are more important than others, so you may want to use more or less glue at your discretion. I haven’t done a lot with toothpick bridges, but it seems to me that lap joints would be the way to go. I’d think that you would want as much overlap in the joints as you can get because the surface area of toothpicks is not much to begin with. But because glue will not hold to end grain, it is a joint that must be held together by mechanical fasteners such as nails or screws.
Generally I like to use Titebond aliphatic resin (carpenters glue) for wood-to-wood or wood-to-foam but it is slow drying, like overnight. Some very cool project, I like working with wood, my grandfather was a violin maker and taught my father how to work with wood and my father taught me. If there is tape goo or excess glue outside the joint then quickly wipe the joint with more solvent. Generally glues cannot be used as a coating and the challenges have started defining how much glue can extend past a joint. Different species of wood have different strength levels, and the exact strength may vary from sample to sample.
Don’t bend it too much, or it could suddenly snap in half and send wood particles flying. Wood is stronger when stressed along the grain (longitudinally) than it is when stressed across the grain (radially and tangentially). Different glues are used when building model airplane kits and the type of glue depends on the joint and the wood type/hardness, as well as the builder’s personal preference. Make sure the base of your tower is larger than its top to create a stable structure.
I use aliphatic resin (a wood glue) when I’m going to have to sand the joint later because CA is too hard. Using full wood outlines on the initial design can help determine which joints to use. Use straight pins around the edges of the sides to keep them in place while the bridge dries. Glue weight is NOT negligible, so it is important to use the appropriate amount, which may take time to discover. If you use a waterproof glue such as epoxy, the glue joint will be strong enough to not require further reinforcement. Expoxy is mainly for extremely high-stress joints (motor mounts, landing gear, wing joiners) and when attaching dissimilar materials like wood-to-metal.
Heat-shrink covering is a relatively modern invention and traditionally balsa model airplanes were (and still are in some cases) covered in lightweight tissue paper (or even silk) with a liquid called ‘dope’ applied to the airframe to stick the tissue. Bridges can be made out of any type of wood except those excluded in the rules, such as particleboard, plywood, bamboo, , etc. Depending on the type of glue you use, the joint is only as strong as the face of the wood.
So I overcame this by building the piece of balsa wood then making the shape of the notch, put that on the balsa wood piece then subtract it. When I finally finished designing the bridge I checked the mass properties and it was5 square inches over. I think an important part of these classes, if it’s not already, would be to analyze each destroyed tower to see where/why it failed.
Chisels can be used for making a variety of cuts in wood, from mortise and tenon joints to dovetail joints. Balsa bridges tend to break for two reasons, a member in compression fails, or the joints pull out. Per Modest’s drawing in post #15, note that all the compressive & tensional loads between the members are carried by the glue. The fiberous nature of balsa wood makes it much stronger in tension than in compression.
Lap joints, on the other hand, use an overlapping method to join two pieces of balsa wood or other material and offer greater stability. The reason I say it is better to go to a local store before buying online, is because you can actually see the wood. Experienced builders try to glue their structure together so the joints are consistently strong without using too much glue. The legs and braces need enough surface area for a glue bond, and compression chords need a large enough cross section to resist buckling. As well as these specifications, there were certain rules like not being able to split or laminate the wood.
But if you have few pieces but still want a strong bridge, bass is the way to go. It’s best if you experiment with both kinds of wood and see what you prefer. White wood glue (PVA), aliphatic resins and cyanoacrylate (CA) glues are commonly used on balsa to balsa joints, but where stronger joints are needed such as landing gear plates and engine bulkheads then two-part epoxy resin is common. Don’t overdo it or you’ll melt the glue in the joint which may cause the sheets to separate.
If you are looking for glue that is waterproof, does not foam like standard polyurethane glue, and has excellent strength then Titebond III is a good choice. For the average 40 size plane, the total glue used will be about 1 ounce more or less, and more isn’t always better. Laminating new wood over a thin spot will usually make the problem worse, for example.
Materials advance over the years and I was just curious if there were any new methods of buliding balsa planes. The difference between the wood suitable for making shipping crates and pallets and the wood suitable for making furniture and toys is fairly large. It is also important that the process used to apply pressure to the glue joint does not apply extra stress to the materials so that they are weakened by the gluing process.