Towers B/C

[Balsa Man]

Baker, your nice words are appreciated; thanks. And my thanks to you and all who share important insights and understanding here.

Over the years, (mainly speaking to the ‘wood structures’ events- towers, bridges, booms) I’ve been struck again and again with how many students…. just aren’t aware of, don’t understand the important, basic concepts. I understand from starting with a few years coaching my own kids, then a couple years coaching an entire team, and a few more years just coaching building events, that it is….. very challenging to a) bring together students who are motivated to put in the time and effort needed to do well, and b) get them access to the correct basic knowledge and understanding needed to do well. S.O. is all about learning and applying science; the wood structures events are about learning and applying engineering , and engineering is about extracting from the full complexity of the underlying/relevant science; getting to and applying a set of approximations that are close enough to ‘work’ for what you’re trying to do- build the lightest structure you can to carry a 15kg load.
In the “real world of science and engineering”, “progress” comes from exploring and understanding “the literature”; the record of what people have discovered, figured out, applied, created. In the S.O. world, the Scioly board is the primary repository of ‘the literature’ on various events.

So, having spoken to the basic mathematical relationships that need to be understood, let me add some thoughts on a few other basic concepts/aspects that, like the mathematical relationships, if you understand and apply them, you will “do well.” What do I mean by “do well”? That depends on you, and it can range from… not being embarrassed and picking up some reasonable points for the team, to winning State/medaling at Nationals. The higher you want to reach, the more thought and work and time its going to take, and the more…. Intensely and correctly you need to apply ‘the basics.’ But even if you just roughly understand and apply the basics, you will do a lot better those who don’t.

Briefly, the concepts/aspects, as applied to towers, are (more detail on all is in the archives to find…):

Estimating & calculating – As discussed in my post on the basic mathematical relationships, with these and simple hand testing of buckling strength, you can ‘run the numbers.’ The rules provide the basic configuration (shape/form); form follows function; legs at the bottom just clearing the base opening, slanting up to a top that fits the load block; draw up a pair of adjacent legs and you have all length dimensions and can calculate forces on legs at full load. Run buckling testing on a variety of potential leg material sticks at full length (typically 36”)- check different (cross-sectional) dimensions and densities (stick weights at a given cross-section/size). You can, with a little thought, set up a spreadsheet, or set of spreadsheets with which you can calculate/estimate/compare key design parameters and tradeoffs- heavier, stiffer legs with a longer bracing interval; lighter, more ‘floppy’ legs with tighter bracing interval. With lengths and weights of all pieces, and an estimate of glue weight, you can calculate an estimated finished tower weight, and compare alternatives.

Precision & Jigs– having settled on your design configuration and sizes of the pieces, the next problem is taking that from paper to three dimensional reality- how to set/align/hold the pieces in place so they can be glued together. That’s what jigs are for- something you build that holds pieces in their correct place/alignment. There are a lot of ways/techniques to do this; I’m sure there’ll be discussion and ideas emerging as the season goes on. The precision with which you build a jig is really important. Your “design configuration” is accurate and symmetrical; forces are equally distributed; the more precisely you construct the jig, the closer to optimal performance of your design you get. Failure (under loading) will first happen at the weakest place; if one leg is a hair longer or shorter than others, if the angle is a hair bigger, one leg will have a higher force on it, and fail first; if one bracing interval is longer than the others, the buckling strength will be lower, and the leg(s) in this interval will fail first. If the jig isn’t perfectly vertical (leans a bit to one side), the leg on the side it leans to will see higher force and break first. A good jig gives you control over a major set of variables. If you build/test a set of towers, where you’re checking (varying) a little higher or lower wood density, and the shape is different, you learn essentially nothing. If the shape is the same, and you vary wood density in a piece/set of pieces, then you’re actually testing the performance of that wood choice. This leads right into the next concept-

Wood selection – the ‘dark art’ of finding the best/optimal pieces of wood is critical to get to the upper end of “doing well.” As discussed earlier, at any given cross-section, the buckling strength of a piece (e.g., a leg) varies with its density; twice as heavy, about twice as strong. BUT, wood is a grown, not manufactured material; no two sticks are the same; different sticks of the same weight/density will and do vary in ‘stiffness’/buckling strength; within a given stick, portions of it will be more and less stiff. Variation in balsa on the order of 20% should be expected. Testing buckling strength of a set of full length sticks will get you in the ball park, then the trick becomes to find a set of sticks (for legs) that are closely matched, and the lightest that get you to the buckling strength needed.

Building skills/techniques – now we’re down to actually building the structure. Important, for sure, but with the steps discussed above in-place, relatively straightforward, and easy to do. Keys include precisely cut joints, just enough glue, careful handling of low density pieces to avoid denting/crimping, and careful…. Finishing- leg ends sitting on the base all in one plane (to a very high precision), with tower really vertical, and leg ends at the top in the same plane, parallel to the base plane.

Last, a comment on the ‘conventional wisdom’ that you’ll run into many places, including on this board; that to succeed you just need to build and break a lot of structures. Without ‘control of variables’, this is, frankly, a waste of time; you can't learn things that will move you toward a more efficient tower. I’ve come, over the years, to believe strongly the best payoff on time comes from following the engineering process I’ve outlined; doing the analysis needed to settle on a design; putting in the time to build precise jig(s) to construct the design (controlling the configuration variables), doing a proper wood selection process, and applying good building skills/techniques.

A quick story from last season to consider – with Regionals fast approaching, a team was… nowhere w/ their bridge. I was asked by a team coach to help; to just help get them to a bridge that met dimensional/competition specs. Recommended they go with…. substantial pieces of bass wood- way more than needed to carry 15kg; helped them figure out how to get a nice precise jig. The result was a 33 gram bridge; at Regionals, it carried full load, and finished mid-pack. They made it to State, and one of the builders wanted to see what could be done with balsa; to see if they could medal at State. Worked through an estimating spreadsheet; it said a ~7.5 gr bridge that should carry full load was doable, using some pretty low density balsa. They built one, to get a feel for the construction challenges dealing with really light/soft wood, then with the carefully selected “competition” pieces of wood, built the State bridge. Neither bridge was tested. At State their bridge weighed just a hair over 7.5 gr, held full load, and missed winning by less than 50 points (out of ~2000).

I’m not suggesting a “no testing needed-just build one and win” approach, particularly if you want to do really well; I’m suggesting that with a real engineering process, applying ‘the basics’, a limited amount of testing will get you to a VERY efficient structure, and is efficient use of the time you have to put into this event.

[jander14indoor]

Hollow tube balsa structural elements are buildable in sizes and weights relevant to Towers. Of course the smaller and lighter the harder.

Boron fiber reinforcement is only used in the MOST weight critical flying classes, not needed for SO. Also a safety hazard in most of the rule writers opinions, they can be tricky to handle, not life threatening, but extremely painful. Just not worth it for this.

Your biggest challenge will be in connecting cross bracing efficiently to that wrong surface. It can be overcome, but it is a building skill challenge.

With that preamble, here's some links to building balsa tubes.
A good article on fairly light rolled tubes, very buildable.
http://www.parmodels.com/Techniques_and ... 0Stick.pdf

This is a compilation of articles on hard core, REALLY light weight indoor rubber powered models. If you dig through you'll find articles about building VERY light weight rolled balsa tubes. On the harder end.
https://indoornewsandviews.files.wordpr ... f-inav.pdf

Another hard core indoor construction hints and tips including rolled motor tubes. May be some repeat/overlap.
https://thef1dblog.files.wordpress.com/ ... ction8.pdf

Hope that helps.

Jeff Anderson
Livonia, MI

[Balsa Man]

Thanks much, Jeff.
Thought you'd have good insights and info.
Agree on the challenge doing bracing on tube surface; a possible approach might be tube stiff enough that it only needs...one or two bracing sets.

[baker]

Along the discussion of making a test stand setup, this is a tip I picked up from here years ago. The students I've worked with always did their testing at my house. We had the Pitsco stand, but we also hung a chain from one of the floor joists that connected to the top of the loading block. When connected to the loading block, the chain would be just one link loose. This way when the tower, bridge or boom would break you could still see what was the weak point. Without this it's hard to see what broke first when you have a pile of tooth picks left over. Also as Balsa Man said, this is not a "one and done" event if you want to be a winner. Over 10 years we must have broken 50 or so devices. I had the students keep a log of what dimensions, densities, weights for each member (tension/compression). Easy to make changes to members with the chain.

[DoctaDave]

The biggest concern I have with using rolled tubes is that they curve A LOT. For example, F1D tailbooms are typically 18" long and are rolled from 4-5# sheets 0.009-0.011" thick. The internal stresses of the wood formed while drying on the form and even the glue shrinkage is enough to make the end of the tube curve a couple inches at the end. This is also after the tube has sat on the form for a few days. Luckily, boron is allowed in F1D so most of the curve can be corrected with that, and the forces in a free flight airplane are much smaller so the curvature isn't a huge issue. But given that tower legs will have to be 60+ cm long, no boron is allowed, and you will need at least 3 tubes/tower (so you can't wait nearly as long for them to dry on the form) it seems that the curvature of the tube will be a much greater issue for towers than it would be for freeflight. Also, curved members are not good for counteracting buckling.

IMO, the problems with just making the tubes themselves makes this impractical. You will pretty much never get them straight, it's an extremely long process, there's a long learning curve with rolling and gluing up balsa tubes this thin, and not to mention the balsa wood is probably not available and very expensive. I have never seen anyone sell indoor balsa sheets over 24" long, and if someone does they will definitely charge a premium for it (>10$ for a 1.5" wide sheet). Although you may be able to work with 1/64" balsa sheets from specializedbalsa.com, the quality and consistency of low density sheets that thin from specialized is questionable in my experience.

[jander14indoor]

Some counter points.
You don't have to make the tubes round. I've made square and even triangular ones. Still gain over solid members in stiffness vs weight.
You don't need dedicated indoor wood. If you look you can find good wood in hobby stores. If you want it thinner than the 1/32 inch commonly available it is pretty simple to sand it uniformly.
You don't necessarily need a continuous tube from bottom to top. Consider one or two joins at the point you want cross bracing with solid balsa. The ends would be round (or whatever) to fit into the tubes and the middle short section could be square or triangular to give good glue surfaces for bracing.

I will say all this is more work. People have been very successful with solid member towers. Before you go this route, you should have a good base in building high level towers with conventional methods.

Jeff Anderson
Livonia, MI

[Balsa Man]

Good insights, guys!

Testing with a way to limit damage to initial failure – this is a very powerful tool for developing/refining a design; a number of ways to implement it. I like the line from a ceiling rafter . We’ve used a ‘safety tower’ over the years to do this- a tripod made of 2x2s higher than the structure/tower- with connecting nut joining threaded top of the eyebolt that’s through the load block to threaded end of another eyebolt hung firmly with wire from the top of the tripod above the tower- adjust so load block can only fall 1/8 to ¼ inch. If you keep the allowed drop small, so damage doesn’t move beyond initial failure, you can patch/splint, and re-test immediately to see what/when next failure mode is.

Viability of tubular legs – agree with/understand the challenges noted, hence my initial negative take. However, some good ideas worth exploring to get around the challenges from Jeff. A little test building and comparison spreadsheet should point to the best way…..

[calvin102111]

I am very inexperienced in the way of tower building, so keep that in mind. The tower I just tested weighed 11.32 g, and held about 11,350 g. This would lead to a score of 1002.65. To the more experienced builders, is this score... decent, for being one of my first towers? I'm pretty sure that I know how I could improve upon the design, after reading through some of this thread.

[bernard]

Test Base, Loading Block Assembly, and Stabilization Stick schematics are on the national website: https://www.soinc.org/towers_b and https://www.soinc.org/towers_c.

Towers Test Base Drawing for 2017 (PDF)
Towers Block and Sticks Drawing (PDF)

[Unome]

Are there any common objects that have a 29 cm diameter?