How much does grain matter in terms of stretch?

[Balsa Man]

I think the two-part configuration dictated by this year’s rules is a major barrier to competitiveness of a tubular leg approach. Last year, with straight legs, I think it could have been done to … a fairly competitive level. I saw some info from a team experimenting with a 3 tubular leg design, and did some calcs from our design spreadsheets for booms, and concluded a…low 7gr C tower (meeting 29cm circle bonus) might be possible.

The issues/challenges are bracing, and the joint where upper and lower leg segments join. Bracing a ‘stick’ leg only involves glueing a brace piece onto a flat leg face. Bracing a tube requires an insert inside the tube; You can’t just brace on the tube wall. The insert can be of pretty low density, but needs to be fully bonded to the inside of the tube…all the way around. A bit tricky to get in the right place, and glue weight not….out of control.

So, at a minimum, you’d need tension brace/tension strap (strong enough to carry something over 2kg) around the base leg ends, a set of ladders at the base section/chimney section joint (strong enough to carry something over 2kg compression), and a ladder at the top. That’s inserts in top and bottom end of both leg segments. The other really tricky part is cut angle on the leg segment ends where upper and lower meet. Critical that tube walls be perfectly aligned. With base legs at a significant angle, and upper legs at a very small angle, what happens if you cut on a plane parallel to the testing base? The exposed cross section of the bottom of the upper leg is close to circular, very slight oval. The exposed cross section of the top of the bottom leg segment is a significantly elongated oval. Put the two together, and the walls only meet up at two points (across the short axis of both ovals). It would take a…pretty massive insert both sides of that joint to ‘work.’