Boomilever for 2013

[fanjiatian]

Thanks SLM!
This is probably a bad question to ask, but how do you figure out the geometry that best reduces bending moments? Would it be through trial and error?

[LKN]

What are the draft rule dimensions for boomilever this year?

[SLM]

Thanks SLM!
This is probably a bad question to ask, but how do you figure out the geometry that best reduces bending moments? Would it be through trial and error?

The magnitude of the bending moment depends on the geometry as well as the cross-sectional size and density of the member and those of its adjacent members. I would use trial and error (with the aid of a structural analysis software) to solve this problem theoretically.

[LKN]

A little off topic, but I have to ask since my last attempt was a disaster.

I plan on retrying to construct a balsa tube. I have the wood soaking now, and plan on wrapping it around a stainless steel handle and then fastening it with zip ties. How do you make sure that the tube is "straight"? Do you cut to size before (leaving room for error) or after it has taken the tube shape? I am assuming to do a very long lap joint to glue the wood together, roughly how much should the wood overlap?

[iwonder]

Not off topic at all, it's not to complicated, cut the wood before you do anything, into as perfect a square as you can manage, as long as you want and wide enough for 1/4-3/8" overlap. Then soak the wood, wrap it around a clean form(handle, in your case), and tie it up with rubber bands, snug, but now too tight.(zip ties would probably crush it), yes, you will have to walk the rubber bands down on longer pieces, be careful not to scratch the wood, at all. Once it's bound, let it dry for a day, take off the rubber bands(leaving it on the form) and slightly open the edge(where the glue will go) try putting a very small layer of glue the length of the gap, and spread it out(not beyond the overlap) with a piece of wood. Before the glue dries, gently press down on the joint(better yet, flip over the form and press the entire thing onto a flat surface), make sure this doesn't crush the wood, and is just light enough to bond the glue. A minute or so later, you can take it off the form, and there's your tube.

Notice hoe particular I was about never crushing/breaking/over-flexing the wood... I've tested two tubes of the exact same dimensions, one accidentally had a very small(impossible to see unless you felt for it) hairline crack, about 1cm long at the top. After testing, the cracked one held 20kg, the uncracked, 50kg. When I attempted to reconstruct them, both had seemed to expand as to be a larger diameter looking down the tube(axially?) and broken, however in the cracked one, the crack had expanded all the way down to the point where it broke, lowering the amount held. Good luck with yours!, love to hear how it turns out...

By the way, mine best one was 3.75" long, weighed 1.2g and held 50kg before failure(compression).

[SLM]

A little off topic, but I have to ask since my last attempt was a disaster.

I plan on retrying to construct a balsa tube. I have the wood soaking now, and plan on wrapping it around a stainless steel handle and then fastening it with zip ties. How do you make sure that the tube is "straight"? Do you cut to size before (leaving room for error) or after it has taken the tube shape? I am assuming to do a very long lap joint to glue the wood together, roughly how much should the wood overlap?

What was the problem you encountered?

Good description by iwonder. Here are a few more comments.

1/32"-thick balsa sheet has worked for us well for building tubular members. We cut ours to size leaving room for minor adjustments. We've used 5 mm of overlap length with success. Creating the long lap joint, however, could be a bit tricky. But, with practice it becomes much easier. If the top or bottom surface of the constructed tube is not flat, carefully sand it.

[iwonder]

SLM, do you buy 1/32" from specialized? I used a sheet sold as 1/32" for mine(from another vendor), but in reality it varied from about .025" to .065", Jake(from specialized) says he keeps sheets within +/-.002" for thickness, does that even sound reasonable? To me it's a little crazy for wood, especially thickness...

[SLM]

SLM, do you buy 1/32" from specialized? I used a sheet sold as 1/32" for mine(from another vendor), but in reality it varied from about .025" to .065", Jake(from specialized) says he keeps sheets within +/-.002" for thickness, does that even sound reasonable? To me it's a little crazy for wood, especially thickness...

Yes, we buy from specialized balsa. So far, we have bought 1/32"-thick sheets twice from them, both times every sheet has been very close to 1/32" in thickness. For us, the challenge is getting balsa sheets and sticks with the density ranges we want. There has not been much consistency there.

[iwonder]

Thanks! Density isn't even an option from where I buy now, so anythings an improvement, right?

[jander14indoor]

Balsa tubes, this is a speciality of the indoor flyers, so you might want to look around the site for old discussions in the Wright Stuff threads.

Alternatively, try this website for lightweight tubes appropriate to SO events: http://www.parmodels.com/Techniques_and ... 0Stick.pdf
For crazy light tubes try: http://www.indoorduration.com/INAVMotor ... uction.htm

Note, to keep things straight you need a form, old aluminum arrows work well. Many hobby shops have tubes and rods of varying sizes that can work.

Don't use rubber bands or plastic ties, use paper and tape as described above, it really works, I couldn't believe how easy it was when I learned.

Don't glue an overlap, wasted weight, and uneven deflection. But do note step 16 where you glide the form out BEFORE you apply glue. And see previous discussions on how to use VERY small amounts of CA glue unless you want to add alot of weight fast, especially when you glue it to your hand (done that).

You ALWAYS have the opportunity to buy balsa by weight if you buy it yourself. Take a scale to the store with you and weigh the sheets. You can almost always (unless you live near me and I've been looking for wood recently) find light wood if you take the time.

Jeff Anderson
Livonia, MI