Boomilever for 2013

[iwonder]

Wow.. Thanks for the links, that's a whole lot nicer than what I've been doing... and I figured out another way to attach the compression member to the loading block

[LKN]

Thanks a lot @jander14indoor, and @iwonder, do you mind sharing?

Another possible main compression member, one large I-beam? (see here http://en.wikipedia.org/wiki/I-beam) Constructed of balsa sheets varying between 1/64 to 1/16, whichever proves to work with testing. With this design I think that the web could be lower density and thinner wood than the flanges, and the flanges thicker, higher density, since the flanges are a built up section meant to resist the vertical buckling mode of the boom. The distal end attachment would be on the outer face of the flanges.

(block)
|--|

(load)

where the hole for the loading block is drilled through the web (longest part of an I) and the block sits on either end of the flanges (the short horizontals of the I).

[iwonder]

Yeah, sure... it's really the testing that makes the difference anyways, right?

I've been looking at using a single, large diameter(1" was what I tested, by the way) compression member, but I couldn't figure out how to attach it to the loading block, the initial idea was posted in the designs(?) thread at the end of last season, basically setting the loading block on top of the tube and attaching the tension members to the side(a low density piece of balsa to set the loading block on)

However, with the article jander14 gave us, talking about thinner members(I thought anything under a 1/2" tube would crack the balsa), it seems much better to use 2 smaller diameter tubes, with a large, low density piece of balsa in the middle to hold the loading block. I'll leave the construction method up to you

As for the I-Beam configuration... remember that the compression member can buckle in any direction, and in fact, vertical buckling is the easiest to stop, by attaching the tension members to the compression members(obviously), so I see possibilities for the I-Beam config if you used low density flanges and a high density web. One thing that may work(someone with more experience, please correct me if I'm wrong) would be to use a high density 1/16" sheet in the web, and put gussets on the flanges where the distal end connects(Is distal end the right term? I thought that refereed to an end where the tension members were misplaced so the compression member acted as a cantilevered beam for a small distance...) to support the loading block. The trick is the attachment of tension members, with large flanges on the sides that have a thin thickness, you really have to glue down the sides of the flanges(one reason for the gussets), but you also want to angle in the tension members for the smallest possible base, so you'd have to very carefully cut slots for the tension members into the flanges and then glue the flanges back together with the tension members as part of them. All in all, it'd be a really cool design to have the loading block nest into an I-Beam... but I've learned that you shouldn't build something because it's cool(I built all 3-legged towers because I could do perfect compound miters on the chimney and every contest I went to I got comments on the build quality... but the tower never held more than 13 kg...), so try it, I'd love to see the thing built.

Reading over your post... I'd advise completely against putting the loading block on the flanges, the thin wood wouldn't do good for the compressive load at that point.

A last suggestion... maybe a compression member made of two rectangular side supports(say, 3/16"x1/2") with the 1/2" vertical, and laminated with ultra high density balsa/bass, then put a truss in between the two side supports, it's yield a very high cross-sectional moment of inertia in both directions, because of the high density, but close in laminations, and the low density, far apart horizontal laminations. One thing to remember, the buckling strength goes up directly with the modulus of elasticity(basically, density(I might be wrong... pretty certain)), but it goes up with the cube of distance from the center(which is why the tube is nice, the wood close to the center does significantly less to improve strength than the outside wood), this, again, lends itself back to a low density, but large I-Beam, like what you describe.

It's a lot(of me rambling) but hopefully that gives you some ideas to start testing out, it sounds like you're on the right track, however. If you feel compelled to share any of your testing data, I wouldn't say no

[Faustina]

With all this talk of large I-beams and tubular compression members, are you guys all assuming that there will be no limit on cross section? If I'm not mistaken, the last time boomilever was an event, the wood could be no greater than 1/4 inch high and 1/4 inch wide, with lamination allowed. Of course, in recent years there has been no limit on cross section. How likely is it that this trend will continue?

[fishman100]

Faustina: There is a 99% chance that there will be no limit on cross section. Like you said, that rule was eliminated ever since 2010 and it has stayed that way ever since. I think that it would be silly to limit the cross section of the pieces because it can limit the creativity and the approach to the event of some builders (see: http://gallery.scioly.org/details.php?image_id=2936; that bridge wouldn't have been legal if there was a limit on the CS). I would be surprised if the CS was limited for boom, especially since some teams might have to use a larger/thicker piece of wood for the attachment base.

Iwonder: Why would you have to cut slots into the flanges? You could just glue the tension members directly onto them, which would also provide a large surface area for gluing (assuming your boom is a sideways I-beam like LKN mentioned).

[fanjiatian]

[quote="iwonder"]
A last suggestion... maybe a compression member made of two rectangular side supports(say, 3/16"x1/2") with the 1/2" vertical, and laminated with ultra high density balsa/bass, then put a truss in between the two side supports, it's yield a very high cross-sectional moment of inertia in both directions, because of the high density, but close in laminations, and the low density, far apart horizontal laminations. One thing to remember, the buckling strength goes up directly with the modulus of elasticity(basically, density(I might be wrong... pretty certain)), but it goes up with the cube of distance from the center(which is why the tube is nice, the wood close to the center does significantly less to improve strength than the outside wood), this, again, lends itself back to a low density, but large I-Beam, like what you describe.
/quote]

Interesting idea. Can you draw a sketch? I'm really bad at visualizing.

[iwonder]

As to the cross section limit, I agree with fishman100, that would be very shocking, also, if I'm not mistaken the limit was added in the second year of the event, it seems that first year builds are less strict and allow for teams to get used to an event before throwing some kind of kink in it(chinook, tower height bonus, etc)

Now, fishman100, there's a reason I choose to cut slots in the flanges, you're right that it'd give more surface area on the glue joint if you just put them on the sides, but, you also want to keep the tension member as straight as possible. To accomplish this, you generally angle in the tension members(because the distal end connection is wider than the base), if you kept the tension members on the side of the I-beam(which is perpendicular) you'd have a bend in the tension members right at the end of the flange, as they pull in to meet the base, this would(as we've seen in an earlier post) cause a premature failure in the member. The alternative, bending in the flanges, causes the gussets to be more complicated(different angles/sizes) and being able to do that would also mean that you over-engineered the I-Beam and it can hold more compression before buckling than it needs to(because you'd reduce the moment of inertia on one side, not on the other). By cutting notches in the flanges, you keep the I-Beam square, it can be lighter because it's not bent in, and the tension members aren't bent in as much, plus, the tension members still have glue down the thin edge of the flange, and, mainly, the distal end connection(which I assumed would be 1/4" thick, to hold the compressive force in the proper mode), 1/4" has proved to be more than enough in the past, so I didn't see a problem with it.

Lastly, I'll be able to draw out my original idea(with the lamination) soon... working on it now.

Alright... done with the sketches... and also

[SLM]

Using an I-beam as a compression member is an interesting idea. It would be useful to generate some performance data (with regard to buckling) for such a member.

As far as compression is concerned, the tubular shape is the most economical one; it resists buckling better than any other, comparable in size, shape.

An I-beam consists of 5 thin plates (the web, and 4 half flanges), as depicted below.


Generally, such thin plates are susceptible to buckling, especially if the member has wide flanges. Although the member may not buckle as a whole, the individual thin (flange) plates could buckle if the compressive stress in the member is relatively high. This kind of buckling is called local buckling and it looks something like this:



If this type of local buckling becomes an issue, the flanges can be braced by placing vertical plates (called stiffeners) between the top and bottom one, like this:

[himlynx]

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?

The rules for the trial event in 2012 state "b. The entire Boomilever (except for the Attachment Base), including gussets, must be constructed of pieces
of wood no larger than ¼ inch high and ¼ inch wide in cross-section."

The wood used for a tube will have a cross section with one dimension greater than 1/4." Unless the rules change in 2013, I don't see how it will be legal.

[iwonder]

There's been some talk about that... It's very likely that the rules will change in order to allow the larger cross section. And SLM, what about x's down the length of an I-Beam instead of plates, would that be lighter and still effective?