Boomilever B/C

[iwonder]

Is there a way to draw designs on Microsoft Word to scale?
(I don't have Autocad or similar software)

Autodesk is very nice about education, if you go to their site you can find the education area, assuming you attend a highschool(no .edu email required) you can get free access to all their software.(yes, all)

[chalker]

I use AutoCad however I think you can draw things on Microsoft publisher?

Try Google Sketchup. It's free and basically the same thing for these purposes. We use it for many of the diagrams that get printed in the rules.

[chalker7]

Sketchup recently changed ownership and it's now Trimble Sketchup. Same product, but don't be confused by the different naming scheme.

[JimY]

So, just to clear it up, there has to be 3/4" between each tension member on the base right? Because of the 3/4" washers?

The washer can be on the back side of the wall rather than on the boom side, so 3/4" is not the closest the tension members can be at the wall. Since a 1/4" bolt uses a 7/16" wrench, that is the narrowest the tension members can be. A more practical minimum would be 1/2 - 7/16 inch to allow for rotation of the bolt head. This width doesn't force the compression member to be the same width. Just angle the tension members in a bit from the distal end to the wall mount. With the length of the tension members being in the 50 cm range, when the compression member width is the same as the loading block, the tension arms angle in only a couple degrees. If you're worried about the glue joint ripping off at on the wall mount because of the angle, instead of making the wall mount a rectangle (top view), make it a slight trapezoid shape by either using a miter saw to cut it or by sanding it to shape. If anyone goes the miter saw route, you should have a coach do the job, as I think this is a sizable risk for students to try doing.

[juicemanman]

So, just to clear it up, there has to be 3/4" between each tension member on the base right? Because of the 3/4" washers?

The washer can be on the back side of the wall rather than on the boom side, so 3/4" is not the closest the tension members can be at the wall. Since a 1/4" bolt uses a 7/16" wrench, that is the narrowest the tension members can be. A more practical minimum would be 1/2 - 7/16 inch to allow for rotation of the bolt head. This width doesn't force the compression member to be the same width. Just angle the tension members in a bit from the distal end to the wall mount. With the length of the tension members being in the 50 cm range, when the compression member width is the same as the loading block, the tension arms angle in only a couple degrees. If you're worried about the glue joint ripping off at on the wall mount because of the angle, instead of making the wall mount a rectangle (top view), make it a slight trapezoid shape by either using a miter saw to cut it or by sanding it to shape. If anyone goes the miter saw route, you should have a coach do the job, as I think this is a sizable risk for students to try doing.

Okay, thanks! That was really helpful!

[noobforce]

Hey guys, I feel stupid for asking this, but would a complete balsa boomilever be recommended?

Last year my towers were completely made out of balsa; I'm asking because the base needs to be a bit stronger, so maybe I should use bass? I don't know what to use for the attachment base.

[iwonder]

It's really hard to recommend certain things/materials to use, it really depends on your design, Aia has a nice wiki page with tips on how to build a boom that involves laminating bass on balsa, but it's really something you should experiment with and find out what woods work better with your design.

[juicemanman]

Hey guys, I feel stupid for asking this, but would a complete balsa boomilever be recommended?

Last year my towers were completely made out of balsa; I'm asking because the base needs to be a bit stronger, so maybe I should use bass? I don't know what to use for the attachment base.

I used mostly balsa, with a little bass where I felt it needed to be stronger (i.e. the distal end)

[noobforce]

Thanks guys! I have a few more questions:

1. The rules say the Testing Wall needs to be at least 40.0 cm wide. Then it says the middle mounting hole must be centered on the wall, and the centers of the other holes placed 10 cm from the center of the middle hole. Does this mean the distance between the middle hole and a side hole is 10 cm, and the distance from one side hole to the other side hole is 10cm? Because if the distance between the center hole and the other side hole is 10cm, the Boomilever would be pretty big..

2. On the Boomilever Wiki Page, it says:
According to the esteemed Bob Monetza: "You'd want to get the point of support as close as possible to the center of the block. If the center of the block is beyond the connection point between the tension and compression chord, it will cause bending of the compression chord, like a diving board."

I understand that if the block is beyond the connection point, the compression chord would become weaker. Then if this is known, why did these boomilevers have the block beyond the connection? Wouldn't it be better to have it before the connection point?
http://scioly.org/phpBB3/gallery/image_ ... ge_id=1356
http://scioly.org/phpBB3/gallery/image_ ... ge_id=1348
http://scioly.org/phpBB3/gallery/image_ ... ge_id=1355

[juicemanman]

Thanks guys! I have a few more questions:

1. The rules say the Testing Wall needs to be at least 40.0 cm wide. Then it says the middle mounting hole must be centered on the wall, and the centers of the other holes placed 10 cm from the center of the middle hole. Does this mean the distance between the middle hole and a side hole is 10 cm, and the distance from one side hole to the other side hole is 10cm? Because if the distance between the center hole and the other side hole is 10cm, the Boomilever would be pretty big..

2. On the Boomilever Wiki Page, it says:
According to the esteemed Bob Monetza: "You'd want to get the point of support as close as possible to the center of the block. If the center of the block is beyond the connection point between the tension and compression chord, it will cause bending of the compression chord, like a diving board."

I understand that if the block is beyond the connection point, the compression chord would become weaker. Then if this is known, why did these boomilevers have the block beyond the connection? Wouldn't it be better to have it before the connection point?
http://scioly.org/phpBB3/gallery/image_ ... ge_id=1356
http://scioly.org/phpBB3/gallery/image_ ... ge_id=1348
http://scioly.org/phpBB3/gallery/image_ ... ge_id=1355

Erm... http://www.scioly.org/phpBB3/viewtopic.php?f=147&t=3876