Scioly.org

Do you guys build a rigid jig? As discussed by other fellows, a jig is the best method for building the towers accurately...

Merry Christmas!

I cut my wood a little longer than I need it, then I sand it down to the correct length, however, I am having problems getting each end to be sanded to the perfect angle. Does anyone have tips on how to ensure that the correct angle is achieved?

We have a tower built but its 25 grams. Is it too heavy ? How do i reduce the weight ?

GeekyGem wrote:We have a tower built but its 25 grams. Is it too heavy ? How do i reduce the weight ?

thats a good weight to start of with
but can have some major improvements
try using different types of wood or less wood overall

GeekyGem wrote:We have a tower built but its 25 grams. Is it too heavy ? How do i reduce the weight ?

well, i dont know what the tower looks like, but there are general ways to reduce weight.

1. first, make sure your base is not 20cm but 15cm rotated at a 45 degree angle. That will save a ton of weight and will also give you a stronger base(because the primary pieces will be shorter and less slanted)

2. use smaller cross sections for the trusses. The primary compression members of the tower are the ones which take most of the force. suppose they are given about 40N of force, the trusses will probably only get about 1N. Therefore, a 1/16in ^2 cross section will suffice for the trusses. Usually, they will never break. It is the primary compression members that break first

3. Use efficient gluing techniques and dont use a heavy glue. In general a tower should not break due to a joint failure. there a treatise on proper gluing techniques and good glues here: http://soinc.org/sites/default/files/up ... weight.pdf

if you do the above, it should be enough to reduce weight significantly. There is more:

4. weigh each piece of wood and do a stiffness test on it. The ideal piece of wood is light and stiff-which is a decent indicator of strength. It really helps to know what fraction of the weight is coming from where.


5. See if you can simplify your design. Of course, less wood=less weight so if a design is successful, then it is worth a try to simplify it(and if the simplification doesnt work, you can just revert back to the old design)

cant think of anything more right now

For gluing technique, get any kind of wire or paper clip, bend it into a shape so that the two end tips are very close together. The capillary action between them will allow you to lift a single drop of glue and precisely apply it. Cut of tip bit by bit after numerous uses due to the glue getting clumped.

Also, 25 grams seems like around what i was getting on bridges when i bought wood from my local hobby store...big mistake. If you can, definitely order wood online, sites like specialized balsa and balsa wood inc actually sell wood specifically for these purposes (free flight/structures), the wood is much lighter. That being said u can take a pocket scale to the store and hand pick wood based on that, though again, it is cheaper on the web anyway.

When stripping balsa, there are some pieces that come out warped. I know I can soak it in water and bend it to a flat shape, but would the strength of the member be affected, like if the grain is warped would that cause a decrease in strength.

Come out warped as in...kinda bent? That happens to me a lot when i strip pieces that are really soft and weak, so there might be a correlation there...usually stiff wood comes out straight as an arrow.

And to really express how much balsa can vary in density even in one sheet, i just finished stripping a 4 inch wide, 1.5 feet long sheet - i stripped one piece (1/8^2) and weighed it - .67 grams. The adjacent piece that i stripped was 1.82 grams....crazy.

yea, bent.

It actually happens when I'm stripping very dense balsa or bass wood. Haven't tried really light balsa but the variations in density of the balsa can contribute to warped balsa.

I use sheets because they're cheaper and you can cut out parts (mostly for copters) and vary your wood sizes, but sticks seem to be more consistent with warping.

When you select your sheets for stripping, do you check that the grain is straight and parallel to the edge? I also strip wood and generally don't have too much trouble with bending unless the grain is wavy along the sheet.

Jeff Anderson
Livonia, MI