Question for wood experts...

[NM_Shooter]

I'm wondering what the best bang for the buck would be for readily available wood, in terms of strength vs. weight.  Aren't a lot of aircraft main wing spars made from spruce?

My daughter needs to build a very small catilevered beam, and place a load on the end.  (okay, I'm busted.  This is not a cabin question).  It can't be more than 1/2 in thick, but must be stiff, strong, and light.  The supporting point is in the middle, and there will be ~40lb load 1.5" on either side of that mounting point.

She's building a truss to lift a load, but is not sure what material to make the mounting plate out of.  I was thinking about getting her a small amount of hobby ply from the local model airplane shop, but thought also that solid spruce might work well.  Weight is an important factor. 

Any thoughts?  Thanks!

[ScottA]

Baltic birch plywood is very strong and stable stuff and can be had in 1/2 and smaller thickness but it's not really light. I'd say basswood might be a better choice. It's often used for fins on model rockets and model airplane parts. Balsa is the strongest wood by weight I belive but it is very soft.

[John_C]

A lot of aircraft & boat parts, think masts & booms, are made from Sitka Spruce because of its high strength - weight ratio. For the highest s/w it also should be quartersawn and have a grain run out of < 1 in 12.  Good Sitka Spruce is not cheap.

Actually a good piece of Doug Fir is also has high s/w properties, and it would be easier to find and cheaper.

If the loads are only 1-1/2" from the center this is a pretty small piece of wood.  Oak, ash or hickory would all be stronger than the spruce or fir and not be very heavy.  They would also be much better at resisting the crushing forces at the fulcrum.

This sounds like a school science project.  What are the parameter.  Lightest piece to support the weight?  Least deflection? 

[NM_Shooter]

She called it a boomilever.  It appears to be a truss structure that mounts on a 1/4"bolt, and has to hold up a 15kg weight at a distance 40cm from the wall.  She is trying to design the mounting plate now, and asked me to buy her some 1/2" square stock for the mounting apparatus.  She built the truss members at school, and then her team member flaked out and has not been coming to assist.  She is now scrambling to get the thing done in time and finish up the wall mount.

She was disappointed that her partner left her hanging, but was still going to show up at the competition.  I told her to expect versions of this same story all through life.

-f-

[John_C]

It's easily done, but you've confused me.   

Is is like a see-saw with weight on each end?  That's what I got from the first post... with the (2) weights 1-1/2" from the fulcrum.

In the second post I get the idea its a boom arm mounted to the wall and one 15kg (33lb) weight is attached about 16" from the wall.  I also get the impression that the 1/2" dimension is a given.  The change from 1-1/2" to  16" makes a big difference.

What she needs to build is ... loosely speaking a small wooden equivalent of this..  correct?