I had thought I understood the crossbeams, but the fact that the main carbon layers are on the top and bottom takes me by surprise. I thought the structure of a crossbeam was similar to an I-beam, where the vertical portion is what makes it stiff and the horizontal faces are just to keep the vertical face from warping out of line.

 

This sounds like the beams are stiffened by the same mechanism as a cored panel, where the seperation of the faces is what creates the stiffness and the vertical faces are just to handle the shear forces.

 

Is there an explanation of why beams are done this way that is simple enough for me to understand, or do I need to go back and take that Mechanics of Deformable bodies class that I managed to skip in colleget because I was in electrical instead of mechanical engineering?

 

- Gardner

On Mon, Aug 8, 2011 at 6:55 PM, Rob Denney <harryproa@gmail.com> wrote:

 

Beams are optimised with the carbon as far from the centre (neutral axis) as possible.  The foam is only there for local stiffness and is not required if there is enough carbon, which there will be on the top and bottom, perhaps not on the sides.  Could make a sheet of carbon and cut it for the top and bottom, then glue it to foam/glass/carbon sides and wrap the whole thing in double bias to hold it together.  As always, the devil is in the detail.

On Tue, Aug 9, 2011 at 2:09 AM, Gardner Pomper <gardner@networknow.org> wrote:

 

As everyone knows, I am always looking for build techniques that require less time and less skill. I have had another idea, and was wondering if someone can tell me if it would be structually sound.

 

I was thinking of building crossbeams by laminating 1" foam with carbon fiber on each side and then cutting the sheet into 8" widths and laminating them together, with the carbon fiber faces vertical, to make an 8"x8" crosssection. Then I could round the ends with a router, and wrap the whole assembly in fiberglass and vac-bag it.

 

It seem like this would be a very easy construction method. It would more expensive because of the foam, but I am mostly concerned with time spent. Would 8 vertical surfaces, each with just 2 layers of carbon fiber be as strong as the usual 2 vertical surfaces with all the carbon just in the front and rear faces?

 

If this technique would work for crossbeams, would it also work for a mast? Not at a round cross-section, but as a central piece, with a fiberglass leading and trailing edge tacked on to make a wing mast. Would this type of construct bend properly?

 

Thanks,

 

- Gardner