Thanks, Airmen,
A lot of good help was forthcoming from this excellent list. To
clarify a few points that were raised:
Why not use a tubular spar? Yes, that would be easy and elegant
but I've heard, in an anecdotal sense, that round spars are not as
stiff as triangular, especially if you know the specific directions
from which most of the stress will be coming. [?] I haven't found a
technical reference that compares spar [or, beam], shapes.
Mixing unequal modulii materials like aluminum and glass plastic
Good point. I used heavy alum foil skin on the underbelly of my 8'
design study model and learned first-hand of what you are saying. For
a large ballooned shape, a plastic skin which stretches, is much
better than stiff aluminum.
Now I'll make a comparative price list of materials for foam
sandwich versus encapsulated tubing and decide which way to make the
big spar for the leading edge of the lifting body dirigible. Thanks,
guys.

On Apr 4, 4:24 pm, "dirigible designer"
wrote:
My dirigible design requires a few long spars curved into C-
shapes. They would be triangular cross-section with their three
faces being about 3 feet wide and 100 feet long. They need to be
light, strong and stiff. [what's new?]
If anyone's has experience building large spars or frame members, I
would like some advice as to how to make them cheap and easy.
My present thoughts favor a thin fiberglass skin around a bundle of
polypropylene tubes, which could get expensive.
I think that EPS blocks carved into the triangular shape and then
covered in glass would require the extra labor of building a glass
internal spar, yuch.
This design has to be a cheap and dirty way for the working guy to
get in the sky.
thanks from Allen