laminated ribs

[Ron Nossaman]

>   Are we saying the same thing in a different language?  ie.  A one inch 
> square rib until  the taper begins?.  Then it becomes ...tapered?
>   Dale

Almost. Crowned top, flat bottom, taper feathering. I cut 'em 
to depth on a table saw, so the thickest part is in the 
middle. I make no attempt to keep the section depth constant 
through the center, and would prefer not to even if I could 
easily do it. They're a taper of one sort or another from the 
center point to just inside where they meet the rim. I want 
them to bend as uniformly as possible, like a bow, so stress 
is evenly distributed and vibration response is from the whole 
rib (and membrane) at once. At least, that's the intent.

Make a 1/3-1/3-1/3 taper-straight-taper test rib, and another 
of the same dimensions with conventional tapering. Support the 
ends and load the center. The taper feathered rib forms a nice 
smooth continuous arc, or if it was constant radius crowned to 
begin with, it deflects to a nearly straight line under the 
right load. The conventionally feathered rib shows a tight 
bend in the feathered sections, a relatively straight run, and 
a relatively tight bend in the center where the load is. The 
tight bends are the high stress areas. while the relatively 
straight sections aren't responding like they could. The 
conventional rib shape just wasn't very well designed as a 
load bearing member, which is why I use the shape I do. It 
does what I think I want it to do.

If I had an easy way to do it, I'd probably crown the bottom 
of the rib too, but the taper is easy and quick with a router 
jig, even though it buries me in chips, and works pretty well.

Ron N