wrote in message
...
snip of misunderstanding

Sorry I wasn't clearer on my intention. I'd only spray about an inch
on the
panels. Not much weight, and still get significant damping.

tom

Balderdash, tom.

I think you are blowing in my ear.

First, just how do you expect to "spray on" a 1 inch thick layer
of foam INSIDE a wing? I can't do it, and I can do anything
(with Duct tape!).

Second, a 100 square foot wing is 14,400 square inches.
One inch thick is 14,400 CUBIC inches, or 8.3 cubic feet.
Top and bottom skins give 28,800 cubic inches or 16.6 cuft.

At the mythical 3 lb/ft^3, that's 50 pounds.

Sorry dude, that boat don't float...

Yeah it will. 8^) I was talking about flat panels on the fuselage. The
Zenith CH701 I would like to build has about 60 square feet of fuselage
area
behind the cabin. At one inch, that amounts to 5 cubic feet. One website
I
visited that made fire retardant spray urethane foam says 1.75 pounds per
cubic foot, so the weight of this addition would be less than nine pounds.

If the panels are really flat, then why spray it on? Low density foam is
available in sheets. I'll bet the consistency is better than you can get
from a can. Bond it on before you rivet. It shouldn't have to go all the
way to the edges to prevent the oilcanning.

It looks like for the 9 lb weight penalty you could increase the thickness
of just those panels by about .010 inches (.15 lbs/sq ft). That would make
it less likely to oilcan and stronger.

Does the foam really need to be continuous? Why not just bond some foam
stiffeners to the inside of the panel every six inches or so? The HP-18
sailplane wing is made from 1/2" foam on four inch centers. That technique
might be more anechoic than a smooth continuous foam surface, too. You
could taper the stiffeners to be thicker in the center of the panel, and get
more stiffness/unit weight that way.

An interesting way to bond the stiffeners might be to use 3M VHB (Very High
Bond) double stick tape. It's used to bond side panels on truck and trailer
bodies. Fast and no goop.

Tim Ward