What concerns me is not the density, it's whether or not this is an approved substitute foam. This isn't a matter of density variations between batches of foam, I'm afraid it simply isn't the same foam. According to the canard pushers, foam is never used to transmit primary or secondary loads so the lesser strength may not be a big issue. However, it also specifically says not to substitute materials with less strength. The smaller cell size might affect the peel strength but the only way to find out would be to test it.
I emailed Wicks concerning this and received a reply yesterday as follows:
"The F200 is the same foam, but it was changed long ago to 1.6 density, in place of 2.0 density. Aircraft Spruce sells the 1.6 also."
As I mentioned in my eariler post, I ordered samples from both suppliers. The foam is very similar but does appear to be a bit different. I weighed and measured the stuff from A/C Spruce and it does seem to be 2 lb/ft^3, not 1.6. It also has slightly larger cells and the color is slightly darker than the stuff from Wicks. They didn't tell me anything about compressive strength but Wicks did give me an MSDS number with which I was able to track down exactly what it was on the Dow website. What I'm worried about is the possibility that the type of foam was changed due to the original Styrofoam FB being discontinued and the new stuff was deemed close enough and never tested. I’m afraid Wicks may have assumed it was “the same foam” as it says in the email above and that they may not have sought RAF’s or Nat’s approval for the substitution. It was easy enough to find the strength of the new foam but I had to do some digging to find the original material’s strength. When I did find it I was alarmed to see that the new stuff has less than 60% of the strength of the original. This is what made me start questioning whether or not it is an approved substitute at all. There are other factors such as fatigue strength and no telling what else that may play a part that would have been tested by RAF if they approved it.
I’m waiting for replies from emails I sent to Wicks and to RAF concerning this issue. I’ll let you know how it turns out.