just reading thru this post with much interest since I recently purchased a Hellcat and have yet to build it. Bigbird, I have used the expansion foam on other projects with much success. I was thinking of using it in the rear of this plane even before reading your post. It doesn't add much weight at all but care must be taken not to add too much because the expansion can actually destroy your airframe. There are different kinds, some that expand more than others also.