Sandbaggin’
As I mentioned previously, part of flight testing is flying at different weights and centers of gravity. I used sand, which was cheap, easy to get, and pre-measured in 50 or 60 lb. bags. It can be messy, though, so if I was going to do more extensive testing or test multiple airplanes, I think I’d find a better solution. It worked well enough, though.
Here’s a picture of a few bags strapped into the rear seats.