I just finished my Guillows Spitfire (kit 504 - 16.5" wingspan). I added some clay. Test glided several times. Removed clay from nose, wound rubber abnd and - IT FLEW!!!. I threw away the kit elastic and prop and added 1 loop of 1/4 inch FAI TAN rubber as well a a 6" prop. The Guillow's supplied prop was 5". I have just glued on the plastic nose and I am waiting for it to dry. When it is set I will try winding the elastic tighter and try for more duration