Looking at flying wings and what may work as a part 103. The XM4 had a gross of 620 lbs and an Ultralight would be happy at 550 gross I found the wing loading for the original and a 95% version would be the same wing loading. Apparently the original didn't stall so that wouldn't be an issue. Fuselage height comes out at 3'6" and span is 15'2". Exterior sheeting is 428 ft2 if done in .016 that is 99lbs. Predator 670 or small block Briggs are 70lbs or so. I like the 4 stroke engines.
