I had the opportunity to see flex this weekend. My New Scratch Built 5.5# Kaos was flying it's 3rd flight and in perfect trim with one other plane in the air. The other plane was a 40% Extra w/ twin 15 cu in turning 32 " carbon graphite prop[]. I was on the south end of the field 100 yards from the field completing a cuban 8 when the 3 D decided to fly south. The carbon graphite prop hit the bottom of the wing dead center of the fuselage and did they flex[:'(]. Pic below. The Extra continued to fly through the flexed Kaos going south and having great difficulty turning. The Extra returned and landed and on the landing the fuselage flexed so the prop could touch the tail. The Extra lost 1/3 of the right wing and broke the spar through the fuse. The fuse totally failed on the touchdown. The Extra and the Kaos both flexed to the trash can. []

It is due to this type of flexing that I will not fly the Herk if other planes are in the air.