I have a modified wing. I cut off the flaps from the wing and fastened the hinges from the underside of the wing. Maybe I changed the depth of the wing a bit. CG states 150 mm in the manual. jkjunior stated here that the CG 170 mm suited him. So I took it to the center and put CG at 160 mm. The first flight was almost the last. The albatross was almost unmanageable and I was very lucky to land without damage. Finally, when Gary looked at his records, he wrote me that he gave his Albatross without a modified wing a CG of 150 mm. He was very sensitive to elevator and therefore added 6 oz (170 g) of lead to his nose. I have a 210 N JetsMunt M210TS turbine, but it's 13.05 oz (370 g) heavier than Gary's Wren 160. I moved the turbine battery from center to nose and had to add 7.54 ounces (214 g) of lead in my nose. CG I now have 140 mm. If you are building this aircraft, I recommend balancing it at 140 mm and if it does not suit you, you can always move the center of gravity backwards. Above all, you will avoid a crisis on your first flight.