Anything in the nose causes a CG problem with this plane. I've seen only a handful truly stock plane postings, most of which used engines in the 20 ounce range. Many of us experimented with servos in the tail for the proper CG. My batt is behind the canopy area installed on a "floor" I constructed between the bottom stringers against a former. The batt is as far back as I could get it based on the length of the lead. I also moved the elevator servo farther rear and built on a box for it (built on to the side of the fuselage.) and used an MK bellcrank to equalize throw. I still ended up with 2 ounces of lead in the tail, but I'm using a 31 ounce Webra 120. Maybe you can do it with no lead. I have no clue what a YS 91 weighs.