Most 40 trainers do not require a 40 size engine. A 25LA is more than enough power to fly that PT-40 unless your flying from a cow pasture with foot high grass. You don't need unlimited vertical performance. The added bonus is a better cg. And yes rubber bands are your problem. If you keep the 46ax on your plane use 8-10 bands per side ,if you use the 25LA 6-8 per side