You are both correct about engine thrust vs CG but in this instance I think I know what Marketvending is trying to say. The plane is tailheavy because the CG recommendation in the instructions is all wrong. The instructions say 100 mm and the folks on this thread say try 90 mm. My second flight described earlier was at 90 mm and the result was the tail was always low and the nose high at all speeds especially slow and in turns it would almost knife edge on its own. It was unstable and slow to respond at low speeds. Also the plane while landing would hop up three or four times when touched down gently. If I guessed I would say my Decathlon's CG is near 85mm now and it performs much better now from take-off to landing. I will play around with it more and report where I finally leave the CG. 80mm might be the key.