It's normal for a plane to climb when power is increased. Increasing power increases speed. Increasing speed increases lift. The plane's weight stays the same, so the only thing it can do is climb. Full scale planes need to be re-trimmed every time the throttle is moved.

Aerobirds are designed to climb in a more pronounced fashion than most planes. Its predecessor, the Firebird XL, did not have elevator control, and depended completely on throttle to climb and dive.

I would trim the airplane to fly straight and level at half throttle. You can adjust the tail according to the instructions to get it to fly level at half throttle with the trim centered, then use the trim to level the plane off as you increase or decrease throttle.