The dihedral causes the plane to level the wing side to side. (roll). It does nothing to keep the plane level front to back (pitch)

Lift is generated by the differential pressure between the top of the wing and the bottom of the wing. There are two contributing factors to that differential pressure. The first is the shape of the air foil, and the second is the angle of attack in flight.

For the classic trainer air-foil (flat bottom, curved top) when the wing has a zero angle of attack, the air flow over the top has a higher relative velocity to the wing than the bottom, and the Bernoulli effect results in a lower pressure on top, creating lift. If the CG (just to keep on the topic of the thread) is coincident with the center of lift, the plane will be balanced level, and should not require any downward force generated by the elevator to keep the plane level. Thus as you increase speed, you'll increase lift, and the plane will tend to climb.

For the Symmetrical airoil (typical for aerobatic planes) the airpath is the same over both surfaces (top and bottom) thus the only way to create lift is by altering the angle of attack. The wing must have a slight positive angle of attack to create a differential pressure, and thus create lift. Therefore, a properly balanced aerobatic plane with a symmetrical airfoil will require some downward force on the elevator to keep the angle of attack positive for the wing (up elevator).

Now invert the planes.

The Trainer is generating a lot of lift, but its in the wrong direction. The angle of attack of the wing must be adjusted to compensate for that. Thus you must push the nose up (by using down elevator) until the effect by the angle of attack overcomes the airfoil.

The aerobatic plane requires only a small amount of down elevator to reverse the condition that was trimmed into the plane for straight and level flight, since it doesn't have the big airfoil effect (the wing looks the same from an aerodynamic standpoint)

Does that sufficiently muddy the water?

Brad