Oleg,

In the non-accelerated flight (eg. level flight at a constant airspeed) of a stable aircraft, the sum of all the vertical forces is zero, the sum of all the horizontal forces is zero and the sum of all the moments is zero (about any point you choose). The plane adopts a pitch attitude that produces an angle of attack of the wing that balances the forces and moments at that airspeed. That angle of attack includes the induced angle of attack which is a function of aspect ratio and coefficient of lift. As the airspeed changes slowly to another airspeed and stabilizes at the new airspeed the pitch attitude of the plane changes to accomodate a new balance of forces. Across the full range of stable pitch attitudes associated with the unstalled angles of attack of the wing, the stabilizer will also adopt a range of angles of attack which contribute to the balance of forces and moments. Since the stab is operating in the downwash of the wing, the stab angle of attack is affected by the wing's downwash angle which increases as the square of the wing's coefficient of lift. The stabs range of angles of attack may carry it through both positive and negative lift conditions.