The picture below shows the location and movement of centre of pressure when angle of attack is increased. Consider the aircraft is trimmed at the second airfoil in the picture. Thus the horizontal stabilizer has to provide a downforce to trim the aircraft. Now if the angle of attack increases, the centre of pressure moves towards the .25c point thus reducing the moment. If the stabilizer is still providing downforce, then the aircraft is unstable. How does the stabilizer give positive static stability?