the plane -- whilst the lift at the front of the plane stays the same. The relative position of the lift hence shifts forward. This could be a good or bad thing depending on the plane. If your paper aeroplane tends to nosedive, then its centre of lift may be behind the centre of gravity along the longitudinal axis of the plane. Shifting the centre of lift to approximately meet the point of the centre of gravity can help it maintain level flight. However, if the centre of lift is already ahead of the centre of gravity -- meaning it has a tendency to pitch upwards -- then shifting the centre of lift yet forward will only exacerbate the problem. 2) Shift the centre of gravity of the plane up or down the vertical axis. If you make a paper aeroplane with fins that point downwards, you are shifting mass down the vertical axis of the plane. This is a good thing for enhancing the keel effect and is why, when I do make paper aeroplanes with fins, I generally make them so they point downwards. The exceptions to this ( paper aeroplanes such as The Piranha ) generally come about because it would be impractical to turn the fins downwards on such planes. 3) Aid directional stability. If the paper aeroplane has fuselage that tapers towards the back (such as The Piranha ), it will tend to lack directional stability unless fins are present. The plane will easily yaw