they are simply made badly If a fin is not parallel to the centre line of the plane, it will increase drag -- as you can see if you look at a plane from the front. (ii) aeroplasticity -or, to use more simple language: the plane changes shape during flight. This is especially a problem for paper aeroplanes like the dart . During the launch phase of the flight, aerodynamic forces will tend to push the fuselage together all the way along the plane; however, as the paper aeroplane starts to slow down, the fuselage will come apart in a V shape at the back. This can turn a pair of fins into a pair of airbrakes. This is why fins should usually be avoided in dart-type planes. Paper Aeroplanes like The Dart change shape slightly during flight. This can turns fins (and other parts of the plane) into airbrakes. 7) Act as accidental elevators . If your paper aeroplane has a fin that is not parallel to the centre line of the plane (either due to imprecise folding or due to the effects of aeroplasticity) AND it is not perfectly vertical  (either due to imprecise folding or aeroplasticity) then it can deflect air upwards or downwards. This can have the same effect as tweaking the backs of the wings upwards or downwards... however, the problem is it's difficult to spot, so you might be creating extra up or down elevator without knowing it. ****************************************\\... Ok, so I think I'm