So we have gravity pulling down at the center of gravity, and air pushing up at the center of lift.

In a properly designed glider, the center of gravity is in front of the center of lift so that the nose of the glider drops. (Check your pencil marks to find out if this is the case for your plane.) The nose slides forward through the air toward the ground, front end first.

But suppose you have a plane in which the center of gravity isn't in front of the center of lift?

One of the great properties of paper is that you can fold it over on itself, creating regions of the plane that are twice or even four times as thick as the single panels of the wings. This allows you to concentrate the weight of the airplane where it helps most, toward the nose.

The folds of paper near the front of the airplane move the center of gravity forward. To see how important this is, try flying your plane backward. It won't fly. The plane definitely has a front end.

Another way to move the centre of gravity forward is to add weight to the nose with tiny pieces of plasticene or blue tack.