Four forces of aerodynamics

Lift: Air rushing over the top of the wings and pushing up from below lifts the plane up into the sky.

Gravity: Gravity pulls the plane down toward the Earth.

Thrust: The engine pushes the plane forward through the air.

Drag: The air rubbing against a plane as it flies slows it down.

Lift and drag work opposite each other top to bottom, just as thrust and drag work against each other front to back.

---

Source: "The Kids' Paper Air Plane Book," by Ken Blackburn and Jeffrey Lammers, Workman Publishing