Four basic forces govern the flight of an aeroplane: Gravity is
the natural force that pulls a plane towards the ground. Lift is
the force that pushes a plane upwards against the force of
gravity. It is created by the movement of a plane's wing through
the air. Drag is the natural force of air opposing an
aeroplane's forward movement. Thrust is the force that opposes
drag and moves a plane forwards. Thrust is created by a plane's
propeller or by its jet engines. When a plane's lift equals the
force of gravity and its thrust equals the drag, the plane is in
level, cruising flight. When any of the four forces changes, the
plane begins to climb, turn, or change its direction or position
in some other way. GRAVITY AND LIFT Gravity tends to keep an
aeroplane on the ground or to pull it to the earth when in
flight. The force of gravity on the ground equals the weight of
the plane on the ground. For a plane to become airborne and to
stay in the air, its wing must create a lifting force greater
than the downward forc