However both the flying squirrel and the flying lemur can glide. Humans experimented and developed ways of flying. They had built their airplane after studying the writings of other aviation pioneers and experimenting with gliders, kites, and wind tunnels. Thrust is created by a plane's propeller or its jet engines. Drag is the natural force of air that resists an airplane's forward movement.
Lift is created by the plane's wings as they move through the air.
And gravity is the natural force that pushes a plane toward the ground. Gravity and lift are opposing forces, as are drag and thrust. When the plane's lift equals the force of gravity and the force of thrust equals the drag, the plane remains in level, cruising flight. When one or more of these four forces change, the plane begins to change its altitude, direction, or speed. This comprehensive site has aviation news and history, a huge photo collection of all types of aircraft, and much, much more.