Aeroplanes fly when two of the four forces acting upon them are greater than the other two. The force of thrust, created by the aeroplanes propellers or jet engines, moves the plane forward. The force of lift is caused by air flowing over the wings. This keeps the plane in the air. The two forces working against thrust and lift are gravity, which pulls the plane towards the Earth, and drag, caused by air resistance, which slows the plane’s forward motion.
Four forces act on a plane in flight. When the plane flies horizontally at a steady speed, lift from the wings exactly balances the plane’s weight and the thrust exactly balances the drag. However, during takeoff, or when the plane is attempting to climb in the sky, the thrust from the engines pushing the plane forward exceeds the drag (air resistance) pulling it back. This creates a lift force, greater than the plane’s weight, which powers the plane higher into the sky.
If you’ve ever watched a jet plane taking off or coming in to land, the first thing you’ll have noticed is the noise of the engines. Jet engines, which are long metal tubes burning a continuous rush of fuel and air, are far noisier (and far more powerful) than traditional propeller engines. You might think engines are the key to making a plane fly, but you’d be wrong. Things can fly quite happily without engines, as gliders (planes with no engines), paper planes, and indeed gliding birds readily show us.
Newton’s third law of motion explains how the engines and wings work together to make a plane move through the sky. The force of the hot exhaust gas shooting backward from the jet engine pushes the plane forward. That creates a moving current of air over the wings. The wings force the air downward and that pushes the plane upward.
