Every aircraft, from a paper plane to an airliner, is pushed and pulled by the same four forces. A force is measured in newtons (N), and each one is drawn as an arrow.

• Weight (W) pulls the aircraft down towards the centre of the Earth. It is mass times gravity: W = m × g, with g = 9.81 m/s². A 1,000 kg light plane weighs 9,810 N.
• Lift (L) is the force of the air on the wing at right angles to the airflow. In level flight that means upwards.
• Thrust (T) is the push forward from a propeller or a jet engine.
• Drag (D) is the air resisting the motion. It points backwards, along the airflow.

Lift and drag both come from the air pushing on the aircraft. Weight is the Earth pulling. Thrust comes from the engine. Notice that the engine does not hold the plane up. That is the job of the wing.