The four forces acting on the plane are:
• Thrust. The force that makes the plane moves in the direction of the motion.
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• Drag: Force that act opposite of the motion.
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• Lift: The force that holds the plane in the air.
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• Weight.
If the pilot wants the plane to stay stable he needs to make sure that the forces on the plane are balanced. In any other case the plane will be going up or down.