Four forces acting on aircraft ?

1. Lift .
2. Drag . 
3. Weight .
4. Thrust .

When aircraft seting on ground only weight is act on aircraft but when aircraft during flight the all forces acting on it . During  take off the thrust force is more than drag force and lift force is more than weight force . In straight level UN-accelerated  flight the all forces are in equilibrium . 

1. Lift : It is a force acting vertically  upward to relative air flow . It is lifting force which lifts the aircraft in air . It is created because of pressure difference on wings . It over comes the force of gravity to lift the aircraft . 
2. Drag : It is a force acting opposite to the aircraft motion . It resist the forward motion of aircraft . To reduce the drag force the manufacture makes the aircraft in streamline shape . It causes because of air resistance . 
3. Weight : It is force acting vertically downward to the relative airflow . It is force of gravity which pull the aircraft towards earth . It cause because of mass of every thing which aircraft contains . 
4. Thrust : It is force which propels the aircraft in direction of motion . It is produced by engines, either it is jet engine or piston engine . It over comes the force of drag .