There are four basic forces that act on an airplane to change its airspeed or altitude.

 

 

 

Thrust

Thrust is produced by the powerplant and propels the aircraft forward. An aircraft accelerates (speeds up) when its thrust is greater than the drag.

Drag

Drag is the force that opposes thrust and may be caused by the disruption of airflow over the wing.  If drag is greater than thrust then the airplane decelerates (slows down).  When thrust equals drag then the aircraft remains in unaccelerated flight, meaning that the airspeed does not change.

Lift

Lift is generated by the wings of the aircraft.  If Lift is greater than the weight of the aircraft then the aircraft will ascend (climb).

Weight

Weight is the gravitational force experienced by the airplane.  This includes the weight of the pilots, passengers, baggage and fuel.  If the weight of the airplane is greater than the lift created by the wings then the airplane will descend.

When lift and weight are balanced, the aircraft remains in straight and level flight or maintains a constant rate of climb or descent.

 

 

 Another Perspective

Becoming a pilot is no easy task. We are always faced with challenges that get in the way of achieving our dreams.  Therefore, many pilots have come up with their own way of demonstrating the four forces of flight (as a joke, of course).