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Forces Acting on an Airplane

in Aerodynamics

There are four forces that act on an airplane at all times during a flight. A pilot’s job is to balance those four forces to make the airplane do as he chooses.

The four forces acting on an airplane are as follows:

Thrust is the forward force generated by the power plant (motor and propeller).  Thrust acts opposite of drag.

Drag is a rearward, retarding force, and is caused by disruption of the airflow by the wing, fuselage and other protrusions from the aircraft.  Drag acts opposite of thrust and is rearward, parallel to the relative wind.

Weight is the combined weight of the airplane and the load it’s carrying.  Weight acts opposite of lift and vertically downward through the airplane’s center of gravity (cg).

Lift is the upward motion created by the air striking the lifting surfaces of the airplane, such as the wings and elevators.

In straight and level flight these forces act equal to, and cancel each other out.  So, thrust equals drag and lift equals weight.  In straight and level flight, lift and weight are equal, but they are also greater than the opposing forces of thrust and drag, as shown in the graphic below.

In an climb, a portion of thrust acts as if it were lift, and a portion of weight acts as if it were drag.

In order for an airplane to move, thrust must be added.  The airplane will continue to gain speed until thrust and drag are equal.

51UFncHi9pL._SX390_BO1,204,203,200_Learn more about airplane aerodynamics with the Illustrated Guide to Aerodynamics. This unique introductory guide, which sold more than 20,000 copies in its first edition, proves that the principles of flight can be easy to understand, even fascinating, to pilots and technicians who want to know how and why an aircraft behaves as it does.


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