A force may be thought of as a push or
pull in a specific direction. This slide shows the forces that act on
the Wright airplane in flight.
Weight is a force that is always directed
toward the center of the earth. The
of the force depends on
the mass of all the airplane parts, plus the amount of fuel, plus the
pilot. The weight is
distributed throughout the airplane. But we can often think of it as
collected and acting through a single point called the center
of gravity. In flight, the airplane
about the center of
gravity, but the direction of the weight force always remains toward
the center of the earth. During a flight, the airplane's weight
constantly changes as the aircraft consumes fuel.
To make an airplane fly, we must generate a force to overcome the
weight. This force is called the lift and is
generated by the motion of the airplane through the air. Lift is an
aerodynamic force ("aero" stands for the air, and
"dynamic" denotes motion). Lift is directed perpendicular
(at right angle) to the flight direction. As with weight, each
part of the aircraft contributes to a single aircraft lift force. But
most aircraft lift is generated by the wings. Aircraft lift acts
through a single point called the
center of pressure.
center of pressure is defined just like the center of gravity, but
distribution around the body instead of the
As the airplane moves through the air, there is another aerodynamic
force present. The air resists the motion of the aircraft; this
resistance force is called the drag of the
airplane. Like lift, there are many factors that affect the magnitude
of the drag force including:
And like lift, we often collect all of the individual components'
drags and combine them into a single aircraft drag magnitude. The
direction of the drag force is always opposed to the flight
direction, and drag acts through the center of pressure.
To overcome drag this airplane has a pair of propellers
to generate a force called thrust.
The propellers are turned by a small
mounted on the top of the
It is often confusing to
remember that aircraft thrust is a reaction to the air being pushed
to the rear by the propellers. The air goes to the back, but the thrust
pushes towards the front. Action <--> reaction is explained by
Newton's Third Law of Motion.
of the airplane through the air depends on the relative
strength and direction of the forces shown above. If the forces are
balanced, the aircraft
velocity. If the forces are unbalanced,
the aircraft accelerates in the direction of the largest force.
You can view a short
of "Orville and Wilbur Wright" explaining how the four forces of weight,
lift, drag and thrust affected the flight of their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page