There are four forces that act on an
aircraft in flight: lift, weight, thrust, and drag. From Newton's
first law of motion we know that an object
at rest will stay at rest, and an object in motion (constant
velocity) will stay in motion unless acted on by an external force.
If there is no net external force, the object will maintain a constant
In an ideal situation, the forces acting on an aircraft in flight can
and produce no net external force.
In this situation the lift
is equal to the weight, and the thrust
is equal to the drag. This flight condition is
called a cruise condition for the aircraft.
While the weight
decreases due to fuel burned, the change is very small relative to
the total aircraft weight. The aircraft maintains a constant airspeed
called the cruise velocity.
Except during take-off, the Wright brothers'
spent most of the flight time in a cruise condition.
If we take into account the relative
velocity of the wind, we can determine the ground speed of a
The ground speed is equal to the airspeed plus the wind speed (vector
of the aircraft is a pure
With a constant ground speed it is relatively easy
to determine the aircraft range, the
distance the airplane can fly with a given load of fuel.
If the forces on the aircraft become
unbalanced, the aircraft moves in the direction of the
greater force, and we can
acceleration of the aircraft from
Newton's second law of motion (F = m
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page