This section is to introduce you to the forces
acting on the airplane in flight.
For a moment, think of an airplane
moving from left to right and the flow
of air moving from right to left.
The weight or force due to gravitypulls down on the plane
opposing the lift
created by air flowing over the wing.
Thrust is generated by
the propeller and opposes drag
caused by air resistance to the
airplane. During take off, thrust
must be greater than drag and lift must
be greater than weight so that the
airplane can become airborne.
For landing thrust must be less than
drag, and lift must be less than weight.
the four forces acting on an aeroplane
An airplane in flight is the centre of a
continuous tug of war between
force or weight, thrust,
and drag. Lift
and Drag are considered aerodynamic
forces because they exist due to the
movement of the aircraft through the air.
The weight pulls down on the plane
opposing the lift created by air flowing
over the wing. Thrust is generated by
the propeller and opposes drag caused by
air resistance to the frontal area of
the airplane. During take off, thrust
must overcome drag and lift must
overcome the weight before the airplane
can become airborne. In level flight at
constant speed, thrust exactly equals
drag and lift exactly equals the weight
or gravity force. For landings thrust
must be reduced below the level of drag
and lift below the level of the gravity
force or weight.
Thrust is a force created by a power
source which gives an airplane forward
motion. It can either "pull" or "push"
an airplane forward. Thrust is that
force which overcomes drag. Conventional
airplanes utilize engines as well as
propellers to obtain thrust.
Drag is the force which delays or slows
the forward movement of an airplane
through the air when the airflow
direction is opposite to the direction
of motion of the airplane. It is the
friction of the air as it meets and
passes over and about an airplane and
its components. The more surface area
exposed to rushing air, the greater the
drag. An airplane's streamlined shape
helps it pass through the air more
Lift is produced by a lower pressure
created on the upper surface of an
airplane's wing compared to the pressure
on the wing's lower surface, causing the
wing to be "lifted" upward. The special
shape of the airplane wing (airfoil)
is designed so that air flowing over it
will have to travel a greater distance
faster, resulting in a lower pressure
area (see illustration) thus lifting the
wing upward. Lift is that force which
opposes the force of gravity (or
Many believe that this explanation is
incorrect because flat wings (such as
seen on balsa wood airplanes, paper
planes and others) also have managed to
Lift is a partial vacuum created above
the surface of an airplane's wing
causing the wing to be "lifted" upward.
The special shape of the airplane wing
(air foil) is designed so that air
flowing over it will have to travel a
greater distance - faster - resulting in
a low pressure area ( see illustration)
thus lifting the wing upward. Lift is
that force which opposes gravity.
The angle of attack is the
angle that the wing presents to oncoming air, and it
controls the thickness of the slice of air the wing is
cutting off. Because it controls the slice, the angle of
attack also controls the amount of lift that the wing
generates (although it is not the only factor).