This section is to introduce you to the
forces acting on the aircraft 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
the four forces acting on an
An airplane in flight is the centre of a
continuous tug of war between fourforces: lift,
gravity 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
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 weight).
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).