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 gravity
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
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.
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Flow is the
smooth, uninterrupted flow of air over
the contour of the wings, fuselage, or
other parts of an aircraft in flight.
Laminar flow is most often found at the
front of a streamlined body and is an
important factor in flight. If the
smooth flow of air is interrupted over a
wing section, turbulence is created
which results in a loss of lift and a
high degree of drag. An airfoil designed
for minimum drag and uninterrupted flow
of the boundary layer is called a
The Laminar flow theory dealt with the
development of a symmetrical airfoil
section which had the same curvature on
both the upper and lower surface. The
design was relatively thin at the
leading edge and progressively widened
to a point of greatest thickness as far
aft as possible. The theory in using an
airfoil of this design was to maintain
the adhesion of the boundary layers of
airflow which are present in flight as
far aft of the leading edge as possible.
on normal airfoils the boundary layer
would be interrupted at high speeds and
the resultant break would cause a
turbulent flow over the remainder of the
foil. This turbulence would be realized
as drag up the point of maximum speed at
which time the control surfaces and
aircraft flying characteristics would be
affected. The formation of the boundary
layer is a process of layers of air
formed one next to the other, ie; the
term laminar is derived from the
lamination principle involved.
next to any surface forms a "boundary
layer", as the flow has zero velocity
right at the surface and some distance
out from the surface it flows at the
same velocity as the local "outside"
flow. If this boundary layer flows in
parallel layers, with no energy transfer
between layers, it is laminar. If there
is energy transfer, it is turbulent.
All boundary layers start off as
laminar. Many influences can act to
destabilize a laminar boundary layer,
causing it to transition to turbulent.
Adverse pressure gradients, surface
roughness, heat and acoustic energy all
examples of destabilizing influences.
Once the boundary layer transitions, the
skin friction goes up. This is the
primary result of a turbulent boundary
layer. The old "lift loss" myth is just
that - a myth.
A favourable pressure gradient is
required to maintain laminar flow.
Laminar flow airfoils are designed to
have long favourable pressure gradients.
All airfoils must have adverse pressure
gradients on their aft end. The usual
definition of a laminar flow airfoil is
that the favourable pressure gradient
ends somewhere between 30 and 75% of
The upper airfoil is typical
for a stunt plane, and the lower airfoil is typical for
supersonic fighters. Note that both are symmetric
on the top and bottom. Stunt planes and supersonic jets
get their lift totally from the angle of attack
of the wing.
angle of attack
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).