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that is parallel to the center of thrust and overcomes drag
providing the aircraft with its forward speed component.
Drag
Drag is the net aerodynamic force parallel to the relative
wind and is generally a sum of two components: induced
drag and parasite drag.
Induced drag
Induced drag is caused from the creation of lift and increases
with angle of attack. Therefore, if the wing is not producing
lift, induced drag is zero. Conversely, induced drag decreases
with airspeed.
Parasite drag
Parasite drag is all drag not caused from the production of
lift. Parasite drag is created by displacement of air by the
aircraft, turbulence generated by the airfoil, and the hindrance
of airflow as it passes over the surface of the aircraft or
components. All of these forces create drag not from the
production of lift but the movement of an object through an
air mass. Parasite drag increases with speed and includes skin
friction drag, interference drag, and form drag.
• Skin Friction Drag
Covering the entire “wetted” surface of the aircraft is a thin
layer of air called a boundary layer. The air molecules on the
surface have zero velocity in relation to the surface; however,
the layer just above moves over the stagnant molecules
below because it is pulled along by a third layer close to
the free stream of air. The velocities of the layers increase
as the distance from the surface increases until free stream
velocity is reached, but all are affected by the free stream.
The distance (total) between the skin surface and where free
stream velocity is reached is called the boundary layer. At
subsonic levels the cumulative layers are about the thickness
of a playing card, yet their motion sliding over one another
creates a drag force. This force retards motion due to the
viscosity of the air and is called skin friction drag. Because
skin friction drag is related to a large surface area its affect
on smaller aircraft is small versus large transport aircraft
where skin friction drag may be considerable.
• Interference Drag
Interference drag is generated by the collision of airstreams
creating eddy currents, turbulence, or restrictions to smooth
flow. For instance, the airflow around a fuselage and around
the wing meet at some point, usually near the wing’s root.
These airflows interfere with each other causing a greater
2-4
Figure 2-4. Newton’s First Law of Motion: the Law of Inertia.
drag than the individual values. This is often the case when
external items are placed on an aircraft. That is, the drag of
each item individually, added to that of the aircraft, are less
than that of the two items when allowed to interfere with
one another.
• Form Drag
Form drag is the drag created because of the shape of a
component or the aircraft. If one were to place a circular
disk in an air stream, the pressure on both the top and bottom
would be equal. However, the airflow starts to break down
as the air flows around the back of the disk. This creates
turbulence and hence a lower pressure results. Because the
total pressure is affected by this reduced pressure, it creates
a drag. Newer aircraft are generally made with consideration
to this by fairing parts along the fuselage (teardrop) so that
turbulence and form drag is reduced.
Total lift must overcome the total weight of the aircraft, which
is comprised of the actual weight and the tail-down force used
to control the aircraft’s pitch attitude. Thrust must overcome
total drag in order to provide forward speed with which to
produce lift. Understanding how the aircraft’s relationship
between these elements and the environment provide proper
interpretation of the aircraft’s instruments.
Newton’s First Law, the Law of Inertia
Newton’s First Law of Motion is the Law of Inertia. It states
that a body at rest will remain at rest, and a body in motion
will remain in motion, at the same speed and in the same
direction until affected by an outside force. The force with
which a body offers resistance to change is called the force of
inertia. Two outside forces are always present on an aircraft
in flight: gravity and drag. The pilot uses pitch and thrust
controls to counter or change these forces to maintain the
desired flight path. If a pilot reduces power while in straightand-
level flight, the aircraft will slow due to drag. However,
as the aircraft slows there is a reduction of lift, which causes
the aircraft to begin a descent due to gravity. [Figure 2-4]
Newton’s Second Law, the Law of Momentum
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