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 hi straight-
and-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
Newton's Second Law of Motion is the Law of Momentum,
which states that a body will accelerate in the same direction
as the force acting upon that body, and the acceleration
will he directly proportional to the net force and inversely
proportional to the mass of the body. Acceleration refers
either to an increase or decrease in velocity, although
deceleration is commonly used to indicate a decrease. This
law governs the aircraft's ability to change flight path and
speed, which are controlled by attitude (both pitch and bank)
and thrust inputs. Speeding up, slowing down, entering
climbs or descents, and turning are examples of accelerations
that the pilot controls in everyday flight. [Figure 2-5]
Newton's Third Law, the Law of Reaction
Newton's Third Law of Motion is the Law of Reaction,
which states that for every action there is an equal and
opposite reaction. As shown in Figure 2-6, the action of
the jet engine's thrust or the pull of the propeller lead to the
reaction of the aircraft's forward motion. This law is also
responsible for a portion of the lift that is produced by a wing,
from the downward deflection of the airflow around it. This
downward force of the relative wind results in an equal but
opposite (upward) lifting force created by the airflow over
the wing. [Figure 2-6]
Atmosphere
The atmosphere is the envelope of air which surrounds the
Earth. A given volume of dry air contains about 78 percent
nitrogen, 21 percent oxygen, and about 1 percent other gases
such as argon, carbon dioxide, and others to a lesser degree.
Although seemingly light, air does have weight and a one
square inch column of the atmosphere at sea level weighs
approximately 14.7 pounds. About one-half of the air by
weight is with in the first 18,000 feet. The remainder of the air
is spread over a vertical distance in excess of 1,000 miles. |
