Lesson
1: AIRFRAME of an airplane
Judging from the
story of Daedalus and Icarus, humans have been interested in aerodynamics and flying for thousands of
years, although flying in a heavier-than-air
machine has been possible only in the last hundred years. Aerodynamics
affects the motion of a large airliner,
a model rocket, a beach ball thrown near the shore, or a kite flying high overhead. The curveball thrown by
big league baseball pitchers gets its
curve from aerodynamics. In this lesson, we will discuss the basics of
aircraft aerodynamics, flight control
surfaces for airplanes and helicopters, and maintaining those flight controls.
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PRINCIPLES OF AERODYNAMICS
What is
aerodynamics? The word comes from two Greek words: aerios, concerning the air,
and dynamis, meaning powerful. Simply
stated, aerodynamics is the study of
forces and the resulting motion of objects through the air.
Forces Exerted On An Aircraft In
Flight
In order to understand how an aircraft
flies, we must first discuss the four forces
that affect the aircraft in flight, figure 1-1. These forces are
manipulated by the pilot, through the
use of engine and flight controls, to produce a desired response from the aircraft.
.
Figure 1-1, Four Forces
LIFT.
One of the
fundamental forces studied in aerodynamics is lift, or the force that keeps an
airplane in the air. Lift is produced by the effect of air passing over an airfoil.
The net force produced is perpendicular to the relative wind, which will be discussed later in this section. Lift
directly opposes gravity.
WEIGHT.
Weight is a
measurement of the force that gravity exerts on a given object. Weight is also the force that offsets
lift, because it acts in the opposite direction. The weight of the airplane must be overcome
by the lift produced by the wings. If an
airplane weighs 70,000 pounds, then the lift produced by its wings must
be greater than 70,000 pounds in order
for the airplane to leave the ground.
THRUST.
Thrust is the force that propels an
airplane forwa
rd through the
air. The airplane’s propulsion system,
either a propeller or jet engine or combination of the two, provides thrust. Thrust is used to
overcome the opposing forces of drag and
inertia. Inertia is a property of matter that causes it to remain
stationary or remain in uniform motion
in a straight line. Consequently, thrust overcomes inertia by setting the aircraft in motion and defeats drag by
keeping the aircraft in motion.
DRAG
. Another important aspect of aerodynamics is the drag, or
resistance, acting on solid bodies
moving through air. The thrust force developed by either the jet engine or the propellers, for example, must
overcome the drag forces exerted by the air
flowing over the airplane. In this section we will discuss two types of
drag: Induced and parasitic.
Induced Drag.
This drag is caused by the development of
lift. As a difference in pressure is
caused about an airfoil, attempts to equalize this pressure at the wing tip cause a circular motion called vortices. This
phenomenon creates, or induces drag.
Parasitic.
The second type of
drag, parasitic, is found in three forms: Skin
friction, interference drag, and form drag.
Skin Friction.
Consider the air
passing over a surface to be made up of thin
sheets of molecules of air. The sheet of air on the surface of the skin
will lodge in the minute pits and
crevices found on all surfaces. As this sheet of air is stopped, a mass is decelerating which will cause a force to be
exerted. The molecules of air do not remain
in the thin sheets but bounce from one sheet to another. Consequently,
as the now slower-moving molecules on
the surface move to the next layer, they slow that layer also. The overall picture will be that of
many sheets of air moving across the skin. The
layer on the skin will be stopped and each adjacent layer will be moving
somewhat faster until a short distance
away all of the layers will be moving at the normal speed.
Interference Drag.
This drag is generated by the collision of
airstreams creating turbulence. It is
reduced by conforming external appendages to surface of the fuselage as much as possible. If you take a clean
aircraft and determine its drag at a given
airspeed and then take an external store like a fuel tank or bomb and
measure its drag at the same airspeed,
the total drag of the aircraft with the external store attached exceeds the sum of the individual drag
values. Interference drag also occurs as a result of external engine nacelles and at the
junction of the wing and tail surfaces with the
fuselage.
Form Drag.
This drag is caused by the relative low
pressure in the area behind any object
in a moving airstream. The difference in pressure between the front and
the rear of the object acts on the
profile area and causes a force aft. An example would be putting your hand out the window of a fast
moving vehicle. Form drag results in a force
aft, which causes your hand to move toward the rear of the vehicle.