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Introduction
The Atmosphere
Newton’s Laws of Motion
Bernoulli’s Principle
Airfoil
Parts of an Airplane
The Four Forces of Flight
Three Axes of Movement
Stability
Control
Aerodynamics
Aerodynamics is the study of objects in motion through
the air and the forces that produce or change such motion.
INTRODUCTION
It is unnecessary that a mechanic be totally versed on
Aerodynamics and Theory of Flight. However he must
understand the relationships between the atmosphere, the
aircraft and the forces acting on it in flight, in order to make
intelligent decisions affecting the flight safety of both airplanes
and helicopters.
The Atmosphere
Air is a mixture of gases composed principally of nitrogen
and oxygen. An aircraft operates in the air, therefore, the
properties of air that affect aircraft control and performance must
be understood.
Pressure – Atmospheric pressure varies with altitude. The
higher an object rises above sea level, the lower the pressure.
Density – It varies directly with the pressure and inversely with
the temperature. With the same horse power, an aircraft can fly
faster at high altitude because of less resistance of air at there.
Humidity – Humidity is the amount of water vapor in the air. It
varies directly with temperature.
Newton's First Law of Motion
According to Newton's first law of motion (inertia), an object at
rest will remain at rest, or an object in motion will continue in
motion at the same speed and in the same direction, until an
outside force acts on it. For an aircraft to taxi or fly, a force
must be applied to it. It would remain at rest without an
outside force. Once the aircraft is moving, another force must
act on it to bring it to a stop. It would continue in motion
without an outside force. This willingness of an object to
remain at rest or to continue in motion is referred to as inertia.
Newton's Second Law of Motion
The second law of motion (force) states that if a object moving
with uniform speed is acted upon by an external force, the
change of motion (acceleration) will be directly proportional to
the amount of force and inversely proportional to the mass of
the object being moved. The motion will take place in the
direction in which the force acts. Simply stated, this means that
an object being pushed by 10 pounds of force will travel faster
than it would if it were pushed by 5 pounds of force. A heavier
object will accelerate more slowly than a lighter object when an
equal force is applied. F = m × a
Newton's Third Law of Motion
The third law of motion (action and reaction) states that for
every action (force) there is an equal and opposite reaction
(force). This law can be demonstrated with a balloon. If you
inflate a balloon with air and release it without securing the
neck, as the air is expelled the balloon moves in the opposite
direction of the air rushing out of it. Figure shows this law of
motion.
ActionReaction
Balloon
Air
BERNOULLI'S PRINCIPLE
Bernoulli's principle states that when a fluid flowing through a
tube reaches a constriction or narrowing of the tube, the
speed of the fluid passing through the constriction is increased
and its pressure is decreased.
Pressure Drop in Venturi Tube
AirfoilAn airfoil is the shape of a wing or blade (of a propeller, rotor or turbine) as seen in cross-section. An aircraft's wings, horizontal, and vertical stabilizers are built with airfoil-shaped cross sections, as are helicopter rotor blades.
- The mean camber line is a line drawn midway between the upper and lower surfaces. - The chord line is a straight line connecting the leading and trailing edges of the airfoil, at the ends of the mean camber line.
Mean camber line
Chord line
kinetic energy(velocity)
potential energy(pressure)
velocity increases
pressure decreases
Airfoil as a Venturi Tube
Lift force appear
Cockpit
Fuselage
Wing
Flap
Aileron
Empennage
Stabilizers
Rudder
Elevator
Engine
Parts of an Airplane
Parts of An Airplane
The forces acting on an airplane in flight are lift, weight, thrust,
and drag. These forces are in equilibrium during straight-and-
level, unaccelerated flight.
The Four Forces of Flight
DRAG
WEIGHT
THRUST
LIFT
Lift is an aerodynamic force
Lift must exceed weight for flight
Generated by motion of aircraft through air
Created by the effects of airflow past wing
Aircraft lift acts through a single point called the center
of pressure.
LiftLift is the force created by the interaction between the wings
and the airflow. It always act upwards. It is considered to be
the 'most important force' as without it, an aircraft cannot
ascend from ground and maintain altitude.
Newton’s Third Law and Lift
Newton’s Second Law and Lift
Lift: Wing Section
Lift Equation: L=CLift Equation: L=CL L ×× ½ ½ ρρ × A × V × A × V22
• The angle of attack is the angle between the chord line
and the average relative wind.
• Greater angle of attack creates more lift (up to a point).
Angle of Attack
High velocity
Low pressure
Low velocity
High pressure
Angle of Attack and Lift Force
• The angle of incidence is the angle between the chord line
and the longitudinal axis of aircraft.
• It is the angle of wing setting.
• When the leading edge of the wing is higher than the
trailing edge, the angle of incidence is said to be positive.
It is negative when the leading edge is lower than the
trailing edge of the wing.
Angle of Incidence
Chord line
Aircraft longitudinal axis
Angle of incidence
Horizontal Component of Lift
• Lift acts through the center of pressure, and perpendicular to the relative wind.
• This creates induced drag.
chord line
average relative wind
total lift
effective lift
induced drag
Lift and Induced Drag
Shape of the Airfoil
• The shape of the airfoil determines the
amount of turbulences or skin friction
that it will produce. The shape of a wing
consequently affects the efficiency of
the wing. A wing may have various
airfoil section from root to tip, with
taper, twist, sweep back and sweep
forward.
Wing Shapes
Weight is not constant
Varies with passengers, cargo, fuel load
Decreases as fuel is consumed or payload off-loaded
Direction is constant toward earth’s center
Acts through a single point called the center of gravity
(the CG)
WeightThis force acts on an aircraft due to the interaction between
the aircraft's body weight and Earth's gravity. Weight is a
downward force.
Forward-acting force opposes drag
Direction of thrust depends on design
Propulsion systems produce thrust
Equal to drag in straight, constant speed flight
ThrustThis force is created by an aircraft's engine and is required for
forward motion.
An aerodynamic force.
Resists forward motion.
Increases with the square of speed.
Two broad drag classifications.
– Parasite drag: drag created by airplane shape.
A result of air viscosity.
– Induced drag: by-product of lift generation.
Caused by the wingtip vortices.
DragThis force acts in reverse direction to that of 'Thrust' and hinders forward motion. Drag is considered as a negative force and all engineers try their best to reduce drag.
Drag Equation: D=CDrag Equation: D=CD D ×× ½ ½ ρρ × A × V × A × V22
Example of Drag Formation
Skin Friction Drag
Three Axes of Movement
Axis of Roll (Longitudinal Axis)
Axis of Pitch (Lateral Axis)
Axis of Yaw (Vertical Axis)
Pitch Around the Lateral Axis
Roll Around Longitudinal Axis
Yaw Around the vertical Axis
There are two types of stability Static Stability - The initial movement of an object after being
disturbed.– Positive Static Stability – returns to position before
displacement.– Neutral Static Stability – tendency to remain in displaced
position.– Negative Static Stability – tends to continue away from
displaced position in same direction. Dynamic Stability - The behavior of the object over time.
– Positive Dynamic Stability – the oscillations or phugoids dampen themselves out.
– Neutral Dynamic Stability – the oscillations or phugoids carry on with out increasing in severity.
– Negative Dynamic Stability – the oscillations or phugoids increase in severity and diverge.
StabilityAn aircraft must have sufficient stability to maintain a uniform flight path and recover from the various upsetting forces also to achieve the best performance.
Static Stability
Positive-Neutral-Negative
Dynamic Stability
Positive Dynamic Stability
Natural Dynamic Stability
Negative Dynamic Stability
Larger wing area
More lift
Smaller wing area
Less lift
Stability recover by a dihedral wing
Stability recover by a sweep back wing
Stability recover by keel effect
CONTROLTo achieve the best performance, the aircraft must have the
proper response to the movement of the controls. Control is the
action taken to make the aircraft follow any desired flight path.
Different Control surfaces are used to control the aircraft about
each of the three axes.
Flight Control Surfaces – Hinged or moveable airfoils
designed to change the attitude of the aircraft during flight.
1. Primary group
- ailerons
- elevators
- rudder
2. Secondary group
- trim tab, spring tab
- servo tab, balance tab
3. Auxiliary group
- wing flaps
- spoilers
- speed brakes
- slats
- leading edge flaps
- slots
Flight Control Surfaces
Flap
Flap
Spoiler
Spoiler
wing flaps spoilers leading edge slats
leading edge slots speed brakes
Ailerons – The ailerons form a part of the wing and are located in
the trailing edge of the wing towards the tips. The control stick is
connected by means of wires or hydraulics to the wings’ ailerons. By
turning the stick, the pilot can change the positions of the ailerons.
ROLLING
Control around the Longitudinal Axis
Rudder – The rudder is a
moveable control surface
attached to the trailing edge of the
vertical stabilizer. The foot pedals
are connected by means of wires
or hydraulics to the rudder of the
tail section. The rudder can also
be used in controlling a bank or
turn in flight.
YAWING
Control around the Vertical Axis
Moving rudder to the right forces tail to the left, nose to the right
Moving rudder to the left forces tail to the right, nose to the left.
Elevators – Elevators are the
movable control surfaces hinged to
the trailing edge of the horizontal
stabilizer. The control stick is
connected by means of wires or
hydraulics to the tail section’s
elevators.
- Stabilator
- Ruddervator
PITCHING
Control around the Lateral Axis