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PRINCIPLES OF FLIGHT
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Contents List.
PRINCIPLES OF FLIGHT
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Lift and Weight
Thrust and Drag
Stability and Control
Stalling
Chapter 5
Chapter 6
Gliding
Helicopters
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Contents List.
Click on a chapter.
PRINCIPLES OF FLIGHT
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Lift and Weight
Thrust and Drag
Stability and Control
Stalling
Chapter 5
Chapter 6
Gliding
Helicopters
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PRINCIPLES OF FLIGHT
Chapter 1
Lift and Weight
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Lift and Weight
How can an aircraft, which is so much heavier than air, be
supported by that air?
Sir Isaac Newton’s third law states:
“To every action, there is an equal and opposite reaction.”
A vehicle weighing 1 tonne is supported by the road pressing
up with a force of 1 tonne.
A boat weighing 10 tonnes is supported by a 10 tonne upward
force from the water, otherwise it would sink.
Lift and Weight
How can an aircraft, which is so much heavier than air, be
supported by that air?
Sir Isaac Newton’s third law states:
“To every action, there is an equal and opposite reaction.”
Aircraft can only remain airborne while they are actually
moving – if they stop moving they cease flying.
The upward force to support the aircraft is only there when
the aircraft is moving forwards through the air.
Lift and Weight
How can an aircraft, which is so much heavier than air, be
supported by that air?
Sir Isaac Newton’s third law states:
“To every action, there is an equal and opposite reaction.”
This upward force is generated by the aircraft’s wings as they
pass through the air.
We must understand how wings moving through the air create
that upward force, known as ‘lift’.
Lift and Weight
In this wind tunnel experiment the air is passing from ‘A’ to
‘C’ through a constriction at ‘B’.
As the amount of air leaving at ‘C’ is the same as the
amount entering at ‘A’, the air must speed up as it passes
the narrowest point ‘B’.
Lift and Weight
Another scientist, Bernoulli, discovered that in areas where
airspeed increases, the air pressure decreases.
In the wind tunnel experiment above, the air pressure
measured at ‘B’ will be less than at ‘A’ and ‘C’.
Lift and Weight
A simple experiment you can do
to demonstrate this principle is
illustrated in this picture.
Blowing along the top of this
sheet of paper causes it to lift
into line with the airflow.
By blowing and speeding up the air over the top, you have
reduced the pressure above the paper, so the normal air
pressure below the paper pushes it up.
Lift and Weight
The top surface of a wing is shaped so that the air which
flows between it and the undisturbed air a little way above
the wing is effectively flowing through a constriction.
undisturbed air
The air flows over the wing at increased speed and
therefore at reduced pressure.
Lift and Weight
All parts of a wing generate lift, some parts more than others.
lift forces
oncoming air
Lift and Weight
The top surface normally generates more lift than the
bottom surface.
lift forces
oncoming air
Lift and Weight
The top surface may generate up to 80% of the total lift.
lift forces
oncoming air
Lift and Weight
The greatest amount of lift on the top surface occurs where
the surface is curved the most.
lift forces
oncoming air
Lift and Weight
All of the lift forces act at 90 degrees to the oncoming airflow.
lift forces
oncoming air
Lift and Weight
All of the lift forces act at 90 degrees to the oncoming airflow.
lift forces
oncoming air
Lift and Weight
Instead of looking at the thousands of lift forces generated by a
wing, we add them together and represent them with a single line.
lift forces
oncoming air
Lift and Weight
Instead of looking at the thousands of lift forces generated by a
wing, we add them together and represent them with a single line.
oncoming air
Lift and Weight
The point at which all the
lift can be said to act is the
‘centre of pressure’.
oncoming air
Lift and Weight
The point at which all the
lift can be said to act is the
‘centre of pressure’.
The idea is similar to
finding the centre of
gravity of a ruler by
balancing it on your finger.
All the small forces of gravity acting on the ruler balance
about the centre of gravity.
Lift and Weight
Several factors affect the amount of lift produced by a wing.
Airspeed
Unsurprisingly, increasing the airspeed increases
the amount of lift produced by a wing.
What might surprise you is that doubling the
airspeed gives four times the lift.
Trebling the airspeed gives nine times the lift!
Lift and Weight
Several factors affect the amount of lift produced by a wing.
Airspeed
Angle of
attack
oncoming air
The angle of attack is the angle between the
chord line of the wing and the oncoming air
(or path of the aircraft).
Lift and Weight
Several factors affect the amount of lift produced by a wing.
Airspeed
Angle of
attack
oncoming air
The chord line of a wing is a straight line
joining the leading edge to the trailing edge.
Lift and Weight
Several factors affect the amount of lift produced by a wing.
Airspeed
Angle of
attack
oncoming air
If the angle of attack is increased the amount
of lift is also increased - until the angle reaches
about 15 degrees when the wing stalls.
Lift and Weight
Several factors affect the amount of lift produced by a wing.
Airspeed
Angle of
attack
Air
density
If the air becomes thinner, or less dense (due
to increases in altitude, temperature or humidity)
the amount of lift is reduced.
Lift and Weight
Several factors affect the amount of lift produced by a wing.
Airspeed
Angle of
attack
Air
density
Wing shape
and area
Designers choose a wing shape and area to
suit the role of the aircraft.
Lift and Weight
Several factors affect the amount of lift produced by a wing.
Airspeed
Angle of
attack
Air
density
Wing shape
and area
Wing ‘X’ is a general purpose wing section.
Lift and Weight
Several factors affect the amount of lift produced by a wing.
Airspeed
Angle of
attack
Air
density
Wing shape
and area
Wing ‘Y’ is a high lift wing section.
Lift and Weight
Several factors affect the amount of lift produced by a wing.
Airspeed
Angle of
attack
Air
density
Wing shape
and area
Sections ‘Z’ and ‘W’ are for high speed.
Lift and Weight
If the lift force is greater than the aircraft’s weight, the aircraft
will climb.
Lift and Weight
If an aircraft suffers a sudden reduction in weight by jettisoning
fuel or dropping cargo, it will climb (unless the pilot responds).
Lift and Weight
If the lift force is less than the aircraft’s weight, the aircraft will
descend.
Lift and Weight
In steady straight and level flight the lift force exactly equals
the force of gravity acting on the aircraft (its weight).
PRINCIPLES OF FLIGHT
Chapter 2
Thrust and Drag
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Thrust and Drag
An aircraft is propelled forwards by its engine or engines.
This force is called thrust.
Thrust and Drag
An aircraft flying through the air encounters resistance from
the air itself.
This force is called drag.
Thrust and Drag
Every part of the aircraft over which the air flows produces
drag which resists forward motion.
Thrust and Drag
If thrust exceeds drag the aircraft will accelerate.
Thrust and Drag
If thrust exceeds drag the aircraft will accelerate.
If drag exceeds thrust the aircraft will decelerate.
Thrust and Drag
If thrust exceeds drag the aircraft will accelerate.
If drag exceeds thrust the aircraft will decelerate.
An aircraft in steady straight and level flight will have
thrust exactly equal to the drag.
Thrust and Drag
As an aircraft’s airspeed increases, then so does the drag.
In the previous chapter we saw lift increase four times for double
the airspeed and increase nine times for treble the airspeed.
Doubling an aircraft’s airspeed will quadruple the drag.
Trebling an aircraft’s airspeed will increase drag by a
factor of nine.
PRINCIPLES OF FLIGHT
Chapter 3
Stability and Control
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Stability and Control
centre of
gravity
There are three axes about which an aircraft rotates.
All three go through the centre of gravity.
Stability and Control
The lateral axis generally runs from wingtip to wingtip.
Remember: L A T – Links Aerofoil Tips.
Stability and Control
The lateral axis generally runs from wingtip to wingtip.
The aircraft moves in the pitching plane about this axis.
Stability and Control
The lateral axis generally runs from wingtip to wingtip.
The aircraft moves in the pitching plane about this axis.
Stability and Control
The longitudinal axis runs from nose to tail.
Remember: aLONG the length of the aircraft.
Stability and Control
The longitudinal axis runs from nose to tail.
The aircraft moves in the rolling plane about this axis.
Stability and Control
The longitudinal axis runs from nose to tail.
The aircraft moves in the rolling plane about this axis.
Stability and Control
The normal axis runs vertically downwards.
The aircraft moves in the yawing plane about this axis.
Stability and Control
The normal axis runs vertically downwards.
The aircraft moves in the yawing plane about this axis.
Stability and Control
An aircraft flying along in steady straight and level flight will
regularly be disturbed by bumpy air (turbulence).
This will cause a wing to drop or the nose of the aircraft to
rise or fall.
A well-designed aircraft will tend to go back to level flight of
its own accord without the pilot having to make adjustments.
This property is called stability.
Stability and Control
The arrows in the diagram below illustrate an aircraft moving
in the pitching plane.
If the nose pitches up the tail goes down. The tailplane will
now be at a positive angle of attack to the oncoming airflow
and create lift.
The aircraft returns to straight and level flight.
Stability and Control
The arrows in the diagram below illustrate an aircraft moving
in the pitching plane.
If the nose pitches down the tail goes up. The tailplane will
now be at a negative angle of attack to the oncoming airflow
creating ‘downwards’ lift.
The aircraft again returns to straight and level flight.
Stability and Control
The arrows in the diagram below illustrate an aircraft moving
in the pitching plane.
Let us examine lateral stability in a little more detail.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-up in turbulence.
Stability and Control
oncoming
air
The tailplane now has a ‘positive angle of attack’ to the
oncoming airflow, generating lift.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
The lift generated returns the aircraft to straight and level
flight.
Stability and Control
In this illustration, the aircraft pitches nose-down in
turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-down in
turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-down in
turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-down in
turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-down in
turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-down in
turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-down in
turbulence.
Stability and Control
In this illustration, the aircraft pitches nose-down in
turbulence.
Stability and Control
oncoming
air
The tailplane now has a ‘negative angle of attack’ to the
oncoming airflow, generating downwards lift.
Stability and Control
The ‘downwards’ lift generated returns the aircraft to straight
and level flight.
Stability and Control
The ‘downwards’ lift generated returns the aircraft to straight
and level flight.
Stability and Control
The ‘downwards’ lift generated returns the aircraft to straight
and level flight.
Stability and Control
The ‘downwards’ lift generated returns the aircraft to straight
and level flight.
Stability and Control
The ‘downwards’ lift generated returns the aircraft to straight
and level flight.
Stability and Control
The ‘downwards’ lift generated returns the aircraft to straight
and level flight.
Stability and Control
The ‘downwards’ lift generated returns the aircraft to straight
and level flight.
Stability and Control
The ‘downwards’ lift generated returns the aircraft to straight
and level flight.
Stability and Control
The arrows in the diagram below illustrate an aircraft moving
in the rolling plane.
Most aircraft are designed with the wings set into the fuselage
at a slight upward angle.
This angle is called dihedral.
Stability and Control
The arrows in the diagram below illustrate an aircraft moving
in the rolling plane.
If a wing drops the aircraft starts to slip towards the lower
wing. This increases the amount of lift produced by that wing.
Part of the higher wing is shielded by the fuselage and
produces less lift.
Stability and Control
The arrows in the diagram below illustrate an aircraft moving
in the rolling plane.
With the lower wing producing more lift than the upper wing,
the aircraft returns to straight and level flight.
Stability in the rolling plane is provided by wing dihedral.
Stability and Control
The arrows in the diagrams below illustrate types of dihedral
on different aircraft.
Stability and Control
High wing aircraft have a natural stability due to the pendulum
effect of the centre of gravity being below the wing.
anhedral
Swept-back wings also have a natural stability and in some
cases the designer has had to take steps to reduce this stability.
Anhedral reduces stability in the rolling plane.
Stability and Control
Stability in the yawing plane,
sometimes known as
‘directional stability’ is
provided by the aircraft’s fin.
Stability and Control
Stability in the yawing plane,
sometimes known as
‘directional stability’ is
provided by the aircraft’s fin.
Stability and Control
If an aircraft is made to yaw
to one side by an air
disturbance,
Stability and Control
If an aircraft is made to yaw
to one side by an air
disturbance,
Stability and Control
If an aircraft is made to yaw
to one side by an air
disturbance,
Stability and Control
If an aircraft is made to yaw
to one side by an air
disturbance,
Stability and Control
oncoming
air
‘weathercock’
effect
If an aircraft is made to yaw
to one side by an air
disturbance,
the airflow will produce a
sideways force on the fin
which will yaw the aircraft
back onto its original heading.
Stability and Control
‘weathercock’
effect
the airflow will produce a
sideways force on the fin
which will yaw the aircraft
back onto its original heading.
Stability and Control
‘weathercock’
effect
the airflow will produce a
sideways force on the fin
which will yaw the aircraft
back onto its original heading.
Stability and Control
‘weathercock’
effect
the airflow will produce a
sideways force on the fin
which will yaw the aircraft
back onto its original heading.
Stability and Control
‘weathercock’
effect
the airflow will produce a
sideways force on the fin
which will yaw the aircraft
back onto its original heading.
Stability and Control
‘weathercock’
effect
the airflow will produce a
sideways force on the fin
which will yaw the aircraft
back onto its original heading.
Stability and Control
Most aircraft have a large fin,
placed as far back as possible
to increase the weathercock
effect and ensure directional
stability.
Stability and Control
Note that aircraft movements, such as
pitching, rolling and yawing, are
described relative to the pilot.
Flying Controls
The control column and
rudder pedals are
connected to the flying
control surfaces.
Flying Controls
The pilot uses elevators
to control the aircraft in
the pitching plane.
elevators
Flying Controls
When the pilot pushes
the control column
forwards, the elevators
move downwards.
elevators
This creates lift, the tail moves up and the nose pitches down.
Flying Controls
When the pilot pulls the
control column back,
the elevators move
upwards.
elevators
This creates downwards lift, the tail moves down, the nose pitches up.
Flying Controls
The rudder is connected
to the rudder pedals.
These are used to
control the aircraft in
the yawing plane.
rudder
Flying Controls
When the pilot pushes
the left rudder pedal, the
rudder moves to the left.
rudder
This deflects the tail to the right and the nose yaws left.
Flying Controls
When the pilot pushes
the right rudder pedal,
the rudder moves to the
right.
rudder
This deflects the tail to the left and the nose yaws right.
Flying Controls
To roll the aircraft, the pilot
uses two moveable parts of
the wing called ailerons.
By moving the control
column left, the left aileron
moves up (decreasing lift)
and the right aileron moves
down (increasing lift).
ailerons
The aircraft rolls to the left.
Flying Controls
To roll the aircraft, the pilot
uses two moveable parts of
the wing called ailerons.
By moving he control column
right, the right aileron moves
up (decreasing lift) and the
left aileron moves down
(increasing lift).
ailerons
The aircraft rolls to the right.
Trimming
During flight an aircraft’s centre of gravity may change for a
number of reasons: when fuel is used, bombs dropped,
ammunition fired etc.
The centre of pressure will also change, usually with changes
of power, speed or attitude.
The pilot would have to maintain a constant pressure on the
control column or rudder pedals to counter the forces created
by these imbalances.
Trimming tabs cancel out these unwanted forces on a
pilot’s flying controls.
Trimming
Elevator trimming
tabs prevent the pilot
having to maintain a
steady forwards or
backwards pressure
on the stick.
elevator
trimming tabs
Trimming
Rudder trimming
tabs prevent the pilot
having to maintain a
steady pressure on
either the left or right
rudder pedals.
rudder
trimming tab
Trimming
Aileron trimming
tabs prevent the pilot
having to maintain a
steady left or right
pressure on the stick.
aileron
trim tab
Flaps
A wing which flies well at the low speeds required for takeoff and landing would be inefficient at high speeds.
The usual solution to this is to design a wing for its main
task then add flaps for use on approach and landing.
There are many types of flap, some of which we will
examine, but all flaps increase the effective camber of the
wing and therefore increase the lift.
Flaps
Simple flap
Split flap
Fowler flap
Flaps
As flaps are lowered, both lift and drag increase.
Flaps
15 degrees of flap improves lift at take-off speeds.
A shorter take-off run is possible because the drag penalty of
15 degrees of flap at low speeds is very small.
Flaps
90 degrees of flap gives a tremendous increase in drag.
The pilot will lower the nose to maintain approach speed, this
increases the approach angle and gives a better forward view.
Slats
Slats are another device to improve handling at low speeds.
flap
slat
When slats are open, air flows through a slot between the
slat and the wing improving the airflow over the wing.
Slats
The turbulence in the airflow over the wing is smoothed
and the wing may not stall until 25 degrees angle of attack.
flap
slat
Although the stalling speed is very much reduced, opening
slats does cause extra drag.
PRINCIPLES OF FLIGHT
Chapter 4
Stalling
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Stalling
To increase the lift produced by a wing the pilot increases
the angle of attack.
Stalling
To increase the lift produced by a wing the pilot increases
the angle of attack.
Stalling
To increase the lift produced by a wing the pilot increases
the angle of attack.
Stalling
To increase the lift produced by a wing the pilot increases
the angle of attack.
Stalling
To increase the lift produced by a wing the pilot increases
the angle of attack.
Stalling
To increase the lift produced by a wing the pilot increases
the angle of attack.
Lift increases with increasing angle of attack up to about
15 degrees.
Stalling
To increase the lift produced by a wing the pilot increases
the angle of attack.
At about 15 degrees angle of attack, the critical angle, the
airflow over the top of the wing becomes turbulent.
Stalling
To increase the lift produced by a wing the pilot increases
the angle of attack.
As the airflow over the top of the wing becomes turbulent
lift is lost.
Stalling
Each particular wing has its own stalling angle and will
always stall when the wing reaches that angle.
For most conventional aircraft the stalling angle is about 15
degrees angle of attack.
Stalling
Although the angle at which a wing stalls does not vary, the
airspeed at which it stalls does change.
Increasing the aircraft’s weight increases its stalling speed
and vice-versa.
Stalling
Although the angle at which a wing stalls does not vary, the
airspeed at which it stalls does change.
Putting an aircraft into a turn will also increase the stalling
speed – the steeper the turn, the higher the stalling speed.
PRINCIPLES OF FLIGHT
Chapter 5
Gliding
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Gliding
Gliders either have no engine, or do not utilise an engine
when gliding. The force of thrust is not part of the equation.
The forces acting on a glider in steady balanced flight are
drag, weight and lift.
Gliders must pitch down and use a part of the ‘weight’
component to replace thrust.
Gliding
‘Gliding Angle’ describes how far a glider can glide from a
given height. The Viking’s is about 1 in 35.
This means that from a height of one kilometre (3280 ft), in
still air, the glider will travel 35 kilometres before landing.
A glider travelling with the wind (downwind) will travel a
much greater distance than one travelling into wind.
For instance, a glider with an airspeed of 35 kts travelling into
a 35 kt wind would appear to slowly lose height over one spot.
Gliding
Gliders are fitted with airbrakes or speedbrakes, panels which
pop out of the wings at 90 degrees to the airflow when selected.
Speedbrakes reduce lift and increase drag significantly.
They are used when approaching to land, to enable the
pilot to land in a smaller area than would otherwise be
possible.
PRINCIPLES OF FLIGHT
Chapter 6
The Helicopter
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The Helicopter
A helicopter creates lift by spinning aerofoil-shaped blades.
The Helicopter
The area swept by the blades or rotors is called the rotor disc.
The Helicopter
Lift can be increased by increasing the angle of attack of all
of the rotors together.
The Helicopter
The rotor disc can be
tilted forwards by
increasing the angle of
attack of the blades as
they sweep the rear
section of the disc.
The Helicopter
The helicopter will then
start to move in a
forwards direction.
The Helicopter
The rotor disc can also be
tilted backwards by
increasing the angle of
attack of the blades as
they sweep the front
section of the disc.
The Helicopter
The helicopter will then
start to fly backwards.
The Helicopter
The helicopter can also
move to the left and right
by tilting the rotor disc in
the relevant direction.
The Helicopter
To alter the pitch of each
rotor as it travels through
its 360 degree cycle, the
pilot uses the cyclic pitch
control.
The Helicopter
In a helicopter,
horizontal flight is
achieved by tilting the
rotor disc using the
cyclic control.
The Helicopter
The Helicopter
Spinning a helicopter’s rotor blades in one direction gives rise
to a ‘torque reaction’, a force in the opposite direction.
The tail rotor of
the helicopter
counters this
‘torque reaction’
and prevents the
helicopter
spinning out of
control.
The Helicopter
The pilot controls the speed of the tail rotor using the rudder
pedals.
Increasing and
decreasing the
speed of the tail
rotor yaws the
helicopter left or
right.
The Helicopter
The ‘collective pitch control’ changes the angle of attack of
all of the rotors together.
It is this ‘collective pitch control’ which
controls the helicopter’s vertical flight.
The Helicopter
When the pilot makes a large upward movement of the
collective lever, more power is required from the engine(s).
The Helicopter
When the pilot makes a large upward movement of the
collective lever, more power is required from the engine(s).
The hand throttle, located on the top of the
collective lever, is opened to supply this power.
PRINCIPLES OF FLIGHT
The End
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Principles of Flight
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