Transcript Slide 1

Principles of Flight
Leading Cadet Training
Introduction
Lecture
1
Definitions
The Leading Edge
Is generally the part of the wing that first contacts the air.
The Trailing Edge
The rear of the wing, where the airflow rejoins
after passing over and under the wing.
The Chord Line
The line passing through the Centres of Curvature
of the Leading and Trailing Edges.
Leading
Edge
Chord Line
THE AEROFOIL
The shape of a wing as seen in cross-section.
Trailing
Edge
Definitions
Free Stream Flow
The Airflow remote from the Aircraft,
and unaffected by its presence.
sometimes called the Relative Airflow.
The Angle of Attack
Symbol α (alpha)
The Angle between the Chord Line
and the Free Stream Flow.
Leading
Edge
Chord Line
α
Free Stream Flow
Trailing
Edge
Definitions
The Mean Camber Line
The line drawn equidistant from the upper
and lower surfaces of the wing.
If the mean camber line lies above the chord line
(as in the illustration)
the aerofoil has a ‘positive camber’;
it is a CAMBERED AEROFOIL
Leading
Edge
Chord Line
Mean Camber
Line
α
Free Stream Flow
Trailing
Edge
Definitions
If the mean camber line is co-incident
(or identical to)
the chord line
it is a SYMMETRICAL AEROFOIL
Leading
Edge
Chord Line
Mean Camber
Line
α
Free Stream Flow
Trailing
Edge
Air is a Substance
The greater the speed of air,
the greater the force it exerts
on its surroundings.
but what happens if the air is still
and an object moves through it ?
Imagine cycling along
a flat road on a calm day.
As your speed increases,
you feel the air against your face.
The faster you go, the greater the force of wind.
Pressure
How is this lifting force to be given to an aircraft?
Hold 2 sheets of paper about 1 finger width apart.
Blow between
the papers.
You would think the papers would be blown apart.
But the opposite happens !
The harder you blow,
the more firmly the papers are drawn together.
Pressure
In a wind tunnel, air speed and pressure are measured.
Air enters the tunnel at ‘A’ and escapes at ‘C’
The amount of air leaving, must equal that entering.
The air speeds up to pass through the narrow point ‘B’.
As speed increases, pressure drops & vice versa.
A
+
+
-
B
-
+
+
C
Pressure
Bernoulli's Principle :
+ _
_
As airspeed decreases,
air pressure increases. +
As airspeed increases,
air pressure decreases.
A
+
+
-
B
-
+
+
C
Pressure
Back to our sheets of paper …
The speed of the air between the paper increases.
So, the air pressure between the paper decreases.
Therefore, the air pressure outside is greater,
pushing the paper together.
Pressure
A wing works in the same way as the Wind-Tunnel.
Because the air is faster over the top surface,
the pressure is decreased.
And the wing produces LIFT.
_
_
Pressure
The Pressure Envelope
Total Reaction (TR)
The consequence or ‘resultant’ of all the aerodynamic forces,
usually on the aerofoil only.
The Centre of Pressure (CP)
The point at which the Total Reaction is said to act.
Where all the forces balance and all lift is said to act.
_
TR
+
o
CP
+
The area of static pressure in the surrounding area.
Pressure
The Pressure Envelope
LIFT - The component of the Total Reaction which is
‘perpendicular’ to the oncoming air and the flight path.
DRAG - The component of the Total Reaction which is
‘parallel’ to the oncoming air and the flight path.
_
LIFT
Oncoming Air
+
TR
o
DRAG
CP
+
Flight Path
Newton’s 3rd Law
“TO EVERY ACTION,
THERE IS AN EQUAL AND OPPOSITE REACTION”
If a car weighing 1 tonne is parked on the road.
Its weight presses down on the road
with a force of 1 tonne
Newton’s Law states that to support the car,
the road must press up with a force of 1 tonne.
Newton’s 3rd Law
Similarly a boat is supported
by an upward force (from the sea)
otherwise it would sink !
Newton’s 3rd Law
The same must apply to an aircraft in flight.
But how can an aircraft be supported by
such a flimsy substance as air?
An aircraft can only stay airborne
as long as it moves.
Weight
If you hold a brick at arm’s length,
you will feel the effort needed to balance
the downward force of gravity
acting on the weight of the brick .
GRAVITY
Less effort from your muscles
(less lift)
makes the brick sink.
More effort from your muscles
(more lift)
and the brick will rise.
EFFORT
Lift and Weight
Lift Greater than Weight
Ascending Flight
Lift Equal to Weight
Straight and Level Flight
Weight Greater than Lift
Descending Flight
Lift
Hold a sheet of paper so the end between your
fingers is horizontal,
and allow the rest to hang down.
Blow along the top of the paper.
and watch the paper lift.
The top surface of an aircraft wing behaves
in a similar manner.
Lift
Air flowing past a stationary object has the same
effect as if the object were moving through the air.
But in real life, the airflow rarely approaches a wing
as shown here.
Lift
The top of an aircraft wing is curved,
which forces the air into a smaller space.
The air flows over the wing at an increased speed And therefore at a reduced pressure,
compared with the surrounding atmosphere.
Lift
The resulting pressure difference
between the air above and below the wing,
tends to lift it up.
Check of Understanding
On an aerofoil the straight line passing through
the Centres of Curvature
of the Leading and Trailing Edges is called . . .
The Chord Line
The Mean Camber Line
The Angle of Attack
The Centre of Pressure
Check of Understanding
If the mean camber line lies above the chord line,
what type of camber does the aerofoil have?
A positive camber
A negative camber
A symmetrical camber
An ovoid camber
Check of Understanding
In the diagram below,
what has happened to the air pressure at point ‘B’?
It is equal to that at ‘A’ and ‘C’
It is higher than at ‘A’ and ‘C’
It is lower than at ‘A’ and ‘C’
It is lower than ‘A’ and higher than ‘C’
Check of Understanding
The Centre of Pressure on an aerofoil is . . .
Where all the forces balance
and all lift is said to act.
Where all the forces balance
and all weight is said to act.
Half way along the chord line
The consequence or ‘resultant’
of all the aerodynamic forces
Check of Understanding
In normal flight,
what happens to the speed of the air
passing over the top surface of a wing?
The speed remains constant
The speed increases
The speed decreases
The speed fluctuates
Check of Understanding
When air flows over the wing at an increased speed,
what happens to the air pressure?
The air pressure increases
The air pressure remains constant
The air pressure decreases
The air pressure fluctuates
Principles of Flight
Leading Cadet Training
End of Presentation