lecture leading cadet training principles of flight 1 introduction
TRANSCRIPT
Lecture
Leading Cadet Training
Principles of Flight
1Introduction
The Leading EdgeIs generally the part of the wing that first contacts the air.
The Trailing EdgeThe rear of the wing, where the airflow rejoins
after passing over and under the wing.
The Chord LineThe line passing through the Centres of Curvature
of the Leading and Trailing Edges.
Definitions
Leading Edge
Trailing Edge
THE AEROFOIL The shape of a wing as seen in cross-section.
Chord Line
Free Stream FlowThe Airflow remote from the Aircraft,
and unaffected by its presence. sometimes called the Relative Airflow.
The Angle of AttackSymbol α (alpha)
The Angle between the Chord Line and the Free Stream Flow.
Definitions
Leading Edge
Trailing Edge
Chord Line
Free Stream Flow
α
DefinitionsThe 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
Trailing Edge
Chord Line
Free Stream Flow
α
Mean Camber Line
Definitions
If the mean camber line is co-incident(or identical to) the chord line
it is a SYMMETRICAL AEROFOIL
Leading Edge
Trailing Edge
Chord Line
Free Stream Flow
α
Mean Camber Line
Air is a Substance
The greater the speed of air, the greater the force it exertson 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.
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
PressureIn a wind tunnel, air speed and pressure are measured.
Air enters the tunnel at ‘A’
A
B
C
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.
++
++
- -- -
Pressure
A
B
C++
++
- -- -
Bernoulli's Principle :
As airspeed increases, air pressure decreases.
As airspeed decreases, air pressure increases.
+
+
__
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.
PressureA 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.
__
The Pressure Envelope
The area of static pressure in the surrounding area.
Pressure
+ +
_TR
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.
CPo
The Pressure EnvelopePressure
+ +
_TR
CPo
Oncoming Air
LIFT
Flight Path
LIFT - The component of the Total Reaction which is ‘perpendicular’ to the oncoming air and the flight path.
DRAG
DRAG - The component of the Total Reaction which is‘parallel’ to the oncoming air and the 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 !
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.
Newton’s 3rd Law
WeightIf 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 .
Less effort from your muscles(less lift)
makes the brick sink.
More effort from your muscles (more lift)
and the brick will rise.
GRAVITY
EFFORT
Lift and WeightLift 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.
Air flowing past a stationary object has the sameeffect as if the object were moving through the air.
But in real life, the airflow rarely approaches a wing as shown here.
Lift
LiftThe 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 UnderstandingOn an aerofoil the straight line passing through
the Centres of Curvature of the Leading and Trailing Edges is called . . .
The Centre of Pressure
The Chord Line
The Angle of Attack
The Mean Camber Line
Check of UnderstandingIf the mean camber line lies above the chord line,
what type of camber does the aerofoil have?
An ovoid camber
A positive camber
A symmetrical camber
A negative camber
Check of UnderstandingIn the diagram below,
what has happened to the air pressure at point ‘B’?
It is lower than ‘A’ and higher than ‘C’
It is equal to that at ‘A’ and ‘C’
It is lower than at ‘A’ and ‘C’
It is higher than at ‘A’ and ‘C’
Check of UnderstandingThe Centre of Pressure on an aerofoil is . . .
The consequence or ‘resultant’of all the aerodynamic forces
Where all the forces balanceand all lift is said to act.
Half way along the chord line
Where all the forces balanceand all weight is said to act.
Check of UnderstandingIn normal flight,
what happens to the speed of the air passing over the top surface of a wing?
The speed fluctuates
The speed remains constant
The speed decreases
The speed increases
Check of UnderstandingWhen air flows over the wing at an increased speed,
what happens to the air pressure?
The air pressure fluctuates
The air pressure increases
The air pressure decreases
The air pressure remains constant
Principles of Flight
End of Presentation
Leading Cadet Training