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Principles Of Flight

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on 17 October 2010

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Transcript of Principles Of Flight

Background
"Every action has an equal and opposite reaction"
- Isaac Newton
Imagine a 1 ton car on the road,
the road pushes back with a force
of 1 ton. This is the equal and
opposite reaction.
But how can air support an aircraft?
(clue, air supports only moving aircraft)
Air is a substance so therefore exerts a force
An example of this force is when you are
cycling on a calm still day, yet feel air rushing
into your face. The faster you move the more
you feel this.
Therefore, when you have air moving past
a stationary object or an object moving
through stationary air a similar force is
exerted
Weight
Imagine lifting up an object such as
a chair in one hand and holding it
at arms length.
You would feel the upwards force
(lift) needed to counteract the
downward gravity of the chair (weight)
Less effort from your muscles (less
lift) makes the chair sink, more
effort (more lift) makes the chair rise.
How is this lifting force given
to the aircraft?
Activity 1. Cut the A4 paper lengthways.
Stand up and hold the sheets so that they
are hanging vertically below your mouth
about 10cm apart. Blow hard between the
sheets of paper. What happens? Is it what
you expected to happen?
They are sucked together.
To find out why this happens
we use a wind tunnel...
Principles of Flight
Wind Tunnel Testing
Please look at diagram on board
In this simple example, air is flowing
over the top and the bottom of the wing.
If air flows past a stationary
object it will have the same effect
as if the object was moving in the
air, therefore the following test
reproduces what happens in flight
There is a change in speed.
The airflow then speeds up across the
top of the wing and the speed of
the air below the wing stays the same.
This is caused by the shape of the
wing.
Because of this speed change
according to Bernoulli's principle,
the pressure changes, where speed increases
pressure decreases. This means that the pressure
on top of the wing is lower than the pressure
below the wing - Lift occurs
If you think back to the
paper experiment, the increase
in speed between the constriction
meant that the pressure between
the paper was lower than outside
and this meant the paper was
pushed together
Lift
Hold some paper between your fingers
and hold it up to your mouth and let
it droop down infront of you. Blow along
the top curved edge...
The paper rose into line with the airflow, the harder you blow the more it will rise, this is exactly the same as an aircrafts wing
The top of a planes wing is shaped so that air between the wing and the undisturbed air above it is forced through constriction
Therefore air flows over the
wing at an increased speed and therefore
a lower pressure, meaning it lifts up,
therefore you have LIFT
However pressure isn't the only reason
for lift but is often the most prevalent,
but DOWNWASH also plays a part, as wings are
often at an angle and therefore this causes
air to flow upwards causing LIFT
Distribution of Lift
The whole surface of the wing is
affected by the airflow. There are
some key points you need to know:
Lift is not evenly distributed
The top service generates more lift
generally than the bottom surface
The greatest amount of lift on
the top surface occurs when it
is curved more
The greateset effect on the wing
is nearer to the front edge than
the rear edge
All lift forces act at 90 degrees
to the direction of airflow
Factors Affecting Lift
Several factors affect the amount
of lift produced by a wing
Altering the airspeed afects the amount
of lift a wing produces. On the runway a
plane doesn't lift, however when the pilot
increases the airspeed they travel down
the runway until there is enough airspeed
to produce lift.
The angle of attack also affects
lift, this is the angle between
airflow and the chordline of the
wing. If the angle of attack is
increased between 0-15 degrees,
lift will increase, anything after
that will lead to a stall
If air becomes thinner or less
dense lift is reduced, this concerns
pilots as they need greater speeds
on take off and may run out of runway
Wing shape and area also affect
lift
Lift and Weight in Straight and Level Flight
In straight and level flight, lift
and weight will be equal
If weight exceeds lift then the
plane will descend
If lift exceeds weight then
the plane will climb