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PRINCIPLES OF
FLIGHT

2. 2) Bernoulli’s Principle 3) Airfoil/Aerofoil Shape
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SCOPE…..
1) 4 Forces of Flight
2) Bernoulli’s Principle
3) Airfoil/Aerofoil Shape
4) How a Plane is Controlled

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4 Forces of Flight...
While an aircraft is flying, there are 4 forces acting on it:
THRUST, DRAG, LIFT and WEIGHT.
Thrust VS Drag while Lift VS Weight. Refer to the pictorial below:
Centre of Gravity, cg

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THRUST VS DRAG
THRUST
A force that is created by the aircraft’s propulsion system, engines.
It is a push or pull that moves the aircraft forward.
For aircraft to start moving, THRUST = DRAG.
For aircraft to accelerate, THRUST > DRAG.

5. How does the aircraft body contributes to drag?
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THRUST VS DRAG
DRAG
Opposes THRUST and acts the opposite direction of THRUST.
It is the resistance of air due to aircraft movement.
The aircraft body itself also contributes to drag.
Discussion
How does the aircraft body contributes to drag?

6. LIFT VS WEIGHT LIFT Generated by airflow over the aircraft body.
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LIFT VS WEIGHT
LIFT
Generated by airflow over the aircraft body.
Wings of aircraft produces most of the aircraft’s lift.
To be able to take off / go airborne / pitch up, LIFT > WEIGHT.

7. LIFT VS WEIGHT WEIGHT Opposite reaction of LIFT.
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LIFT VS WEIGHT
WEIGHT
Opposite reaction of LIFT.
It is a vertical downward force towards the ground.
Includes mass of aircraft, fuel, cargo or any loads on aircraft.
Weight distribution in aircraft is not constant. i.e. Passengers walking about, fuel consumption.

8. HOW IS THIS APPLICABLE TO FLIGHT?
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BERNOULLI'S PRINCIPLE...
David Bernoulli is a Swiss scientist in the 18th century. He discovered that as the velocity of fluid increases, its pressure decreases.
Venturi Tube
As the fluid enters the narrow
part of the tube, it speeds up.
This decreases the fluid pressure.
As the molecules speed up, it uses
more energy, thus less energy is
used to exert pressure. 1 1 2
The speed of the fluid decreases as it
exits the narrow part of the tube. There
is more energy to exert pressure as
less energy is used for the speed of
the molecules. 2
HOW IS THIS APPLICABLE TO FLIGHT?

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THE AIRFOIL SHAPE...
In most aircraft, you will see this shape if seen from the wing tip. There’s a lot of different types of airfoil shape, which is designed for the aircraft, depending on its role.
To facilitate better understanding of how aircraft wings generate lift for the whole aircraft, we will look at the most common design of airfoil.

10. Cross-section of the wing…
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Cross-section of the wing…
For easier understanding, Bernoulli’s principle will be used to explain how airfoil shape generates lift.

11. HOW A PLANE IS CONTROLLED...
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HOW A PLANE IS CONTROLLED...
Movements of an aircraft plays along 3 axis. Different movements about or along different axis will bring about different kind of movements.
AXIS
MOVEMENT
Longitudinal
Roll
Lateral
Pitch
Vertical
Yaw
From the above, we get 3 different kinds of moments:
Rolling, Pitching and Yawing moments.

12. Air particles accelerate as it goes along the upper camber, due
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How LIFT is produced…
Airflow is separated. 1 2
Air particles accelerate as it
goes along the upper camber, due
to the longer distance it has to travel
as compared to that of lower camber.
As the velocity of air increases,
the pressure against the upper
camber decreases. 2 4 1 3
Since pressure against the upper
camber is more than that against the
lower camber, the wing is being
pushed upwards by the higher
pressure. 4
Velocity of air remains about the
same as before. Thus, pressure
exerted against the lower camber
would be greater then the pressure
exerted on the upper camber. 3

13. Rolling Moment OUTCOME?
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Rolling Moment
When the starboard (right side of aircraft) aileron is up:
- Port (left side of aircraft) aileron will move downwards.
- Greater pressure at the bottom of port wing compared to top; pushing the port wing upwards.
- Greater pressure on top of starboard wing compared to bottom; pushing the starboard wing downwards.
OUTCOME?
Aircraft will roll to the right.
It is a rotation about the longitudinal axis
Controlled by ailerons

14. Pitching Moment OUTCOME? When the elevator is up:
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Pitching Moment
When the elevator is up:
- Pressure on top of the tailplane is greater as compared to the bottom.
- This pushes the tailplane down.
OUTCOME?
Aircraft will pitch up.
It is a rotation about the lateral axis
Controlled by elevator

15. Yawing Moment OUTCOME? It is a rotation about the vertical axis
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Yawing Moment
It is a rotation about the vertical axis
Controlled by rudder
When the rudder is moved to the left:
- Pressure on the left of the vertical stabiliser is more as compared to the right.
OUTCOME?
Aircraft will yaw to the right.

16. RESEARCH ACTIVITY Find out how these flight controls work: Elevons
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RESEARCH ACTIVITY
Find out how these flight controls work:
Elevons
Ruddervator
Spoilerons
Flaperons

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