Bernoulli's principle states that as the speed of a fluid increases, the pressure decreases. Airfoils are shaped to take advantage of this by making the air flow faster over the top surface, creating lower pressure and lift. As angle of attack increases, lift increases but drag also increases. Parasite drag is caused by the shape of the aircraft while induced drag is caused by the parts that produce lift, like wingtips. Streamlining the aircraft reduces drag to maximize flight distance or efficiency.

1. Theory of Flight
6.05 Lift and Drag
References:
Transport Canada Study and Reference Guide Glider Pilot
FTGU pages 91-98

2. Review
1. Name the 4 forces acting on an aircraft.
2. What are Newton’s 3 laws?
3. Is an aircraft considered to be in a state of
equilibrium when it is on the ground.

3. 6.05 Lift & Drag
• MTPs:
– Bernoulli’s Principle- how lift is created
– Airfoil terms
– Angle of attack versus lift
– Angle of Incidence
– Types of airfoils
– Types of Drag
– Wingtip Vortices
– Streamlining

4. Bernoulli’s Principle
• Bernoulli’s Principle
– An increase in the speed of a fluid occurs
simultaneously with a decrease in pressure or
a decrease in the fluid's potential energy.

5. Bernoulli’s Principle
Speed Pressure
Speed Pressure

6. Bernoulli’s Principle
– Airfoils are shaped in a way to manipulate air
pressure based on Bernoulli’s principle.
– Air moves faster over the upper surface of the wing,
decreasing pressure (or increasing suction)
– Air moves slower over the lower surface of the wing
and creates a higher pressure, or pushing force.

7. Airfoil Terms
Terms related to airfoils
• Relative Airflow
• Angle of Attack
• Centre of Pressure

8. Relative Airflow
– The direction of the airflow with respect to the wing.
– It is created by the movement of the airplane as well as the
movement of air.
– Relative airflow is usually opposite to the flight path.
– Relative airflow itself is not related to lift

9. Angle of Attack
–The angle at which the airfoil meets the
relative airflow

10. Centre of Pressure
• Is the average of the pressure on an airfoil
– Sum of the distributed pressures on the airfoil acting
through a single straight line
• Moves forward on an airfoil as the angle of attack is
increased

11. Centre of Pressure

12. A of A versus Lift
• Now we know how we get lift but how much can we get?
• One factor affecting lift is Angle of Attack
– As the angle of attack increases, what happens to lift?
– BUT… As the angle of attack increases, what
happens to drag?
Lift Increases
Drag Increases

13. A of A versus Lift
Angle of
Attack
Lift/
Drag

14. Angle of Incidence
– The angle of incidence is the angle between
the mean aerodynamic chord of the wing and
the longitudinal axis of the aircraft
– The angle of incidence impacts forward
visibility, take-off and landing characteristics,
and drag in level flight

15. Types of Airfoils
• Airfoil
A two-dimensional surface designed to obtain a reaction from
the air through which it moves.
The shape of an airfoil will affect how air flows over the wing
Airfoil – 2D vs. Wing – 3D

16. Types of Airfoils
• What are some functions
of airfoils?
– Create lift
– Create thrust
– Attachment point for the
control surfaces (including
flaps, slots, slats, etc)
– House fuel tanks and
instrument components
–Stability

17. Confirmation Check

18. Confirmation
1. Label where the high pressure and low pressure are:
2. Fill in the blanks
__________________: The angle at which the airfoil meets the
relative airflow
__________________: airflow with respect to the wing
__________________:the average of the pressure on an airfoil

19. Confirmation
3. Lift acts 90o to the ________________.
4. Bernoulli’s principle states that:
An ____________ (increase/decrease) in the speed of a fluid occurs
simultaneously with a _____________________
(increase/decrease) in pressure or a _______________
(increase/decrease) in the fluid's potential energy
5. As lift increases, drag _______________.

20. Drag
– Is the resistance an airplane experiences in
moving forward through the air

21. Types of Drag
There are 2 main types of Drag:
1. Parasite Drag– This drag is further broken
down
– Form drag
– Skin Friction
2. Induced Drag

22. Parasite Drag
Drag of all those parts the airplane which
DO NOT contribute to lift
– This drag is hard to eliminate but it can be
minimized
• Form drag is caused by the shape of the
aircraft
– Landing gear
– Antennas
– Struts
– Wing tip fuel tanks

23. Parasite Drag
• Skin Friction
– The tendency of air
flowing over the body
to cling to its surface
– What causes it to
resist motion?
• Ice on the wings
• Dirt build up

24. Induced Drag
– Caused by those parts of an airplane which
are active in producing lift
– Cannot be eliminated
Lift Drag

25. Induced Drag
• Induced drag can be reduced with a high
aspect ratio

26. Wing Tip Vortices
• Over the top of the wing the
airflow goes inward
• Under the bottom of the wing
the airflow moves outward
Due to the
pressure changes
This creates a swirling motion off the wing tip called a vortex

27. Streamlining
• The less drag you have…
– Flying a glider: the further you can
fly
– Flying an airplane: the less fuel you
use
• Therefore streamlining is important
– A design device by which a body is
shaped to minimize drag

28. Confirmation
1. Fill in the diagram
2. Is it possible to eliminate induced drag? Why
or why not?

29. Wingtip Vortices