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Theory of Flight
6.05 Lift and DragReferences: Transport Canada Study and Reference Guide Glider Pilot FTGU pages 91-98
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.
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
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.
Bernoulli’s Principle
Speed Pressure
Speed Pressure
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.
Airfoil Terms
Terms related to airfoils• Relative Airflow• Angle of Attack• Centre of Pressure
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
Angle of Attack
– The angle at which the airfoil meets the relative airflow
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
Centre of Pressure
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
A of A versus Lift
Angle of Attack
Lift/ Drag
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
Types of Airfoils
• AirfoilA 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 Airfoil – 2D vs. vs. Wing – 3DWing – 3D
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
Confirmation Check
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
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 _______________.
Drag
– Is the resistance an airplane experiences in moving forward through the air
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
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
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
Induced Drag
– Caused by those parts of an airplane which are active in producing lift
– Cannot be eliminated
Lift Drag
Induced Drag
• Induced drag can be reduced with a high aspect ratio
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 vortexThis creates a swirling motion off the wing tip called a vortex
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
Confirmation
1. Fill in the diagram
2. Is it possible to eliminate induced drag? Why or why not?
Wingtip Vortices