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