Principles of FlightLesson 1 – Lift, Weight, Thrust,

Drag

Principles of FlightLift and Weight

Lift

• How do objects support themselves?– Newton’s Third Law

“Every action has an equal and opposite reaction”

But what does that mean?

• If you push against something, it pushes back with the same force!

• It’s how the ground supports you• If a 10 tonne ship pushes down on the

water, the water pushes back with 10 tonnes of force – the boat floats!

• If it can work in water, why can’t it work in the air?

• Air and water are both fluids, air is just very thin!

So how do we create lift in air?

• If you put your hand out of your car window the air will try and move it back

• If you increase the speed of the car, your hand will move back more

• So….the faster something moves through the air, the more force the air exerts on it

• Now, the complicated maths bit!

Ok, that’s lots of equations but so what?

• Basically all these things come together to explain how lift is created

• The main thing to remember is:

Faster moving air gives a lower pressure

So how does this relate to flying

• By placing an aerofoil section in an airflow, the speed of the air can be changed

This is what it looks like in real life

How is lift distributed?

• Lift is not the same all the way across the wing

How is lift distributed?

• The lift is assumed to act at one point along the wing known as the Centre of Pressure

Things that affect lift 1 – Angle of Attack

• The angle of the wing relative to the airflow

Things that affect lift 1 – Angle of Attack

• Lift increase with AofA until it stalls

Things that affect lift 2 - Airspeed

• Lift can be calculated using a simple equation:– Lift = ½ ρ V2 A CL

• The important thing in this equation is that the lift is related to the square of the speed (V)

Things that affect lift 3 – Air Density

• Looking at the lift equation another thing that can affect the lift is the air density– Lift = ½ ρ V2 A CL

• The higher the air density the greater the lift

Things that affect lift 4 – Wing Area

• The final part of the lift equation that can affect the lift is the wing area– Lift = ½ ρ V2 A CL

• The larger the wing area, the greater the lift

Other things that affect the wing shape

• Wings are designed for their application

• One design criteria used is the aspect ratio

• Aspect ratio = wing span2 / wing area• High aspect ratios give good lift• Low aspect ratios give good roll rate

Examples of aspect ratios

Wing cross sections

• Different aerofoils perform better at different speeds

But what happens at low speeds

• High speed wings don’t work well at low speeds

• The low speed performance is improved by using flaps

So what is the enemy of lift?

• Weight!• Aircraft are made as light as possible

to reduce the amount of lift required

Lift and weight acting on an aircraft

Principles of FlightThrust and Drag

How do we get the air to move over the wing?

• We need thrust!• How do we get thrust?

What is the enemy of thrust?

• Drag!• Different shapes produce different

amounts of drag• Try putting your hand out of the car

window• Your hand gives more resistance when

your palm is flat to the airflow rather than side on

Drag

How can you reduce drag?

• You can streamline the object!

What’s a good streamlined shape?

• How streamlined a shape is can be defined by its fineness ratio

• This is the ratio of the width of the object to the length

Drag

• The drag caused by the shape of an object can be worked out in the same way as the lift– Drag = ½ ρ V2 A CD

• This means that all the same affects are present as for lift!

Thrust and drag acting on an aircraft

All the forces acting on the aircraft