The Fly Higher Tutorial IV THE SCIENCE OF FLIGHT
In order for an aircraft to fly we must have two things: 1) Thrust 2) Lift Aerodynamics The Basics Representation of the balance of forces These act against the two opposing forces which slow the aircraft and keep it on the ground, or pull it downward, towards the ground: 1) Drag 2) Weight
What can you remember? You should have met at least some of these terms before. Can you remember what they are? Define them in your own words and remind yourself of any relevant science: Weight Lift Thrust Drag These are all FORCES. Can you quote the formal definition of a force? How is it measured? What is the difference between weight and mass?
Newton s Three Laws of Motion Can you remember them? Much of the next section rests on the work of Sir Isaac Newton and his Three Laws of Motion. These are very important to Physics. Can you remember what they are? Sir Isaac Newton (1643 1727)
Newton s Three Laws of Motion The first law: An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a net or unbalanced force The second law: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object or force = mass x acceleration (f = ma) The third law: For every action, there is an equal and opposite reaction. 3 rd Law in action
Lift and Drag LIFT is the aerodynamic force perpendicular to the direction of the airflow. It is the presence of lift that raises an aircraft off the ground. DRAG is the aerodynamic force parallel to the direction of the airflow. Drag is the enemy of flight and must be overcome for any aircraft to fly.
Thrust and Weight THRUST is required to move the vehicle forward (and is provided by the engines). This must not only be greater than the air resistance or DRAG, but powerful enough to move the aeroplane at sufficient speed for the wings to create the necessary LIFT. WEIGHT is the total weight of the aircraft including all passengers, crew, fuel and cargo. The force, LIFT, must overcome the opposing force, WEIGHT, if the aeroplane is to fly.
The Basics In slide 2, you saw the force diagram of an aircraft in the air. Slide 2 Force Diagram Draw a similar force diagram for an aircraft: a) standing, motionless, on the tarmac b) Slowly taxi-ing, without lift, across the airport.
The creation of Lift As the air flows passed the wing, the pressure on the top surface of the wing is reduced whilst the pressure on the bottom surface is increased LOW PRESSURE HIGH PRESSURE
The creation of Lift LOW PRESSURE HIGH PRESSURE This pressure difference results in a net force pushing the wing both upwards and backwards. The upward force always acts at right angles to the direction of the airflow hence LIFT!
What about DRAG? DRAG is the resisting, backward force experienced by the aircraft as it pushes against the air, moving it out of the way of the aircraft s direction of travel. There is also additional drag caused by the friction between air and the skin of the aircraft. Jet power to move us forward!
The shape of the wing The Aerofoil The shape and size of the wing is crucially important! The basic shape is called an AEROFOIL. Varying the exact aerofoil shape (so adjusting the thickness, camber and chord) will achieve different results. Arguably, the Wright Brothers real achievement was to find the wing shape that worked for the weight and speed of their aircraft, and to control her by using flaps that adjusted that wing shape for different phases of the flight.
The Coefficient of Lift Every wing shape can be tested in a wind tunnel and its lift efficiency measured by a Coefficient of Lift. The Lift achieved can then be calculated from a formula that also takes account of the wing size and the speed of the aircraft. L = Lift V = Velocity (m/s) S = Wing area (m²) ρ = Density of air = 1.225 kg/m³ C L = Coefficient of Lift L = ½ ρ V 2 S C L
Other factors Note that Lift depends very significantly upon the VELOCITY (V 2 in the formula) with which the aircraft can move. This is all the more important before take-off when the aircraft is travelling on the ground and faces an additional resisting force. What would this force be? Do you know how it is calculated? As lift depends so significantly upon speed, and the weight (in part) upon fuel and the engines themselves, ENGINE TECHNOLOGY is of vital importance to the aeronautics industry.
Other factors Reducing the opposing forces most obviously WEIGHT is hugely important, too. So although the MATERIALS with which the aircraft is built must be extremely strong, they must not be heavy. Light-weight but superstrong materials (such as Aluminium Alloys) have been crucial to the development of modern aircraft.
: Air Density Notice the LIFT formula included something you had probably NOT thought of before: AIR DENSITY. What do you think this means? Air density is symbolised in the formula by Greek letter rho, ρ In almost all of Europe at sea-level ρ = 1.225 kg/m³. Elsewhere it varies due the physical height of the land. Discussion: Would it be easier or more difficult for an aircraft to take off from a highaltitude airport such as Daocheng Airport in China (at 648m or 14,472 ft.) than from a sea-level one, such as Schiphol in the Netherlands?
How ELSE can we increase LIFT? AIRFLOW 6 10 16 We can also gain more LIFT from a wing by altering the angle at which it is struck by the oncoming airflow. We call this the AoA Angle of Attack.
Increasing LIFT Here is a plot of LIFT generated by a wing at ever increasing angles of attack (AoA s). You can see from the blue line that LIFT increases proportionately to the AoA until, suddenly, it falls away. This has major implications for an aircraft s take-off. Why?
Angle of Attack But, if you remember, there are TWO forces created by the aerofoil as it passes through the air. What was the other? What do you think will be its effect as the Angle of Attack is increased?
Angle of Attack DRAG! The enemy of flight! If we create too much drag by raising the AoA, we create new problems. We can see this by examining experiment charts of DRAG generated at varying AoAs
Increasing DRAG Here is another plot, of DRAG generated by a wing at ever increasing angles of attack. You can see from the red line that DRAG grows exponentially as the AoA increases.
Too much DRAG? Putting these two plots together shows that at some point the DRAG will overcome the LIFT at a certain angle of attack. This is called the STALLING POINT. What do think will happen to an aircraft that reaches this point?
All in the Design The basic aerofoil shape can be seen in the construction of a wing. Although the central part of the wing is fixed the trailing and leading edges move. Why?
All in the Design 1. 2. 3. The moving parts are fixed onto the wing in order to help manoeuvre the aircraft by altering the Aerofoil shape of the wing, as shown. Can you tell which shape is for landing, for cruise and for take-off? Can you suggest why they are the shape they are?
Manoeuvring the Aircraft It is the combination of these control surfaces that enable the aircraft to change direction through the air. There the three ways an aircraft moves to alter its position Roll this is where the aircraft can roll around the central axis of its body in the air. Pitch This is where the aircraft angles itself up or down to rise and fall in the air, this also affects the AoA. Yaw This allows the aircraft o move left and right in a horizontal axis only
Manoeuvring the Aircraft Roll this is controlled at the end trailing (back) surfaces of the wings and can move up and down in opposite direction to roll the aircraft. Pitch Controlled by the Tail area it angles moves up and down (again on the trailing edge) Yaw This is the steering (left or right) and is a horizontal surface at the back of the tail section.
Manoeuvring the Aircraft What part of the aircraft moves which axis of the aircraft? ROLL = Ailerons Move up or down in opposite directions YAW = Rudder movement, left or right PITCH = Elevators move up or down together
And FINALLY... Aeronautics is a complex but fascinating study. This presentation has only just touched upon the topic. Nonetheless, the basic principles outlined here apply to all fixed-wing aircraft and are the bedrock underpinning The Science of Flight. And those basic principles are, as we have seen, to be found in the science you are studying at school. We hope you will want to find out more and research further on the internet there is a host of information available at many different levels of scientific understanding. Perhaps, one day YOU will be an aeronautics engineer!
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