1. Principles of Flight Spitfire

2. Chapter 1 – Weight and Lift How is an aircraft, which is much heavier than the air it flies in, supported by the air? C130

3. Sir Isaac Newton formulated laws to explain the movement of objects. Sir Isaac Newton formulated laws to explain the movement of objects. Newton’s Third Law states that ‘ to every action there is an equal and opposite reaction’. Newton’s Third Law states that ‘ to every action there is an equal and opposite reaction’. For example: Imagine a car weighing 10,000N parked on the road. The cars weight presses down on the road with a force of 10,000N – and from Newton’s third law the road must be supporting the car and pushing up with an equal and opposite force of 10,000N. For example: Imagine a car weighing 10,000N parked on the road. The cars weight presses down on the road with a force of 10,000N – and from Newton’s third law the road must be supporting the car and pushing up with an equal and opposite force of 10,000N.

4. How does this apply to an aircraft flying? How does this apply to an aircraft flying? How can an aircraft which might weigh many tonnes be supported by such a flimsy substance such as air? How can an aircraft which might weigh many tonnes be supported by such a flimsy substance such as air? One clue is that an aircraft can only stay airborne as long as it is moving. (except a Harrier of course!) One clue is that an aircraft can only stay airborne as long as it is moving. (except a Harrier of course!)

5. Air is a substance Moving air can exert forces on objects – e.g. in hurricanes The greater the speed, the greater the force it exerts. Conclusion: Air is undoubtedly a very real substance, invisible as it is!

6. Moving objects through air Is there a force if an object is moving through stationary air? Is there a force if an object is moving through stationary air? Imagine cycling along a flat road on a perfectly calm day. As your speed increases, you will begin to feel the air on your face. The faster you pedal the greater the force of this apparent wind. Imagine cycling along a flat road on a perfectly calm day. As your speed increases, you will begin to feel the air on your face. The faster you pedal the greater the force of this apparent wind. Whether you have air moving past a stationary object or an object moving through stationary air, similar forces are experienced. Whether you have air moving past a stationary object or an object moving through stationary air, similar forces are experienced.

7. Every object has weight. Every object has weight. The weight of an aircraft must be supported or lifted by a force. The weight of an aircraft must be supported or lifted by a force. How is this lifting force to be given to an aircraft? How is this lifting force to be given to an aircraft?

8. Try this: Hold two sheets of paper with the edges vertical and about finger width apart. What will happen if you blow between the two pieces of paper?

9. What causes this to happen? Wind Tunnel Test Wind Tunnel Test The air goes in at point A, passes the constriction at point B, and comes out at point C. The air goes in at point A, passes the constriction at point B, and comes out at point C. What are the speeds and pressures at points A, B and C? What are the speeds and pressures at points A, B and C?

10. Bernoulli’s Principle The same amount of air enters B as is leaving, therefore the speed of air at B must increase to pass through the narrowest part of the tube. The increased speed of the air at B, creates a streamlined flow (non-turbulent) which exerts less pressure on the tube. Conclusion: Air moving a increased speeds exerts less pressure or creates an area of lower pressure.

11. Can you now explain why the two pieces of paper move together when you blow air between them?

12. Proving the theory Try this: Hold a half sheet of A4 (halved lengthways) such that the end between you fingers is horizontal, but allow the rest of the paper to curve downwards under its own weight. Then blow along the curved top of the paper only. What happens?

13. Observation: The paper rises in line with the air flow. By speeding up the air over top you have reduced the pressure above the paper, so the air pressure under the paper pushes it up. What happens when you blow harder? The harder you blow the more it rises.

14. Lift The top surface of an aircraft’s wing is shaped such that air which flows between it and the undisturbed air a little way above the wing is, in effect, being forced through a constrictions. The air flows over the wing at an increased speed-and therefore at a reduced pressure-compared with the surrounding atmosphere

15. The resulting pressure difference between the air above and below the wing tends to lift it up. The resulting pressure difference between the air above and below the wing tends to lift it up. This is not the only thing that contributes to lift. In real life, the airflow rarely approaches a wing as shown above. This is not the only thing that contributes to lift. In real life, the airflow rarely approaches a wing as shown above. In most flight conditions, the wing is inclined to the airflow at a slight positive angle so that it deflects some of the airflow downwards. In most flight conditions, the wing is inclined to the airflow at a slight positive angle so that it deflects some of the airflow downwards. This results in an upward force, though not normally as much as the lift generated by the top surface of the wing This results in an upward force, though not normally as much as the lift generated by the top surface of the wing

16. Distribution of lift There are pressure forces acting all over the wing- and there can be lift forces all over the wing There are pressure forces acting all over the wing- and there can be lift forces all over the wing The length of each arrow indicates the amount of lift at that point on the wing’s surface. The length of each arrow indicates the amount of lift at that point on the wing’s surface. Pressure patterns near A wing

17. Note that: Note that: Lift is not distributed evenly around the wing Lift is not distributed evenly around the wing The top surface generates more lift than the bottom surface (sometimes as much as 80% of the total!) The top surface generates more lift than the bottom surface (sometimes as much as 80% of the total!) The greatest amount of lift on the top surface occurs where the surface is curved the most. The greatest amount of lift on the top surface occurs where the surface is curved the most. The greatest effect, on both top and bottom surfaces, is nearer the front edge of the wing than the rear (1/3 of the way from the front) The greatest effect, on both top and bottom surfaces, is nearer the front edge of the wing than the rear (1/3 of the way from the front) All lift forces act at 90° to the direction of the airflow. All lift forces act at 90° to the direction of the airflow.

18. Centre of Pressure We normally add all the forces together and represent them by a single straight line. We normally add all the forces together and represent them by a single straight line. This force is drawn from the point at which all the forces balance. This force is drawn from the point at which all the forces balance. This is the point which all the lift can be said to act and it is called the ‘centre of pressure’. This is the point which all the lift can be said to act and it is called the ‘centre of pressure’.

19. How Lift Varies Several factors affect the amount of lift produced by a wing Several factors affect the amount of lift produced by a wing A) Air speed – greater airspeed creates greater lift (doubling the airspeed, quadruples the lift!) A) Air speed – greater airspeed creates greater lift (doubling the airspeed, quadruples the lift!)

20. B) Angle of attack- This is the angle between the chord line of the wing and the oncoming air. A pilot can alter this by altering the pitch attitude of the aircraft- by easing forward or pulling back on the controls. The lift will increase until the angle reaches about 15°. Beyond this point, the lift rapidly decreases and the wing has stalled

22. C) Air Density – If air becomes thinner or less dense ( at increased height temperature or humidity), the amount of lift is reduced. To calculate the take off run the pilot would use tables which allow for engine performance, weight, wind speed and direction, and of course the air temperature, humidity and density. C) Air Density – If air becomes thinner or less dense ( at increased height temperature or humidity), the amount of lift is reduced. To calculate the take off run the pilot would use tables which allow for engine performance, weight, wind speed and direction, and of course the air temperature, humidity and density.

23. D) Wing shape and area- The shape of the wing section and the plan area of the wing are calculated by the aircraft designer to suit the aircraft’s role and required performance. A high-lift section would be used where good lift at low speeds is all- important

24. General Purpose section would suit a moderately fast aircraft, not intended to approach the speed of sound. More lift for any given wing section and airspeed can be found by increasing the plan area of the wing- that is, by making the wing bigger.

25. Unfortunately a bigger wing is heavier and also more resistant to rapid manoeuvres- not acceptable for air combat aircraft. Hinged flaps fitted to most light aircraft have the effect of increasing the camber of the wing when they are lowered. More advanced Fowler flaps slide backwards to increase the wing area, as well as hinging downwards

26. Lift and Weight in Straight and Level Flight The lift force equals the force of gravity acting on the aircraft (its weight) If the lift is greater than the weight, an aircraft will climb If the weight is more than the lift, it will descend.

27. Questions