PRINCIPLES OF FLIGHT Click on ‘Slide Show’ then ‘View Show’ to start.

PRINCIPLES OF FLIGHT Contents List. Chapter 1 Lift and Weight Chapter 2Thrust and Drag Chapter 3Stability and Control Chapter 4Stalling Chapter 5Gliding Chapter 6Helicopters exit

PRINCIPLES OF FLIGHT Chapter 1 Lift and Weight Chapter 2Thrust and Drag Chapter 3Stability and Control Chapter 4Stalling Chapter 5Gliding Chapter 6Helicopters exit Contents List. Click on a chapter.

PRINCIPLES OF FLIGHT Chapter 1 Lift and Weight exit Return to contents list

Lift and Weight How can an aircraft, which is so much heavier than air, be supported by that air? Sir Isaac Newton’s third law states: “To every action, there is an equal and opposite reaction.” A vehicle weighing 1 tonne is supported by the road pressing up with a force of 1 tonne. A boat weighing 10 tonnes is supported by a 10 tonne upward force from the water, otherwise it would sink.

Lift and Weight How can an aircraft, which is so much heavier than air, be supported by that air? Sir Isaac Newton’s third law states: “To every action, there is an equal and opposite reaction.” Aircraft can only remain airborne while they are actually moving – if they stop moving they cease flying. The upward force to support the aircraft is only there when the aircraft is moving forwards through the air.

Lift and Weight How can an aircraft, which is so much heavier than air, be supported by that air? Sir Isaac Newton’s third law states: “To every action, there is an equal and opposite reaction.” This upward force is generated by the aircraft’s wings as they pass through the air. We must understand how wings moving through the air create that upward force, known as ‘lift’.

Lift and Weight In this wind tunnel experiment the air is passing from ‘A’ to ‘C’ through a constriction at ‘B’. As the amount of air leaving at ‘C’ is the same as the amount entering at ‘A’, the air must speed up as it passes the narrowest point ‘B’.

Lift and Weight Another scientist, Bernoulli, discovered that in areas where airspeed increases, the air pressure decreases. In the wind tunnel experiment above, the air pressure measured at ‘B’ will be less than at ‘A’ and ‘C’.

Lift and Weight A simple experiment you can do to demonstrate this principle is illustrated in this picture. Blowing along the top of this sheet of paper causes it to lift into line with the airflow. By blowing and speeding up the air over the top, you have reduced the pressure above the paper, so the normal air pressure below the paper pushes it up.

Lift and Weight The top surface of a wing is shaped so that the air which flows between it and the undisturbed air a little way above the wing is effectively flowing through a constriction. The air flows over the wing at increased speed and therefore at reduced pressure. undisturbed air

Lift and Weight All parts of a wing generate lift, some parts more than others. oncoming air lift forces

Lift and Weight The top surface normally generates more lift than the bottom surface. oncoming air lift forces

Lift and Weight The top surface may generate up to 80% of the total lift. oncoming air lift forces

Lift and Weight The greatest amount of lift on the top surface occurs where the surface is curved the most. oncoming air lift forces

Lift and Weight All of the lift forces act at 90 degrees to the oncoming airflow. oncoming air lift forces

Lift and Weight Instead of looking at the thousands of lift forces generated by a wing, we add them together and represent them with a single line. oncoming air lift forces

Lift and Weight Instead of looking at the thousands of lift forces generated by a wing, we add them together and represent them with a single line. oncoming air

Lift and Weight oncoming air The point at which all the lift can be said to act is the ‘centre of pressure’.

Lift and Weight The point at which all the lift can be said to act is the ‘centre of pressure’. The idea is similar to finding the centre of gravity of a ruler by balancing it on your finger. All the small forces of gravity acting on the ruler balance about the centre of gravity.

Lift and Weight Several factors affect the amount of lift produced by a wing. AirspeedUnsurprisingly, increasing the airspeed increases the amount of lift produced by a wing. What might surprise you is that doubling the airspeed gives four times the lift. Trebling the airspeed gives nine times the lift!

Lift and Weight Several factors affect the amount of lift produced by a wing. Airspeed Angle of attack The angle of attack is the angle between the chord line of the wing and the oncoming air (or path of the aircraft). oncoming air

Lift and Weight Several factors affect the amount of lift produced by a wing. Airspeed Angle of attack The chord line of a wing is a straight line joining the leading edge to the trailing edge. oncoming air

Lift and Weight Several factors affect the amount of lift produced by a wing. Airspeed Angle of attack If the angle of attack is increased the amount of lift is also increased oncoming air - until the angle reaches about 15 degrees when the wing stalls.

Lift and Weight Several factors affect the amount of lift produced by a wing. Airspeed Angle of attack Air density If the air becomes thinner, or less dense (due to increases in altitude, temperature or humidity) the amount of lift is reduced.

Lift and Weight Several factors affect the amount of lift produced by a wing. Airspeed Angle of attack Air density Designers choose a wing shape and area to suit the role of the aircraft. Wing shape and area

Lift and Weight Several factors affect the amount of lift produced by a wing. Airspeed Angle of attack Air density Wing ‘X’ is a general purpose wing section.Wing shape and area

Lift and Weight Several factors affect the amount of lift produced by a wing. Airspeed Angle of attack Air density Wing ‘Y’ is a high lift wing section.Wing shape and area

Lift and Weight Several factors affect the amount of lift produced by a wing. Airspeed Angle of attack Air density Sections ‘Z’ and ‘W’ are for high speed.Wing shape and area

Lift and Weight If the lift force is greater than the aircraft’s weight, the aircraft will climb.

Lift and Weight If an aircraft suffers a sudden reduction in weight by jettisoning fuel or dropping cargo, it will climb (unless the pilot responds).

Lift and Weight If the lift force is less than the aircraft’s weight, the aircraft will descend.

Lift and Weight In steady straight and level flight the lift force exactly equals the force of gravity acting on the aircraft (its weight).

PRINCIPLES OF FLIGHT Chapter 2 Thrust and Drag exit Return to contents list

Thrust and Drag An aircraft is propelled forwards by its engine or engines. This force is called thrust.

Thrust and Drag An aircraft flying through the air encounters resistance from the air itself. This force is called drag.

Thrust and Drag Every part of the aircraft over which the air flows produces drag which resists forward motion.

Thrust and Drag If thrust exceeds drag the aircraft will accelerate.

Thrust and Drag If thrust exceeds drag the aircraft will accelerate. If drag exceeds thrust the aircraft will decelerate.

Thrust and Drag If thrust exceeds drag the aircraft will accelerate. If drag exceeds thrust the aircraft will decelerate. An aircraft in steady straight and level flight will have thrust exactly equal to the drag.

Thrust and Drag As an aircraft’s airspeed increases, then so does the drag. In the previous chapter we saw lift increase four times for double the airspeed and increase nine times for treble the airspeed. Doubling an aircraft’s airspeed will quadruple the drag. Trebling an aircraft’s airspeed will increase drag by a factor of nine.

PRINCIPLES OF FLIGHT Chapter 3 Stability and Control exit Return to contents list

Stability and Control There are three axes about which an aircraft rotates. All three go through the centre of gravity. centre of gravity

Stability and Control The lateral axis generally runs from wingtip to wingtip. Remember: L A T – Links Aerofoil Tips.

Stability and Control The lateral axis generally runs from wingtip to wingtip. The aircraft moves in the pitching plane about this axis.

Stability and Control The longitudinal axis runs from nose to tail. Remember: aLONG the length of the aircraft.

Stability and Control The longitudinal axis runs from nose to tail. The aircraft moves in the rolling plane about this axis.

Stability and Control The normal axis runs vertically downwards. The aircraft moves in the yawing plane about this axis.

Stability and Control An aircraft flying along in steady straight and level flight will regularly be disturbed by bumpy air (turbulence). This will cause a wing to drop or the nose of the aircraft to rise or fall. A well-designed aircraft will tend to go back to level flight of its own accord without the pilot having to make adjustments. This property is called stability.

Stability and Control The arrows in the diagram below illustrate an aircraft moving in the pitching plane. If the nose pitches up the tail goes down. The tailplane will now be at a positive angle of attack to the oncoming airflow and create lift. The aircraft returns to straight and level flight.

Stability and Control The arrows in the diagram below illustrate an aircraft moving in the pitching plane. If the nose pitches down the tail goes up. The tailplane will now be at a negative angle of attack to the oncoming airflow creating ‘downwards’ lift. The aircraft again returns to straight and level flight.

Stability and Control The arrows in the diagram below illustrate an aircraft moving in the pitching plane. Let us examine lateral stability in a little more detail.

Stability and Control In this illustration, the aircraft pitches nose-up in turbulence.

Stability and Control The tailplane now has a ‘positive angle of attack’ to the oncoming airflow, generating lift. oncoming air

Stability and Control The lift generated returns the aircraft to straight and level flight.

Stability and Control In this illustration, the aircraft pitches nose-down in turbulence.

Stability and Control The tailplane now has a ‘negative angle of attack’ to the oncoming airflow, generating downwards lift. oncoming air

Stability and Control The ‘downwards’ lift generated returns the aircraft to straight and level flight.

Stability and Control The arrows in the diagram below illustrate an aircraft moving in the rolling plane. Most aircraft are designed with the wings set into the fuselage at a slight upward angle. This angle is called dihedral.

Stability and Control The arrows in the diagram below illustrate an aircraft moving in the rolling plane. If a wing drops the aircraft starts to slip towards the lower wing. This increases the amount of lift produced by that wing. Part of the higher wing is shielded by the fuselage and produces less lift.

Stability and Control The arrows in the diagram below illustrate an aircraft moving in the rolling plane. With the lower wing producing more lift than the upper wing, the aircraft returns to straight and level flight. Stability in the rolling plane is provided by wing dihedral.

Stability and Control The arrows in the diagrams below illustrate types of dihedral on different aircraft.

Stability and Control High wing aircraft have a natural stability due to the pendulum effect of the centre of gravity being below the wing. Swept-back wings also have a natural stability and in some cases the designer has had to take steps to reduce this stability. Anhedral reduces stability in the rolling plane. anhedral

Stability and Control Stability in the yawing plane, sometimes known as ‘directional stability’ is provided by the aircraft’s fin.

Stability and Control If an aircraft is made to yaw to one side by an air disturbance,

Stability and Control If an aircraft is made to yaw to one side by an air disturbance, the airflow will produce a sideways force on the fin which will yaw the aircraft back onto its original heading. oncoming air ‘weathercock’ effect

Stability and Control the airflow will produce a sideways force on the fin which will yaw the aircraft back onto its original heading. ‘weathercock’ effect

Stability and Control ‘weathercock’ effect the airflow will produce a sideways force on the fin which will yaw the aircraft back onto its original heading.

Stability and Control Most aircraft have a large fin, placed as far back as possible to increase the weathercock effect and ensure directional stability.

Stability and Control Note that aircraft movements, such as pitching, rolling and yawing, are described relative to the pilot.

Flying Controls The control column and rudder pedals are connected to the flying control surfaces.

Flying Controls elevators The pilot uses elevators to control the aircraft in the pitching plane.

Flying Controls elevators When the pilot pushes the control column forwards, the elevators move downwards. This creates lift, the tail moves up and the nose pitches down.

Flying Controls elevators This creates downwards lift, the tail moves down, the nose pitches up. When the pilot pulls the control column back, the elevators move upwards.

Flying Controls The rudder is connected to the rudder pedals. These are used to control the aircraft in the yawing plane. rudder

Flying Controls When the pilot pushes the left rudder pedal, the rudder moves to the left. rudder This deflects the tail to the right and the nose yaws left.

Flying Controls rudder When the pilot pushes the right rudder pedal, the rudder moves to the right. This deflects the tail to the left and the nose yaws right.

Flying Controls To roll the aircraft, the pilot uses two moveable parts of the wing called ailerons. ailerons By moving the control column left, the left aileron moves up (decreasing lift) and the right aileron moves down (increasing lift). The aircraft rolls to the left.

Flying Controls To roll the aircraft, the pilot uses two moveable parts of the wing called ailerons. ailerons By moving he control column right, the right aileron moves up (decreasing lift) and the left aileron moves down (increasing lift). The aircraft rolls to the right.

Trimming During flight an aircraft’s centre of gravity may change for a number of reasons: when fuel is used, bombs dropped, ammunition fired etc. The centre of pressure will also change, usually with changes of power, speed or attitude. The pilot would have to maintain a constant pressure on the control column or rudder pedals to counter the forces created by these imbalances. Trimming tabs cancel out these unwanted forces on a pilot’s flying controls.

Trimming elevator trimming tabs Elevator trimming tabs prevent the pilot having to maintain a steady forwards or backwards pressure on the stick.

Trimming Rudder trimming tabs prevent the pilot having to maintain a steady pressure on either the left or right rudder pedals. rudder trimming tab

Trimming Aileron trimming tabs prevent the pilot having to maintain a steady left or right pressure on the stick. aileron trim tab

Flaps A wing which flies well at the low speeds required for take- off and landing would be inefficient at high speeds. The usual solution to this is to design a wing for its main task then add flaps for use on approach and landing. There are many types of flap, some of which we will examine, but all flaps increase the effective camber of the wing and therefore increase the lift.

Flaps Simple flap Split flap Fowler flap

Flaps As flaps are lowered, both lift and drag increase.

Flaps 15 degrees of flap improves lift at take-off speeds. A shorter take-off run is possible because the drag penalty of 15 degrees of flap at low speeds is very small.

Flaps 90 degrees of flap gives a tremendous increase in drag. The pilot will lower the nose to maintain approach speed, this increases the approach angle and gives a better forward view.

Slats Slats are another device to improve handling at low speeds. When slats are open, air flows through a slot between the slat and the wing improving the airflow over the wing. slat flap

Slats The turbulence in the airflow over the wing is smoothed and the wing may not stall until 25 degrees angle of attack. Although the stalling speed is very much reduced, opening slats does cause extra drag. slat flap

PRINCIPLES OF FLIGHT Chapter 4 Stalling exit Return to contents list

Stalling To increase the lift produced by a wing the pilot increases the angle of attack.

Stalling To increase the lift produced by a wing the pilot increases the angle of attack. Lift increases with increasing angle of attack up to about 15 degrees.

Stalling To increase the lift produced by a wing the pilot increases the angle of attack. At about 15 degrees angle of attack, the critical angle, the airflow over the top of the wing becomes turbulent.

Stalling To increase the lift produced by a wing the pilot increases the angle of attack. As the airflow over the top of the wing becomes turbulent lift is lost.

Stalling Each particular wing has its own stalling angle and will always stall when the wing reaches that angle. For most conventional aircraft the stalling angle is about 15 degrees angle of attack.

Stalling Although the angle at which a wing stalls does not vary, the airspeed at which it stalls does change. Increasing the aircraft’s weight increases its stalling speed and vice-versa.

Stalling Although the angle at which a wing stalls does not vary, the airspeed at which it stalls does change. Putting an aircraft into a turn will also increase the stalling speed – the steeper the turn, the higher the stalling speed.

PRINCIPLES OF FLIGHT Chapter 5 Gliding exit Return to contents list

Gliding Gliders either have no engine, or do not utilise an engine when gliding. The force of thrust is not part of the equation. The forces acting on a glider in steady balanced flight are drag, weight and lift. Gliders must pitch down and use a part of the ‘weight’ component to replace thrust.

Gliding ‘Gliding Angle’ describes how far a glider can glide from a given height. The Viking’s is about 1 in 35. This means that from a height of one kilometre (3280 ft), in still air, the glider will travel 35 kilometres before landing. A glider travelling with the wind (downwind) will travel a much greater distance than one travelling into wind. For instance, a glider with an airspeed of 35 kts travelling into a 35 kt wind would appear to slowly lose height over one spot.

Gliding Gliders are fitted with airbrakes or speedbrakes, panels which pop out of the wings at 90 degrees to the airflow when selected. Speedbrakes reduce lift and increase drag significantly. They are used when approaching to land, to enable the pilot to land in a smaller area than would otherwise be possible.

PRINCIPLES OF FLIGHT Chapter 6 The Helicopter exit Return to contents list

The Helicopter A helicopter creates lift by spinning aerofoil-shaped blades.

The Helicopter The area swept by the blades or rotors is called the rotor disc.

The Helicopter Lift can be increased by increasing the angle of attack of all of the rotors together.

The Helicopter The rotor disc can be tilted forwards by increasing the angle of attack of the blades as they sweep the rear section of the disc.

The Helicopter The helicopter will then start to move in a forwards direction.

The Helicopter The rotor disc can also be tilted backwards by increasing the angle of attack of the blades as they sweep the front section of the disc.

The Helicopter The helicopter will then start to fly backwards.

The Helicopter The helicopter can also move to the left and right by tilting the rotor disc in the relevant direction.

The Helicopter To alter the pitch of each rotor as it travels through its 360 degree cycle, the pilot uses the cyclic pitch control.

The Helicopter In a helicopter, horizontal flight is achieved by tilting the rotor disc using the cyclic control.

The Helicopter

Spinning a helicopter’s rotor blades in one direction gives rise to a ‘torque reaction’, a force in the opposite direction. The tail rotor of the helicopter counters this ‘torque reaction’ and prevents the helicopter spinning out of control.

The Helicopter The pilot controls the speed of the tail rotor using the rudder pedals. Increasing and decreasing the speed of the tail rotor yaws the helicopter left or right.

The Helicopter The ‘collective pitch control’ changes the angle of attack of all of the rotors together. It is this ‘collective pitch control’ which controls the helicopter’s vertical flight.

The Helicopter When the pilot makes a large upward movement of the collective lever, more power is required from the engine(s).

The Helicopter When the pilot makes a large upward movement of the collective lever, more power is required from the engine(s). The hand throttle, located on the top of the collective lever, is opened to supply this power.

PRINCIPLES OF FLIGHT The End exit Return to contents list

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