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Propellers Chapter 7. Aim To understand principal of operations of propeller systems.

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Presentation on theme: "Propellers Chapter 7. Aim To understand principal of operations of propeller systems."— Presentation transcript:

1 Propellers Chapter 7

2 Aim To understand principal of operations of propeller systems

3 Objectives 1.Review aerodynamic properties of propellers 2.Review propeller performance considerations 3.Discuss principals of operation of constant speed propellers 4.Describe factors affecting propeller stress and materials used in construction

4 1. Aerodynamic properties Blade Angle The blade angle is the angle between the chord of the blade and the plane of rotation The blade angle is not constant throughout the blade length Chord Line Plane of rotation L.E. T.E. Blade face Blade backBlade Angle For a fixed pitch propeller this is angle is set at the time of manufacturing

5 1. Aerodynamic properties Propeller Pitch Propeller pitch is a linear equivalent measure of blade angle Both blade angle and radius are measured at a standard radius of 75% blade length Chord Line Plane of rotation Coarse Pitch Chord Line Plane of rotation Fine Pitch

6 1. Aerodynamic properties Forces acting on a propeller The propeller has two speeds: 1.RPM is produced by the engine 2.TAS is due to the forward movement of the aircraft These two speeds result in a helical direction of travel, opposite the direction of travel is the relative airflow (RAF) Between the relative airflow and the chord is the angle of attack, this angle of attack creates two aerodynamic forces on the propeller: 1.Propeller torque opposes the engine torque (RPM) and acts parallel to the plane of rotation 2.Thrust acts perpendicular to the plane of rotation The resultant of these two forces is termed the total reaction Chord Line RPM RAF AoA TAS Total Reaction Thrust Propeller Torque

7 1. Aerodynamic properties Centrifugal Twisting Moments Centrifugal force acts directly away from the centre of rotation along the full length of the blade The centrifugal force attempts to stretch the tip from the hub As the centrifugal force does not align with the pitch change axis, we can break it down into two forces which act out from the leading and trailing edge Pitch Change Axis T.E. L.E. XX

8 1. Aerodynamic properties Centrifugal Twisting Moments The two forces acting from the leading edge and trailing edge creates the twisting moment The twisting moment attempts to change the pitch of the propeller towards fine pitch Pitch Change Axis T.E. L.E. X X

9 1. Aerodynamic properties Aerodynamic Twisting Moments The total reaction force does not act through the propellers pitch change axis, just like a wing it will typically act through the centre of pressure around 1/3 chord As centre of pressure is forward of the pitch change axis it tends to move the blade toward coarse, counteracting the CTM Aerodynamic twisting moments are not as strong as centrifugal twisting moments Total Reaction Thrust Propeller Torque

10 1. Aerodynamic properties Windmilling propeller If the engine fails the propeller will windmill (drive the engine) When the propeller is windmilling the aerodynamic and centrifugal twisting act in the same direction, towards fine pitch This is not what we want to happen as it will increase the drag produced by the blade The drag produced by a windmilling propeller is equal to the drag produced by a solid disc of the same radius Total Reaction Thrust Propeller Torque

11 1. Aerodynamic properties Feathered propeller Following an engine failure the propeller should be feathered to stop it from windmilling The angle of attack of the propeller will be slightly negative to produce zero thrust as the propeller is cambered Propeller Torque Total Reaction RAF

12 2. Propeller efficiency Solidity The blade fitted to an engine must be carefully matched to the power output of the engine in order to convert power into thrust Solidity can be increased by: Increasing the number of blades Increasing the chord of the blades Although increasing the chord is often the easier option, the corresponding decrease in aspect ratio will make the blade less efficient Solidity is the ratio of the propeller disk at a given radius to the circumference of a solid disk of the same radius and can be measured by: Radius Chord Increasing the blade length will increase the tip speed, many propeller blades come close to or exceed the speed of sound. As an object approaches the speed of sound the shockwaves created increase the drag on the object, kinetic energy will be turned into sound and heat, reducing efficiency

13 2. Propeller efficiency Fixed Pitch Propellers As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the same can be said for a propeller blade The angle of attack of the propeller blade is determined by the RPM and the TAS Chord Line RPM RAF AoA TAS

14 2. Propeller efficiency Fixed Pitch Propellers As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the same can be said for a propeller blade The angle of attack of the propeller blade is determined by the RPM and the TAS If the RPM increases the AoA will increase Chord Line RPM RAF AoA TAS

15 2. Propeller efficiency Fixed Pitch Propellers As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the same can be said for a propeller blade The angle of attack of the propeller blade is determined by the RPM and the TAS If the RPM increases the AoA will increase If the TAS reduces the AoA will increase Chord Line RPM AoA RAF TAS

16 2. Propeller efficiency Fixed Pitch Propellers As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the same can be said for a propeller blade The angle of attack of the propeller blade is determined by the RPM and the TAS If the RPM increases the AoA will increase If the TAS reduces the AoA will increase If the RPM decrease the AoA will decrease Chord Line RPM AoA RAF TAS

17 2. Propeller efficiency Fixed Pitch Propellers As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the same can be said for a propeller blade The angle of attack of the propeller blade is determined by the RPM and the TAS If the RPM increases the AoA will increase If the TAS reduces the AoA will increase If the RPM decrease the AoA will decrease If the TAS increases the AoA will decrease Chord Line RPM AoA RAF TAS Any decrease in AoA will cause a corresponding decrease in prop torque, this will allow the RPM to increase on a fixed pitch propeller, this can be seen if we lower the nose of the aircraft allowing TAS to increase A fixed pitch propeller will be most efficient at one RPM and TAS combination, this is set by the manufacturer and will typically be set for cruise

18 2. Propeller efficiency Variable Pitch Propellers By varying the pitch of the propeller we are able to operate the propeller at the optimum AoA over a wider range of RPM and TAS settings Efficiency TAS 100% Envelope of max efficiency Blade angle Fine Pitch Coarse Pitch

19 3. Constant Speed Propellers Constant Speed Propellers In an aircraft fitted with a variable pitch propeller the pitch control lever controls engine RPM and the throttle controls manifold air pressure The system utilizes a constant speed unit and a pitch change mechanism to maintain the RPM set by the pilot

20 3. Constant Speed Propellers Constant Speed Units Constant speed units (sometimes referred to as propeller governors) are normally fitted to the front of the engine and will incorporate a geared boost pump to provide the pitch change mechanism with high pressure oil The CSU utilizes flyweights which are rotated by the engine and are subject to centrifugal forces which tends to make them fly outwards They are prevented from dong so by the speeder spring, the pilot sets the tension on this spring via the pitch control lever If the system is maintaining the set RPM the pilot valve will maintain pressure in the propeller hub, this is known as On-speed

21 3. Constant Speed Propellers Constant Speed Units If the RPM tries to decrease (Under-speed) the flyweights will fly inwards and open the pilot valve allowing high pressure oil to travel to the pitch change mechanism decreasing the blade angle. As the RPM begins to increase back to the selected RPM the pilot valve will close until the system returns to the on-speed condition

22 3. Constant Speed Propellers Constant Speed Units If the RPM tries to increase (Over-speed) the flyweights will fly outwards and raise the pilot valve allowing oil to return to the sump increasing blade angle. As the RPM decreases back to selected RPM the pilot valve will close until the system returns to the on- speed condition

23 3. Constant Speed Propellers Pitch Change Mechanism There are a number of different systems utilized by aircraft manufacturers to adjust the pitch of the propeller, the most common is the use of hydraulic pistons Centrifugal twisting moments are used to assist in moving the blade to fine pitch, this is often augmented by the use of springs, counter weights or air/nitrogen charges Typically engine oil is used as the hydraulic fluid as is boosted to the required pressure via a pump In this system when an increase in pitch is required the CSU pumps high pressure oil into the cylinder and the piston coarsens the blade via mechanical linkage. When a decrease in pitch is required pressure is released by the CSU and oil is allowed to return to the sump as the centrifugal twisting moments move the blade to the fine position

24 3. Constant Speed Propellers Pitch Change Mechanism Here the piston is fixed and the moveable dome coarsens the blade pitch, again to fine the blade the CSU will release the oil pressure and the blade will fine via the centrifugal twisting moments This system can be seen as the opposite of the pervious two, a counterweight is introduced which is offset from the pitch change axis, this counterweight is used to coarsen the pitch. The hydraulic system is used to move the blade to fine

25 3. Constant Speed Propellers Feathering Systems As previously mentioned, a windmilling propeller will produce drag equal to a disk of the same radius In a twin engine aircraft this excess drag will cause a yaw and rolling moment towards the failed engine, depending on the airspeed and power produced by the live engine the rudder may not have enough authority to overcome this The type of feathering system depends on the type of pitch change mechanism used, a large number of aircraft incorporate an auto feather system In some aircraft the aircraft must be feathered before the RPM drops below a certain value, beyond this the feathering system will not be able to overcome the combination of centrifugal and aerodynamic twisting moments

26 4. Propeller Stress Propeller Failure Propeller stress or failure can be caused by a number of factors including: Over Speeding – Increased centrifugal forces resulting in excess stress on the propeller hub Lightning Damage – Can result in burn damage Vibration – When the propeller is producing thrust aerodynamic and mechanical forces cause the blade to vibrate. High tip speeds cause excess drag on the blade tips, especially on high powered aircraft where the blade tips can exceed the speed of sound. Power pulses from piston engines have the potential to set up standing waves in the propeller. The resulting bending loads may cause blade sections to shear off.

27 4. Propeller Stress Propeller Failure Nicks and fatigue cracks – Chips in the propeller may lead to a fatigue crack and eventual failure. The crack will develop in a chord wise direction and will be visible from the back of the blade, it may not be visible from the front of the blade until just before failure due to thrust bending loads. Any nicks should be filed back by a qualified LAME, typically an equivalent amount will be filed from both blades to reduce vibration Fatigue of the hub – It can be hard to detect fatigue in the hub due to its size. Caution must be taken when selecting the cleaning products used on aircraft as some general purpose cleaners promote fatigue and corrosion in the propeller, especially in the hub

28 4. Propeller Stress Propeller Materials Propellers on older aircraft were typically made of wood, as engines became more powerful metal propellers were used The disadvantages of metal propellers include: Heavier than wooden propellers Harder to manufacture Increased tendency to vibrate Cost more The advantages of metal propellers include: Ease of maintenance should the prop be chipped Resistance to weathering Low drag Low service requirements Ease of storage

29 4. Propeller Stress Propeller Materials Most modern aircraft propeller blades are constructed from aluminium Wood may still be used on small aircraft, such as the jabiru, where low power engines are used A number of composite materials have been developed, including Kevlar composite, however these are not yet widely used

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