PTC B1.1 Notes - Sub Module 17.1 (Fundamentals)

PIATrainingCentre(PTC) Module17PROPELLERCategory A/B1

ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 Mar 2014

MODULE17:PROPELLERSubModule17.1FUNDAMENTALSSubModule17.2PROPELLERCONSTRUCTIONSubModule17.3PROPELLERPITCHCONTROL

SubModule17.4PROPELLERSYNCHRONISINGSubModule17.5PROPELLERICEPROTECTIONSubModule17.6PROPELLERMAINTENANCESubModule17.7PROPELLERSTORAGEANDPRESERVATION



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ListofAmendments

Amendment No. Sub-Module & Pages: Issue Date: Date Inserted: Inserted By: Date Removed: Removed By:

Issue 01, Rev-00 All 31 March 2014

PIATrainingCentre(PTC) Module17PROPELLERCategory A/B1 Sub Module 17.1 Fundamentals

ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.1 Mar 2014

MODULE17

SubModule17.1

FUNDAMENTALS


ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.1 - i Mar 2014

ContentsINTRODUCTION1

BASICPRINCIPLES3

BLADEELEMENTTHEORY9

HIGH/LOWBLADEANGLE17

REVERSEBLADEANGLE18

ANGLEOFATTACK19

ROTATIONALSPEED(RPM)21

PROPELLERSLIP23

PROPELLEREFFICIENCY26

POWERABSORPTION27

AERODYNAMIC,CENTRIFUGALANDTHRUSTFORCES29

TORQUE33

RELATIVEAIRFLOWONBLADEANGLEOFATTACK34

VIBRATIONANDRESONANCE35


ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.1 - ii Mar 2014



ISO90012008Certified ForTrainingPurposeOnly

PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.1 - 1 Mar 2014

INTRODUCTION In the beginning, use of a turbine engine in aircraft was for the turbine to drive the propeller. Turbojet engines showed so much promise that some believed they would make propellers obsolete. Fortunately, this has proven to be untrue. Turboprop power plants fill an important place between turbojet or turbofan engines and reciprocating engines. They combine the high propulsive efficiency with the low weight and high time between overhauls of the turbine engine. The gas-turbine engine with a reduction gear assembly and a propeller has been in use for many years, and has proved to be a most efficient power source for aircraft operating at speeds of 300 to 450 mph. These engines provide the best specific fuel consumption (SFC) of any gas-turbine engine, and they perform well from sea level to comparatively high altitudes (over 20,000 ft). At higher speeds and/or altitudes, the efficiency of the propeller deteriorates rapidly because of the development of shock waves on the blade tips. Although various names have been applied to gas-turbine engine/propeller combinations, the most widely used name is Turboprop. Another popular name is Propjet. The whole purpose of a propeller is to provide the thrust required to move the aircraft forward. The aircraft propeller consists of 2 or more blades and a central hub to which the blades are attached. Each blade of an aircraft propeller is essentially a rotating wing. As a result of their construction, propeller blades produce forces that create thrust to pull or push the aeroplane through the air.

Power to rotate an aircrafts propeller blades is provided by the engine. On low-horsepower piston-type engines, the propeller is mounted on a shaft that is usually an extension of the crankshaft. On high-horsepower engines, such as a turboprop engines, the propeller is mounted on a propeller shaft driven by a turbine through a reduction gearbox (Figure 17.1.01). In either case, the engine rotates the aerofoils of the blades through the air at high speeds, and the propeller transforms the rotary power of the engine into thrust.




Figure 17.1.01




BASIC PRINCIPLES The Aerofoil The aerofoil is a particular streamlined shape which, when moving through the atmosphere, will produce a force approximately at right angles to the direction of movement. When the aerofoil is the wing of an aircraft, we call the force produced lift, but when the aerofoil is the blade of a propeller we call this force thrust. It is the thrust produced by the propeller that moves the aircraft forward and the lift of the wings that support the aircraft in the air. A typical aerofoil is shown in Figure 17.1.02.

Figure 17.1.02

When an aerofoil moves through the air its special streamlined shape causes a particular airflow pattern to develop. Air passing over the curved aerofoil surface is caused to increase in velocity relative to the velocity of the air flowing over the flat surface and, as a consequence, the pressure of the air over the curved surface is reduced relative to the pressure of the air flowing over the flat surface. This relative change in pressure creates a resultant net force as shown in Figure 17.1.03. Since the propeller blade and the wing of an aero plane are similar in shape, each propeller blade may be considered as a rotating wing. It is true that it is a small wing that has been reduced in length, width and thickness, but it is still a wing in shape. At one end this small wing is shaped into a shank, thus forming a propeller blade. When the blade starts rotating, air flows around the blade just as it flows around the wing of an aeroplane, except that the wing, which is approximately horizontal, is lifted upward, whereas the blade is lifted forward.




Figure 17.1.03




Producing Thrust The propeller has a number of blades of an aerofoil shape that will produce thrust when the propeller turns and the blades move through the air. The low pressure created in front of the blades attracts more air towards the propeller and this in turn is thrown rearwards by the movement of the blades until the propeller is moving a column of air towards the rear, as shown in the figures 17.1.04(a) and 17.1.04(b). The amount of useful thrust produced by a propeller depends upon the amount of air that the propeller can move and the increase in velocity that it can add to the moving air mass. From the equation: Force = mass x acceleration, the thrust produced by an aircraft propeller is:

Thrust = )( 01 vvm Where: m = mass airflow

1v = velocity of the propeller wake 0v = velocity of the aircraft

Compared with a pure turbojet engine, the mass airflow of the propeller engine is large and the increase in velocity small.




Figure 17.1.04(a)

Figure 17.1.04(b)




Propeller Blade Description The identification of the various parts of the propeller blade is shown in Figure 17.1.05(a) and 17.1.05(b).

Figure 17.1.05(a)




Figure 17.1.05(b)




BLADE ELEMENT THEORY The thrust produced by a propeller blade is determined by five things:

1. The shape of the aerofoil section 2. The area of the aerofoil section 3. The angle of attack 4. The density of the air 5. The speed the aerofoil is moving through the air

The blade element theory considers a propeller blade to be made of an infinite number of aerofoil sections, with each section located a specific distance from the axis of rotation of the propeller. Each blade can be marked off in one inch segments known as blade stations. Each blade element travels at a different speed because of its distance from the centre of the hub and to prevent the thrust from increasing along the length of the blade as its speed increases, the cross-sectional shape of the blade and its blade, or pitch, angle, vary from a thick, high pitch angle near the low-speed shank to a thin, low pitch angle at the high-speed tip. By using the blade element theory, a propeller designer can select the proper aerofoil section and pitch angle to provide the optimum thrust distribution along the blade. This is named propeller blade twist, as shown in the Figure 17.1.06.

Pitch Distribution The pitch distribution (blade twist), as shown in fig Figure 17.1.06, and the change in aerofoil shape along the length of the blade is necessary, because each section moves through the air at a different velocity, with the slowest speeds near the hub and the highest speeds near the tip. To illustrate the difference in the speed of aerofoil sections at a fixed RPM, consider the 3 blade stations indicated on the propeller shown in Figure 17.1.07. If the propeller is rotating at 1800 RPM, the 18-inch station will travel 9.42 feet per revolution (193 mph), while the 36-inch station will travel 18.84 feet per revolution or 385 mph. And the 48-inch station will move 25.13 feet per revolution, or 514 mph. The aerofoil that gives the best lift at 193 mph is inefficient at 514 mph. Thus the aerofoil is changed gradually along the length of the blade. This progressive change in blade angle ensures that the angle of attack remains constant along the total length of the blade.




Figure 17.1.06

Figure 17.1.07




Thrust and Torque The resultant force produced by the propeller as it moves through the air can be resolved into thrust and torque. Thrust is the component of the resultant force acting at right angles to the Plane of Rotation. Torque is the component of the resultant force acting in the Plane of Rotation, opposing the engine torque. The thrust and torque produced by a propeller depend on:

1. Air density 2. Angle of attack 3. Propeller speed

1. Air Density: Increase in density increases the thrust, however, the denser air offers greater resistance to the propeller, i.e., increased torque. 2. Angle of Attack: Any increase in the angle of attack to just below the stalling speed will produce more thrust and torque. The optimum angle of attack will give the best thrust/torque ratio.

3. Propeller Speed: Thrust and torque alter in direct proportion to the propeller speed.

Relative Airflow (Figure 17.1.08) The relative airflow is the resultant of two component airflows:

(i) The airflow due to rotation (the propeller speed) (ii) The airflow due to the forward speed of the aircraft




Figure 17.1.08




Blade Angle The angle, normally acute, between the pressure face (or chord line) of an element of propeller blade and the plane of rotation. (Note: An element is a particular section of the blade.) Adjustment of the blade angle relative to the plane in which the propeller is rotating is used to vary the thrust output of the propeller. Helix Angle (Angle of Advance) The angle between the resultant direction of the airflow and the plane of rotation is called the Angle of Advance or helix angle (Figure 17.1.09). It is a different angle at each section (element) of the blade. The sections near the tip move on a helix of much greater diameter and they also move at a much greater velocity than those near the root.




Figure 17.1.09




Blade Twist (Wash Out) The linear velocity due to rotational speed of any part of the blade depends on the distance from the propeller axis of rotation. The forward speed of the propeller is the same at all parts of the blade. The relative airflow is dependent on those two velocities (rotational speed and forward speed) and so it can vary along the length of the blade producing varying angles of attack on a blade of constant angle. In order to maintain a constant angle of attack along the blade the blade angle is reduced from root to tip. This reduction in blade angle from root to tip is known as Wash out.




Figure 17.1.10




HIGH/LOW BLADE ANGLE The roots of propeller blade can be rotated about the pitch change axis by a mechanism in the hub to vary the blade angle by approximately 110. Movement of the blade is controlled by a Propeller Control Unit (PCU) that directs hydraulic pressure to turn the blade. During flight, PCU controls the angle of blades for fine pitch and coarse pitch. Coarse/High Blade Angle At coarse pitch, greater mass of air is accelerated for lower engine RPM, resulting in saving fuel and engine wear during cruising of flight. Flight Fine / Low Blade Angle At this position the angle of attack is small; so accelerates a smaller mass of air per revolution. This position is the minimum blade angle allowed in flight. It allows the engine to turn at higher speed, like take off RPM. Although the mass airflow is smaller for high RPM, the slip stream velocity is high and with low forward aircraft speed the thrust is also high.

Ground Fine / Superfine / Very Low Blade Angle During starting and taxiing in fixed shaft engines, when power available from the turbines is insufficient to drive the propeller efficiently, this position is used to off-load the engine. When the propeller is in the ground fine pitch position just after touch-down, it also acts as an effective brake, being propeller discs producing drag in the airflow. This mode of blade position is only available when the aircraft is on ground.

Figure 17.1.11




REVERSE BLADE ANGLE Some variable pitch propellers have a range of blade angles that allows the blade to pass through superfine into a negative blade angle range. This allows engine power to be absorbed by the propeller whilst it creates a reverse thrust. This is called a Power on brake. The facility is used to provide effective aircraft braking after landing and is particularly useful if for example the runway is wet as if relieves the aircraft wheel brakes of much of their load and reduces the possibility of the aircraft wheels locking up and skidding. Because the propeller has to first pass back through the zero pitch setting lo reach reverse pitch, precautions must be taken to avoid an engine over-speed condition resulting from the momentary low torque experienced at the zero blade angle position. The facility has a secondary advantage in that the aircraft can be taxied in reverse if required. Maximum in reverse pitch is the bottom end of the ground range. This blade angle range cannot be selected when the aircraft is airborne.

Figure 17.1.12




ANGLE OF ATTACK One of the factors on which thrust produced by a propeller depends is the blade's angle of attack. Angle of attack is the acute angle between the chord line of a propeller blade and the relative wind. Angle of attack relates to the blade pitch angle, but it is not a fixed angle. It varies with (i) the forward speed of the airplane and (ii) the RPM of the engine. Effect of Forward Speed An increase in airspeed will decrease the angle of attack. As the angle of attack decreases, the load on the propeller (torque) is reduced. Reduced propeller torque allows the engine rpm to increase until the propeller and engine torque match each other. So, with a fixed pitch propeller, an increase in relative airspeed, due to a dive or turn into wind, would result in a reduced angle of attack, reduced drag torque, and an increased engine rpm without any alteration to the throttle setting. This has a limiting affect on the aircrafts performance, i.e. the maximum speed of the aircraft is limited to prevent an engine over-speed.

Effect of Engine Speed (RPM) Changes in engine speed also vary the relative airflow and so affect the angle of attack on a fixed pitch propeller. An increase in engine speed will increase the angle of attack. Increased engine speed means more power available, the aircraft would accelerate and the angle of attack would be restored to its original value. Best Angle of Attack Typically, the most effective angle of attack for a propeller blade is between 2 and 4. Any angle of attack exceeding 15 is ineffective because of the possibility of a propeller stall. Typically propellers with a fixed blade angle are designed to produce an angle of attack between 2 and 4 at either climb or cruise airspeed with a specific RPM setting. The optimum angle may vary from propeller to propeller according to blade design; however, it is usually around 4.




Figure 17.1.13

Figure 17.1.14




ROTATIONAL SPEED (RPM) The rotational speed (RPM) of the propeller is another factor on which propeller thrust and torque depend. If the propeller speed increases, the thrust and torque produced by that propeller also increases. But, on the other hand, an increase in RPM also increases the angle of attack and if the angle reaches or exceeds 15, blade stall will occur and thrust will be lost. Also, the RPM of the propeller is limited by the propeller tip speed reaching to Mach 1, which results in fluttering and loss of efficiency of the propeller. However, in a variable pitch constant speed propeller, if the engine power is increased, the RPM would normally rise and the blade angle of attack would increase and consequently the torque will increase to balance the increase in power and the engine RPM will not increase. The blade angle then adjusts to maintain the correct angle of attack as the aircraft forward speed increases. All the time, the blade angle adjusts to maintain the correct angle of attack whilst the engine RPM remains constant because its changing power output is being balanced by the changes in torque.




Equivalent Shaft Horsepower One horsepower is equal to 33,000 foot pounds of work done per minute, which is the same as 550 foot pounds per second or 375 mile pounds per hour. Shaft horsepower (shp), is the horsepower delivered to the propeller shaft and can be calculated using the formula:

shp = actual propeller rpm x torque x K

where K is the torque-meter constant (K = 2 33,000) With a turboprop engine, some jet velocity is left at the jet nozzle (net thrust developed at the engine exhaust) after the turbines have extracted the required energy for driving the compressor, reduction gear and accessories etc. This velocity can be calculated as net thrust (Fn), that also aids in propelling the aircraft. If shaft horsepower and net thrust are added together a new term equivalent shaft horsepower (eshp) results. However the net thrust must be converted to equivalent horsepower. Under static conditions, one shp is approx. equal to 2.5 lbs of thrust. The formula for calculating eshp is:

eshp (static) = shp + Fn / 2 . 5

In flight, the eshp considers the thrust produced by the propeller, which is found by multiplying the net thrust in pounds by the speed of the aircraft in mph. Divide this by 375 times the propeller efficiency, which is considered to be 80%.

eshp (flight) = shp + Fn x v / 375 x Where: v = aircraft speed (mph) = propeller efficiency; an industry standard of 80% 375 = a constant; mile pounds per hour for one horsepower Example: Find the equivalent shaft horsepower produced by a turboprop aircraft that has the following specifications:

Airspeed = 260 mph shp indicated on the cockpit gauge = 525 shp Net thrust = 195 lbs

eshp (flight) = shp + (195) (260) / (375) (0.8)

eshp (flight) = 525 + 169 eshp (flight) = 694 Under these conditions, the engine is producing 694 eshp.




PROPELLER SLIP In order to obtain thrust, the propeller blade must be set at a certain angle to its plane of rotation, in the same manner that the wing of an aero plane is set at an angle to its forward path. While the propeller is rotating in forward flight, each section of the blade has a motion that combines the forward movement of the aeroplane with the circular or rotary movement of the propeller. Therefore, any section of the blade has a path through the air that is shaped like a spiral or a corkscrew, as shown in Figure 17.1.15.

Figure 17.1.15

The amount of bite (amount of air) taken by each blade is determined by its blade angle, as shown in Figure 17.1.16. An imaginary point on a section near the tip of the blade traces the largest spiral, a point on a section midway along the blade traces a smaller spiral and a point on the section near the shank of the blade traces the smallest spiral of all. In one turn of the blade, all sections move forward the same distance, but the sections near the tip of the blade move a greater circular distance than the sections near the hub.

Figure 17.1.16




If the spiral paths made by various points on sections of the blades are traced, with the sections at their most effective angles, then each individual section must be designed and constructed so that the angles gradually decrease towards the tip of the blade and increase towards the shank. This gradual change of blade section angles is called pitch distribution and accounts for the pronounced twist of the propeller blade. Geometric Pitch Since the pitch angle of a propeller blade varies along its length, a particular blade station must be chosen to specify the pitch of a blade. This is normally done by specifying the angle and the blade station, e.g. 14 at the 42-inch station. Rather than using blade angles at a reference station, some propeller manufacturers express pitch in inches at 75% of the radius. This is the geometric pitch, or the distance this particular element would move forward in one revolution along a helix, or spiral, equal to its blade angle. The geometric pitch is found by the formula:

Geometric Pitch = tan(pitch angle) x 2 r where r = radius of the blade element (blade station) Example: A propeller with a blade angle of 14 at the 42-inch station has a geometric pitch of 65.9 inches. Geometric Pitch = tan pitch angle x 2 r

= tan 14 x 6.28 x 42 = 65.9 inches

Effective Pitch The effective pitch is the actual distance the aero plane moves forward during one revolution (360) of the propeller in flight. Figure 17.1.17 shows two different pitch positions. The black aerofoil drawn across the hub of each represents the cross section of the propeller to illustrate the blade setting. When there is a small blade angle, there is a low pitch and the aero plane does not move very far forward in one revolution of the propeller. When there is a large blade angle, there is a high pitch and the aero plane moves further forward during a single revolution of the propeller.

Figure 17.1.17




Slip Slip is defined as the difference between the geometric pitch and the effective pitch of a propeller (Figure 17.1.18). It may be expressed as percentage of the mean geometric pitch or as a linear dimension.

%100GPAPRGPSlip

where GP = Geometric pitch APR = Advance per revolution

Example: If a propeller has a pitch of 50 inches, in theory, it should move forward 50 inches in one revolution. But, if the aircraft actually moves forward only 35 inches in one revolution, then the slip is 30% and the effective pitch is 70%. Although the terms blade angle and pitch are often used to express the same thing, pitch will vary relative to the forward speed of the aircraft, whereas blade angle can be locked in any position regardless of forward speed.

Figure 17.1.18




PROPELLER EFFICIENCY The thrust horsepower is the actual amount of horsepower that an engine-propeller unit transforms into thrust. This is less than the shaft horsepower developed by the engine, since the propellers are never 100% efficient. Propeller efficiency varies from approx. 50% to 90% depending on how much the propeller slips. Some of the work performed by the engine is lost in the production of noise. Normally, about half of the noise made by the propeller-driven engine is made by the propeller itself. When the propeller blade tips approach the speed of sound, vibrations are produced that cause the noise. When the blades operate in the transonic range, they not only produce noise, but the drag becomes excessive and the efficiency drops off dramatically. For the propeller disc to be as large as possible while keeping the tips below the speed of sound, most high-powered engines are geared so the propeller turns slower than the engine driveshaft. The maximum propeller efficiency that has been obtained in practice under the most ideal conditions, using conventional engines and propellers, has been only about 92%. And, in order to obtain this efficiency, it has been necessary to use thin aerofoil sections near the tips of the blades and very sharp leading and trailing edges.

Since the efficiency of any machine is the ratio of the useful power output to the power input, propulsive efficiency is the ratio of thrust horsepower [work done by propeller] to shaft horsepower [work done by engine]. The usual symbol for propulsive efficiency is the Greek letter (eta). The efficiency of the propeller is the ratio of the thrust horsepower to the shaft horsepower:

%100shpThp

where Thp = thrust horsepower shp = shaft horsepower

Example: The drag on an aircraft traveling at 200 mph is 1125 lbs. The engine produces 750 shp. Calculate the propeller efficiency (one hp = 375 mile pounds per hour). In level flight, drag is equal to thrust

Thp = (Thrust) (Aircraft Speed) / 375

= 1125 X 200 / 375 = 600

shp = 750

Propeller Efficiency () = 600 x 100 = 80% 750




POWER ABSORPTION When engine power is changed into thrust by the propeller, the drag or torque created by the propeller being forced through the air limits the engine speed. For maximum efficiency, the propeller must be able to absorb all the engine power available. Power can be absorbed by propeller design but each method used has its limitations and a compromise has to be made for the final propeller design.




Number of Blades The number of blades has been an option for propeller engineers. The logical choice for fixed pitch wood and forged metal propellers is two blades that have the advantage of ease of construction and balancing, low manufacturing cost and efficient operation. When more thrust is needed the blade area can be increased by lengthening the blades, but only to a point at which the tip speeds approach the speed of sound, or if tip clearance from the structure or ground is a factor. To keep the blades short, more blades can be used. Three and four-bladed fixed pitch propellers have been constructed, but usually, propellers with more than 2 blades are made so their pitch can be adjusted. Some modern propellers have 4, 5 or 6 blades; and Propfan and Unducted Fan propellers have as many as 12. Solidity Solidity is calculated at the blade master station which is about 0.7 of the blade length from root to tip.

The greater the solidity, the greater will be the power which can be absorbed by the propeller. Figure 17.1.19 shows the disc area swept by the propeller.

Figure 17.1.19




AERODYNAMIC, CENTRIFUGAL AND THRUST FORCES 1. Centrifugal Force Centrifugal force puts the greatest stress on a propeller as it tries to pull the blades out of the hub (Figure 17.1.20). It is not uncommon for the centrifugal force to be several thousand times the weight of the blade. For example, a 25 pound propeller blade turning at 2700 RPM may exert a force of 50 tons (100 000 pounds) on the blade root.

Figure 17.1.20

2. Thrust Bending Force Thrust bending force is caused by the aerodynamic lift produced by the aerofoil shape of the blade as it moves through the air (Figure 17.1.21). It tries to bend the blade forward and the force is at its greatest near the tip. Centrifugal force, trying to pull the blade out straight, opposes some of the thrust bending force.

Figure 17.1.21




3. Aerodynamic Turning Moment Centrifugal force, thrust bending force, and torque bending force require a propeller to be strong and heavy, and they serve no useful function. But 2 twisting forces are useful in the pitch change mechanism of controllable pitch propellers. Aerodynamic Turning Moment (ATM) tries to increase the blade angle. The axis of rotation of a blade is near the centre of its chord line, and the centre of pressure is between the axis and the leading edge. Figure 17.1.22 shows how the aerodynamic force acting through the centre of pressure ahead of the axis of rotation tries to rotate the blade to a higher pitch angle.

Figure 17.1.22

4. Centrifugal Turning Moment Centrifugal Turning Moment (CTM) tries to decrease the blade angle. As the propeller turns, centrifugal force acts on all the blade components and tries to force them to rotate in the same plane as the blades axis of rotation. This rotates the blade to a lower-pitch angle. CTM opposes ATM, but its effect is greater, and the net result of the twisting forces is a force that tries to move the blades to a lower pitch (Figure 17.1.23).

Figure 17.1.23




Many controllable-pitch propellers have counterweights that are on arms clamped around the blade shank, and provide a Counterweight Turning Moment that opposes the CTM. The centrifugal effect is to try to move the counterweights into the plane of rotation and, therefore, the blades towards coarse pitch. Unless a propeller is balanced so that each blade produces the same centrifugal force, aerodynamic forces and CTM, then severe vibration will occur. Therefore, each propeller is subjected to a comprehensive balancing process before it can be fitted to the engine of an aircraft.

5. Gyroscopic Effect A rotating propeller has the properties of a gyro. If the plane of rotation is changed, a moment will be produced at right angles to the applied moment. For example, if an aircraft with a right handed propeller (clockwise rotation viewed from rear) is yawed to the right, it will experience a nose down pitching moment due to the gyroscopic effect of the propeller. Similarly, if the aircraft is pitched nose up it will experience a yaw to the right. On most aircraft the gyroscope effects are small and easily controlled.

Figure 17.1.24




6. Asymmetric Effect With an aircraft in a nose up attitude (high angle of attack) and in straight flight, the axis of the propeller will be inclined upwards to the direction of flight. This causes the down moving blade to have a greater effective angle of attack than the up going blade and, therefore, develops a greater thrust.

Figure 17.1.25




TORQUE Torque bending force tries to bend a propeller blade in its plane of rotation opposite to the direction of the rotation (Figure 17.1.26).

Figure 17.1.26

Newtons third law tells us that to every action there is an equal and opposite reaction. The force rotating the propeller is torque so it follows that there must be a reaction force in the opposite direction of rotation. In a single engine aircraft this torque reaction is attempting to rotate the aircraft about the propeller shaft axis. This can create a rolling (and secondary yawing) tendency, especially at take-off. Torque effect can be eliminated by mounting two propellers, one behind the other, each driven by its own shaft but in opposite directions. The shafts are concentric and as the propellers rotate in opposite directions; their torque effects cancel each other out. Such an arrangement is referred to as counter-rotating propellers. Contra-rotating propellers, on the other hand, are two propellers mounted in tandem but driven by a single engine through a gearbox to ensure the propellers rotate in opposite directions and likewise their torque effects are cancelled out. Contra-rotating propellers also increase thrust generating capability when high power is required.




RELATIVE AIRFLOW ON BLADE ANGLE OF ATTACK Torque is a force that a propeller experiences because of the air resistance it encounters as it rotates. It is in fact the drag force acting on the propeller blades. If the torque balances the engine shaft power (shp) then the propeller will run at the speed where this is achieved. Any increase in engine power will cause the propeller rotational speed to increase until the torque balances the power again. If the propeller blade angle is increased then the torque will increase and this will require an increase in engine power if RPM is to be maintained. Recall that the relative airflow in both direction and velocity is governed by the rotational speed of the propeller and the forward speed of the aircraft. The angle of attack of a propeller blade is the angle formed between the blade chord line and the relative airflow. Blade angle and angle of attack are different in different flight conditions. These will be discussed in more details in sub-module 17.3.




VIBRATION AND RESONANCE When a propeller is producing thrust, aerodynamic and mechanical forces are present which cause the blades to vibrate. Vibration caused by the propeller also produces resonance, which is produced by the echoing beat of the out of balance propeller. Vibration caused by mechanical inputs is generally the cause of an out of balance blade on a propeller assembly. Propellers, spinners, and power train dynamic components are, manufactured to strict tolerances; however, due to centrifugal force, aerodynamic loading and individual component tolerances, some residual out-of-balance moment may be present in the whole assembly, which will lead to an increase in vibration. The process of tracking and balancing propellers may be generally required on the following occasions:

After major component replacement As a fault diagnosis/troubleshooting aid Whenever required by the maintenance manual or

maintenance procedures When looking at how the process of tracking and balancing is accomplished, it is first necessary to determine what vibration is.

Vibration is a rapid oscillation that may be caused by rotating assemblies, when they are out of balance or by external influences such as aerodynamic forces. These oscillations will be felt with a certain force, known as displacement or amplitude, and at a certain rate, i.e. vibrations per minute, known as frequency. Vibration is generally expressed as frequency per second, and is measured in Hertz (Hz). One hertz is therefore equal to one cycle per second, that is a movement from the baseline in one direction and continuing in the opposite direction until a return to the baseline is achieved, as shown in figure 17.1.27. If not compensated for in the design, vibration may cause excessive flexing, work-hardening of the metal, and result in sections of the propeller blade breaking off during operation.




Figure 17.1.27




Aerodynamic forces have a great vibration effect at the tip of a blade where the effects of transonic speeds cause buffeting and vibration. The vibrations may be decreased by use of the proper aerofoil shape and tip designs. Mechanical vibrations are generated by power pulses in a piston engine and are considered to be more destructive in their effect than aerodynamic vibration. These engine power pulses cause a propeller blade to vibrate and set up standing wave patterns that cause metal fatigue and failure. The location and number of stress points changes with different rpm settings, but the most critical location for these stress concentrations is about six inches in from the top of the blades. Most airframe-engine-propeller combinations have no problem in eliminating the detrimental effects of these vibrational stresses; however, some combinations are sensitive to certain rpm ranges and have this CRITICAL RANGE indicated on the tachometer by a red arc. The engine should not be operated in the critical range except as necessary to pass through it to set a higher or lower rpm.

If the engine is operated in the critical range, there is a possibility of structural failure in the aircraft due to the vibrational stresses set up. Propellers driven by a gas turbine turbo-prop are not affected by a critical range as there are not power pulses in a gas turbine engine. As the operation cycle is continuous, there is no reciprocating motion in the engine, thus a smoother operating engine producing lower vibration frequencies and hence lower resonance. This means that there are much less fatigue and operational stresses transmitted to the propeller. The above mentioned technical data means that a turbo-prop propeller should remain in service longer, more time between overhaul (TBO). When a propeller produces thrust, aerodynamic and mechanical forces are present which cause the blade to vibrate. If this is not compensated for in the design, this vibration may cause excessive flexing and work-hardening of the metal and may even result in sections of the propeller blade breaking off in flight. Aerodynamic forces cause vibrations at the tip of a blade where the effects of transonic speeds cause buffeting and vibration.

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PTC B1.1 Notes - Sub Module 17.1 (Fundamentals)