EASA Module 17

M 17 Propeller

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EASA Part-66 Training Handbook

Module 17

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Propeller 17.1

Fundamentals

17.1.1

Blade Element Theory

17.1.1 .1

Basic Principles Refer to Figure 1.

A propeller is a rotating airfoil that consists of 2 or more blades. These blades are attached to a central hub which is mounted on an engine crankshaft. The function of the propeller is to convert engine power into useful thrust. The blades have a leading edge, a trailing edge, a tip, a shank, a face and a back as shown in Figure 2, details a) and b).

Blade Angle Refer to Figure 3. 'Blade angle' (or: pitch angle) is the angle between the propeller's plane of rotation and the chord line of the propeller's airfoil section (i.e. of the blades). The cord line is an imaginary line from the leading edge of the blade to its trailing edge. An increase in blade angle increases the thrust. A reduction of the blade angle results in less thrust.

Angle of Attack To produce thrust, the airfoil section of a propeller must be slightly tilted in relation to the direction of airflow over it. This is known as the 'angle of attack'. It is part of the blade angle. Both are equal when the propeller is not turning. Refer to Figure 4. The angle of attack is a product of the aircraft's forward speed and the rotational speed of the propeller. For any given blade angle, as the forward speed of the aircraft increases, the angle of attack decreases until it finally reduces the amount of thrust available and limits the aircraft's forward speed. Efficiency can be regained by increasing the propeller speed or by increasing the blade angle. For mechanical and aerodynamic reasons, both propeller rpm (revolutions per minute) and blade angle are limited. Refer to Figure 5. The possible blade angles range from a full reverse, negative blade angle to a fully streamlined feathered position. When the turboprop engine is at idle, the engine is at minimum load. At ground idle (GI), the blade angle will be almost zero and the thrust is at a minimum.

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As the power lever is moved toward the take-off position, the propeller blade angle becomes positive thereby creating a forward thrust in order to move the aircraft. Under conditions of emergency shut-down in the air, the blade angle is set to its maximum, i.e. it is set to approx. 90". When the engine is shut down, it is important to streamline the blade into the direction of flight (to stop windmilling of the engine and reduce drag). Refer to Figure 6 . In order to maintain a constant angle of attack at a constant engine power, the blade angle must be increased when the aircraft speed increases. With the blade angle being at 'full reverse' after touch down, the angle of attack will decrease as aircraft speed decreases. Propeller Pitch Refer to Figure 7 'Pitch' is the distance in inches (") that a propeller section moves forward during one revolution. 'Pitch distribution' is the gradual twist in the propeller blade from shank to tip. 'Geometric pitch' of a propeller is based on the blade angle at the 75-% blade station. Note:

In the example shown in Figure 2, detail a), the 7 5 % station is at 42" from the hub. Geometric pitch is a theoretical value because it does not take into account any losses caused by inefficiency. 'Effective pitch' is the distance the aircraft actually moves forward during one revolution of the propeller. It may vary from zero (when the aircraft is stationary on the ground) to approx. 90 % of the geometric pitch during the most efficient flight conditions. The difference between geometric pitch and effective pitch is called 'slip'.

Example:

17.1.1.2

If a propeller has a pitch of 50" it should (theoretically) move forward 50" during one revolution. But, if the aircraft actually moves forward only 35" during one revolution, the effective pitch is 35", and the propeller has an efficiency of 70 % in pitch.

Production of Thrust Basically, the propeller produces thrust by giving a momentum to a large mass of air and accelerating it rearwards. The amount of thrust produced depends largely upon the amount of air that the propeller can move and on the amount of velocity given to the moving air. Refer to Figure 8. To gain efficiency, the propeller blades have a cross-section of a special airfoil shape. They must also be set at a certain angle to the direction of motion (the angle of attack). This ensures that, when the propeller turns, the tilted blades move the air and force large amounts of it towards the rear. Useful thrust is produced to move the aircraft forward.

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When an aircraft is flying, the blade tips move on a spiral path (see Figure 8, detail b)). The distance moved forward along the flight path during one propeller revolution is the same for all sections of the propeller blade. Refer to Figure 2 again. The blade sections nearest to the tip travel greater distances through the air than the sections at or near the blade root. To produce an even thrust along the whole length of the blade, the blade angle is varied from a large angle at the root to a small angle at the tip. This variation of the blade angle produces a 'twist' in the propeller blade that is called 'blade twist'.

17.1 .I .3

Forces Acting on the Propeller When a propeller rotates, many forces interact and cause tension, twisting and bending stresses affecting the propeller.

Centrifugal Force The force which causes the greatest stress on a propeller is the centrifugal force (see Figure 9, detail a)). Centrifugal force can best be described as the force which tries to pull the blades out of the hub. The amount of stress created by centrifugal force may be greater than 7,500 times the weight of the propeller blade.

Torque Bending Force Torque bending forces try to bend the propeller blade back into the direction opposite the direction of rotation (see Figure 9, detail b)).

Thrust Bending Force The thrust bending force attempts to bend the propeller blades forward at the tips. This occurs because the lift toward the tip of the blade flexes the thin blade sections forward. The thrust bending force opposes the centrifugal force to a certain degree (see Figure 9, detail c)).

Aerodynamic Twisting Moment The aerodynamic twisting moment tries to twist a blade to a higher angle. This force is produced because the axis of rotation of the blade is at the midpoint of the chord line, while the center of the lift of the blade is forward of this axis. This force tries to increase the blade angle. The aerodynamic twisting moment is used in some designs to help feather the propeller (see Figure 9, detail d)).

Centrifugal Twisting Moment The centrifugal twisting moment tries to decrease the blade angle, and opposes the aerodynamic twisting moment (see Figure 9, detail e)). This tendency to decrease the blade angle is produced because all the parts of a rotating propeller try to move in the same plane of rotation as the blade's center line. At operational rpm, this force is greater than the aerodynamic twisting moment. It is used in some designs to decrease the blade angle.


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Vibrational Force and Critical Range When a propeller produces thrust, aerodynamic and mechanical forces are present which cause the blade to vibrate. If this is not allowed 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. Mechanical vibrations are caused by the power pulses in a piston engine. They are considered to be more destructive in their effect than aerodynamic vibration. These 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 change with different rpm settings. But the most critical location for these stress concentrations is approx. 6" from the tip of the blades. Most airframe/engine/propeller combinations have eliminated the adverse effects of these vibrational stresses by careful design. But some combinations are sensitive to certain propeller speeds. This critical range is indicated on the airspeed indicator 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, a structural failure in the aircraft becomes possible because of the vibrational stresses set up.

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Fabric sheathing

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Hub assembly

Tip

Figure 1

Typical Wooden Aircraft Propeller


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a) Typical

Blade butt

Designations Front of aircraft

Chord / line

A

7

1

Blade

/ back

Blade angle

I Axis ofirotation

I

Figure 2

Designations and Blade Cross-section of a Propeller



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a) Blade angle

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/

Plane of rotation

Blade cord line

-

/

I

Blade angle

b) Angle of attack

Plane of rotation

d

.

\

Figure 3

/

'

Relative airflow

Blade Angle and Angle of Attack

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Pitch angle

Angle of attack

Relative wind

Engine speed

Figure 4

Blade Angle and Angle of Attack versus Relative Wind

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Power lever position in

reverse

ground idle

take-off

flight idle

zero

positive

streamlined (90") (feathered)

minimum

forward

Pitch angle is

negative

Thrust effect is

reverse

Figure 5

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zero

Range of Propeller Blade Angles



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Plane of rotation

Figure 6

Blade Angle and Angle of Attack in 'Reverse' Position




Figure 7

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Effective and Geometric Pitch of a Propeller


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a) Airflow by propeller revolution

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--

+-------A

Angle of attack

Flight path

I) Direction of propeller rotation

I

b) Propeller revolutions produce a spiral path

Flight path

T

-

F

I . revolution 2. revolution 3. revolution

Figure 8

Production of Thrust

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a) Centrifugal force

b) Torque bending forces

c) Thrust bending forces Thrust load

d) Aerodynamic twisting moment

e) Centrifugal twisting moment

Center of rotation

Center of pressure

Figure 9

Forces Acting on the Propeller

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M o d u l e 17

17.1.2

Aerodynamic Performance of Propellers

17.1.2.1

Introduction The propeller has the task to transform the power of the engine into thrust in the most efficient way. Due to the high rotational speed and the high power to absorb, propellers for pylon racing engines usually are made from wood or composite materials. Most injection-moulded propellers cannot sustain the high loads which makes them insecure and dangerous. When propellers are made from composite materials (epoxy resin and carbonlglasslkevlar rovings), moulds are used which permit a very accurate reproduction of the master propeller. Wooden propellers are lighter and reduce the vibration levels, but are more delicate to handle and cannot easily be duplicated. Also composite materials are better suited for the rather thin airfoils near the propeller tips.

17.1.2.2

Geometry o f Propellers

Propeller Velocity Calculation Refer to Figures 1 and 2. Similar to a wing, a propeller can be defined by one overall dimension (the diameter D) and local dimensions like the chord length c and the local blade angle (twist) which are depending on their radial position r. In contrast to a wing, a propeller shows a strong variation of the twist distribution along the radius. The local incoming flow (inflow), seen by a segment of the propeller, consists of 2 parts:

0 the axial velocity component v due to the movement of the aircraft the circumferential component caused by the rotation of the propeller. The rotational component depends on the rotation speed and the radial position, where the blade section is located; at the axis this component is zero, whereas at the blade tip it reaches its maximum value of

vtip = x . n . D where n D

= rotation speed (rpm)

= diameter.

The total velocity is the vectorial sum of the axial and the circumferential component:

Example:

= 0.0, a propeller having a diameter of D = 0.18 m and On the ground, with avil, rotating at a speed of = 25,000 rpm reaches a tip velocity of:

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If the aircraft flies at a speed of 60 mls, the tip speed increases to

Because the speed of sound is approx. 334 mls, this tip speed equals Mach 0.73

Propeller Pitch Calculation Instead of the twist or blade angle distribution, the term 'pitch' is often used. The pitch of a metal screw is the distance the screw would travel through its nut in axial direction, when it performs one rotation. Unfortunately, this is only valid for a perfect screw, mathematically a 'helix', which has the same pitch all along the radius, otherwise the screw would get stuck. Propellers usually do not have a constant pitch along the radius, so that a given pitch is only an approximate measure to describe a propeller's geometry. Comparing Figures 1 and 2 the pitch P of the inflow at the propeller tip can be calculated as follows:

v

(x . n . D)

-

P

-

(x D)

pitch circumference of one rotation

which can be solved for P to be

Airfoil Thickness Due to the high Mach number, compressibility effects (recompression shocks, causing additional drag) reduce the efficiency of the propeller. A practical way to keep the drag of an airfoil at acceptable levels is the use of thinner and less cambered airfoils. To avoid excessive drag, a certain critical camber and thickness should not be exceeded. The Mach number, at which the flow reaches supersonic speed at some point on the airfoil, is called the critical Mach number. Sometimes it might be acceptable to have a small supersonic region at the propeller tip, because a reduction of the diameter (to avoid supersonic tips) also decreases the performance. But in general, a propeller should be designed to avoid supersonic flow by choosing the right airfoil 'thin-ness' and the right diameter. The analysis of compressibility effects on propeller performance is a very complex matter, and cannot be handled here, but, concluding from experimental data, it is possible to develop a rule of thumb. The diagram in Figure 3 can be used to find the maximum allowable thickness and camber for a given Mach number and vice versa.




Example:

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Assume that the propeller has a Clark-Y-like airfoil (flat lower side), which means, that the camber is approx. half the airfoil thickness and using the result from the propeller velocity calculation, with a tip Mach number of M = 0.73, we enter the diagram at the left, draw a line straight up, until it meets the curved line. From the intersection, we draw a horizontal line up to the line corresponding to the camber of the airfoil. For the special case of Clark-Y-like airfoils, we can also use the dotted line as an end point. Dropping down from this intersection, we find the maximum thickness to be 5 %. If we choose a thicker section, we will reach supersonic flow at the wing tip, degrading the performance. If the chord length at the tip is 10 mm, the airfoil there should have a thickness of 0.5 mm, which is very difficult to manufacture and rather unpracticable though.

17.1.2.3

Momentum Theory

Thrust The thrust of a propeller depends on U

the volume of air accelerated per time unit

U

the amount of the acceleration

U

the density of the medium.

Based on momentum considerations, it can be expressed by the following formula:

where:

T

=

thrust

(in N)

D

=

propeller diameter

(in m)

v

=

velocity of incoming flow

(in m/s)

Av = additional velocity, acceleration by propeller p Note:

=

density (rho) of fluid

(in m/s) (in kg/m3)

Air has a density of 1.225 kg/m3 Examining the quite simple formula reveals, that the thrust T increases when the diameter D increases or when the density of the medium increases. The acceleration of a propeller depends on the velocity v, thus it is generally not true that increasing the velocity v increases the thrust. But it can be said, that increasing the additional velocity, increases the thrust. For a propeller of a fixed diameter, working in a certain medium at a certain speed, thrust depends on the velocity increase only.

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Power Power is defined as force times distance per time. Using the available thrust T to drive a vehicle at a certain speed v (which already is distance per time) we can calculate the propulsive power (sometimes also called available power) from:

Now, thrust is the one thing, the power to create this thrust the other. Of course we want to create as much thrust as possible from the smallest amount of power, which can be expressed by the term efficiency. Efficiency The efficiency 7 of a propeller is defined as the ratio of available power to the engine power which is

Pa - T.V rl = ----- - .'engine

'engine

Note, that this definition for efficiency contains the velocity v, which means, that the efficiency approaches zero as the flight speed goes to zero, because the thrust cannot become infinitely large. So this definition is not useful for the special case of static thrust. Neglecting rotational losses, the power absorbed by the propeller can also be expressed by

which can be used to combine the equations above into a relation between the velocity and the efficiency for a given power and diameter:

When using the graphs in Figure 4, we can find the efficiency .rl for given values of power P, diameter D and density p. This efficiency could be achieved by an optimum propeller in its design point, if there were no induced and friction losses. It is the upper limit of what can be expected from a perfect propeller. In reality, the efficiency will be 10 to 15 % less than this value. Only highly efficient propellers, operating under light load conditions P / D ~come close to this theoretical limit. For a given power P, it is always desirable to use the largest possible propeller diameter D, which may be limited by mechanical restrictions (landing gear height) or aerodynamic constraints (tip Mach number). That is why most man- or solarpowered aircraft use large, slowly turning propellers. These catch a large volume of air and accelerate it only slightly to achieve the maximum efficiency. Conclusions Using the quite simple momentum theory, we can already deduct important information about the performance of propellers. We can study the influence of the propeller diameter on efficiency as well as how it depends on flight speed or the density of the air (corresponding to a certain altitude).

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For the design or the analysis of a propeller, more sophisticated models are necessary, but the momentum theory always gives a good estimate for the maximum efficiency which we can expect. It is possible to extend the momentum theory to include rotational losses, which results in an additional efficiency loss of 2 to 5 % for typical propellers. These losses depend on the velocity of rotation and favour low-torque, high-speed conditions.

17.1.2.4

Static T h r u s t o f Propellers As long as an aircraft does not move, its propeller operates under static conditions. There is no air moving towards the propeller due to the flight speed, the propeller creates its own inflow instead. A propeller, with its chord and twist distribution designed for the operating point under flight conditions, does not perform very well under static conditions. As opposed to a larger helicopter rotor, the flow around the relatively small propeller is heavily distorted and even may be partially separated. From the momentum theory of propellers we learn, that the efficiency at lower speeds is strongly dependent on the power loading (power per disk area), and this ratio for a propeller is much higher than that for a helicopter rotor. We are able to achieve approx. 80 to 90 % of the thrust, as predicted by the momentum theory for the design point, but we can reach only 50 % or less of the predicted ideal thrust under static conditions. Static thrust depends also on the inflow, influenced by the environment of the propeller (fuselage, crosswind, ground clearance). Measurements of static thrust can be easily done. But the theoretical treatment is very complicated and only possible with a lower degree of confidence than calculations in the vicinity of the design point. Due to local flow separation, the behaviour of propellers under static conditions can be very sensitive with respect to blade angle settings and airfoil shape. To get a picture of the bandwidth of static thrust, several older reports have been examined. The results are combined in Figure 5 which shows the static thrust coefficient versus blade angle for different propellers having 2, 3, 4, 6 and 8 blades. The true static thrust depends on blade form and blade angle of the blade and the generic graph gives you a rather wide band of results. One important aspect seems to be the observation of a critical blade angle approx. 25". For increased angles, a large part of the blade seems to stall. This effect can be seen on some propellers for high-speed model aircraft with large pitch values. After launching the model, it takes some time for the propeller to 'catch on', even when engine and exhaust system are properly tuned. For high static thrust values, a smaller number of blades seems to be better, because (for the same power consumption) they have a wider chord, creating a stronger circulation, being less prone to separation. A hovering helicopter would have a very small blade angle (approx. 5") resulting in large static thrust values.




17.1.2.5

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Aerodynamic Characteristics of Propellers A propeller creates a thrust force out of the supplied power. The magnitude of this force is not constant for a given propeller, but depends on the velocity of the incoming air and the rotational velocity of the propeller itself. Thus tests of propellers usually cover a wide range of operating conditions. Propellers having the same shape, but are scaled by a size factor, behave similar. In order to make a comparison of propellers of different size easier, aerodynamicists try to get rid of the units. Then it is possible to use the results of a small-scale wind tunnel model to predict the performance of a full-scale propeller. Similar to airfoils and wings, the performance of propellers can be described by dimensionless (normalised) coefficients. While an airfoil can be characterised by relations between angle of attack, lift coefficient and drag coefficient, a propeller can be described in terms of advance ratio, thrust coefficient and power coefficient. These coefficients are helpful for the comparison of propellers of differing diameters, tested under different operating conditions. Thrust coefficient

Power coefficient

CP

-

Advance ratio

v/nD

=

P

p

. n3 . D5

v n.D

Efficiency 7

where v velocity

(in mls)

D diameter

(in rn)

n

revolutions per second (in 11s)

p

density of air

P

power

T

thrust

(in kg/m3) (in W) (in N)

It should be noted, that the definition of the efficiency includes the velocity v. Thus the efficiency goes to zero when the flight speed approaches zero; of course, this does not mean, that the thrust goes to zero. Usually the power and thrust coefficients are plotted versus the advance ratio. The efficiency of a high-speed aircraft propeller, as calculated from these coefficients, is shown in Figure 6.

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17.1.2.6

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Performance Considerations

Shape of Propeller Tips Propeller tips can be rounded, swept or square. Various tips are often used to meet blade vibration resonance or special design conditions. The tip shape is also a function of aesthetics, noise requirements, flight performance, repairability and ground clearance.

Propeller Diameter Propeller diameters are a function of engine and airframe limitations. Larger propeller diameters are preferred for low airspeed operation, while smaller diameters are best for high airspeeds. For example, the diameter of a fixed-pitch propeller is often large to favour low airspeed operation, while the blade size is small to favour higher airspeeds and faster turning at low airspeeds. The diameter and blade size of a constant-speed propeller is often larger (than a fixed-pitch), due to the variability of blade angles.

Engine Power and Speed For fixed-pitch propellers, at a fixed throttle setting, propeller and engine speed increases or decreases with the airspeed. At a constant airspeed, fixed-pitch propeller and engine speed change if power is increased or decreased. A constant-speed propeller uses a governor to provide constant speed at the selected throttle setting. The blade angle automatically increases or decreases as the speed setting or engine power changes. With a fixed speed and power setting, the blade angle automatically changes as airspeed increases or decreases.

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Dimensions of a Propeller

:h

Local blade angle (twist) Diameter

+-

\

'*-

Radius

Blade segment of a propeller

Axial velocity component

Circumferential component

Figure 1

Geometry of Propellers


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Flight path

/ t l revolution+l

revolution4

Pitch = advance per 1 revolution

Helix model

Figure 2

Helix Model of a Propeller




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Figure 4

Optimum Efficiency According to Momentum Theory versus Flight Speed for Different Power Loadings

40

2 cu. -

50

Blade angle ["I

4.

I

Figure 5

Static Thrust Coefficient versus Blade Angle for Different Propellers


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17.2

Propeller C o n s t r u c t i o n

17.2.1

Material and Classification of Propellers

17.2.1.1

Material G u i d e

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General The propeller has the task to transform the power of the engine into thrust in the most efficient way. Due to the high rotational speed and the high power to absorb, propellers for pylon racing engines usually are made from wood or composite materials. Most injection-moulded propellers cannot sustain the high loads which makes them insecure and dangerous. When propellers are made from composite materials (epoxy resin and carbon/glass/kevlar rovings), moulds are used which permit a very accurate reproduction of the master propeller. Wooden propellers are lighter and reduce the vibration levels, but are more delicate to handle and cannot easily be duplicated. Also composite materials are better suited for the rather thin airfoils near the propeller tips. Metal Whilst it may seem that metal would be the perfect material, they are prone to metal fatigue and if bent will stay bent and not return to their original shape. These factors produce a high risk situation. Metal is just too dangerous and for this reason they are quite rightly banned from use. Carbon Fibre (CRE) Carbon fibre is a wondrous material, it is light and strong (when used in conjunction with a good resin system). It is very important that any carbon (or glass) propeller by made using the correct resin. Carbon fibre propellers will flex less under load, maintaining their efficiency, producing an increase in rpm and / or decrease in noise. Some modellers believe a glass propeller is quieter than carbon due to it's 'softer' sound .... this may be true to the ear, but generally the carbon is quieter on the noise meter. The greater strength of carbon fibre also allows the user to thin down (file or sand) the propeller. A thinner propeller will almost always perform better. Carbon is the easiest of all materials to work with. The disadvantages of carbon are it's expense and sometimes brittle nature (this is only a problem with small, thin racing propellers). Many carbon propellers are made with a core of glass fibre in the middle with the carbon on the outside faces (much in the same way of a balsa covered foam wing), this offers advantages in cost and reduction of the brittle nature of carbon. When moulding carbon (or glass) propellers, the aim is to pack in the highest concentration of carbon to resin as possible. For this reason the mould is overfilled resulting in the excess escaping the mould in the form of what is known as 'flashing'. The down side to this method is that it becomes impossible to produce a perfectly balanced or finished propeller. All carbon propellers are black.

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As a matter of interest, Aramid fibres (kevlar) is far too flexible to make good rigid propellers. Glass Fibre Glass reinforced epoxy (GRE) propellers are similar to carbon, they differ in being slightly heavier and not quite as strong but less brittle. They are also cheaper in material cost. It must be said however that most of the expense of a GRE or CRE propeller is in the labour, it can take from 15 to 115 minutes to make a propeller, depending on the size. Glass propellers can be any colour (pigment in the resin). Generally a glass fibre propeller will be able to deliver the performance required except at high rpm where carbon fibre propellers should be used. Glass propellers can be slightly quieter than carbon, if sufficiently rigid in the glass form .... carbon propellers often have a slightly metallic 'ring' to them. Note:

Fibre contents for CRE or GRE propellers is usually between 55% and 65%, the more the better.

Wood The most common of propellers until the advent of good plastics and fibres. Generally made of good strong maples etc, the wood propeller has the advantage of light weight and suitability for any size of propeller. The disadvantages of wood are it's ease of breakage and are sometimes prone to warping. The light weight of most woods can be a problem when operating 4 stroke engines, which prefer a heavier propeller for smooth running. Laminated Wood Modern laminated woods are almost as good as fibre filled epoxy propellers, being almost as strong and possibly quieter, but with the disadvantage of being machined to shape which prevents the optimisation of the design. They are also very expensive. Nylon Along with the advent of plastics came the nylon propeller. These propellers are made by pressure injecting molten nylon into a mould, which when cooled, is opened to reveal a finished propeller. The advantages of these propellers are that due to the fact they take less than a minute to make and are all the same. The disadvantages of nylon are it's lack of strength , weight and flexibility. For these reasons the performance of a nylon propeller is less than better materials. When used for larger propellers the weight of the propeller combined with the low strength will actually stretch the propeller to the point of breakage. It is possible for a 15" propeller to stretch 114" in operation. Never use a nylon propeller on a high performance engine.

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Nylon, Glass Filled The modern 'plastic' propeller uses a nylon filled with very short lengths of glass fibres. The length of fibres varies from manufacturer to manufacturer, ranging from very short, to claims of being reasonably long, but nowhere near being the full length continuous strands of a 'glass' or 'carbon' propeller. These propellers are a big improvement on straight nylon, but still suffer from the same problems, but to a lesser degree. The downside to the manufacture of these propellers, is that the more glass fibres in the nylon the more brittle and breakable the product becomes. Common glass fibre contents vary from an industry 'normal' of 30% up to about 50% for high glass content propellers. For the average modeller, glass filled nylon propellers are the 'normal' propeller. They are cheap, and the better brands perform very well. It must be said however some brands generally the older designs) are not very good in design or quality. All nylon based propellers should be accurate and close to perfect balance. Please note: There are many different carbons, glass's, resins, woods and nylon type products available. The above can only be regarded as a summary of the different 'styles' of materials.

Application of a Composite Propeller Blade Refer to Figure 1. The composite blade is composed of a titanium blade shank retention section into which is molded a low density foam core that supports built-up layers of composite laminate. An erosion shield of electroformed nickel is incorporated into the fabrication to protect the blade leading edge from impact damage. The Erosion shields are adhesively bonded to the blades. Also, a stainless steel wire mesh is incorporated into the fabrication of the blade surface to limit lightning strike damage. Filament windings of composite material provide blade retention of the blade material to the internal metal plug. The composite laminates which are an integral component of the blade also provide a retention load path directly under the bearing. The blade is matched by its vertical balance. Then the composite blade is balanced in the horizontal plane during production by the addition of lead wool to a centrally located balance tube in the metal blade shank (which may protrude into the blade's foam core). A finish covering of polyurethane paint protects the entire blade from erosion as well as ultraviolet damage. A de-ice boot on the external surface of the blade is used for de-icing protection.

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17.2.1.2

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Classification o f Propellers There are various types or classes of propeller. The simplest of them are the fixed-pitch and the ground-adjustable propellers. The complexity of propeller systems increases from these simpler forms to the feathering and reversing propeller systems used on turboprop aircraft.

Note:

In this Lesson, only the basic types are described. No attempt has been made to include all propeller systems that may exist.

Fixed-pitch Propellers Refer to Figure 2. Fixed-pitch propellers are simple propellers whose blade angle cannot be changed during normal operation (see Figure 2, detail a)). They are usually made of wood or aluminium alloy and are usually found on light, single-engine aircraft. Fixed-pitch propellers are most efficient at one certain rotational and forward speed. But they are designed to fit a set of conditions of both aircraft and engine speeds. Any change in these conditions will reduce the efficiency of both the propeller and the engine. Ground-adjustable Propellers Ground-adjustable propellers are similar to fixed-pitch propellers in that their blade angles cannot be changed in flight. But the propeller is made so that the blade angles can be changed on the ground (see Figure 2, detail b). The pitch (i.e. the blade angle) can be adjusted to give the desired flight characteristics, i.e. a low blade angle if the aircraft is used for operation from short airstrips or a high blade angle if high-speed cruise is important. This type of propeller was widely used on aircraft built between the 1920s and the 1940s. The blades of a ground-adjustable propeller can be rotated in the hub to change the blade angles. The hub is made in 2 halves that must be slightly separated to loosen the blades to rotate them. The hub is held together with clamps or bolts to prevent the blades from rotating during operation. The propeller blades may be made of wood, aluminium or steel with shoulders machined to the root to hold the blades in the hub against the centrifugal operating loads. The hub of the propeller is made of aluminium or steel, with the 2 halves machined as a matching pair. Grooves in the hub mate with the shoulders on the blades. When steel blades are used, the hub is usually held together with bolts. When wood or aluminium alloy blades are used, the hub halves are held together with bolts or clamp rings (see Figure 2, detail b)). Controllable-pitch Propellers The controllable-pitch propeller allows a change of pitch while the propeller is rotating. This allows the propeller to select a blade angle that gives the best performance for the particular flight condition. The number of pitch positions may be limited, or the pitch may be adjusted to any angle between the minimum and the maximum pitch settings of a given propeller.

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The use of a controllable-pitch propeller makes it possible to attain the desired engine speed for a particular flight condition. As an airfoil is moved through the air, it produces 2 forces: 0 lift and

drag. An increasing blade angle increases the angle of attack and produces more lift and drag. This action increases the horsepower required to turn the propeller at a given rpm. Since the engine still produces the same horsepower, the propeller slows down. If the blade angle is decreased, the propeller will speed up. Thus, the engine rpm can be controlled by increasing or decreasing the blade angle. Constant-speed Propellers In automatic propeller systems, the control device adjusts the pitch without needing the pilot to maintain a specific preset engine rpm. Example:

If engine speed increases, the control device will automatically increase the blade angle until the desired rpm has been re-established. Refer to Figure 3. A good automatic control system responds to such small variations of rpm that, for all practical purposes, a constant rpm is maintained. Automatic propellers are usually termed 'constant-speed propellers'. Additional refinements, such as pitch reversal and feathering features, are included in some propeller systems to still further improve their operational characteristics.

Reverse-pitch Propellers Refer to Figure 4. Reversing propeller systems are refinements of the constant-speed feathering systems. The propeller blades can be rotated to a negative angle to produce reverse thrust. Air is forced forward instead of backward and permits shorter landing roll and improved ground manoeuvring characteristics. Feathering Propellers Refer to Figure 5. Most multi-engine aircraft are equipped with feathering propellers. These are constant-speed propeller systems which also have the capability of being feathered. When a propeller is feathered, its blades are rotated so that they present the smallest cross-sectional area to the wind. In this position, the drag associated with a windmilling propeller is reduced. Feathering propellers must be used on multi-engine aircraft to reduce propeller drag during 'engine-offlfailure-in-flight' operations.

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Metal blade plug

a,

Low-density foam core

''1

Erosion shield Blade retention windings I

\ Retention laminates Composite material

Solid unidirectional composite material

Laminated layers of composite material

/

Solid unidirectional composite material \

'\

\

Erosion shield

Figure 1

Laminated layers of composite material

\ Low-density foam core

Cross-section of a Composite Propeller Blade

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a) Typical fixed-pitch propeller installation

Rear bulkhead

Front bulkhead

b) Propeller hub for ground-adjustable propellers

Hub with clamp rings

d d

r"Q v

Hub with bolts

Figure 2

Fixed-pitch and Ground-adjustable Propeller

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Figure 3

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Typical Constant-speed Propeller




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Piston unit

Screws, feathering

Figure 4

Typical Feathering and Reversing Propeller

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stop

Figure 5

McCAULEY Feathering Propeller


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17.2.2

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Propeller Installation The method used to attach the propeller to the engine crankshaft varies with the design of the crankshaft. Basically, there are 3 types of crankshaft used on aircraft engines: 0 flanged crankshaft

Note:

17.2.2.1

o

tapered crankshaft

U

splined crankshaft.

The installation procedures for all 3 types are described below. But, as with all other maintenance, the manufacturer's maintenance instructions should be followed for each particular installation.

Flanged-shaft lnstallation Refer to Figure 1. Flanged propeller shafts are used on most horizontally opposed and on some turboprop engines. The front of the crankshaft is formed into a flange 4" to 8" (approx. 100 mm to 200 mm) across and perpendicular to the crankshaft center-line. Mounting bolt holes and dowel pin holes are machined into the flange and (on some flanges) threaded inserts are pressed into the bolt holes.

Preparation for lnstallation Before the propeller is installed, the flange is to be inspected for corrosion, nicks, burrs and other surface defects. Any defects should be repaired in accordance with the engine manufacturer's recommendations. Light corrosion can be removed with very fine sandpaper (crocus paper). If a bent flange is suspected, a run-out inspection should be performed. The bolt holes and threaded inserts should be clean and in good condition. When the flange area is clean and smooth, a light coat of oil or anti-seize compound is applied to prevent corrosion and to make removal of the propeller easy. Next the mounting surface of the propeller is to be inspected and prepared in the same way as the flange. The attaching bolts should be in good condition and be inspected for cracks either with the dye-penetrant or magnetic particle inspection process. Washers and nuts should also be inspected, and new fibre lock nuts used if they are required in the installation.

lnstallation The propeller is now ready to be mounted on the crankshaft. If dowel pins are used, the propeller will fit on the shaft in only one position. But if there is no dowel, the propeller is installed in the position specified in the aircraft or engine maintenance manual, as the propeller position is critical for maximum engine life in some installations.




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If no position is specified on a 4-cylinder horizontally opposed engine, the propeller should be installed so that the blades are at the '10-o'clock' and '4-o'clock' positions when the engine is stopped (see Figure I , detail b)). This reduces vibration in many instances and puts the propeller in position for hand-propping the engine. Other engine configurations, if not dowelled, are normally not affected by the installation position. The bolts, washers and nuts are installed, according to the particular installation. All bolts must be tightened slightly, then an alternating torquing sequence is to be used (see Figure 2, detail a)) to tighten the bolts to the desired value. This value is usually 35 foot-pounds or higher for metal propellers and approx. 25 foot-pounds for wooden propellers. Note:

Always consult the maintenance manual! When a 'skull cap' spinner is used, the mounting bracket is installed with 2 of the propeller mounting bolts (see Figure 1, detail b)). If a full spinner is used, a rear bulkhead is slipped on the flange before the propeller is installed and a front bulkhead is installed on the front of the boss, before the bolts are slipped through the boss. After the bolts are tightened and safetied, the spinner is installed with machine screws through the spinner into nut plates on the bulkheads (see Figure 2, detail b)). When a propeller is installed, a faceplate is normally placed on the front of the propeller boss before installing the bolts. This faceplate distributes the compression load of the bolts over the entire surface of the boss. After the bolts have been installed and properly torqued, the propeller is tracked and safetied.

17.2.2.2

Tapered-shaft Installations Refer to Figure 3. Tapered-shaft crankshafts are found on older engines of low horsepower. This type of crankshaft requires a hub to attach the propeller to the shaft.

Pre-installation Checks Before the propeller is installed, the taper of the shaft is carefully to be inspected for corrosion, thread condition, cracks and wear in the areas of the keyway. The keyway is critical, since cracks can develop in the corners of the keyway and result in breaking of the crankshaft. A dye-penetrant inspection of the keyway area is advisable each time the propeller is removed. If surface irregularities are found, the defects are dressed or polished out if allowed by the manufacturer. The hub components and mounting hardware are to be inspected for wear, cracks, corrosion and warpage. Hub and bolts are to be inspected with dye-penetrant or magnetic particle inspection methods. The fit of the hub on the crankshaft should be checked by the use of a transfer ink, such as Prussian blue. The Prussian blue is applied in a thin, even coating onto the tapered area of the crankshaft. Then, with the key installed in the keyway, the hub is installed on the shaft and the retaining nut tightened to the recommended installation torque. The hub is to be removed and the amount of dye transferred from the crankshaft to the hub is to be noted.

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The dye transfer should indicate a minimum contact area of approx. 70 %. If a contact area of less than 70 % is indicated, the hub and crankshaft should be checked for surface irregularities such as dirt, wear and corrosion. The surfaces may be lapped to fit by removing the key from the crankshaft and lapping the hub to the crankshaft, using a fine-grit polishing compound, until a minimum of 70 % contact area is achieved. Note:

The engine manufacturer's instructions should be checked for specific information about the lapping procedure. This inspection and corrective action may be done with the propeller installed on the hub. When sufficient contact area is obtained, the hub and shaft should be cleaned of the dye and the polishing compound.

Installation A very light coat of oil or anti-seize compound is applied to the crankshaft. It must be ensured that the key is installed properly. Propeller and hub assembly are placed on the shaft. It must be ensured that the threads on the shaft and nut are clean and dry. Then the retaining nut is to be installed and the nut torqued to the correct value. The puller snap ring is to be installed. The propeller should be tracked and safety-wired. Removal To remove the propeller from the tapered shaft, the safety wire has to be removed. The retaining nut must be backed off with a bar to pull the propeller from the shaft. Asnap ring is installed inside the hub so that the retaining nut can act as a puller to loosen the hub from the shaft as the nut is unscrewed. If no snap ring is installed, removal of the hub may be very difficult.

17.2.2.3

S p l i n e d - s h a f t Installations Splined crankshafts are found on most radial engines and some horizontally opposed, in-line and even turboprop engines. The splined shaft has grooves and splines of equal dimensions and a 'master' (or: double-width) spline so that a hub will fit on the shaft in only one position. A typical splined shaft is shown in Figure 3, detail d).

Pre-installation Checks The crankshaft must be inspected for cracks, surface defects and corrosion. Any defects are to be repaired in accordance with the engine manufacturer's instructions. The splines on the crankshaft and on the hub are inspected for wear by using a 'golno-go' gauge which is 0.002" (0.05 mm) larger than the maximum space allowed between the splines. The crankshaft or spline is serviceable if the gauge cannot be inserted between the splines for more than 20 % of the spline length. If it goes more than 20 % of the way in, the hub or the crankshaft is excessively worn and must be replaced.

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Refer to Figure 4. The cones that are used to center the hub on the crankshaft should be inspected for general condition. The rear cone is made of bronze and is split to allow flexibility during installation and to ensure a tight fit when it is installed. The front cone is made in 2 pieces and is a matched set. The 2 halves are marked with a serial number to identify the mates in a set. Typical Trial Installation The rear cone and (in some installations) a bronze spacer are slipped on the crankshaft and pushed all the way back on the shaft. A thin coat of Prussian blue is applied to the rear cone. The hub is slided on the shaft. In doing so care must be taken that the hub is aligned on the master spline. The hub is pushed back against the rear cone. The front cone halves are coated with Prussian blue and placed around the lip of the retaining nut. The nut is installed in the hub and tightened to the proper torque. The retaining nut and front cone are removed and the amount of Prussian blue transferred to the hub is noted. A minimum of 70 % contact is required. The hub is removed from the crankshaft and the transfer of dye from the rear cone noted. Again, a minimum of 70 % contact is required. If contact is insufficient, the hub is to be lapped to the cones by using special lapping fixtures. If no dye is transferred from the rear cone during the transfer check, a condition known as 'rear cone bottoming' exists. This happens when the apex (or: point) of the rear cone contacts the land on the rear seat of the hub before the hub can seat on the rear cone (see Figure 4, detail c)). This rear cone bottoming is corrected by removing up to 0.0625" (1.6 mm) from the apex of the cone with sandpaper on a surface plate. 'Front cone bottoming' occurs when the apex of the front cone bottoms on the splines of the crankshaft before it seats on the hub. Front cone bottoming is indicated when the hub is loose on the shaft after the retaining nut has been torqued and there is no transfer of Prussian blue to the front hub seat. The front cone bottoming is corrected by using a spacer with a thickness of not more than 0.125" (3.2 mm) behind the rear cone. This moves the hub forward so that the front cone can seat properly. Installation The propeller is installed on the hub in the same way as described for a taperedshaft installation. The position of the propeller on the hub in relation to the master spline is critical. Some installations require that one blade aligns with the master spline while other installations require that the blades be perpendicular to the master spline position. So it must be ensured that the engine maintenance manual is consulted for the requirements of a particular installation. Once the propeller is mounted on the hub, the crankshaft will be coated with oil or an anti-seize compound and propeller and hub assembly are slided in place on the shaft. The front cone is installed and torqued. Then the retaining nut is installed. The propeller is tracked and then the installation safety-wired. Propeller removal is done in the same way as a tapered-shaft installation.

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a) Typical crankshaft flanges for mounting a propeller

Flange with dowel pin holes

Flange with threaded inserts

b) Position of propeller installation on a 4-cylinder opposed engine

c) Typical skull cap spinner installation

Figure 1

Propeller, Flange-mounted Directly to the Crankshaft (Light Aircraft)




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a) Torque sequence for tightening propeller retaining bolts

b) Spinner installation

I Spinner bulkhead wjth thread-holes

Figure 2

I \ Screw Counterweight for interrupter

Installation of Flanged Shaft and Spinner


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b) Tapered shaft used on some smaller aircraft engines

a) Propeller hub used to mount a wooden propeller on a tapered or spline shaft engine

Flange plate

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Safety holes

Face plate

c) The snap ring is installed inside the hub of a propeller mounted on a splined or tapered shaft to aid in pulling the propeller from the shaft Retaining nut

Hub ,

ring

-

--h

d) Spiined propeller shaft showing the master spline / Master spline

Figure 3

Propeller Installation on a Tapered and a Splined Shaft


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b) Propeller retaining nut and the front cone to secure a propeller to the splined shaft

a) Front and rear cone for mounting a propeller on a splined shaft

Front cone showing the serial numbers

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Rear cone

c ) Rear cone bottoming I 3

Point of contact d) Front cone bottoming

Hub /

Point of contact

Figure 4

Propeller Installation and Securing with Front and Rear Cone on a Splined Shaft




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17.3

Propeller Pitch Control

17.3.1

Propeller and Power Management of Turboprop Engines

17.3.1.1

Propeller System

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Fundamentals Refer to Figure 1. A propeller system consists of far more than just blades mounted in a hub at the front of the typically configured aircraft. First of all, the propeller hub serves as a device to contain the blades (centrifugal force can range from 15,000 Ibs. to as much as 50,000 Ibs. for normal operation). Secondly, the hub assembly contains the propeller blade pitch change mechanism that on command from a governing device accomplishes a pitch change. Of fundamental importance in the design of this pitch change mechanism is the manner in which it defaults upon loss of control input (usually engine oil supply for the typical hydraulically actuated system).

Twin-engine Aircraft A twin-engine aircraft will default to a feather command and a single-engine aircraft will typically default to a low pitch command. There are exceptions as with most things.

Single -engine Aircraft A single-engine aircraft system with a reciprocating engine may be so defined as to default to high pitch on loss of engine oil pressure if the installation is aerobatic. Therefore, when oil pressure is lost during an aerobatic maneuver, the propeller will coarsen pitch thus resulting in a reduced rpm thus preventing an engine overspeed. Another exception is the single-engine turboprop powered aircraft where the propeller will typically default to a feather command on loss of control input.

Pitch Control The pitch control command typically comes from a hydraulically operated fly-ball governor mounted on the engine. The governor senses whether the enginelpropeller is running at the correct rpm and either supplies or maintains oil flow to the propeller, or drains oil to the engine crankcase. Of course, it is very important that the engine-mounted governor be properly matched to the propeller for proper pitch change command and response. In addition to controlling the propeller, this governor typically contains a pump that increases the oil pressure to a higher level than that which the engine supplies to a value sufficient to control the propeller.

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Turbine-po wered Aircraft On turbine-powered aircraft, it is normal to include an overspeed governor into the propeller control system that backs up the primary propeller governor. Therefore, if the primary governor fails in such a manner as to command a reduced-pitch (increased rpm) command, its command can be overridden by the overspeed governor whose setting is just above normal maximum rpm, and therefore the overspeed governor can provide sufficient rpm control to so as to allow the aircraft to land safely.

17.3.1.2

Propeller Pitch Control System

Fundamentals Variable-pitch Propellers Constant-speed Propeller System With a constant-speed (constant-rpm) system, the pilot selects the propeller and engine speed for any situation and the system automatically maintains that rpm under varying conditions of aircraft attitude and engine power, thereby permitting operation of propeller and engine at the most efficient rpm. Speed is controlled by varying the pitch of the propeller blades, i.e. the angle of the blades with relation to the plane of rotation. When the pilot increases power in flight, the blade angle is increased, the torque required to spin the propeller is increased and, for any given rpm setting, aircraft speed and torque on the engine will increase. For economic cruising, the pilot can throttle back to the desired manifold pressure for cruise conditions and decrease the pitch of the propeller, while maintaining the pilot-selected rpm. Full-feathering Propeller System

A full-feathering propeller system is normally used only on twin-engine aircraft. If one of the engines fails in flight, the propeller on the idle engine can rotate or windmill, causing increased drag. To prevent this, the propeller can be feathered (turned to a very high pitch), with the blades almost parallel to the airstream. This eliminates asymmetric drag forces caused by windmilling when an engine is shut down. A propeller that can be pitched to this position is called a full-feathering propeller Changing Pitch Pitch is changed hydraulically in a single-acting system, using engine oil controlled by the propeller governor to change the pitch of the propeller blades. In constant-speed systems, the pitch is increased with oil pressure. In full-feathering systems, the pitch is decreased with oil pressure.

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To prevent accidentally moving the propellers to the feathered position during powered flight, which would overload and damage an engine that is still running, the controls have detents at the low-rpm (high-pitch) end. Governing Mode The governing mode occurs during flight. The constant-speed unit is maintaining the rpm that has been selected by the pilot through the condition lever. Blades increase pitch to decrease rprn. Blades decrease pitch to increase rpm. The range of blade angle change is from 'feather' to 'flight idle'. Beta Mode The beta mode occurs during ground operations (i.e. taxiing, reverse). The pilot manually controls the blade angle of the propeller by movement of the power lever within the beta range. The range of blade angle change is from 'flight idle' to 'maximum reverse'. Propeller Governor Refer to Figure 2. Besides the propeller, the other major component of the system is the governor.The propeller governor is an rpm-sensing device which responds to a change in system rpm by directing oil to or releasing oil from the propeller to change the blade angle and return the system rpm to the original value. The governor may be set up for a specific rpm by the cockpit propeller control. A typical propeller governor is divided into 3 parts: @ head body base. The head of the governor contains

o

the flyweights the flyweight cup the speeder spring a speeder rack-and-pinion mechanism a control pulley.

A flange for the pulley adjustment stop screw is cast at the side of the head. Some head designs incorporate a balance spring above the speeder rack to set the governor to cruise rpm if the control cable breaks. The body of the governor contains the propeller oil flow control mechanism. This mechanism is composed of the pilot valve, oil passages and the pressure relief valve. This valve is usually set to 180 psi to 200 psi. The base contains the booster pump, the mounting surface for installation in the engine and oil passages. The passages direct engine oil to the pump and return oil from the propeller to the engine sump. The main components, i.e. head, body and base, are held together with studs and nuts.




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Principle of Operation A typical governor contains a drive shaft which is connected to the engine drive train. The drive shaft rotates at a speed that is proportional to the engine rpm. An oil pump drive gear is mounted on the drive shaft. It meshes with an oil pump idler gear. These gears take oil at the engine oil system's pressure and boost it to the propeller's operating pressure. Excess pressure built up by the booster pump is returned to the inlet side of the pump by a pressure relief valve. The boosted oil is routed through passages in the governor to a pilot valve which fits in the center of the hollow drive shaft. This pilot valve can be moved up and down in the drive shaft. It directs oil through ports in the drive shaft to or from the propeller to vary the blade angle. The position of the pilot valve is determined by the action of the flyweights attached to the end of the drive shaft. The flyweights are designed to tilt outward when the rprn increases and inward when rprn decreases. When the flyweights tilt outward, they raise the pilot valve, and when they tilt inward, the pilot valve is lowered. The movement of the pilot valve in response to changes in rprn directs the oil flow to adjust the blade angle to maintain the selected rpm. The action of the flyweights is opposed by a speeder spring located above the flyweights. The tension of the spring can be adjusted by the pilot via a control cable (pulley) and a speeder rack. When a higher rprn is desired, the cockpit control is moved forward to compress the speeder spring. This increased spring compression tilts the flyweights inward and the pilot valve is lowered. This causes the blade angle to decrease. The rprn will increase until the centrifugal force on the flyweights overcomes the force of the speeder spring and returns the pilot valve to the neutral position. The opposite action occurs if the cockpit control is moved aft: when the speeder spring compression is reduced, the flyweights tilt outward, the pilot valve is raised and the blade angle increases (because oil is bled off) until the engine slows down and the centrifugal force on the flyweights decreases. The pilot valve then returns to its neutral position. Refer to Figure 3. Whenever the flyweights tilt outward and the pilot valve is raised, the governor is said to be in an 'overspeed condition'. This means that the propeller rprn is h~gherthan the setting of the speeder spring calls for (the propeller blades need to be coarsened). Refer to Figure 4. When the flyweights tilt inward, the governor is in an 'underspeed condition', i.e., the rprn is lower than the setting of the speeder spring calls for (the blades need to be fined). Refer to Figure 5. When the propeller rpm is the same as the governor setting is calling for, the governor is in its 'on-speed condition'. The same governing action of the flyweights and pilot valve occurs with changing flight conditions.

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Refer to Figure 4 again If the aircraft is in a 'cruise' condition and the aircraft begins to climb, the airspeed will decrease. This causes an increase in the angle of attack of the propeller blades. Now, more drag is produced and the propeller rprn decreases. The governor senses this decrease in rpm by the reduced centrifugal force on the flyweights. They tilt inward, lower the pilot valve and produce an underspeed condition. When the pilot valve is lowered, the blade angle is reduced and the rprn increases to its original value. The system returns to the on-speed condition. Refer to Figure 3 again. If the aircraft is placed in a dive from cruising flight, an overspeed condition is created. Now, the governor will cause an increase in blade angle to return the system to the on-speed condition. A change in throttle setting will have the same effect as placing the aircraft in a climb or dive. Increasing the throttle will cause an increase in the blade angle to prevent an increase in rpm. Retarding the throttle will result in a decrease in blade angle.

Single- and Dual-acting Propellers

17.3.1.3

The turboprop propeller is operated by a gas turbine engine through a reduction gear assembly. It has proved to be an extremely efficient power source. Turboprop engines are used on aircraft ranging in size from the large 4-engine transports to relatively small single-engine business and utility aircraft. Because the engine and propeller must work together to produce the required thrust for a turboprop installation, there are a few unique relationships. Refer to Figure 6. Basically, a turboprop engine is equipped with either a single- or a dual-acting propeller. The differences between single- and dual-acting propeller operations are as follows: Single-acting Propeller Operation Inherent Forces Aerodynamic twisting moment Centrifugal twisting moment. Controlling Force High-pressure oil is ported to one side of a piston against a spring and counterweights to achieve a change in blade angle. The oil pressure from the governor opposes the centrifugal force of the counterweights and the feathering spring. Failure Modes Loss of oil pressure causes the propeller to go to a high blade angle (feather) due to counterweights and feathering spring.

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Dual-acting Propeller Operation Inherent Forces 3

Aerodynamic twisting moment

0 Centrifugal twisting moment. Controlling Force

o

High-pressure oil is ported to one side or the other of a piston to achieve a change in blade angle.

n Oil pressure vs oil pressure. Failure Modes Loss of oil pressure to the propeller results in the propeller going 'uncontrolled' to a low blade angle which is usually prevented by a pitch lock. Loss of oil pressure In the control system results in the propeller going to a high blade angle (feather) if oil is available for actuation of the propeller. Governing Mode and Beta Mode

Governing Mode The governing mode occurs during flight. The constant-speed unit maintains the rpm that has been selected by the pilot through the condition lever. The blades increase pitch to decrease rpm. The blades decrease pitch to increase rpm. The range of blade angle change is from feather to flight idle. Beta Mode The beta mode operates during ground operations (i.e. taxiing, reverse). The pilot manually controls the blade angle of the propeller by moving the power lever within the beta range. The range of blade angle change is from flight idle to maximum reverse.

17.3.1.4

Single-acting Propeller and Power Management of a Single-shaft Engine The turboprop engine described as an example in the following Chapter is the AiResearch GARRElT TPE 331 single-shaft engine.

Note:

Power Management of the TPE 331 The cockpit controls of the TPE 331 engine are shown in Figure 7. On the left there is the power lever. The power lever is primarily used to control output power. It depends on the mode of operation whether this is fuel or torque.




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Both functions have a direct relationship to temperature. The speed lever, on the right, is sometimes called the 'condition' or 'rprn lever'. It basically serves the function to select the engine operating speed. Refer to Figure 8. To maintain constant speed, engine power and propeller load must be balanced. Power management is nothing else but controlling these 2 factors. The function of the power management system is to provide a means for controlling load and power. To do this a basic system using one lever to control power and a separate lever to control propeller load can be used. Moving the fuel lever will cause rpm to increase or decrease, but a real thrust will not be produced until the pitch lever is moved. The same can be said about using the pitch lever alone. Rpm fluctuations can be caused by varying propeller load, but a real power will not be produced. If the operator were extremely good, both levers could be worked simultaneously (Figure 9, detail a)). This would keep rpm constant and give useful thrust with power. To reach this aim, 4 controlling devices are incorporated. To control engine power a manual fuel valve (MFV) and an underspeed governor (USG) are used, both of which are contained in the fuel control unit (FCU). To control the propeller load a propeller pitch control (PPC) and a propeller governor (PG) are utilised. These components are mounted on the aft side of the gearbox. Rather than depending on the skilful movement of 2 levers, the TPE 331 incorporates the 2 functions in one lever through the use of 2 specially cut cams (Figure 10, detail a)). These cams are located in the manual fuel valve and the propeller pitch control and are controlled by the power lever. Movement of the power lever causes the cams to effect their respective components, while the cuts of the cams ensure that both cams are not effective at the same time.

Example:

When the power lever is advanced forward from the flight idle detent, the propeller pitch control cam will momentarily hold the propeller at a fixed pitch. The manual fuel valve cam will then cause fuel to increase. When the power is adjusted according to the set power lever position, then the pitch control cam basically becomes ineffective. The constant-speed theory pointed out earlier implies that the engine operates at one rprn when load and power are equal. Because of different operating conditions, such as taxiing or cruise, it is necessary to operate at other rpm besides 100 %, i.e. for noise reduction, fuel economy or operation at minimum load. To accomplish this the TPE 331 includes a speed control function through the speed lever. The only function of the speed lever is to set engine operating rpm. In doing so the speed lever gets support by the underspeed fuel governor and the propeller governor, linked to it as shown in Figure 10, detail b).

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The speed lever 'calibrates' or sets each governor rpm limit. With the speed lever in the low or taxi position, D the underspeed governor is set to 65 %

- 73 % rprn

the propeller governor is set to 94.5 % - 96.5 % rprn depending upon the aircraft application. When the speed lever is advanced to the high or take-off position the underspeed governor is set to 97 % rprn the propeller governor is set to 100 % rpm. Example:

The speed lever is placed in the 'high' position. The engine is operating at 97 %. As the power lever is advanced ahead of the flight idle detent this causes the power lever cams to react by momentarily holding a fixed pitch and increasing fuel (the schematic drawing in Figure 11 shows the operation with the power and speed levers). With the increase in rprn the propeller governor setting of 100 % is reached. The propeller governor then takes control of the blade angle and increases it to maintain the selected rprn by equalling load with power demand. This is known as 'propeller governing mode'. Because of the cut of the cams the propeller pitch control has no effect and so the propeller governor has automatic load control. The underspeed governor is effectively overridden by the manual fuel valve. The prop governing (or in-flight) mode is shown in Figure 12, detail a). The range of operation of the power lever is from flight idle to maximum. The effective component is the manual fuel valve. Normal propeller governing operation of the speed lever is from cruise to take-off, or high. The increased fuel demand drives the rprn to the setting of the propeller governor which changes the blade angle to the propeller to equal the demanded power. In 'beta' or ground mode of operation (shown in Figure 12, detail b)) the pilot has manual control of propeller load through the propeller pitch control. The range of operation of the power lever is from flight idle to reverse. The manual fuel valve cam is cut such that it now has effect in this area. Speed control is a function of the underspeed governor. Normal range of the speed lever in beta mode is from low to high. For beta operation, the effect of bringing the power lever behind flight idle is that fuel is reduced to the point where rprn drops below the setting of the propeller governor. The underspeed governor then assumes control of fuel to maintain the selected minimum rprn. If the speed lever were high, this rpm would be 97 %. The need for a beta mode derives itself from the need to manually demand a reverse pitch to bring the aircraft to a stop after landing. Also directional thrust control is needed to provide adequate control of the aircraft for taxiing. Refer to Figure 13. In flight, the manual fuel valve is the effective component giving a manual power demand.




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The propeller governor is the other effective component responsible for automatic load control to equal with the manual power demand and maintain the selected engine rpm. This mode is known as 'propeller governing mode'. For ground operation, the propeller pitch control provides manual load control, the underspeed governor automatically controls fuel flow in response to load changes. This equals power and load to maintain selected engine rpm. This mode is called 'beta mode'.

Mode

Controlling elements

Flight

manual fuel valve ( M N ) and propeller governor (PG)

Beta

I

propeller pitch control (PPC) and underspeed governor (USG).

Propeller and Propeller Control System Refer to Figure 14. The propeller commonly used on the TPE 331 is a 3- or 4-blade HARTZELL steel hub reversing propeller. The propeller is spring-loaded and counterweighted to the feather position and uses engine oil pressurised by the governor to decrease the blade angle. The propeller is flange-mounted on the drive shaft and it locks at a flat angle of approx. 2" when the engine is shut down on the ground. This prevents excessive strain on the engine starter when the engine is being started. The propeller is constructed similarly to the feathering steel hub designs. The principal additional component is the beta tube, which passes through the center of the propeller and serves as an oil passage and follow-up device during propeller operation. Refer to Figure 15. To minimise the aerodynamic load of the propeller on starter and power supply, propellers used on the TPE 331 series engines have 'start locks' to hold the propellers at or near a flat pitch angle during ground starting of the engine. The start locks are spring-loaded pins that hold the propeller blades to a flat position. A plate that engages the locking pin is at the base of each propeller blade hub. The HARTZELL propeller is always installed or removed with the blades in the 'feather' position. This prevents the load of the heavy feather spring from distorting the start lock arrangement. The single-acting propeller is spring-loaded to the feather position. During engine operation, this spring force is overcome by oil metered from the power management system. If the operator fails to put the propeller on the start locks during shut-down, the blades must be moved from a feathered position to a flat pitch for ground starts. A nacelle-mounted propeller unfeathering pump is used to put the propeller on the start locks. The procedure for this is to position the power lever to the reverse position and actuate the unfeathering pump switch. Oil is then routed to the propeller pitch control through the oil transfer tube to the propeller. Pressure overcomes the spring force and causes the propeller to rotate towards the reverse position. When the start latches engage, the unfeathering pump is de-activated.




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The oil transfer tube (beta tube, shown in Figure 15, detail b)) is threaded into the propeller piston and extends aft, through the engine propeller shaft and into the propeller pitch control. The tube portion housed within the propeller pitch control ported sleeve has oil ports through which propeller governor discharge oil is routed to the propeller dome. During beta mode, the oil transfer tube is positioned by power lever movement of the servo-valve in the propeller pitch control which meters oil pressure to the piston to position blade pitch angle. During propeller governor mode, the governor meters oil pressure through the propeller pitch control and oil transfer tube directly to the propeller piston.

System Operation A schematic drawing of the complete propeller control system of a TPE 331 engine is shown in Figure 16. The 2 basic operating modes of the TPE 331 system are the beta mode, meaning any ground operation including start, taxi and reverse operation (shown in Figure 17), and the alpha mode, which is any flight operation from take-off to landing. Typically, beta mode includes operation from 65 % to 95 % rprn and alpha mode includes operation from 95 % to 100 % of the system rated rpm. When the engine is started, the power lever is set to the ground idle position and the speed lever is in the start position. When the engine starts, the propeller latches are retracted by reversing the propeller with the power levers. The propeller moves to a zero-degree blade angle as the propeller pitch control is positioned by the power lever over the beta tube. The beta tube is attached to the propeller piston and it moves forward with the piston as the propeller moves to the low blade angle. The propeller blade angle stops changing when the beta tube moves forward to the neutral position in the propeller pitch control Refer to Figure 18. The speed lever is used to set the desired rprn through the underspeed governor during ground operation, and the power lever is used to valley the blade angle to cause the aircraft to move forward or rearward. If the power lever is moved forward, the propeller pitch control moves rearward, so that the oil ports on the end of the beta tube are open to the gear reduction case and the oil in the propeller is forced out by the springs and counterweights. As the blade angle increases, the propeller piston is moving inward until the beta tube returns to its neutral position in the propeller pitch control unit. This causes a proportional response of the propeller to the power lever movement. With the increase in blade angle, the engine starts to slow down, but the underspeed governor, which is set by the speed lever, adjusts the fuel flow to the engine to maintain the selected rpm. When the power lever is moved rearward, the propeller pitch control moves forward over the beta tube and governor oil pressure flows out to the propeller piston and causes a decrease in blade angle. As the piston moves outward, the beta tube moves with it and returns to the neutral position as the blade angle changes. With this lower blade angle, the engine rprn will try to change, but the underspeed governor will reduce the fuel flow to maintain the selected rpm. In the alpha mode of operation, flight operation, the speed lever is moved to a high rprn setting between 95 % and 100 % and the power lever is moved to the flight idle position.

For training only


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When this is done, the underspeed governor is fully opened and no longer affects system operation. Rpm control is now accomplished through the propeller governor. When the power lever is moved to flight idle, the propeller pitch control moves forward so that the beta tube is fully in the propeller pitch control and it no longer functions to adjust the blade angle. The power lever now controls the fuel flow through the engine fuel control unit. With a fixed power lever setting in the alpha mode, the propeller governor is adjusted by the speed lever to set the system rpm in the same manner as for any constantspeed system. Refer to Figure 19. With a fixed speed lever setting in the alpha mode, the power lever adjusts the fuel control unit to control the amount of fuel delivered to the engine. When the power lever is moved forward, fuel flow will increase and the propeller blade angle will be increased by the propeller governor to absorb the increased engine power and maintain the set rpm. When the power lever is moved aft, fuel flow will decrease and the propeller blade angle will decrease by the action of the propeller governor to maintain the selected rpm. Whenever it is desired to feather the propeller, the feather handle is pulled or the speed lever is moved full aft, depending on the aircraft design. This action shifts the feathering valve located on the rear of the gear reduction assembly and releases the oil pressure from the propeller, returning the oil to the engine sump. Refer to Figure 20. The springs and counterweights on the propeller force the oil out of the propeller and the blades go into the feather angle. To unfeather the propeller, the electric unfeathering pump is turned on with a toggle switch in the cockpit and oil pressure is directed to the propeller to reduce the blade angle. This causes the propeller to start windmilling in flight and an air start can be accomplished. On the ground, the propeller can be unfeathered in the same manner before starting the engine.

Installation and Adjustment The propeller is installed following the basic procedure used for the installation of other flange-shaft propellers. The beta tube is installed through the propeller piston after the propeller is installed and is booted to the forward part of the piston. To adjust the reverse blade angle, the beta tube is adjusted in or out of the piston to set the neutral position on the propeller pitch control unit. This angle must be adjusted according to the aircraft service manual during propeller installation. Propeller governor, propeller pitch control, feather valve and fuel control unit are mounted on the engine gear reduction assembly in accordance with the engine service manual. The interconnection between speed lever, underspeed governor and propeller governor is rigged and adjusted according to the manual pertaining to the particular aircraft and engine model used. The same holds true for the interconnection between power lever, fuel control unit and propeller pitch control.

For training only

(c) by L~nk& Learn Aviation Training GmbH 551132

E A S A Part-66 Training Handbook

Module 17

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System Maintenance

When the propeller is inspected and repaired the basic procedures set forth for other versions of the HARTZELL steel hub propeller have to be followed. When the beta tube is removed or installed care has to be taken that the tube's surface will not be damaged. The beta tube is of trued roundness and is machined to close tolerances. The propeller control units must be inspected for leaks, security and damage. The linkages between these units are to be checked for freedom of movement, security and damage. Any defective seals should be replaced, rigging adjusted and all nuts and bolts secured as appropriate for the installation. The engine or aircraft maintenance manuals are to be used for specific instructions concerning each aircraft mode. Basic trouble-shooting procedures as previously discussed apply to the HARTZELL reversing propeller system. If the proper propeller response does not occur and there is no obvious defect, the system must be checked for proper rigging. In the beat mode, if the rpm is not constant, the underspeed governor on the fuel control unit should be investigated. If the blade angle does not properly respond to power lever movement, propeller pitch control is to be checked. In the alpha mode, if the rpm is not constant, the propeller governor must be checked. If power does not change smoothly, the fuel control unit is to be checked.

17.3.1.5

Propeller and Power Management of a Multiple-shaft Engine

Note:

The turboprop engine described as an example in the following Chapter is the PRATT & WHITNEY PT6 multiple-shaft engine.

Power Management System of the PRATT & WHITNEY PT6 Multiple-shaft Engine

The PT6 engine is a free-turbine turboprop engine that produces more than 600 HP at 38,000 rprn. A geared reduction mechanism couples the engine power turbine to the propeller drive shaft with the propeller rotating at 2,200 rprn at 100 % rpm. Note:

The 'free-turbine' design means that the power turbine is not mechanically connected to the engine compressor. The hot gases generated by the gas generator section of the engine flow through the power turbine wheel and cause the power turbine and the propeller to rotate. Another turbine section is mechanically linked to the compressor section and is used to drive the compressor. During engine start it is possible that the compressor and its turbine rotate while the propeller and the power turbine remain stationary. The starter motor is not under load from the propeller and the power turbine. For this reason, the propeller can be shut down in 'feather' and does not need a low blade angle latch/lock mechanism for engine starting.




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Propeller Refer to Figure 21. The propeller commonly used with the PT6 is a 3- or 4-blade HARTZELL steel-hub reversing propeller. The propeller is flange-mounted on the engine and is springloaded and counterweighted to the 'feather' position, with oil pressure being used to decrease the blade angle. A beta-slipring assembly on the rear of the propeller serves as a follow-up mechanism, giving proportional propeller response to control inputs in the beta mode.

Governor The propeller governor used with the PT6 is basically the same as other governors discussed for constant-speed operation. It uses a speeder spring and flyweights to control a pilot valve which directs the oil flow to and from the propeller (shown in Figure 22, detail a)). A lift rod is incorporated in the governor to allow the propeller to feather. For beta mode operation, the governor contains a beta control valve operated by the power lever linkage. It directs oil pressure generated by the governor boost pump to the propeller, or relieves oil from the propeller to change the blade angle.

System Components A propeller overspeed governor is mounted on the gear reduction assembly and it releases oil from the propeller whenever the propeller rpm exceeds 100 %. The release of oil pressure results in a higher blade angle and reduction in rpm. The overspeed governor is adjusted by the overhaul facility; it cannot be adjusted during flight. There are no cockpit controls to this governor except for a test mode in some aircraft. The overspeed governor is shown in Figure 22, detail b).

A power turbine governor is installed on the gear reduction assembly as a safety back-up in case the other propeller governing devices fail. If the power turbine speed reaches about 105 %, the power turbine governor will reduce the fuel flow to the engine. Refer to Figure 23. The power turbine governor is not controllable from the cockpit. The engine fuel control unit is mounted on the rear of the engine and is linked through a cam assembly to the beta control valve on the propeller governor and also to the beta slipring on the propeller. This interconnection with the fuel control unit is used during beta mode operation.

Cockpit Controls The cockpit controls of the PT6 turboprop installation consist of a power lever controlling engine power output in all modes and propeller blade angle in the beta mode. There is also a propeller control lever which adjusts the system rpm when in the alpha mode and a fuel cut-off lever which turns the fuel on or off at the fuel control. The power lever is linked to the cam assembly on the side of the engine and from there, rearward to the fuel control unit and forward to the propeller governor beta control valve. The power lever adjusts both engine fuel flow and propeller blade angle when operating in the beta mode which is reverse to flight idle.

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M o d u l e 17

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In the alpha mode, the lever controls only the fuel flow to the engine. The propeller control lever adjusts system rprn in the alpha mode through conventional governor operation. Full aft movement of the lever raises the lifl rod in the governor and causes the propeller to feather. The fuel cut-off lever turns the fuel to the engine on and off at the engine fuel control unit. Some designs have an intermediate position called 'Lo-idle' to limit system power while operating on the ground.

System Operation Beta mode operation is generally in the range of 50 % to 85 % rpm. In this range, the power lever is used to control both fuel flow and propeller blade angle. When the power lever is moved forward, the cam assembly on the side of the engine causes the fuel flow to the engine to increase. At the same time, the l~nkageto the propeller governor moves the beta control valve forward out of the governor body and oil pressure in the propeller is released. Figure 24 shows a schematic drawing of the power management PT6. As the propeller cylinder moves rearward in response to the loss of oil, the slipring on the rear of the cylinder moves rearward and returns the beta control valve through the carbon block and linkage to a neutral position. When the power lever is moved rearward, fuel flow is reduced and the beta control valve moves into the governor body, directing oil pressure to the propeller to decrease the blade angle. And as the propeller cylinder moves forward, the beta control valve returns to its neutral position by the action of the slipring, carbon block and linkage. If the power lever is moved aft of the zero thrust position, fuel flow will increase and the blade angle goes negative to allow a variable reverse thrust. This change in fuel flow is caused by the cam mechanism on the side of the engine. During operation in the beta mode, the propeller governor constant-speed mechanism is in underspeed and the pilot valve is lowered. The governor oil pump supplies the oil pressure for propeller operation in the beta mode. In the alpha mode, the system rprn is high enough for the propeller governor to operate and the system is in a constant-speed mode of operation. When the power lever is moved forward, more fuel flows to the engine to increase the horsepower and the propeller governor causes an increase in propeller blade angle to absorb the power increase and maintain the selected system rpm. If the power lever is moved aft, the blade angle will be decreased by the governor to maintain the selected rpm. To feather the propeller, the propeller control lever is moved full aft. This raises the pilot valve in the governor by a lift rod and releases all of the oil pressure in the propeller. The springs and counterweights in the propeller will take it to 'feather'. To unfeather the propeller, the engine is started. As the engine begins to rotate, the power turbine will rotate and the governor or beta control valve will take the propeller to the selected blade angle or governor rprn setting. If the propeller rprn exceeds 100 %, the propeller overspeed governor will raise its pilot valve and release oil from the propeller to increase blade angle and prevent overspeeding of the propeller. The overspeed governor is automatic and is not controllable in flight.




M o d u l e 17

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The power turbine governor prevents excessive overspeeding of the propeller by reducing fuel flow to the engine at approximately 105 % rpm. This governor is not controllable in flight and is automatic in operation.

17.3.1.6

Dual-acting Propeller and Power Management of t h e PRAlT & WHlTNEY PW-119 Multiple-shaft Turboprop E n g i n e

Propeller and Control System The dual-acting propeller system consists of 2 major systems (Figure 25, detail a)): LI propeller control system. Dual-acting propeller components are (Figure 25, detail b)): propeller hub assembly pitch change components fl composite blades fl hydraulic unit n pitch lock. Functional Description The dual-acting propeller (HARTZELL HD-E6C-3, 6-blade) is a constant-speed, hydraulic-actuated propeller with feathering and reversing capabilities, designed for use with PRAlT & WHlTNEY PW-119 series turboprop engines. The propeller is constructed of a 2-piece aluminium hub with composite blades. A 105"-pitch range is available with adjustable reverse and feather blade angles. The low pitch blade angle is controlled by the propeller control unit (PCU). The propeller uses a pitch lock unit to protect the propeller from damaging overspeeds resulting from uncommanded movement of the blades toward 'low pitchl/'reverse' positions. Operation Blade pitch is controlled by the main pistonlcylinder combination mounted on the forward flange of the propeller hub. High-pressure oil is supplied from the high-pressure pump to the propeller by the oil transfer tube. This oil is metered to either side of the main piston via the propeller control valve. The oil transfer tube and control rod are locked together during propeller installation. Forward and aft movement of the oil transfer tube, as commanded by the PCU, is transmitted to the control rod. The linear movement of the control rod meters high-pressure oil to either the high-pitch or low-pitch side of the propeller pitch. High-pressure oil forced into the cavity between the piston and the hub moves the piston forward from high to low pitch. High-pressure oil forced into the cavity between the piston and the cylinder moves the piston aft, toward the engine, from low to high pitch.




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Refer to Figure 26. The linear motion of the piston is transmitted to each blade assembly through a pitch change rod, a slotted fork and a blade pitch change knob unit. Blade pitch is controlled by the knob wh~chis bolted and pinned to the shank of the blade. A roller bearing on the end of the knob minimises friction. Each blade is supported by a blade retention split-bearing which permits pitch change. Blade Movement

On Speed Refer to Figure 27 The propeller control valve meters oil to one side of the piston while draining an equal amount of oil from the opposite side. The control valve is spring-loaded toward the feather position by the forward control spring. The valve sleeve is retained and located in the pitch change rod by a pin riding in a slot. The sleeve location in relation to the pitch change rod is maintained by the aft control spring holding the sleeve against the pin. The aft control spring is weaker than the forward control spring. During normal operation the aft control spring and valve sleeve remain stationary with the sleeve at its forward limit, both ports open. Underspeed/Re verse

Refer to Figure 28. The operation of the PCU servo-piston toward low pitch moves the oil transfer tube, and, in turn, the control rod forward. This movement of the control rod compresses the forward control spring slightly, porting high-pressure oil to the reverse side of the piston while draining oil from the feather side.

Refer to Figure 29. When oil is allowed to drain from the servo-cylinder during the commanded higher blade angle, the forward control spring overcomes the force of the servo-piston, moving the valve aft. This action ports high-pressure oil to the feather side of the piston while draining oil form the reverse side. Valve Sleeve Shifted Toward Feather

Refer to Figure 30. In the event that the control valve spool seized to the valve sleeve, not allowing the valve to return to the feather position, the forward control spring would overcome the force of the aft control spring. The spring would be compressed, allowing the valve housing itself to move aft, draining oil from the reverse side of the piston.

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Hub Assembly Refer to Figure 31. The hub assembly of the propeller consists of the following items: hub unit mounting bolt lubrication fittings. Hydraulic Unit The hydraulic unit consists of the following items: piston aluminium cylinder aluminium hardcoated anodised cylinder control valve. Pitch Lock Refer to Figure 32. In the dual-acting system there are no built-in forces that tend to feather the propeller in the event of any loss in supplying oil pressure to the propeller, i.e. counterweights and feathering spring, as used in single-acting propellers. To prevent an overspeed condition, a pitch lock mechanism has been fitted. The pitch lock is designed to be a one-way locking device. Locking occurs any time the propeller piston moves toward low pitch and the transfer tubelcontrol rod combination (i.e. PCU input) command higher pitch or are stationary. If system oil pressure is lost, the forward control spring will push the control rod toward feather, however since there is no system pressure, the propeller cannot feather. Due to the centrifugal twisting moment on the blades the propeller will have a natural tendency to change pitch toward a lower blade angle. The pitch lock has been designed specifically for this condition and will lock the blade pitch within 2" of travel, preventing a severe overspeed. If system pressure is subsequently regained, the hydraulic forces can disengage the pitch lock and full function of the propeller is restored. If system pressure is restored by means of the auxiliary pump, the hydraulic forces can disengage the pitch lock and feather the propeller.

17.3.1.7

Automatic Feathering Systems An automatic feathering system is used on some multi-engine aircraft to feather a propeller automatically if the engine fails. The system is normally armed for take-off and landing, but is turned off during cruising flight.

System Components Refer to Figure 33. The system master switch is located at the pilot's or flight engineer's console. It is normally covered by a guard. When the switch is turned on, an indicator light illuminates to indicate that the system is armed.

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M o d u l e 17

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A throttle switch is used to arm the circuit by closing a microswitch when the throttle is advanced to a specific position, for example to 75 % of full throttle movement (depending on the aircraft). The circuit is open when the throttles are below this setting. The system does not auto-feather below 75 %. A torque pressure switch is used to sense the power output of the engine. This switch will close a contact whenever the engine power falls below a specific level. The amount of torque pressure loss required for the system to operate varies according to the engine's size and aircraft design. In most circuits, a time delay unit is used to prevent auto-feathering if a momentary interruption in power occurs. The power loss must exceed 1 s to 2 s for the system to auto-feather. This value also varies with aircraft designs. The feather control is activated by the system when one engine fails. When the control is actuated by the auto-feather systems, a red light in the cockpit will indicate which propeller is feathered. A blocking relay is used in the system to prevent auto-feathering of more than one engine. This relay may be electrically located between the master switch and the throttle switch or may be incorporated in some other part of the circuit. If one engine auto-feathers, some systems can be reset in order to re-arm the auto-feather system if another engine fails. The feathering system can also be operated manually. The pilot can feather any engine at any time, regardless of whether or not a propeller has been auto-feathered. A test switch is used to by-pass certain portions of the circuit so that the system operation can be checked on the ground without developing high-power settings.

System Operation Before take-off and landing, the system is armed by turning on the system master switch. As power is increased for take-off (or during a missed landing approach) the throttle switch closes and the torque pressure switch is armed (the torque pressure switch contacts are open). If a loss of engine power occurs, the torque pressure switch closes. After the prescribed time interval, the time delay unit completes the circuit and energises the feather control. At the same time, the blocking relay is actuated to break the circuit for the auto-feather system on the other engines. System Maintenance System components should be inspected and maintained in accordance with the aircraft's maintenance manual. Units can be removed and replaced as necessary to ensure correct system operation. An operational check of the system is the best way to discover possible defects and to ensure faultless operation. Such an operational check may be carried out according to the following procedure: O Start the engines and arm the system with the auto-feather system master switch. Advance the throttles to develop the required torque to arm the torque pressure switch.

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Hold the test switch in the 'test' position for the engine to be checked and retard the throttle to idle. This should cause the torque pressure switch and the time delay relay to close and to start the feathering operation by activating the feather control and by turning on the corresponding light in the cockpit. Release the test switch and de-activate the feather control to prevent the propeller feathering. Note:

Some components (for example the blocking relay and the throttle switch) are not covered by this check. If the system indicator light does not turn on when the system is armed, the bulb may be burned out, the system master switch may be open or electric power may not be supplied to the system. If the system operates properly during a ground test, but will not auto-feather in flight, the components not included In the test circuit may be open or incorrectly adjusted.

For training only

( c ) by Link & Learn Aviation Training GmbH 6311 32

Propeller blade /

,Propeller hub

Figure 1

Propeller Control System (Allison)

LINK & LEARN

Module 17


a) Basic configuration of a typical propeller governor

Booster

gear pump-

Relief valve

Speeder spring

valve

Flyweights

Propeller line

b) Cross-section of propeller governor

Oil 1

adjusting 14

'D

1

Synchronizer coil

Figure 2

Propeller Governor (c) by Link & Learn Aviation Training GmbH

For train~ngonly 651132


Module 17

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Relief valve

Figure 3

Propeller Governor in Overspeed Condition




Figure 4

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Propeller Governor in Underspeed Condition



Module 17


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Flywe head

Drive sha-'tf

Relief valve '

Figure 5

1111 k

Sliding

w metering valve

Propeller Governor in On-speed Condition




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Dual-acting propeller

Figure 6

Single- and Dual-acting Propellers


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Power lever rpm lever (speed condition) Low Rev

Engine parameters RPM

FUEL

lndicated rpm

Flow rate

90 PERCENT 30 80 RPM 40 70 60 50

PPHxlOO

0

POWER

TEMPERATURE

16 TORQUE ENGINE 2

LBs/FTx1OO6 10

Indicated torque

m

8

Figure 7

ITT

7 G 5

a

"a

t!

Relationship between Control Lever and Engine Operating Parameters

For training only



LINK & LEARN

Module 17

a) Direction of forces for maintaining constant speed Excess turbine power and propeller load must be balanced

b) Basic power management to control power and load

XI

lever

x

I

I

Fuel

b

h lever

:MFV

II I

I

Engine

Figure 8

PP

Controlling Load and Power


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a) Power management linkage

Underspeed governor

Prop pitch control b) Fuel and propeller controls MFV \

USG

\

m 0

a '

2

Prop pitch control

Prop governor

Figure 9

Main Parts of Power Management


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Module 17

a) Engine power control Power lever Beta mode

Prop governing mode 0

4

Rev

Max

I

Fuel control unit

I

Propeller

Controls:

Controls:

- Blade angle (load)

- Fuel (turbine power)

- Direction of thrust

b) Engine rpm control

rpm lever

Low rpm

High

Fuel control unit governor

_ _ _

_

I I I _ I _ _

Controls: - Fuel (turbine power)

_

__

Q

governor

Controls: - Blade angle (load) -J %

z Beta mode

Prop governing mode

Figure 10 Power and Speed Lever Connection


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a) Changing from 'beta' mode to 'prop governing' mode Ground operation

I IOff'

rpm lever

Power lever

Rev

b) Changing from 'prop governing' mode to 'beta' mode

Ground operation 'On' Power lever

rpm lever

Low rDm

Rev

9B % rprn

Figure 11 Changing of Modes


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Module 17

a) 'Prop governing' mode

Rev

rpm lever

Limit

Power lever

Max ---------------

J

% rpm

b) 'Beta' mode

Power lever

'On' rpm lever

b Low rpm

Q\, .,.

High rPm

% rpm

Figure 12 Lever Positions in Ground and Flight Mode



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Module 17


a) Linkage interconnection to the propeller and fuel controls

Power lever

p-

Prop governing mode r-----

I

Beta mode

(fuel) MFV

\o

PPC (prop)

PG (prop)

Speed lever

USG (fuel)

b) System components of engine and propeller controls

Propeller

controller Unfeathering pump LPropeller governor

" L ~ o r q u sensor e Prop pitch actuator

Figure 13 Engine and Propeller Controls

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Figure 14 HARTZELL Single-acting Propeller




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a) HARTZELL propeller with start locks engaged

Start locks

b) Installation of beta tube and unfeathering pump to the PPC

Figure 15 Control of Propeller in 'Feather' and 'Start' Positions


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Propeller - -- -- --governor ---

From lube oil tank

Figure 16 TPE 331 Engine Power Control System

Propeller governor

a Drain oil pressure Engine oil pressure Governor oil pressure

Dump case Lube oil pressure

'Max sweed' stow Relief valve -1 300-350 PSlG

Feathering valve Manual P L X er

Pro eller oil (be&) tube

Propeller\

'

7

,

J

-

\

\

1

Propeller pitch control

ntrol el valv Power lever L-

Figure 17 Increasing Propeller Blade Angle in the Beta Mode

A

U

Propeller governor

r----p-

@@3 Engine oil pressure @El Governor oil pressure

Dump case

I

-pear

pump

\_'Min

n

Lube oil pressure

speed' stop

rol inated setting 'Max soeed' s t o ~

Relief valve J 300-350 PSlG Manual Beta tube opening blocked Propeller oil (beta) tube

FL% er 7

-

-

J

7 Propeller pitch control

ntrol el valve Power lever L

Figure 18 Effect of the Beta Tube in the Beta Mode

I

Figure 21 HARTZELL Propeller of a PT6 Engine

Module 17


LINK & LEARN

a) Propeller governor of the PT6 engine Propeller s select lever

p

e

~

,

d

~

valve

Oil dump to reduction propeller gearbox

propeller

b) Over,speedgovernor of the PT6 installation

Flyweight

m

PI VI

UI

propellerIt( From

Figure 22 Main Propeller Control Units of a PT6 Engine



LO-idle position

Take-off position

Reverse position

Rear clevis end

Figure 23 Side View of a PT6 Engine

Propeller speed select lever

Propeller oil transfer housing Carbon block

/

Low

Piston seal

Reverse return springs

Figure 24 PT6 Propeller System and Power Management Configuration


Module 17

LINK & LEARN

a) Propeller and control system

Auxiliary pump.

High-pressure pump \

Propeller control unit

b) Propeller cross-section

Figure 25 Dual-acting Propeller and Controls


For training only

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'I]

a

'D

tD

'I]

0

-cu. 7

-.3 3 m

1 Forward Oil transfer tube removed Hub Feather s t o ~

\

Valve sleeve

Piston \

Valve

Forward c o n t r o b spring

/

\

Pitch change rod Feather side of cylinder

Reverse side of cylinder open to drain

Figure 30 Propeller Control Valve in 'Toward Feather' Position

Aft control spring

-

-, ru. 2. 3

(D

Feathering

Time

Propeller feathering button and red light Green light

Automatic featherina ter

I

Feathering Pump

Blocking relay

Battery

Figure 33 Basic Automatic Feathering System


Module 17

LINK & LEARN

Propeller Synchronising 17.4.1

Synchronising and Synchrophasing

Propeller Synchronising Systems A propeller synchronising system is used to set all propellers of an aircraft to exactly the same speed (in rpm) in order to eliminate excess noise and vibration. It is used for all flight operations except for take-off and landing. Refer to Figure 1.

A master engine is used to establish the rpm to which the other engine, called the slave engine, is adjusted. A frequency generator built into each propeller governor generates a signal that is proportional to the rpm of that engine.

Refer to Figure 2.

A comparison circuit in the control box compares the rpm signal of the slave engine with the rpm signal of the master engine. If necessary, it sends a correcting signal to the slave engine governor's control mechanism. The comparison unit has a limited range of operation. The slave engine must be within approx. ? 100 rpm of the master engine to enable synchronisation.

17.4.1.2

Synchrophasing Systems

Description Refer to Figure 3. Synchrophasing is a more detailed method of synchronisation. It allows the pilot to set the angular difference in the plane of rotation between the blades of the slave engine(s) and the blades of the master engine. Synchrophasing is used to reduce the noise and vibration created by the engines and propellers. The synchrophase angle can be varied by the pilot to be adjusted to different flight conditions and to still achieve a minimum noise level.

A pulse generator is keyed to the same blade of each propeller (e.g. blade 1). This generator generates a signal to determine the relative position of each blade 1 at any given instant. The signals of both (or all) engines are compared with each other. If a difference occurs a signal is sent to the governor of the slave engine to cause it to adopt the phase angle selected by the pilot.

A manual propeller phase control in the cockpit allows the pilot to select that phase angle which reduces noise and vibration to a minimum.




M o d u l e 17

LINK & LEARN

Synchrophaser The synchrophaser consists of an airtight case containing a comparator and processing circuits for the propeller synchrophasing signals.

Speed Pick-up The speed pick-ups are mounted on a mounting bracket attached to either the reduction gear (as shown in Figure 4) or, in the case of aircraft fitted with propeller de-icing, on the de-icing brush block assembly. The speed pick-ups consist of a cylindrical housing which contains a coil.

Operation

Synchrophasing System Refer to Figure 5. The governor coil of the faster engine is weakened. It allows the flyweight to move outwards. The plunger piston of the pilot valve is lifted which reduces the control shaft orifice and restricts the flow of oil from the propeller servo. The pitch of the blades is increased and the engine speed decreases. The reverse procedure occurs at the same time on the slower engine until the speeds are synchronised.

Synchrophaser If the engines are manually synchronised to within + 2 % of the rpm and the control knob is set to ON, the engines will be synchronised to a speed difference of zero. To minimise the propeller noise, the propellers can be set in phase with each other by turning the control knob to the PHASE SELECT range. The system can be turned off for take-off, landing, single engine operation and in case of malfunction.

Speed Pick-ups Refer to Figure 6. On each rotation of the propeller shaft, a permanent magnet induces a pulse in the pick-up coil. The resultant AC voltage is proportional to the propeller rpm. It is supplied to the synchrophaser where it is processed into a synchrophasing control signal. On aircraft without propeller de-icing system, the magnet is mounted on the spinner bulkhead or on the slipring. On aircraft with propeller de-icing, it is mounted on the slipring.



Module 17


LINK & LEARN

a) Light twin-engine aircraft (push-pull type)

Puller propeller (front)

Pusher propeller (rear)

b) Synchrophasing allows the pilot to adjust the phase angle between the propellers (here: pusher and puller propeller) of the various engines to reduce noise and vibration to a minimum

Phase

0"

c) Synchrophasing control panel of a light twin-engine aircraft Indicator lamp

Figure 3

/

\

Function switch

Principles of Synchrophasing




Module 17

LINK & LEARN

4 5 Legend: 1 wiring connector 3 Pulse transmitter bracket 2 Lock nut 4 Mounting bracket (reduction gear) 5 Speed pick-up (synchrophasing system)

Figure 4

Speed Pick-up Installation

For training only


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Relief valve orifice

Figure 5

Master Engine Propeller Governor


For training only

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17.5

Propeller I c e P r o t e c t i o n

17.5.1

Propeller De-icing

17.5.1.I

Introduction

Module

17

LINK & LEARN

Ice in almost any form is a serious hazard to aircraft in flight and it must either be removed before a flight can be safely conducted or the build up of ice must be prevented during flight. There are 2 types of ice protection systems used for propeller chemical anti-icing electrothermal de-icing systems.

Chemical Anti-icing Propeller anti-icing, shown in Figure 1, uses isopropyl alcohol which is sprayed onto the leading edges of the propeller blades, preventing icing. The alcohol is stored in a tank from which it is pumped to the propeller when needed. The pump is driven by an electric motor which is controlled by a rheostat, the pilot can control the amount of alcohol flowing to the propeller. Each propeller has a slinger ring that uses centrifugal force to distribute the alcohol to the blade nozzles. The length of time this system can be used is limited by the amount of alcohol the tank can carry.

Electrothermal Propeller De-icing Refer to Figure 2. Many of modern propellers installed on both reciprocating and turboprop engines are de-iced with an electrothermal de-icer system. Rubber boots with heater wires embedded in the rubber are bonded to the leading edges of the propeller blades, and electrical current is passed through these wires to heat the rubber and melt any ice that has formed, so centrifugal force and wind can carry the ice away. The boots in some installations are made in 2 sections on each blade. current flows for about a 112 minute through the outboard section of all blades and then for the same time through the heaters on the inboard section of all of the blades. The time the current flows has been proven by flight tests to be sufficient to allow ice to form over the inactive section and long enough to loosen the ice form the section that is receiving the current.

Components The complete propeller de-icer system consists of following components: Electrically heated de-icers bonded to the propeller blades. Slip ring and brush block assemblies that carry the current to the rotating propeller. i~ Timer to control the heating time and sequence of the de-icing cycle. A n ammeter to indicate the operation of the system. All of the wiring, switches and circuit breakers necessary to conduct electrical power from the aircraft electrical system into the de-icer system.




Module 17

LINK & LEARN

Function The slip ring assembly is mounted on the propeller either through a specially adapted starter gear, or attached to the spinner bulkhead or the crankshaft flange. The brush block is mounted on the engine so the 3 brushes will ride squarely on the slip rings. The timer controls the sequence of current to each of the de-icers. The sequence of heating is important, to provide the best loosening of the ice so it can be carried away by the centrifugal force. It is also important that the same portion of each blade be heated at the same time, to prevent an out-of-balance condition. The ammeter monitors the operation of the system and assures the pilot that each heater element is taking the required amount of current. In this way, the pilot knows that there is even de-icing of the propellers.

17.5.1.2

Application of a Propeller De-icing System The following description of a typical ice system is based on the system installed in the Dornier DO 328 aircraft.

Note:

Refer to Figure 3. The propeller de-icing system prevents and eliminates any ice formation on the propeller blades. This is done by electrically heated de-icer mats on the leading edges of each propeller blade. To prevent propeller imbalance due to uneven shedding of ice, the de-icing process is controlled by timers which ensure even de-icing. The systems for the left and right propellers are independent of each other. If a propeller stops rotating, the electrical power is automatically removed to prevent overheating of the carbon-fibre propeller blades. Electrical power for the de-icer boots is controlled by the timers. It is transmitted to the de-icer mats via slip rings. The de-icing process is cyclic and is also controlled by the timers. The propellers are de-iced in 2 steps. During each step, 3 symmetric de-icer mats are powered. Refer to Figure 4. The controls for the propeller de-icing system are located at the ice protection panel. They are labelled PROP with 3 positions, BELOW -10 "C, OFF and ABOVE -10 " C . Brush Block Refer to Figure 5. The brush block transfers electrical power for the de-icer boots from the aircraft harness to the rotating propellers. This is done by pressing carbon brushes under load from constant pressure springs, onto the rotating slip rings mounted on the hub of the propeller. The brush block is mounted in front of the engine gearbox. Slip Ring The slip ring is used to transfer electrical power from the non-rotating brush block to the rotating propeller de-icer. The slip ring consists of an aluminium carrier which houses copper alloy rings. The assembly is fitted onto the rear of the propeller hub.




Module 17

LINK & L E A R N

Metal Oxide Varistor Metal oxide varistor (MOV) modules are installed, to protect the aircraft's electrical power supply if the propellers are struck by lightning. If any propeller is struck by lightning, the reverse current in the system is routed to the aircraft ground.

Timer The timers control and transmit the electrical power for the de-icer boots via slip rings. Furthermore, they control the timing of the de-icing process. The system is energised by 2 3-position switches on the PROP section of the ice protection panel. Each timerlmonitor incorporates circuitry to monitor the propeller rotation speed. It prevents any output to the de-icer mats if the propeller speed drops below 675 rpm + 25 rpm. This is done to avoid any damage to the carbon-fibre propeller blades by overheating. Warning circuits are also included to detect any faults and to provide signals for the CAWS. There are also circuits used to govern the load control of the de-icing supply voltage.

Operation Refer to Figure 6. When either PROP selector switch is set to BELOW -10 "C, 28 V DC is supplied to the timer via the circuit breaker ICE PROT PROP LHIRH. Via the timer outputs, the 2 sets of de-icer mats are heated alternately for 25 s. These heating phases are separated by an 'off' interval of 17.5 s. This cycle is repeated until either PROP selector switch is set to OFF. The mode BELOW -10 "C is disabled at temperatures of +5 "C and above. When either PROP selector switch is set to ABOVE -10 "C, 28 V DC is supplied to the timer via the circuit breaker ICE PROT PROP LHIRH. Then the timer sequence starts. Via the timer outputs, the 2 sets of de-icer mats are heated alternately for 7 s separated by an 'off' interval of 29 s. The cycle is repeated until either PROP selector switch is set to OFF.

Figure 7 shows an electrical schematic of a single propeller heating system.

For training only



Module 17

LINK & LEARN

Power output

lgi -

Signal to onloff indication lamp (or ammeter)

-

7

"O"

Power Input

Power Input

q'

off

Otf

Below

-,ooc

Above

Below

Above

1ooc

28 V DC ESS bus

115 Vl200 V AC

Figure 3

Signal to onloff indication lamp (or ammeter)

-

115 Vl200 V AC

Twin Propeller De-icing System


For training only

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Module 17

1 1

Normal operation (dark) - Operation of the engine air intake boots is disabled ON (white) - When pressed, engine air intake boots are operated

LINK & LEARN

Detector button When pushed, the FAlL and ICE lights are illuminated to indicate the test sequence. Normal operation (dark) - ice detector is operating. ICE (amber) - ice detector detected ice build-up. FAlL (amber)

WSHLD 112 button Normal operation (dark) - windshield heating is off. ON (white) - windshield heating is on (pushed in) FAlL (amber) - windshield heating has failed.

Probes 112 and STBY button Normal operation (dark) - probes are continuously heated (pushed in). FAIL (amber) - probe heatings have failed. OFF (amber) - probe heatings are manually switched off. Horns leftlright button Normal operation (dark) - elevator and rudder horn heatings are disabled. FAlL (amber) - elevator and rudder horn heatings have failed. ON (white) - elevator and rudder horn heatings are on.

Figure 4

Ice Protection Control Panel of a Twin Torboprop Aircraft (c) by Link & Learn Aviation Training GmbH

For training only 111/132

a h

0

t

LINK & LEARN

Module 17


Carbon

-

-

-- -

iI Figure 5

MOV module

Components of the Propeller De-icing System



EASA Part-66 Training Hand book

Module 17

17.6

Propeller Maintenance

17.6.1

Propeller Inspection and Maintenance

17.6.1.1

Propeller Maintenance Regulations

LINK & LEARN

Authorised Maintenance Personnel The inspection, adjustment, installation and minor repair of a propeller and its related parts and appliances on the engine are within the responsibility of the powerplant technician. He may also perform the 100-h inspection of the propeller and its related components. A propeller repair specialist may perform or supervise the major overhaul and repair of propellers and their related parts and appliances for which he is certificated. The repair and overhaul must be performed at and by a certificated repair station or (within certain limits) by the holder of a 'commercial operator' or 'air carrier' certificate. A technician who holds an lnspection Authorisation (IA) may perform the annual inspection of a propeller. But he may not approve a propeller for return to service after major repairs and alterations to propellers or their related parts and appliances have been carried out. Only a certificated facility, such as a propeller repair station, may return a propeller or accessory to service after a major repair or alteration/ modification.

Preventive Maintenance The following types of preventive maintenance may be associated with propellers and their systems: replacement of defective safety wiring or cotter keys lubrication which does not require disassembly other than removal of nonstructural items such as cover plates, cowlings and fairings application of preservative or protective material (paint, wax etc.) to components where no disassembly is required and where the coating is not prohibited.

Major Alterations and Repairs The following are major propeller alterations (if not otherwise specified in the propeller specifications/documentation): changes in the blade or hub design changes in the governor or control design installation of a governor or feathering system installation of a propeller de-icing system installation of parts not approved for the propeller.


For training only

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Module 17

When a propeller or control device is overhauled by a repair facility, a maintenance release tag must be attached to the item to certify that the item is approved for return to service (RTS).

17.6.1.2

Propeller Inspection

Annual and 100-hour Inspections When performing a 100-h or annual inspection, the following areas relating to propellers and their controls must be inspected: engine controls to be checked for defects, range of movement and correct locking lines, hoses and clamps to be checked for leaks, condition and looseness accessories to be checked for defects in security of mounting all systems to be checked for correct installation, general condition, defects and insecure attachment propeller assembly to be checked for cracks, nicks, binds and oil leakage bolts to be checked for correct torque and locking anti-icing devices to be checked for correct operation and defects control mechanisms to be checked for correct operation, secure mounting and freedom of movement. These inspections are the minimum required by current regulations. The powerplant technician should always refer to the manufacturer's manuals for specific inspection procedures.

Inspection, Maintenance and Repair of Aluminium Alloy Blades One advantage of aluminium propellers is the low cost of maintenance owing to the one-piece construction of the propellers and the hardness of the metal from which they are made. However, when damage does occur, it is usually critical and may result in blade failure. For this reason, the blades must be carefully inspected and any damage must be repaired before further flight. Before inspecting a propeller, it is to be cleaned with a solution of mild soap and water to remove all of the dirt, grease and grass stains. Then the blades are to be inspected for pitting, nicks, dents, cracks and corrosion, especially at the leading edges and faces. A 4x-magnifying glass will aid in these inspections. A dye-penetrant inspection should be performed if cracks are suspected. A majority of the surface defects that occur on the blades can be repaired by the powerplant technician. Figures 1 and 2 show the allowable repairs to a metal propeller blade. Defects on the leading and trailing edges of a blade may be dressed out by using round and half-round files. The repair should blend in smoothly with the edge and should not leave any sharp edges or angles. An airfoil section should be maintained. The approx. max. allowable size of a repaired edge defect is 0.125 inch (") (3.2 mm) in depth and not more than 1.5" (38 mm) in length.

For training only



Module 17

LINK & LEARN

Repairs to the face and back of a blade are performed with a spoon-like riffle file which is used to dish out the damaged area. The maximum allowable repair size of a surface defect is 0.0625" (1.6 mm) deep, 0.375" (9.5 mm) wide and 1" (25.4 mm) long. All repairs are finished by polishing with very fine sandpaper (crocus paper) by moving the paper in a direction parallel to the length of the blade. Then, the surface is to be treated with ALODINE, paint or some other appropriate protective coating. The hub boss is to be checked for damage and corrosion inside the center bore and on the surfaces which mount on the crankshaft. The bolt holes should be inspected for damage, security and dimensions. Light corrosion in the boss can be cleaned with sandpaper (crocus paper) and then painted or treated to prevent the recurrence of corrosion. Propellers with damage, dimensional wear or heavy corrosion in the boss area should be transferred for repair to a repair station. Propellers with damage in the shank area of a blade should be transferred for corrective action to an overhaul facility. Since all forces acting on the propeller are concentrated on the shank, any damage in this area is critical. If a blade has been bent, the angle of the bend and the blade station of the bend center are to be measured. By using the proper chart, the repairability of the blade can be determined. To determine the serviceability, the center of the bend has to be found and measured from the center of the hub to determine the blade station of the bend center. Next, the blade has to be marked 1" (25.4 mm) on each side of the center of the bend and the degree of bend measured by using a protractor similar to the one shown in Figure 3, detail a). Be sure that the protractor is tangent to the 1"-lines when measuring the angle.

Note:

The chart approved by the propeller manufacturer is to be used to determine if the bend is repairable. When reading the chart, any bend above the graph line is not repairable. If the proper chart is not available, the measurements have to be taken and an overhaul facility is to be contacted for a decision before sending the propeller to them for straightening. After the propeller has been repaired, the surface finish must be restored. The face of each blade is painted with one coat of zinc chromate primer and 2 coats of flat black lacquer from the 6"-station to the tip. The back of the blade should have the last 4" (100 mm) of the tip painted with one coat of zinc chromate primer and 2 coats of a high visibility colour (red, yellow or orange). The colour scheme on the back of the blade of some aircraft differs from that described here, so the original colour scheme may be duplicated if desired.




17.6.1.3

LINK & LEARN

Module 17

Vibration When powerplant vibration is encountered, it is sometimes difficult to determine whether it is the result of engine vibration or propeller vibration. In most cases, the reason for the vibration can be determined by observing the propeller hub, dome or spinner while the engine is running within the 1,200-rpm to 1,500-rpm range. The hub or spinner is observed and it is determined whether or not the propeller hub rotates on an absolutely horizontal plane. If the propeller hub appears to swing in a slight orbit, the vibration will normally be caused by the propeller. If the propeller hub does not appear to rotate in an orbit, the difficulty will probably be caused by engine vibration. If the hub appears to rotate in a horizontal plane, then engine vibration is the most likely cause. When propeller vibration is the reason for excessive powerplant vibration, the normal causes are: propeller blade imbalance

u propeller blades not tracking variation in propeller blade angle settings. First the propeller blade tracking is to be checked and then the low-pitch bladeangle setting to determine if they are the cause of vibration. If both the propeller tracking and the low blade angle setting are incorrect, the propeller is statically or dynamically unbalanced. It should be replaced or rebalanced by the manufacturer. In recent years, electronic equipment for dynamically balancing propeller installations has come into common use. This equipment allows very accurate balancing and reduction of vibration.

17.6.1.4

Blade Tracking Once the propeller is installed and torqued, the track is checked. The track of the propeller is defined as the path that the tips of the blades follow as they rotate with the aircraft being stationary. For light aircraft with propellers of up to approx. 6 feet (1.83 m) in diameter, metal propellers can be out of track not more than 0.0625" (1.6 mm). The track of a wooden propeller may not be out more than 0.125" (3.2 mm). Refer to Figure 4. Before the propeller can be tracked, the aircraft must be made stationary by chocking the wheels so that the aircraft will not move. A fixed reference point is to be placed within 0.25" (6.35 mm) of the propeller arc. This may be done by placing a board on blocks under the propeller arc and taping a piece of paper to the board so that the track of each blade can be marked.


For training only

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Module 17

LINK & LEARN

The propeller is rotated by hand until one blade is pointing down at the paper. This position is marked on the paper. Then the propeller is turned so that the track of the next blade can be marked on the paper. This procedure is to be repeated for each blade. The maximum difference in track for all of the blades should not exceed the limits above. If the propeller is out of limits, the cause should be investigated and the condition corrected. Probably the easiest item to check is the propeller torque. If all bolts are properly torqued, it will probably be necessary to remove the propeller, to inspect it for dirt or damage and to check the crankshaft for alignment.

17.6.1.5

Safetying the Propeller Once a propeller is properly tracked and torqued, it can be safetied. There are numerous ways to safety a propeller installation because of the many different types of installations. For this reason, only the more commonly used methods of safetying will be discussed in the following. Refer to Figure 5.

A flanged-shaft installation has the largest variety of safetying methods because of its many variations. If the flange has threaded inserts installed, the propeller is held by bolts screwed into the inserts. The bolt heads are drilled and safetied with 0.041" (1 mm)-stainless steel safety wire, using standard safety wire procedures. If threaded inserts are not pressed into the flange, bolts and nuts are used. Some installations use fibre lock nuts which require no safetying. But the nuts should be replaced each time the propeller is removed. Other installations use castellated nuts and drilled bolts and the nuts are safetied to the bolts with cotter pins. The retaining nuts for tapered and splined shaft installations are safetied in the same way. A clevis pin is installed through the safety holes in the retaining nut and crankshaft. This pin is to be positioned with the head toward the center of the crankshaft so that the centrifugal force will hold the pin in the hole.

17.6.1.6

Checking and Adjusting the Blade Angle Refer to Figure 6. The universal propeller protractor can be used to check propeller blade angles when the propeller is on a balancing stand or installed on the engine. The frame of a typical universal protractor is made of aluminium alloy. 3 sides of it are at 90" to each other.

A bubble spirit level is mounted on one corner of the front of the frame. This level swings out to indicate when the protractor is levelled. A movable ring is located inside the frame. It is used to set the zero reference angle for blade angle measurements. The ring is engraved with vernier index marks, which allow readings as small as one tenth of a degree. A center disc is engraved with a degree scale from zero to 180" (both positive and negative). It contains a spirit level to indicate when the disc is levelled.


For training only

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Module 17

LINK & LEARN

When using the propeller protractor and before measuring the angle of a propeller blade, the reference blade station is determined using the aircraft manufacturer's maintenance manual. This reference station is marked on the blade with chalk or with a grease pencil. Refer to Figure 7. The next step is to establish the reference plane from the engine crankshaft center-line, rather than the airframe attitude, because some engines are canted in the aircraft. To set the protractor to zero, the ring-to-frame lock is loosened and the zeros on the disc and the ring aligned. Then the disc-to-ring lock is engaged. One edge of the protractor is placed on a flat surface on the propeller hub (that is parallel to, or perpendicular to, the crankshaft center-line) and the ring adjuster turned until the spirit level in the center of the disc is levelled. The corner level should also be levelled. Now, the ring-to-frame lock is tightened and the disc-to-ring lock released. The protractor is now aligned with the engine crankshaft. One blade of the propeller is placed horizontally and moved out to the reference station marked on the face of the blade to measure the blade angle. Stand on the same side of the aircraft, facing the same direction, as done when you had established the zero with the protractor. Otherwise the measurement will be incorrect.

Note:

The edge of the protractor is placed on the face of the blade at the reference station and the disc adjuster is turned until the spirit level centers. Then the blade angle is read, using the zero line on the ring as the index. Tenths of degrees can be read from the vernier scale. Each blade is to be rotated to the same horizontal position, and then the angle is measured. If the face of the propeller blade is curved, masking tape must be used to attach a piece of 0.125" (3.2 mm) drill rod 0.5" (12.7 mm) from the leading and trailing edges. The angle is measured with the protractor resting on the rods.

17.6.1.7

Balancing Propeller imbalance, which is a source of vibration in an aircraft, may be either static or dynamic.

Static Imbalance Static propeller imbalance occurs when the center of gravity of the propeller does not coincide with the axis of rotation. Static balancing can be done by the EI suspension method knife-edge method. Note:

The suspension method is used less frequently than the simpler and more accurate knife-edge method.

For training only



Module 17

LINK & LEARN

Suspension Method Refer to Figure 8. In the suspension method, the propeller (or part) is hung by a cord. Any imbalance is determined by the eccentricity between a disc firmly attached to the cord and a cylinder attached to the assembly (or part) being tested.

Knife -edge Method Refer to Figure 9. The knife-edge test stand has 2 hardened steel edges mounted to allow the free rotation of an assembled propeller between them. The knife-edge test stand must be located in a room or area that is free from any air motion, and preferably removed from any source of heavy vibration. During a propeller static balance check, all blades must be at the same angle. Before conducting the balance check, check that each blade has been set to the same blade angle. The standard method of checking propeller assembly balance involves the following sequence of operations: 1. Insert a bushing in the engine shaft hole of the propeller.

2. Insert a mandrel or arbor through the bushing. 3. Place the propeller assembly so that the ends of the arbor are supported by the balance stand knife-edges. The propeller must be free to rotate.

If the propeller is properly balanced (statically) it will remain in any position in which it is placed. Check 2-bladed propeller assemblies for balance, first with the blades in vertical position and then with the blades in horizontal position as shown in Figure 9, detail a). Repeat the vertical position check with the blade positions reversed; that is, the blade which was checked in the downward position is now placed in the upward position. Check a 3-bladed propeller assembly with each blade in a downward vertical position as shown in Figure 9, detail b). Unless specified by the manufacturer, an acceptable balance check requires that the propeller assembly has no tendency to rotate in any of the positions previously described. If the propeller balances perfectly in all described positions, it should also balance perfectly in all intermediate positions. When necessary, check for balance in intermediate positions to verify static balance. When a propeller assembly is checked for static balance and there is a definite tendency of the assembly to rotate, certain corrections to remove the imbalance are allowed: the addition of permanent fixed weights at acceptable locations when the total weight of the propeller assembly or parts is under allowable limits the removal of weight at acceptable locations when the total weight of the propeller assembly or parts is equal to the allowable limit.




M o d u l e 17

LINK & LEARN

The location for removal or addition of weight for propeller imbalance correction has been determined by the propeller manufacturer. The method and point of application of imbalance corrections must be checked to see that they are according to the applicable drawings. Dynamic Imbalance Dynamic imbalance results when the centers of gravity of similar propeller elements (such as blades or counterweights) do not follow the same plane of rotation. Since the length of the propeller assembly along the engine crankshaft is short (in comparison to its diameter), and since the blades are secured to the hub so that they lie in the same plane perpendicular to the running axis, the dynamic imbalance resulting from improper mass distribution is negligible (provided the track tolerance requirements are met). Dynamic balance checks are now done with the propeller, spinner and related equipment installed on the aircraft and with the engine running. Electronic equipment may be used to locate an imbalance and to determine the amount of weight required to correct the condition. Another type of propeller imbalance, aerodynamic imbalance, results when the thrust (or pull) of the blades is unequal. This type of imbalance can be largely eliminated by checking the blade contour and the blade-angle setting.

17.6.1.8

Servicing Propellers Propeller servicing includes cleaning, lubricating and replenishing of operating oil supplies.

Cleaning Propeller Blades Aluminium and steel propeller blades and hubs are usually cleaned by washing the blades with a suitable cleaning solvent, using a brush or cloth. Acid or caustic materials should not be used. Power buffers, steel wool, steel brushes or any other tools or substances that may scratch the blade should be avoided. If a high polish is desired, a number of good grades of commercial metal polish are available. After completing the polishing operation, all traces of polish should be removed. When the blades are clean, they should be coated with a clean film of engine oil or a suitable equivalent. To clean wooden propellers, warm water and a mild soap can be used together with brushes or cloth. If a propeller has been subjected to salt water, it should be flushed with fresh water until all traces of salt have been removed. This should be accomplished as soon as possible after the salt water has been splashed on the propeller, regardless of whether the propeller parts are made of aluminium alloy, steel or wood. After flushing, all parts should be dried thoroughly. Metal parts should be coated with clean engine oil or a suitable equivalent.

For training only


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Propeller Lubrication Propeller lubrication procedures, together with oil and grease specifications, are usually published in the manufacturer's instructions. Experience indicates that water sometimes gets into the propeller blade bearing assembly on some models of propellers. For this reason, the propeller manufacturer's greasing schedule must be followed to ensure proper lubrication of moving parts.


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Maximum thickness of blade is at a point approx. 0.3 of cord length as shown

Caution: Do not destroy maximum thickness of section if possible!

-0.3

Correct method

of cord length

f

1 1 4

L- Original section

blade -wOr-d

Notes: A = maintain original radius B = rework cor~tourto point of maximum thickness C = radius is too lar e D = contour is too b unt

Damaged portion

I

y

u

Incorrect method

Figure 1

Allowable Repairs to a Metal Propeller Blade (Leading-edge Damage)

Module 17


LINK & LEARN

Surface gouge

Before

I After

I

Edge nick

Figure 2

Surface crack

Allowable Repairs to a Metal Propeller Blade (Typical Surface Defects)


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Module 17

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a) Method of measuring a propeller blade bend using a protractor

Angle of bend

Measure at a point of tangency taken one inch each side CL of bend

b) Typical chart for determining the allowable amount of bend

a

0

8

10 12 14 16 18 20 22 24 26 28 30 32 34

Blade station

Figure 3

Measurement on Metal Propeller Blades




Module 17

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a) Method of tracking a propeller by using a reference board

b) Adjusting the propeller track by the use of shims

Figure 4

Adjustment of Propeller Track


For training only

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a) Securing of propeller bolt heads with safety wire Wire must Bolt with /- oull in the lockina wire Washer I

b) Use of cotter pins to secure propeller bolts and retaining nuts on tapered or splined propeller shafts Cotter

Clevis pin

nut

Figure 5

Methods of Securing a Propeller

For training only


Degree scale (on disc) (shown incomplete)

Nut (loosenlrotate to adjust)

Figure 6

-

\

Corner spirit level (on frame; folded-in)

\-- True measuring edge

Universal Propeller Protractor


Module 17

LINK & LEARN

a) Correct method of using a propeller protractor

b) Correcting for blade curvature by using a universal protractor Note: Tape rods on reference station on thrust side of blade.

Blade cross-section

Figure 7

Checking and Adjusting the Blade Angle


For training only

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a) Positions of a 2-blade propeller during balance check

Horizontal balance check

Vertical balance check

b) Positions of a 3-blade propeller during balance check 1

Figure 9

2

3

Methods of Propeller Balancing (Knife-edge Method)



EASA Module 17

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