A m17 Propeller

(1)

M1

M17

7 PR

PROP

OPE

ELL

LLER

ER

EASA Part-66

CAT A

P66 M17 A E P66 M17 A E DAC DAC NOV2 NOV2 29.11.2012 29.11.2012

(2)

Training Manual

For training purposes and internal

use only.



Copyright by Lufthansa Technical Training (LTT).

LTT is the owner of all rights to training documents and

training software.

Any

use

outside

the

training

measures,

especially

reproduction and/or copying of training documents and

software

−−

also extracts there of

−−

in any format at all

(photocopying, using electronic systems or with the aid

of other methods) is prohibited.

Passing on training material and training software to

third parties for the purpose of reproduction and/or

copying is prohibited without the express written

consent of LTT.

Copyright endorsements, trademarks or brands may

not be removed.

A tape or vi

deo recording

of training c

ourses or

similar

services is only permissible with

the written consent of

LTT.

In other

respects, legal requirements

, especially under

copyright and

criminal law, apply.

Lufthansa Technical Training

Dept HAM US

Lufthansa Base Hamburg

Weg beim Jäger 193

22335 Hamburg

Germany

T

el

:

+4

9 (

0)

40

5

07

07 0 2

0 2

52

0

0 Fa

Fa

x:

+4

9 (

0)

40

50

70

47

46

46 www.Lufthansa-Technical-Training.com

www.Lufthansa-Technical-Training.com



 The date given in the column ”Revision” on the face of The date given in the column ”Revision” on the face of

this cover is binding for the complete Training Manual.



 Dates and author’s ID, which may be given at the baseDates and author’s ID, which may be given at the base

of the individual pages, are for

of the individual pages, are for information about theinformation about the

latest revision of that page(s) only.



 The LTT production process ensures that the TrainingThe LTT production process ensures that the Training

Manual contains a complete set of

Manual contains a complete set of all necessary pagesall necessary pages

in the latest finalized revision.

Revision Identification:

(3)

Training Manual

For training purposes and internal

use only.



Copyright by Lufthansa Technical Training (LTT).

LTT is the owner of all rights to training documents and

training software.

Any

use

outside

the

training

measures,

especially

reproduction and/or copying of training documents and

software

−−

also extracts there of

−−

in any format at all

(photocopying, using electronic systems or with the aid

of other methods) is prohibited.

Passing on training material and training software to

third parties for the purpose of reproduction and/or

copying is prohibited without the express written

consent of LTT.

Copyright endorsements, trademarks or brands may

not be removed.

A tape or vi

deo recording

of training c

ourses or

similar

services is only permissible with

the written consent of

LTT.

In other

respects, legal requirements

, especially under

copyright and

criminal law, apply.

Lufthansa Technical Training

Dept HAM US

Lufthansa Base Hamburg

Weg beim Jäger 193

22335 Hamburg

Germany

T

el

:

+4

9 (

0)

40

5

07

07 0 2

0 2

52

0

0 Fa

Fa

x:

+4

9 (

0)

40

50

70

47

46

46 www.Lufthansa-Technical-Training.com

www.Lufthansa-Technical-Training.com



 The date given in the column ”Revision” on the face of The date given in the column ”Revision” on the face of

this cover is binding for the complete Training Manual.



 Dates and author’s ID, which may be given at the baseDates and author’s ID, which may be given at the base

of the individual pages, are for

of the individual pages, are for information about theinformation about the

latest revision of that page(s) only.



 The LTT production process ensures that the TrainingThe LTT production process ensures that the Training

Manual contains a complete set of

Manual contains a complete set of all necessary pagesall necessary pages

in the latest finalized revision.

Revision Identification:

(4)

F F O O R R T T R R A A I I N N I I N N G G P P U U R R P P O O S S E E S S O O N N L L Y Y ! !

P

PR

RO

OP

PE

EL

LL

LE

ER

R

E

EA

AS

SA

A

P

PA

AR

RT

T

6

66

6 M17

M17

M

1

7

7 P

P

R

O

P

E

L

E

R

L L u u f f t t h h a a n n s s a a T T e e c c h h n n i i c c a a l l T T r r a a i i n n i i n n g g

(5)

F O R T R A I N I N G P U R P O S E S O N L Y !

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

M17.1

PROPELLER FUNDAMENTALS

GENERAL

The propeller is driven by an engine with a performance measured in shaft horse power or brake horse power). It accelerates a mass of air and the reaction produces thrust.

Propellers can also be used as aerodynamic brakes by reversing the direction of air acceleration.

The propeller consists of a propeller hub and two or more propeller blades. The propeller is connected to the propeller shaft by the hub.

The propeller blades have an aerodynamic profile. When they move through the air (rotation of the propeller), an air mass is accelerated by the difference in pressure on the surfaces of the blades.

The following terms apply to the propeller blade:

leading edge

trailing edge

blade root

and blade tip.

As the geometry of the blade changes from the root to the tip, details on chord length, chord thickness and blade angle refer to a particular reference station. This reference station is normally located from 0.7R - 0.75R.

HOW THE PROPELLER WORKS

PRODUCTION OF THRUST

The way the propeller works is based on the reactive principle. The air mass flowing through the propeller plane is accelerated by the difference ∆v. The reason for this acceleration of the air mass is the change in pressure in front of and behind the propeller plane, which occurs as a result of the air flowing around the propeller blade airfoil. As a reaction to the accelerating forces, propeller thrust (Fs) is created. As the air mass in the propeller plane also receives an accelerating component in the direction of the circumference, the air mass spirals away from the propeller plane. Because of the higher velocity of the propeller wash behind the propeller plane, its cross−section is reduced there.

As the pressure differences on the propeller blade airfoils are small by nature, the acceleration of the air mass is also small. This leads to low downwash speeds with high propulsive efficiency at low to medium airspeeds (mach 0.5 to 0.6).

propeller plane

Figure 1

Propellerstream

L u f t h a n s a T e c h n i c a l T r a i n i n g

(6)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

Spinner anti-icing tip BLADE TIP reference station chord length BLADE TIP HUB LEADING EDGE

AIRFOIL BLADE ROOT

BLADE ROOT TRAILING EDGE L u f t h a n s a T e c h n i c a l T r a i n i n g

(7)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

ANGLES OF THE PROPELLER BLADE Blade Angle (ϕ)

The angle between the propeller chord and the rotational plane of the propeller is the blade angle or the angle of incidence. The blade angle is not constant over the whole length of the propeller (see aerodynamic twist).

In practice the angle always refers to the pressure side of the blade, even if the profile chord differs from this. As the blade angle is not constant over the whole length of the blade, a particular part of the blade is termed the reference station. This station is generally at 3/4R of the propeller.

Angle of Attack (α)

The angle of attack is the angle between the profile chord line and the relative air flow towards it. With the angle of incidence running appropriately the length of the blade, the desired lift distribution is achieved from the resulting angles of attack. As the propeller moves on a plane which is perpendicular to the forward movement of the aircraft, two velocities, perpendicular to each other, are definitive for the angle of attack:

the relative air flow velocity, resulting from aircraft airspeed (v)

the relative air flow velocity, resulting from propeller peripheral speed (u). Both velocities produce the resultant relative velocity (w) and determine direction and magnitude of the velocity (w).

Angle of Advance (β)

The angle of advance(β) is the angle between the rotational plane of the propeller and the relative velocity (w). The angle of advance increases with increasing airspeed (v). L u f t h a n s a T e c h n i c a l T r a i n i n g

(8)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

resulting velocity w chord propeller plane bladeangle angle of attack angle of advance air speed peripheral speed u α β L u f t h a n s a T e c h n i c a l T r a i n i n g

(9)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

AIRFLOW ONTO THE PROPELLER BLADE Influences on the Angle of Attack

A change in airspeed or a change in peripheral speed (depending on RPM) results immediately in a change of resultant relative air flow direction and velocity. This can even lead to a negative angle of attack, for example during descent with idle power. The propeller would then drive the engine (windmilling). This would mean negative torque for the engine.

As a certain angle of attack is optimal for any given propeller, a fixed propeller only works optimally within a given speed range.

Thus fixed propellers are good for climbing performance, or optimized for towing or for high cruising speeds.

(10)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

Change of Airspeed v

Change of Peripheral Speed u

Influences on the Angle of Pitch

(11)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

GEOMETRY OF THE PROPELLER BLADE Blade Shapes and Profiles (Airfoil Sections)

For every speed range there is an optimal profile shape with regard to lift and drag. Thick profiles are used for low speeds and thin ones (usually laminar profiles) for high speeds. At the same time the profile changes from thick at the root area to thin at the blade tips. This is of advantage regarding static stress. In the root area, where the forces are higher, we find a thicker material cross −section, so that the stresses affecting the material do not exceed the permissible range.

The blade shape depends on the purpose of the propeller, whereby performance, airspeed and diameter play a role. The higher the circle load is, the wider the propellers which should be used. For reasons of reducing noise, propeller tips should be elliptical.

Blade Twist

The further the profile section of the propeller blade is from its rotational axis, the greater will be the peripheral speed at constant rotational speed.

If a nearly constant angle of pitch is to be retained, the propeller blade must be twisted.

The angle of incidence must become smaller the further it is from the axis in order to keep a nearly constant angle of pitch. In practice the angle of incidence running the length on the blade determines the angle of pitch in such a way that an optimal distribution of lift results.

In addition to the angle of incidence, the profile shape also changes for static and aerodynamic reasons. Center of Hub Blade Butt Blade Shank Tip Section 6 “ S e c t i o n s Blade Angle

Figure 5

Twisted Blade

(12)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

(13)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

Geometric Pitch Geometric Pitch

If the propeller were to spiral through the air on a course, where the angle of pitch If the propeller were to spiral through the air on a course, where the angle of pitch equalled the

equalled the blade angleblade angle, the propeller would, in one rotation, have moved, the propeller would, in one rotation, have moved forward axially by the ”geometric pitch”.

forward axially by the ”geometric pitch”.

If the aircraft moved through the air according to the geometric propeller pitch, the If the aircraft moved through the air according to the geometric propeller pitch, the propeller angle of attack would be zero.

propeller angle of attack would be zero.

To calculate the geometric pitch of a propeller based on the blade angle, you use To calculate the geometric pitch of a propeller based on the blade angle, you use the blade angle at the reference station on the

the blade angle at the reference station on the blade. This is normally 3/4 of theblade. This is normally 3/4 of the propeller radius.

propeller radius.

Effective Pitch Effective Pitch

The actual helical path on which the propeller moves through the air has an angle The actual helical path on which the propeller moves through the air has an angle of pitch which corresponds to the angle of advance.

of pitch which corresponds to the angle of advance.

With one rotation of the propeller the aircraft moves forward by the effective pitch. With one rotation of the propeller the aircraft moves forward by the effective pitch. The effective pitch can be calculated by replacing the blade angle by the angle of The effective pitch can be calculated by replacing the blade angle by the angle of advance in the above equation.

advance in the above equation.

Slip Slip

Slip is geometric pitch minus effective pitch. It is given in percentage of geometric Slip is geometric pitch minus effective pitch. It is given in percentage of geometric pitch.

pitch.

PROPELLER PITCH AND EFFICIENCY PROPELLER PITCH AND EFFICIENCY Propeller Efficiency

Propeller Efficiency

Propeller efficiency is basically the performance produced by the propeller in Propeller efficiency is basically the performance produced by the propeller in relationship to its motive performance.

relationship to its motive performance.

Motive performance is the same as the output power of the engine (brake power). Motive performance is the same as the output power of the engine (brake power). The performance produced is the thrust performance of the propeller. Thrust The performance produced is the thrust performance of the propeller. Thrust performance can be calculated from thrust and airspeed. Propeller efficiency can performance can be calculated from thrust and airspeed. Propeller efficiency can also be calculated by dividing effective pitch by geometric pitch.

also be calculated by dividing effective pitch by geometric pitch. Propeller efficiency ranges from 0.8 to 0.9 (80%

Propeller efficiency ranges from 0.8 to 0.9 (80% −− 90%). 90%).

(14)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

effective pitch = angle of advance

β

geometric pitch = blade angle

ϕ

F

(15)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

Aerodynamic Forces on the Propeller Blade Aerodynamic Forces on the Propeller Blade

When air flows towards the propeller blade with the resultant (w), resultant air force When air flows towards the propeller blade with the resultant (w), resultant air force (F

(FRR) is produced. With regard to the ) is produced. With regard to the propeller element it is termedpropeller element it is termed∆∆FFR.R.This canThis can

be split into its components

be split into its components∆∆FFLLandand∆∆FFDD. The quotient of. The quotient of∆∆FFDD and and∆∆FFLLresults inresults in

the lift/drag ratio. As with air flowing around a wing, here the drag

the lift/drag ratio. As with air flowing around a wing, here the drag ∆∆FFDD isis

considerably lower than lift

considerably lower than lift∆∆FFLL. The resultant airforce can also be. The resultant airforce can also be

divided in such a way that the component

divided in such a way that the component∆∆FFss lies in the direction of flight and lies in the direction of flight and∆∆FFTT

in the propeller rotational plane. The component

in the propeller rotational plane. The component∆∆FFss represents the share of thrust represents the share of thrust

and

and∆∆FFTT is the tangential force component. If is the tangential force component. If∆∆FFTT is multiplied by the effective lever is multiplied by the effective lever

to the propeller’s axis of rotation, the result is

to the propeller’s axis of rotation, the result is the share ofthe share of propeller brake moment. The sum of all partial forces

propeller brake moment. The sum of all partial forces∆∆FFss over the radial extent over the radial extent

of all propeller blades results in the propeller thrust. If the torque of all partial forces of all propeller blades results in the propeller thrust. If the torque of all partial forces

∆

∆FFTT are added together over the are added together over the same area, we arrive at the same area, we arrive at the resultant propellerresultant propeller

torque or the brake moment of air

torque or the brake moment of air forces affecting the propeller.forces affecting the propeller.

At constant rotational speed the sum of propeller brake moment and engine At constant rotational speed the sum of propeller brake moment and engine torque is zero.

torque is zero.

The reason for the air forces created on the

The reason for the air forces created on the profile is the difference in pressureprofile is the difference in pressure on the profile, arising from the air flowing around it. As

on the profile, arising from the air flowing around it. As the acceleration of air inthe acceleration of air in the propeller wash is caused by the difference in pressure, the resultant air the propeller wash is caused by the difference in pressure, the resultant air force

force∆∆FFRR can be looked upon as being the force which is can be looked upon as being the force which is reactive to thereactive to the

accelerating forces of the air. accelerating forces of the air.

(16)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

flight direction

turn direction

(17)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

PROPELLER BRAKE MOMENT

Brake Moment with Changing Airspeed

The Brake moment is produced by the partial force∆FT, which affects the propeller

blades. As∆FT is a component of the resultant air force ∆FR, ∆FT is to a great extent

directly dependent on the angle of pitch. Thus the propeller blade angle of pitch has a direct influence on the brake moment.

With constant rotational speed the angle of pitch can be influenced by changes in airspeed or blade angle (pitch).

When the airspeed increases, the partial force∆FT becomes smaller, as does the

brake moment. If the engine continues to supply the same motive power and the propeller is not adjusted, the rotational speed will increase until the moments return to equilibrium. Accelerating to very high airspeeds, an engine with fixed propeller can exceed its maximum permissible rotational speed. In such a case a timely reduction in power is necessary.

Brake Moment when Changing the Blade Angle

A reduction in blade angle (pitch) leads to a reduction on the partial force∆FT and

thus to a reduction in the brake moment. With constant motive power the rotational speed will increase. An increase in pitch has the opposite effect.

If the pitch is adjusted to a changing airspeed, the magnitude of brake moment can be maintained. This leads to a constant rotational speed without changing engine power and to almost constant propeller thrust FS.

In this way propeller efficiency improves for the whole of the aircraft’s speed range. Thus with the same engine power higher airspeeds can be achieved than in the case of a fixed propeller.

Brake Moment when Windmilling

If with constant pitch airspeed increases rapidly or rotational speed is greatly reduced, a flow of air to the propeller occurs which causes the propeller to windmill. In this case the partial force∆FT works in the direction of rotation and drive the

propeller.

As thrust∆Fs is relatively large in this situation and directed against the direction

of flight, the aircraft drag is considerably increased by a windmilling propeller. The drag caused by the propeller is greatly reduced if it is put in the feathering position.

Brake Moment at Reverse Thrust

If the blade angle is reduced so far that the angle of attack is less than the zero lift angle of attack, thrust acting against the direction of flight results. The partial force∆FT acts contrary to the direction of rotation, so that the brake moment it

causes must be overcome by the drive. The brake moments which occur very quickly become very large when the blade angle is reduced. So corresponding engine power must be available to maintain the rotational speed.

As the air mass flowing through the propeller plane is not accelerated but decelerated, maximum achievable brake thrust increases with airspeed and can even exceed take−off thrust.

(18)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

direction of turn

Windmilling

direction of turn

Reverse

(19)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

EFFECT ON THE AIRCRAFT

Effect of Engine Torque on the Aircraft

The counter moments caused by engine torque tries to turn the aircraft around the longitudinal axis against the propeller’s rotation. Due to this moment the main landing gear on that side is pushed strongly towards the ground when taxiing and especially on take−off. This leads to an asymmetric distribution of roll resistance and produces a yaw moment around the aircraft’s vertical axis, in other words causing a certain run−off tendency at take−off. To compensate for the roll moment during flight, aerodynamic means are normally used. An example of this would be a small trim strip or a trim tab on one of the ailerons.

Exact compensation is only possible for one particular speed and engine power. Normally cruising speed is chosen. At greater and lower airspeeds the pilot must make corrections with small deflections of the aileron.

The Twist Effect of the Propeller Wash.

The propeller does not only accelerate the air backwards but also causes a twist in the propeller wash. Due to this twist the flow of air to the vertical stabilizer is asymmetric and produces a stabilizer load (FQ) or a yaw moment around the aircraft’s vertical axis. At the same time a roll moment around the aircraft’s longitudinal axis is created. If the propeller is rotating clockwise, as seen by the pilot, these moments will make the aircraft slew to the left. This tendency is heightened by engine torque.

To compensate the vertical stabilizer is normally mounted obliquely by 1° to 2° to the aircraft’s longitudinal axis. This aerodynamic compensation is only perfect for one operational regime (normally during cruise).

In addition, there are other effects of the propeller wash which are of note. If the propeller is mounted in front of a wing and rotates clockwise (as seen from behind), the propeller wash is deflected to the left. Rotating anti-clockwise, the deflection is to the right. The main reason for this is the circulation around the wing, which through the superpositioning with the propeller airstream increases the rate of flow in the upper propeller semi-circle while reducing it in the lower.

In a homogenous parallel stream these changes in velocity would lead to a downwards deflection. But as the propeller wash is twisted it causes, in the same way as a gyro, a pitching motion known as precession.

The described deflection of the propeller wash to one side can, depending on how the tailplane and fin are arranged, lead to a change in the direction of air flowing to these parts.

Figure 10

Twisted Fin

(20)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

twist effect due to propeller wash

twist effect due to engine torque

(21)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

PROPELLER NOISE

The Components of Propeller Noise

If we analyses propeller noise, we can distinguish between the following components according to their origins.

1.

A. Rotation Noise

The rotating pressure field of the propeller produces rotation noise. At mach numbers of the blade tips between M = 0.5 and M = 0.85 and an undisturbed flow of air to the blade this noise exceeds all other noise components. B. Vortex Noise

This noise is caused by the vortices leaving the blade tip and blade trailing edge. Its maximum value is found in the plane of rotation of the propeller. C. Displacement Noise

The origin of this noise is the displacement of the air by the propeller blades as they have a finite thickness. It first becomes critical at higher mach numbers at the propeller tips. At blade tip mach numbers above 0.9 this noise source equals that of rotation noise.

D. Blade Vibration Noise

This noise occurs with periodic stalls, for example when the stall limit of the blade is alternately exceeded and fallen below. The rotors of helicopters are a good example of this phenomenon.

E. Noise caused by inconsistent Airflow

Normally the vortices leave the trailing edge and blade tips in such a way that they do not affect the following propeller blade. The latter can then work in an undisturbed airflow. This is not the case with variable pitch propellers when the angle of pitch is negative and the propeller has zero thrust. Then the vortices of the preceding blade hit the leading edge of the following blade. This results in noise. A similar occurrence is possible if the airflow on the preceding blade stalls as a result of excessive load.

(22)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

Brake Power

Propeller Diameter

Speed of Sound 330 m/s

RPM

2 ₋

Blade

3 ₋

Blade

4 −

Blade

db

_(A)

min

-1

(23)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

G Influence of Propeller Blade Shape

With the same power, blade area, profile type, camber, profile section, ratio and diameter a scimitar−shaped propeller produces the least noise, and one with straight tips the most. This favorable effect of the sabre −shape is due to the increasing outward sweep of the propeller blade as the locally occurring effective mach number is reduced by the factor cos ϕ (ϕ =angle of sweep). The following list shows by about how much propeller noise can be changed according to various influencing factors:

blade tip shape: 3 − 6 dB

profile type: 2 − 3 dB

blade contour: 1 − 2 dB

blade twist: 1 − 2 dB

profile camber: 1 − 2 dB

profile section ratio: 1 − 2 dB

G Influence of Material

If the blades are not made of metal but of wood or composite construction, they have a more favourable vibrational behavior due to better self −damping properties. The noise caused by blade vibrations is lower in the case of such blades. Also by using composite construction more aerodynamic and low−noise blade shapes can be realised without problems regarding strength and stiffness occurring.

The SAAB 2000 propeller is a good example of this. Its construction was optimized with a view to the influences described above. In order to keep noise development as low as possible, this composite propeller rotates when cruising at only 950 rpm.

Figure 13

Different Shapes of Propellers

(24)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

swept propeller

a) Scimitar Shape

b) Elliptical Shape,

with rounded Tips

c) Straight Tips

(25)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

PROPELLERBELASTUNGEN

BLADE SHAPES AND PROFILES (AIRFOIL SECTIONS)

For every speed range there is an optimal profile shape with regard to lift and drag. Thick profiles are used for low speeds and thin ones (usually laminar profiles) for high speeds. At the same time the profile changes from thick at the root area to thin at the blade tips. This is of advantage regarding static stress. In the root area, where the forces are higher, we find a thicker material cross −section, so that the stresses affecting the material do not exceed the permissible range.

The blade shape depends on the purpose of the propeller, whereby performance, airspeed and diameter play a role. The higher the circle load is, the wider the propellers which should be used. For reasons of reducing noise, propeller tips should be elliptical.

PROPELLER LOADS

The components of the propeller are subject to very high loads when in operation. We differentiate between static and dynamic loads.

G Static Loads

Centrifugal force is the main static load on the propeller.

Furthermore the propeller is subject to loads from brake moment and the thrust acting on the blades. Torque loads affect the propeller because of the off −centre shift in the centre of pressure and from the blade’s mass distribution together with the centrifugal force.

The static loads are superimposing at the blade root. Thus the greatest stress from static loads occurs in the region of the blade root.

Damage and repair work, for example the blending of strike damage, are not permitted in this area.

As the blades are attached to the hub, this too is subject to high loads, and thus high stresses also affect its material.

Clark Y

RAF 6

NACA 16

Laminar-Profil NACA 16 RAF 6 Clark Y d t

Figure 15

NACA Shapes

(26)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

Bending by thrust loads

M

torque loads due to mass distribution and

centrifugal force

centrifugal

force

thrust FS

centre of

pressure

torque loads by difference of point

of rotation and pressure

thrust distribution

Bending by braking moment

point of rotation

(27)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

F. Dynamic loads

Maximum dynamic loads occur in the range of the natural frequency of the propeller. The vibrations are excited by the inconstant drive RPM of piston engines as a result of the operating stroke phases of the individual cylinders or by vibrations of the propeller gearbox. Additionally unfavourable aero dynamic conditions cause vibrations.

The natural frequency of the propeller blades depends on blade length, blade shape, blade root and material. The basic frequency ranges from 20 Hz (metal) to 60 Hz (wood). The blade’s natural frequencies also change over the RPM range due to differing centrifugal loads.

At a distance of about 20% of the blade radius from the blade tip the highest vibrational loads occur. This region is therefore particularly susceptible. Nicks caused by scratching, corrosion and strikes affect the durability of metal propellers particularly severely.

For this reason it is essential to look out for such damage during a blade inspection. Damage is to be rectified in accordance with the manufacturer’s manual. L u f t h a n s a T e c h n i c a l T r a i n i n g

(28)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

R= 0,8

1.Order

3.Order

2.Order

nodal point

Point of max. Vibration Loads (Outer Nodal Point)

unsymetr.

symetr.

(29)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

B. Dynamic loads due to Resonance B. Dynamic loads due to Resonance

The exciting frequency for propeller blade vibrations changes with RPM. The exciting frequency for propeller blade vibrations changes with RPM. TheThe combination of engine and propeller is chosen in such a way that the vibrational combination of engine and propeller is chosen in such a way that the vibrational behaviour of the combination is not critical in the operational range of the behaviour of the combination is not critical in the operational range of the engine.

engine.

With some propellers the frequency excited by a certain RPM range may lie With some propellers the frequency excited by a certain RPM range may lie within the natural frequency range of the propeller. With 2

within the natural frequency range of the propeller. With 2−−blade metalblade metal propellers used on small aircraft this resonance is found at about 2100 propellers used on small aircraft this resonance is found at about 2100 −− 2200 2200 propeller RPM range. This range is therefore not suitable for

propeller RPM range. This range is therefore not suitable for continuous operation and should be avoided.

continuous operation and should be avoided.

In order to have a picture of the vibrational behaviour of the

In order to have a picture of the vibrational behaviour of the propeller, apropeller, a resonance diagram is constructed. The horizontal line shows engine RPM resonance diagram is constructed. The horizontal line shows engine RPM (min

(min−−11) and the vertical line the calculated frequencies (min) and the vertical line the calculated frequencies (min−−11). Frequency lines). Frequency lines

1

1 −− 4 and the line of the natural frequency of the 4 and the line of the natural frequency of the propeller are drawn on thepropeller are drawn on the diagram. The natural frequency line of the propeller must not cut through the diagram. The natural frequency line of the propeller must not cut through the lines of exciting frequencies in the

lines of exciting frequencies in the operating range.operating range.

propeller rpm propeller rpm

max. design load max. design load

off limit off limit rpm range rpm range

F

Fiig

gu

urre

e 1

18

8 K

Ke

ee

ep o

p ou

ut Z

t Zo

on

ne

e

(30)

M 17 PROPELLER

M 17.1 FUNDAMENTALS

EASA PART 66

M17

resonance diagram

natural frequency 1

natural frequency 2

engine rpm engine rpm ff o o p p e e r r a a t t i i n n g g r r a a n n g g e e L L u u f f t t h h a a n n s s a a T T e e c c h h n n i i c c a a l l T T r r a a i i n n i i n n g g

Fi

(31)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

M

1

7

7 .

.

2

2 P

P

R

O

P

E

L

E

R C

O

N

S

T

R

U

C

T

I

O

N

GENERAL

Propellers are designed as either pusher or puller (tractor) propellers, which are Propellers are designed as either pusher or puller (tractor) propellers, which are then subdivided into fixed pitch propellers, adjustable pitch propellers and variable then subdivided into fixed pitch propellers, adjustable pitch propellers and variable pitch propellers.

pitch propellers.

Variable pitch propellers are further categorized according to the method of pitch Variable pitch propellers are further categorized according to the method of pitch changing, for example hydraulic, mechanical or electrical, according to the type changing, for example hydraulic, mechanical or electrical, according to the type of change, e. g. changing to a particular angle or a particular RPM or according of change, e. g. changing to a particular angle or a particular RPM or according to the scope of change. In this respect there are propellers which, in addition to to the scope of change. In this respect there are propellers which, in addition to normal change of pitch, can also be feathered and/or put

normal change of pitch, can also be feathered and/or put into reverse thrust.into reverse thrust.

FIXED PITCH PROPELLERS

Fixed pitch propellers are used for up to about 200 kW (250 hp) performance and Fixed pitch propellers are used for up to about 200 kW (250 hp) performance and speeds of up to

speeds of up to 250 km/h (160 mph). The blade angle (pitch) cannot be changed250 km/h (160 mph). The blade angle (pitch) cannot be changed and is determined in accordance with the purpose it

and is determined in accordance with the purpose it is to be used for. is to be used for. For steepFor steep climbing and towing low (fine) pitch is needed and for more gradual climb and climbing and towing low (fine) pitch is needed and for more gradual climb and cruising flight a higher (coarse) pitch is preferred. Greater efficiency can only be cruising flight a higher (coarse) pitch is preferred. Greater efficiency can only be achieved over a small range of speeds. Fixed propellers are favourable with regard achieved over a small range of speeds. Fixed propellers are favourable with regard to production and maintenance costs.

to production and maintenance costs.

These propellers are generally manufactured from forged light alloys

These propellers are generally manufactured from forged light alloys or layers of or layers of bonded wooden strips (typically birch).

bonded wooden strips (typically birch).

The fixed pitch propeller has a thick hub to create a smooth transition from the thick The fixed pitch propeller has a thick hub to create a smooth transition from the thick airfoil section at the blade root (with its high blade angle) to the hub. In most cases airfoil section at the blade root (with its high blade angle) to the hub. In most cases these propellers can be attached directly to the engine

these propellers can be attached directly to the engine with bolts. To maintain awith bolts. To maintain a larger distance from the engine flange, which allows for a more favourable engine larger distance from the engine flange, which allows for a more favourable engine cowling, spacers are used, which are

cowling, spacers are used, which are available in different thicknesses.available in different thicknesses.

ADJUSTABLE PITCH PROPELLERS

The blade angle of an adjustable pitch propeller can be changed on the ground The blade angle of an adjustable pitch propeller can be changed on the ground when the engine is shut down. The blades are clamped in the hub. When the when the engine is shut down. The blades are clamped in the hub. When the clamping bolts are loosened, the blades can be turned in the hub. There are clamping bolts are loosened, the blades can be turned in the hub. There are generally adjusting marks in the blade and the hub.

generally adjusting marks in the blade and the hub.

The hub is usually made of forged light alloy or steel. The blades are manufactured The hub is usually made of forged light alloy or steel. The blades are manufactured from forged light alloy or wood.

from forged light alloy or wood.

Wooden blades are either made in one piece from laminated wood or as a Wooden blades are either made in one piece from laminated wood or as a combination, with

combination, with kunstharzpressholz kunstharzpressholz (Synthetic Resin Compressed Wood)(Synthetic Resin Compressed Wood)

at the blade root and a light

at the blade root and a light wood (e. g. spruce) for the body of the wood (e. g. spruce) for the body of the blade.blade. The certification of these propellers requires a great deal of time

The certification of these propellers requires a great deal of time−−consuming workconsuming work and a vibration examination. They have not become very popular and are used and a vibration examination. They have not become very popular and are used only in special cases.

only in special cases.

Fi

Figu

gure

re 20

20 Ad

Adju

just

stab

able P

le Pit

itch

ch Pr

Prop

opel

elle

lerr

(32)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

SPINNER DOME

FORWARD SPINNER BULKHEAD PROPELLER DOWEL PIN

CRANK SHAFT

RING GEAR

ASSEMBLY

SPACER REAR SPINNER BULKHEAD

Propeller Installation

(33)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

VARIABLE PITCH PROPELLERS

GENERAL CONSTRUCTION

In the case of a variable pitch propeller the blade angle can be changed during operation. In this way it can be adjusted for different operating conditions. This type of propeller is therefore more efficient over a wider range of speeds. Nowadays hydraulically controlled variable pitch propellers are almost exclusively in use, except for motorized gliders, the propellers of which are often adjusted mechanically (3 position propeller) or electrically.

The blades of a variable pitch propeller are mounted on ball, roller or needle bearings in the hub and can be turned to adjust the blade angle. They can be made of forged light alloys, steel, fibre reinforced plastics, or of a wooden composite construction. The components for adjusting the blade angle are normally found on the front of the hub but in some cases they are inside the hub itself.

The main parts are the pitch change piston and the pitch change cylinder, whereby either the piston or the cylinder can move axially. The axial movement of the piston or cylinder is converted into a rotational movement of the blade via pins, bevels or linkages. The oil needed for the hydraulic action is taken from the pressurized oil in the engine lubrication system. It is supplied to the pitch change piston and cylinder via a valve on the governor and through the hollow propeller shaft.

PITCH CHANGING MECHANISM

(McCauley BLACKMAC)

Figure 22

Pitch changing Propeller

(34)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

(35)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

PITCH CHANGE RANGE

In the range between the low (fine) pitch stop (for low airspeeds) and the high (coarse) pitch stop (for high airspeeds) the propeller can be adjusted to any angle. In the case of multiple engine aircraft and motorized gliders an engine should produce as little drag as possible when it is shut down. Therefore their blades can also be moved into the feathering position (least drag).

With large aircraft the production of reverse thrust is intended to shorten the distance on landing. For this purpose the propellers are turned to reverse pitch, where air is accelerated forwards while the propellers continue to turn in the same direction. Thus reverse thrust is produced.

The following types of propeller commonly have hydraulic pitch change mechanisms:

Constant speed propellers (pitch change from low (fine) to high (coarse) pitch)

Constant speed propellers with feathering position

Constant speed propellers with feathering and reverse (for turboprop engines)

(36)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

Pitch Change Range

Feather Position

High Pitch

Low Pitch

Reverse

(37)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

SINGLE ACTING PROPELLERS

Some propeller systems operate in such a way that oil pressure changes the pitch in one direction only. Movement in the opposite direction is the result of spring force and the torsion moments of the blades themselves. Propellers which have such a pitch change mechanism are called single acting propellers.

Single Acting Propellers for Single-Engine Aircraft

With these propellers the oil pressure moves the blades in the direction of high (coarse) pitch and the spring moves it towards low pitch. After engine shut−down the blades are in the lowest (fine) pitch stop position, which is optimal for restarting the engine. Should the engine fail during flight, this blade position is favourable for windmilling, which makes it easier to restart the engine.

(38)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

Oil Pressure increases

Pitch

Spring Force decreases Pitch

Pitch Changing Mechanism

for Single Engine Aircraft

(39)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

Single Acting Propellers for Multi-Engine Aircraft

If single acting propellers are used on multi-engine aircraft, oil pressure moves the blades in the direction of low (fine) pitch. The springs and torsional moments of the blades move the blades towards high pitch. If engine failure occurs during flight with decreasing oil pressure the blades move in the high (coarse) pitch direction. In this way they have already covered part of the transition to the feather position.

FLYWEIGHTS

PITCH CHANGE-MECHANISM

BLADE

HUB

Figure 26

Pitch changing Propeller

(40)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

flyweight

in-creases pitch

oil pressure

decreases pitch

(41)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

PITCH CHANGE MOMENTS FROM CENTRIFUGAL FORCE (FLY WEIGHTS)

The centrifugal force of the propeller blade mass produces a pitch change moment which turns the blade in the direction of low (fine) pitch. The creation of this natural pitch change moment (or flymoment) is due to the distribution of the propeller blade mass.

The mass elements not lying on the blade axis create a proportion of centrifugal force, the effect of which is at a small angle away from the blade axis. Thus this force has a component in radial direction FZR and one in tangential direction FZT.

The latter component is at right angles to the blade axis. This tangential force component affects the blade laterally to its axis. This means that the force components work with a lever on the blade axis, on which the blade turns, and therefore produce torque in the direction of low (fine) pitch.

If the propeller blade is to turn towards high (coarse) pitch as a result of centrifugal force (for propellers with feathering position) then a flyweight must be attached to the blade root. The creation of the pitch change moment from the centrifugal force of the flyweight is based on the same principles as for the propeller blade. The torque in the direction of high (coarse) pitch produced by the flyweight is as a rule twice the amount of the natural torque in the direction of low pitch.

(42)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

(43)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

DOUBLE ACTING PROPELLERS

Large propellers are generally constructed with pitch change mechanisms where oil pressure leads to pitch change in both directions. These are called double acting propellers. The valve for controlling the flow of oil to the two ends of the piston is mounted either behind the gearbox or in the propeller hub.

COARSE

FINE PITCH OIL

COARSE PITCH OIL

Figure 29

Moving Cylinder Propeller

If the control valve, as in the Dowdy propeller of the Fokker 50, is mounted behind the gearbox in the PCU, the propeller shaft must have two oil transfer tubes, one for the front and one for the back of the piston. These oil tubes are constructed as coaxial tubes (here beta tube).

COARSE

FINE PITCH OIL

COARSE PITCH OIL

Figure 30

Moving Piston Propeller

(44)

M 17 PROPELLER

M 17.2 CONSTRUCTION

EASA PART 66

M17

COUNTERWEIGHT BLADE ASSEMBLY CYLINDER PISTON OPERATING PIN ASSEMBLY BLADE ROOT HUB DOWEL HOLE BETA TUBE CROSSHEAD YOKE PROPELLER CYLINDER CROSSHEAD SHAFT CROSSHEAD YOKE HUB PISTON L u f t h a n s a T e c h n i c a l T r a i n i n g