16 turboprops (1)

Issue 3 – Jan 2004 Page 16-1
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16 TURBO-PROP ENGINES
16.1 INTRODUCTION
The earliest concept of the use of a turbine engine in aircraft was for the turbine to
drive the propeller. Turbojet engines showed so much promise that some believed
they would make propellers obsolete. Fortunately, this has proven to be untrue.
Turboprop powerplants fill an important niche between turbojet or turbofan engines
and reciprocating engines. They combine the high propulsive efficiency with the low
weight and high time between overhauls of the turbine engine.
The gas-turbine engine in combination with a reduction gear assembly and a
propeller has been in use for many years, and has proved to be a most efficient
power source for aircraft operating at speeds of 300 to 450 mph [482.70 to 724.05
km/h]. These engines provide the best specific fuel consumption of any gas-turbine
engine, and they perform well from sea level to comparatively high altitudes (over
20,000 ft [6096 m]). At higher speeds and altitudes, the efficiency of the propeller
deteriorates rapidly because of the development of shock waves on the blade tips.
Although various names have been applied to gas-turbine engine/propeller
combinations, the most widely used name is turboprop. Another popular name is
‘propjet’.
16.2 TYPES OF TURBOPROP ENGINES
The power section of a turboprop engine is similar to that of a turbojet engine.
However, there are some important differences, and the most important of these
differences can be found in the turbine section. In the turbojet engine, the turbine
section is designed to extract only enough energy from the hot gases to drive the
compressor and accessories. The turboprop engine, on the other hand, has a turbine
section that extracts as much as 75 to 85 percent of the total power output to drive
the propeller.
The turbine section of the turboprop usually has more stages than that of the turbojet
engine; in addition, the turbine blade design of the turboprop is such that the turbines
extract more energy from the hot gas stream of the exhaust. In the turboprop engine,
the compressor, combustion section and the compressor turbine comprise what is
often called the gas generator or gas producer. The gas generator produces the
high velocity gases that drive the power turbine. The gas generator section
performs only one function: converting fuel energy into high-speed rotational energy.
Current turbo-prop engines can be categorised according to the method used to
achieve propeller drive; these categories are:
a. Coupled Power Turbine (or, Fixed Shaft Engine).
b. Free Turbine.
c. Compounded Engine.

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16.2.1 COUPLED POWER TURBINE
A different method of converting the high-speed rotational energy from the gas
generator into useable shaft horsepower is shown schematically in Figure 16.1. In
this case, the gas generator has an additional (third) turbine wheel. This additional
turbine capability utilises the excess hot gas energy (that is, energy in excess of that
required to drive the engine’s compressor section) to drive the propeller.
In a coupled power turbine, the shaft is mechanically connected to the gearbox so
that the high- speed low-torque rotational energy transmitted into the gearbox from
the turbine can then be converted to the low-speed high-torque power required to
drive the propeller. An example of the fixed shaft engine configuration is illustrated in
Figure 16.2.
Coupled (fixed shaft) type engine
Fig 16.1
Garrett TPE331 turboprop engine
Fig 16.2

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16.2.2 FREE POWER TURBINE
In this arrangement, a gas turbine acts simply as a gas generator to supply high-
energy gases to an independent free power turbine as shown in the Figure 16.3. An
additional turbine wheel is placed in the exhaust stream from the gas generator and
the primary effort is directed towards driving the propeller. The gases are expanded
across the free turbine, which is connected to the propeller drive shaft via reduction
gearing. The free turbine arrangement is very flexible; it is easy to start due to the
absence of propeller drag and the propeller and gas producer shafts can assume
their optimum speeds independently.
An example of the free turbine engine configuration is illustrated in Figure 16.4.
Free power turbine-type power conversion.
Fig 16.3
Pratt & Whitney PT6 Free Power Turbine Engine.
Fig 16.4

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16.2.3 COMPOUNDED ENGINE
The compounded engine arrangement features a two-spool engine, with the propeller
drive directly connected to the low-pressure spool as shown in Figure 16.5.
16.3 REDUCTION GEARING
The power turbine shaft of a turbo-prop engine normally rotates at around 8,000 to
10,000 rpm, although rpm of over 40,000 are found in some engines of small
diameter. However, the rotational speed of the propeller is dictated by the limiting tip
velocity. A large reduction of shaft speed must be provided in order to match the
power turbine to the propeller. The reduction gearing must provide a propeller shaft
speed which can be utilised effectively by the propeller; gearing ratios of between 6
and 20:1 are typical. Figure 16.5a shows a modern turboprop engine.
Compounded engine arrangement
Fig 16.5
A modern turboprop
Fig 16.5a

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In the direct coupled power turbine and compounded engines, the shaft bearing the
compressor and turbine assemblies drives the propeller directly through a reduction
gearbox. In the free turbine arrangement reduction gearing on the turbine shaft is
still necessary; this is because the turbine operates at high speed for maximum
efficiency. The reduction gearing accounts for a large proportion (up to 25%) of the
total weight of a turbo-prop engine and also increases its complexity; power losses of
the order of 3 to 4% are incurred in the gearing (eg. on a turbo-prop producing 6,000
eshp, some 200 shp is lost through the gearing).
16.3.1 SIMPLE SPUR ‘EPICYCLIC’
A gear train consisting of a sun (driving) gear meshing with and driving three or more
equi-spaced gears known as ‘Planet Pinions’. These pinions are mounted on a
carrier and rotate independently on their own axles. Surrounding the gear train is an
internally toothed ‘Annulus Gear’ in mesh with the Planet Pinions, as shown in Figure
16.6.
If the annulus is fixed, rotation of the sun wheel causes the planet pinions to rotate
about their axes within the annulus gear, this causes the planet carrier to rotate in the
same direction as sun wheel but at a lower speed. With the propeller shaft secured
to the planet pinion carrier, a speed reduction is obtained with the turbine shaft (input
shaft) and propeller shaft (output shaft) in the same axis and rotating in the same
direction, as shown in Figure 16.7.
An epicyclic gear.
Fig 16.6

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If the annulus is free, rotation of the sun wheel causes the planet pinions to rotate
about their axles within the annulus gear. With the planet pinion carrier fixed and the
propeller shaft attached to the annulus gear, rotation of the planet pinions causes the
annulus gear and propeller to rotate in the opposite direction to the sun wheel and at
a reduced speed. (Figure 16.8.)
Epicyclic gear train with fixed annulus ring gear.
Fig 16.7
Epicyclic gear train with fixed planet gear carrier.
Fig 16.8

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16.3.2 COMPOUND SPUR EPICYCLIC
Compound epicyclic reduction gears enable a greater reduction in speed to be
obtained without resorting to larger components. They may be of either the fixed or
free annulus type. An illustration of a compound spur gear is shown in Figure 16.9.
Compound spur gear
Fig 16.19

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16.3.3 GEAR TRAIN/EPICYCLIC
Some turbo-props will use a gear train or a combination of gear train and epicyclic.
An example of this arrangement is shown in the cutaway illustration of a Garrett 331
engine in Figure 16.20.
16.4 TURBO-PROP PERFORMANCE
The turbo-prop has a higher propulsive efficiency than the turbo-jet up to speeds of
approximately 575 mph and higher than a turbo-fan engine up to approximately 450
mph. Compared with a piston engine of equivalent power, the turbo-prop has a
higher power to weight ratio and a greater fatigue life because of the reduced
vibration level from the gas turbine rotating assemblies.
Combined epicyclic gear train.
Fig 16.20

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16.5 INTRODUCTION TO PROPELLERS
Many of the earliest aeroplanes failed to fly because of the inability of their propellers
to convert the power of the engine, as little as it was, into thrust. And one of the
reasons the Wright flyer succeeded in 1903 was the careful design of its propellers.
The weak 12-horsepower engine drove 2 propellers that together produced 90
pounds of thrust, enough to fly the 750-pound aeroplane. The 81/2-foot diameter
laminated spruce propellers, turning at 330 RPM, had an efficiency of 66% which
was certainly commendable in view of the fact that the best efficiency of a modern
propeller is only about 90%.
The whole purpose of a propeller is to provide the thrust required to move the aircraft
forward. The aircraft propeller consists of 2 or more blades and a central hub to
which the blades are attached. Each blade of an aircraft propeller is essentially a
rotating wing. As a result of their construction, propeller blades produce forces that
create thrust to pull or push the aeroplane through the air.
Power to rotate an aircraft’s propeller blades is provided by the engine. On low-
horsepower piston-type engines, the propeller is mounted on a shaft that is usually
an extension of the crankshaft. On high-horsepower engines, such as a turboprop
engine, the propeller is mounted on a propeller shaft driven by a turbine through a
reduction gearbox. In either case, the engine rotates the aerofoils of the blades
through the air at high speeds, and the propeller transforms the rotary power of the
engine into thrust.
The basic principle of propellers has changed very little since 1903, but technology
has undergone many revolutionary advances in aerodynamics as well as materials
and construction methods.
16.5.1 PROPELLER PRINCIPLES – THE AEROFOIL
The aerofoil is a particular streamlined shape which, when moving through the
atmosphere, will produce a force approximately at right angles to the direction of
movement. When the aerofoil is the wing of an aircraft, we call the force produced
‘lift’, but when the aerofoil is the blade of a propeller we call this force ‘thrust’. It is the
thrust produced by the propeller that moves the aircraft forward and the lift of the
wings that support the aircraft in the air. A typical aerofoil is shown in Figure 16.21.

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When an aerofoil moves through the air its special streamlined shape causes a
particular airflow pattern to develop. Air passing over the curved aerofoil surface is
caused to increase in velocity relative to the velocity of the air flowing over the flat
surface and, as a consequence, the pressure of the air over the curved surface is
reduced relative to the pressure of the air flowing over the flat surface. This relative
change in pressure creates a resultant net force as shown in Figure 16.22.
Only the air that passes over the curved and flat surfaces will exhibit relative changes
in velocity and pressure, and the air that is some distance in front of the leading edge
will remain undisturbed.
Since the propeller blade and the wing of an aeroplane are similar in shape, each
propeller blade may be considered as a rotating wing. It is true that it is a small wing
Airflow reaction to a moving aerofoil.
Fig 16.22
Typical aerofoil section
Fig 16.21
Leading
edge
Flat
undersurface
Curved or cambered
Top surface
Trailing
edge

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that has been reduced in length, width and thickness, but it is still a wing in shape. At
one end this small wing is shaped into a shank, thus forming a propeller blade.
When the blade starts rotating, air flows around the blade just as it flows around the
wing of an aeroplane, except that the wing, which is approximately horizontal, is lifted
upward, whereas the blade is ‘lifted’ forward. Figure 16.23 shows the typical aerofoil
section of a propeller.
16.5.2 PROPELLER GLOSSARY OF TERMS
Thrust - The aerodynamic force produced by a propeller or turbojet engine as
it forces a mass of air to the rear of the aircraft. A propeller produces its thrust
by accelerating a large mass of air by a relatively small amount.
Torque – A force that produces or tries to produce rotation.
Propeller Torque – Acts in the plane of rotation and is the resistance to
rotation offered by the propeller and opposes engine torque.
Air Density – An increase in air density increases the thrust. However, denser
air offers greater resistance to propeller rotation, i.e. increased torque.
Plane of Rotation – The plane in which the propeller blade rotates. The plane
of rotation is perpendicular to the propeller shaft.
Propeller Speed – Thrust and torque change in direct proportion to the
propeller speed.
Typical aerofoil section of a propeller.
Fig 16.23

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Horsepower – The most commonly used unit of mechanical power. One
horsepower is equal to 33000 foot pounds of work done in one minute, or 375
mile pounds per hour.
Shaft Horsepower – The horsepower actually available at a rotating shaft.
Equivalent Shaft Horsepower – A measure of the power produced by a
turboprop engine that combines the shaft horsepower being delivered to the
propeller and the thrust being developed at the engine exhaust.
Thrust Horsepower – The horsepower equivalent of the thrust produced by a
turbojet engine.
Angle of Attack – Any increase in the angle of attack to just below the stalling
speed will produce more thrust and torque. The optimum angle of attack will
give the best thrust to torque ratio.
Best Angle of Attack – To obtain the best lift versus drag, then the most
efficient angle of attack will be between 2° and 4° positive. The actual blade
angle necessary to maintain this small angle of attack varies with the forward
speed of the aircraft and rotational speed of the propeller.
Relative Airflow – The relative airflow is the resultant of 2 component
airflows:
 The airflow due to the rotational speed of the propeller
 The airflow due to the forward speed of the aircraft
Blade Station - A reference position on a blade that is a specified distance
from the centre of the hub.
Geometric Pitch – The distance a propeller would advance in one revolution
if it were rotating in a solid.
Effective Pitch – The actual distance a propeller advances in one revolution
through the air.
Pitch Distribution – A gradual twist in the propeller blade from shank to tip.

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16.5.3 PROPELLER BLADE DESCRIPTION
The identification of the various parts of the propeller blade are shown in Figure
16.24.
Propeller general terms
Fig 16.24
spinner
hub
root
trailing edge
leading edge
tip

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16.6 COMPARISON OF AEROFOIL AND BLADE FORCES
The illustrations in Figure 16.25 below show the comparison between the
aerodynamic forces generated from an aircraft aerofoil and from a propeller blade.
AEROFOIL
Aerodynamic forces on a propeller blade
Fig 16.25
PROPELLER
direction
of flight

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16.6.1 PRODUCING THRUST
The propeller has a number of blades of an aerofoil shape that will produce thrust
when the propeller turns and the blades move through the air. The low pressure
created in front of the blades attracts more air towards the propeller and this in turn is
thrown rearwards by the movement of the blades until the propeller is moving a
column of air towards the rear (Figure 16.26). The amount of useful thrust produced
by a propeller depends upon the amount of air that the propeller can move and the
increase in velocity that it can add to the moving air mass.
From the equation: Force = mass x acceleration
Thrust = m [v2 – v1]
where: m = mass airflow
v2 = velocity of the propeller wake
v1 = velocity of the aircraft
Compared with a jet engine, the mass airflow of the propeller is large and the
increase in velocity small.
Flight
path
Propeller forces
Fig 16.26
air
flow
VELOCITY
CHANGE
[ ms-1 ]
AIRFLOW
[
kgs
-1
]

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16.7 PROPELLER THEORY
The thrust produced by a propeller blade is determined by 5 things:
 The shape of the aerofoil section
 The area of the aerofoil section
 The angle of attack
 The density of the air
 The speed the aerofoil is moving through the air
There are 2 aspects of the overall theory that explain the operation of a propeller:
 The momentum theory
 The blade-element theory
The momentum theory considers a propeller blade an aerofoil that, when rotated by
the engine, produces a pressure differential between its back and face which
accelerates and deflects the air. The amount of thrust is determined by the change in
momentum of air passing through the propeller, multiplied by the area of the propeller
disc. The amount of thrust produced depends on the aerofoil shape, RPM and angle
of attack of the propeller blade sections.
The blade element theory considers a propeller blade to be made of an infinite
number of aerofoil sections, with each section located a specific distance from the
axis of rotation of the propeller. Each blade can be marked off in one inch segments
known as blade stations. The cross section of each blade station will show that the
low-speed aerofoils are used near the hub and high-speed aerofoils towards the tip.
By using the blade element theory, a propeller designer can select the proper aerofoil
section and pitch angle to provide the optimum thrust distribution along the blade.

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16.7.1 PITCH DISTRIBUTION
The pitch distribution (blade twist), as shown in fig Figure 16.27, and the change in
aerofoil shape along the length of the blade is necessary, because each section
moves through the air at a different velocity, with the slowest speeds near the hub
and the highest speeds near the tip.
Pitch distribution in a typical aircraft propeller
Fig 16.27

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To illustrate the difference in the speed of aerofoil sections at a fixed RPM, consider
the 3 blade stations indicated on the propeller shown in Figure 16.28. If the propeller
is rotating at 1800 RPM, the 18-inch station will travel 9.42 feet per revolution (193
mph), while the 36-inch station will travel 18.84 feet per revolution or 385 mph. And
the 48-inch station will move 25.13 feet per revolution, or 514 mph.
The aerofoil that gives the best lift at 193 mph is inefficient at 514 mph. Thus the
aerofoil is changed gradually along the length of the blade. This progressive change
in blade angle ensures that the angle of attack remains constant along the total
length of the blade.
Comparative velocities at 3 blade stations
Fig 16.28

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16.7.2 BLADE ANGLE
Technically, the blade angle is defined as the angle between the face or chord of a
particular blade section and the plane in which the propeller blades rotate. Figure
16.29. illustrates a 4-bladed propeller (only 2 blades are shown for simplicity)
indicating the blade angle, plane of rotation, blade face, longitudinal axis and the
nose of the aeroplane.
In order to obtain thrust, the propeller blade must be set at a certain angle to its plane
of rotation, in the same manner that the wing of an aeroplane is set at an angle to its
forward path. While the propeller is rotating in forward flight, each section of the
blade has a motion that combines the forward movement of the aeroplane with the
circular or rotary movement of the propeller. Therefore, any section of the blade has
a path through the air that is shaped like a spiral or a corkscrew, as shown in Figure
16.30.
Four-bladed propeller.
Fig 16.29

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The amount of bite (amount of air) taken by each blade is determined by its blade
angle, as shown in Figure 16.31.
Spiral movement of propeller.
Fig 16.30
Propeller blade angle.
Fig 16.31

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An imaginary point on a section near the tip of the blade traces the largest spiral, a
point on a section midway along the blade traces a smaller spiral and a point on the
section near the shank of the blade traces the smallest spiral of all. In one turn of the
blade, all sections move forward the same distance, but the sections near the tip of
the blade move a greater circular distance than the sections near the hub.
16.8 PROPELLER PITCH
If the spiral paths made by various points on sections of the blades are traced, with
the sections at their most effective angles, then each individual section must be
designed and constructed so that the angles gradually decrease towards the tip of
the blade and increase towards the shank. This gradual change of blade section
angles is called pitch distribution and accounts for the pronounced twist of the
propeller blade.
16.8.1 GEOMETRIC PITCH
Since the pitch angle of a propeller blade varies along its length, a particular blade
station must be chosen to specify the pitch of a blade. This is normally done by
specifying the angle and the blade station, e.g. 14° at the 42-inch station.
Rather than using blade angles at a reference station, some propeller manufacturers
express pitch in inches at 75% of the radius. This is the geometric pitch, or the
distance this particular element would move forward in one revolution along a helix,
or spiral, equal to its blade angle.
The geometric pitch is found by the formula:
Geometric Pitch = Tan pitch angle x 2  r
Where: Tan pitch angle = the tangent of the pitch angle
2 = a constant, 6.28
r = radius of the blade element (blade station)
A propeller with a blade angle of 14° at the 42-inch station has a geometric pitch of
65.9 inches.
Geometric Pitch = Tan pitch angle x 2  r
= Tan 14° x 6.28 x 42
= 0.25 x 6.28 x 42
= 65.9 inches

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16.8.2 EFFECTIVE PITCH
The effective pitch is the actual distance the aeroplane moves forward during one
revolution (360°) of the propeller in flight. ‘Pitch’ is not a synonym for ‘blade angle’ but
the two terms are commonly used interchangeably because they are so closely
related. Figure 16.32. shows two different pitch positions. The black aerofoil drawn
across the hub of each represents the cross section of the propeller to illustrate the
blade setting.
When there is a small blade angle, there is a low pitch and the aeroplane does not
move very far forward in one revolution of the propeller. When there is a large blade
angle, there is a high pitch and the aeroplane moves further forward during a single
revolution of the propeller.
16.8.3 SLIP
Slip is defined as the difference between the geometric pitch and the effective pitch
of a propeller (Figure 16.33). It may be expressed as percentage of the mean
geometric pitch or as a linear dimension.
Low pitch and high pitch.
Fig 16.32
Geometric pitch – advance per revolution
Geometric pitch
x 100 %
Slip =

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If a propeller has a pitch of 50 inches, in theory, it should move forward 50 inches in
one revolution. But, if the aircraft actually moves forward only 35 inches in one
revolution, then the effective pitch and the propeller is 70% effective.
Although the terms blade angle and pitch are often used to express the same thing,
pitch will vary relative to the forward speed of the aircraft, whereas blade angle can
be locked in any position regardless of forward speed. Figure 16.34 compares the
advance per revolution (effective pitch) with the geometric pitch, in relation to aircraft
forward speed and propeller rotation.
Effective and geometric pitch.
Fig 16.33.
Comparison of geometric and effective pitch.
Fig 16.34.

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16.9 THE ANGLE OF ATTACK
An aerofoil encourages a smooth airflow when it moves through the atmosphere but
it becomes a thrust producer only when it is inclined at an angle to the airflow (Figure
16.35). The angle at which the aerofoil strikes the air is called the angle of attack: the
best results are obtained when this angle is about 4°.
Thrust produced by a propeller, in the same way as lift produced by a wing, is
determined by the blades angle of attack. Angle of attack relates to the blade pitch
angle, but it is not a fixed angle. It varies with the forward speed of the aircraft and
the RPM of the engine. As shown in Figure 16.36, any change in the forward or
rotational velocities alter the angle of attack. An increase in forward velocity
decreases the angle of attack and an increase in rotational velocity increases the
angle of attack.
The angle of attack varying with aircraft forward speed and engine RPM.
Fig 16.36
Propeller blade angles.
Fig 16.35

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16.10 EQUIVALENT SHAFT HORSE POWER
One horsepower is equal to 33,000 foot pounds of work done per minute, which is
the same as 550 foot pounds per second or 375 mile pounds per hour. Shaft
horsepower (shp), is the horsepower delivered to the propeller shaft and can be
calculated using the formula :.
shp = actual propeller rpm x torque x K
Where K is the torque-meter constant ( K = 2   33,000 )
With a turboprop engine, some jet velocity is left at the jet nozzle (net thrust
developed at the engine exhaust) after the turbines have extracted the required
energy for driving the compressor, reduction gear and accessories etc. This velocity
can be calculated as net thrust ( Fn ), that also aids in propelling the aircraft. If shaft
horsepower and net thrust are added together, a new term, ‘equivalent shaft
horsepower’ (eshp) results. However the net thrust must be converted to equivalent
horsepower. Under static conditions, one shp is approx. equal to 2.5 lbs of thrust.
The formula for calculating eshp is:
eshp (static) = shp +
In flight, the ehsp considers the thrust produced by the propeller, which is found by
multiplying the net thrust in pounds by the speed of the aircraft in mph. Divide this by
375 times the propeller efficiency, which is considered to be 80%.
eshp (flight) = shp +
where: v = aircraft speed (mph)
 = propeller efficiency; an industry standard of 80%
375 = a constant; mile pounds per hour for one horsepower
Example: Find the equivalent shaft horsepower produced by a turboprop aircraft that
has the following specifications:
Airspeed = 260 mph
Shaft horsepower indicated on the cockpit gauge = 525 shp
Net thrust = 195 lbs
eshp (flight) = shp +
eshp (flight) = 525 +
= 525 + 169 = 694
Under these conditions, the engine is producing 694 eshp
Fn
2.5
Fn x v
375 x 
Fn x v
375 x 
195 x 260
375 x 

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16.11 PROPELLER EFFICIENCY
The thrust horsepower is the actual amount of horsepower that an engine-propeller
unit transforms into thrust. This is less than the shaft horsepower developed by the
engine, since the propellers are never 100% efficient. Propeller efficiency varies from
approx. 50% to 90% depending on how much the propeller ‘slips’.
Some of the work performed by the engine is lost in the production of noise.
Normally, about half of the noise made by the propeller-driven engine is made by the
propeller itself. When the propeller blade tips approach the speed of sound,
vibrations are produced that cause the noise. When the blades operate in the
transonic range, they not only produce noise, but the drag becomes excessive and
the efficiency drops off dramatically. For the propeller disc to be as large as possible
while keeping the tips below the speed of sound, most high-powered engines are
geared so the propeller turns slower than the engine driveshaft.
The maximum propeller efficiency that has been obtained in practice under the most
ideal conditions, using conventional engines and propellers, has been only about
92%. And, in order to obtain this efficiency, it has been necessary to use thin aerofoil
sections near the tips of the blades and very sharp leading and trailing edges.
Since the efficiency of any machine is the ratio of the useful power output to the
power input, propulsive efficiency is the ratio of thrust horsepower [work done by
propeller] to shaft horsepower [work done by engine]. The usual symbol for
propulsive efficiency is the Greek letter  (eta).
The efficiency of the propeller is the ratio of the thrust horsepower to the shaft
horsepower:
thrust horsepower
propeller efficiency = x 100
shaft horsepower
Example: The drag on an aircraft travelling at 200 mph is 1125 lbs. The engine
produces 750 shp. Calculate the propeller efficiency (one hp = 375 mile pounds per
hour).
In level flight, drag is equal to thrust
Thrust x aircraft speed 1125 x 200
Thrust horsepower = = = 600
375 375
Shaft horsepower = 750
600
 propeller efficiency = x 100 = 80 %
750

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16.12 POWER ABSORPTION
When engine power is changed into thrust by the propeller, the drag or torque
created by the propeller being forced through the air limits the engine speed. For
maximum efficiency, the propeller must be able to absorb all the engine power
available.
Power can be absorbed by propeller design but each method used has its limitations
and a compromise has to be made for the final propeller design.
Power Absorbed By: Limitations
Increasing blade angle Reduction in thrust / torque ratio.
Blades ‘stall’ at low forward speed.
Blade length increased High tip speeds – reduced efficiency.
Propeller clearance of ground and aircraft
structure.
Higher propeller speed High tip speeds – reduced efficiency.
Altering the blade camber Reduced aerodynamic efficiency.
Increasing the blade chord Increased weight, increased turning moment
loading.
Increasing the number of blades Increased weight, structural difficulties at
propeller hub.
Contra rotating propellers Complicated pitch change mechanism,
expense and maintenance
16.12.1 NUMBER OF BLADES
The number of blades has been an option for propeller engineers. The logical choice
for fixed pitch wood and forged-metal propellers is 2 blades, that have the advantage
of ease of construction and balancing, low manufacturing cost and efficient operation.
When more thrust is needed the blade area can be increased by lengthening the
blades, but only to a point at which the tip speeds approach the speed of sound, or if
tip clearance from the structure or ground is a factor. To keep the blades short, more
blades can be used. Three and four-bladed fixed pitch propellers have been
constructed, but usually, propellers with more than 2 blades are made so their pitch
can be adjusted. Some modern propellers have 4, 5 or 6 blades; and Propfan and
Unducted Fan propellers have as many as 12.
16.12.2 SOLIDITY
Solidity is calculated at the blade master station which is about 0.7 of the blade
length from root to tip.
number of blades x blade chord
Solidity =
2  x radius at blade master station

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The greater the solidity, the greater the power which can be absorbed by the
propeller. Figure 16.37 shows the disc area swept by the propeller.
16.13 FORCES ACTING ON PROPELLERS
The propeller is one of the most highly stressed components in an aeroplane, and 5
basic forces act on a propeller turning at high speed. These are:
 Centrifugal force
 Thrust bending force
 Torque bending force
 Aerodynamic turning moment (ATM)
 Centrifugal turning moment (CTM)
Note: ATM and CTM may also be referred to as Aerodynamic Twisting Force (ATF)
and Centrifugal Twisting Force (CTF).
16.13.1 CENTRIFUGAL FORCE
Centrifugal force puts the greatest stress on a propeller as it tries to pull the blades
out of the hub (Figure 16.38). It is not uncommon for the centrifugal force to be
several thousand times the weight of the blade. For example, a 25 pound propeller
blade turning at 2700 RPM may exert a force of 50 tons (100,000 pounds) on the
blade root.
Propeller disc area.
Fig 16.37

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16.13.2 THRUST BENDING FORCE
Thrust bending force is caused by the aerodynamic lift produced by the aerofoil
shape of the blade as it moves through the air (Figure 16.39). It tries to bend the
blade forward and the force is at its greatest near the tip. Centrifugal force, trying to
pull the blade out straight, opposes some of the thrust bending force.
Centrifugal force.
Fig 16.38
Thrust bending force.
Fig 16.39

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16.13.3 TORQUE BENDING FORCE
Torque bending force tries to bend a propeller blade in its plane of rotation opposite
to the direction of the rotation (Figure 16.40).
16.13.4 AERODYNAMIC TURNING MOMENT
Centrifugal force, thrust bending force, and torque bending force require a propeller
to be strong and heavy, and they serve no useful function. But 2 twisting forces are
useful in the pitch change mechanism of controllable pitch propellers.
Aerodynamic Turning Moment (ATM) tries to increase the blade angle. The axis of
rotation of a blade is near the centre of its chord line, and the centre of pressure is
between the axis and the leading edge. Figure 16.41 shows how the aerodynamic
force acting through the centre of pressure ahead of the axis of rotation tries to rotate
the blade to a higher pitch angle.
Torque bending force
Fig 16.40
ATM tries to increase the blade turning force.
Fig 16.41

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16.13.5 CENTRIFUGAL TURNING MOMENT
Centrifugal Turning Moment (CTM) tries to decrease the blade angle. As the
propeller turns, centrifugal force acts on all the blade components and tries to force
them to rotate in the same plane as the blade’s axis of rotation. This rotates the blade
to a lower-pitch angle. CTM opposes ATM, but its effect is greater, and the net result
of the twisting forces is a force that tries to move the blades to a lower pitch (Figure
16.42).
Many controllable-pitch propellers have counterweights that are on arms clamped
around the blade shank, and provide a Counterweight Turning Moment that opposes
the CTM. The centrifugal effect is to try to move the counterweights into the plane of
rotation and, therefore, the blades towards coarse pitch.
CTM tries to decrease the blade pitch angle
Fig 16.42

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Unless a propeller is balanced so that each blade produces the same centrifugal
force, aerodynamic forces and CTM, then severe vibration will occur. Therefore, each
propeller is subjected to a comprehensive balancing process before it can be fitted to
the engine of an aircraft.
16.13.6 VIBRATION AND CRITICAL RANGE
When a propeller produces thrust, aerodynamic and mechanical forces are present
which cause the blade to vibrate. If this is not compensated for in the design, this
vibration may cause excessive flexing and work-hardening of the metal and may
even result in sections of the propeller blade breaking off in flight.
Aerodynamic forces cause vibrations at the tip of a blade where the effects of
transonic speeds cause buffeting and vibration.
16.13.7 GYROSCOPIC EFFECT
A rotating propeller has the properties of a gyro. If the plane of rotation is changed, a
moment will be produced at right angles to the applied moment. For example, if an
aircraft with a right handed propeller (clockwise rotation viewed from rear) is yawed
to the right, it will experience a nose down pitching moment due to the gyroscopic
effect of the propeller. Similarly, if the aircraft is pitched nose up it will experience a
yaw to the right. On most aircraft the gyroscope effects are small and easily
controlled.
16.13.8 ASYMMETRIC EFFECT
With an aircraft in a nose up attitude (high angle of attack) and in straight flight, the
axis of the propeller will be inclined upwards to the direction of flight. This causes the
down moving blade to have a greater effective angle of attack than the up going
blade and, therefore, develops a greater thrust. (Figures 16.43a and 16.43b).
Asymmetric Effect
Fig 16.43a

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16.14 PROPELLER MATERIALS AND CONSTRUCTION
For decades, propellers used on low-powered engines were made of laminated
hardwood and had a fixed pitch. When more power had to be absorbed, propellers
made of metal became widely used, with forged aluminium alloy being the most
popular metal.
Some of the most modern blades are made of composite materials. Composite
blades are much lighter than metal blades and capable of absorbing the same
amount of power. The lighter blades impose less centrifugal loading on the hub,
allowing it to be made lighter. They have a very low notch sensitivity, and their foam
cores absorb much of the vibration that would damage metal propellers. While
composite blades currently cost more than metal blades, their greater efficiency and
longer life make them much more cost effective.
[Notch Sensitivity: a measure of the loss of strength of a material caused by the
presence of a notch, or a V-shaped cut]
16.14.1 METAL PROPELLERS
Improvements in metallurgy and manufacturing techniques have enabled metal
propellers to replace wood propellers for modern commercially manufactured aircraft.
Figure 16.44 shows a metal construction propeller blade.
Metal propellers are forged from high-strength aluminium alloy, and after being
ground to their finished dimensions and pitch, are anodised to protect them from
corrosion. Metal propellers cost more than wood for the same engine and aeroplane,
Asymmetric Effect
Fig 16.43b

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but their increased durability, resistance to weathering, and ability to be straightened
after minor damage have made them more cost effective in the long term.
[Anodise: a hard, airtight, unbroken oxide film electrolytically deposited on an alloy
surface]
Some propellers have blades made of steel with the blade halves stamped of thin
sheet steel and brazed together along the leading and trailing edges. The blade shell
is then installed over a tubular steel shank. A few propellers with hollow steel blades
are still flying, but these are usually found only on special-purpose aeroplanes.
16.14.2 COMPOSITE PROPELLER BLADES
Laminated wood, forged aluminium alloy, and brazed sheet steel propellers have
been standard for decades. But the powerful turboprop engines and the demands for
higher-speed flight and quieter operation have caused propeller manufacturers to
exploit the advantages of modern advanced composite materials.
Composite materials used in the propeller manufacturing consist of 2 constituents:
the fibres and the matrix. The fibres most generally used are glass, graphite and
aramid (Kevlar), and the matrix is a thermosetting resin such as epoxy.
The strength and stiffness of the blades are determined by the material, diameter and
orientation of the fibres. The matrix material supports the fibres, holds them in place
and completely encapsulates them for environmental protection. Because the fibres
have strength only parallel to their length, they are arranged in such a way that they
can sustain tensile loads.
Metal blade construction
Fig 16.44

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[Graphite Fibres: an advanced composite fibre made by drawing filaments of carbon
at a high temperature and in a controlled atmosphere.]
[Aramid Fibres: fibres made from an organic compound of carbon, hydrogen,
oxygen and nitrogen. It has high strength and low density and is flexible under load.
Kevlar: the registered trade name by Du Pont for a patented aramid fibre.
Matrix: the material that bonds the fibres together in an advanced composite
structure.]
16.14.3 HARTZELL BLADE CONSTRUCTION
The typical Hartzell composite propeller, like that in Figures 16.45 and 16.46, has a
machined aluminium alloy shank, and moulded into this shank is a low density foam
core. Slots are cut into the foam core and unidirectional Kevlar shear webs are
inserted. The leading and trailing edges are solid sections made of unidirectional
Kevlar and laminations of pre-impregnated material are cut and laid up over the core
foundation to provide the correct blade thickness, aerofoil shape, pitch distribution,
planform and ply orientation.
The outer shell is held in place on the aluminium alloy shank by Kevlar filaments
impregnated with epoxy resin wound around the portion of the shell that grips the
shank. Some Hartzell blades have a stainless steel mesh under the final layer of
Kevlar to protect against abrasion, and a nickel leading edge erosion shield is
bonded in place. The entire blade is put into a blade press and cured under
computer-controlled heat and pressure.
Cross section of a Hartzell composite blade
Fig 16.45
Plan view of a Hartzell composite blade
Fig 16.46

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Figure 16.47 shows the method of blade retention of a Hartzell composite propeller
blade.
16.14.4 HAMILTON-STANDARD BLADE CONSTRUCTION
The Hamilton-Standard blade has tremendous strength and fatigue resistance
because of its solid aluminium alloy spar enclosed in a glass fibre shell (Figure
16.48). The spar is machined to its correct configuration and placed in a mould
cavity, and the core foam is injected around it. The foam is cured and removed from
the mould. Glass fibre cloth, with the correct number of plies and the proper ply
orientation, is then laid over the cured core. The complete item is then placed in a
second mould that has the shape of the finished blade. The resin matrix is injected to
impregnate all the fibres, and is cured with heat and pressure.
Method of blade retention
Fig 16.47
Cross section of a Hamilton-Standard composite blade
Fig 16.48

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16.14.5 DOWTY ROTOL BLADE CONSTRUCTION
The Dowty Rotol composite propeller blade has 2 carbon fibre spars that run the
length of the blade on both the face and back and come smoothly together at the
blade root (Figure 16.49). The carbon fibres and pre-impregnated glass fibre cloth
are laid with the correct number of plies and the correct ply orientation and are
placed in a mould. Polyurethane foam is injected into the inside of the blade, and the
entire unit is cured under heat and pressure.
The Dowty Rotol blade is secured in the hub by expanding the carbon fibre spars
with tapered glass fibre wedges and locking them between the inner and outer
sleeves (Figure 16.50).
Cross section of a Dowty Rotol composite blade
Fig 16.49
Method of blade retention of a Dowty Rotol composite blade
Fig 16.50

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16.15 HUB OPERATING MECHANISM AND CONSTRUCTION
The cutaway drawing in Figure 16.51 illustrates the operating mechanism and
construction of a Dart turboprop propeller hub. The hub consists of an operating pin
mounted on the face of each blade root to provide blade rotation. An oil transfer tube
is positioned in the centre of the cross-head hub and carries oil to the piston chamber
that would be attached to the forward end of the cross-head. Two rows of taper roller
bearings between the hub shoulder and the blade root provide for low-friction rotation
of each blade and absorbs the centrifugal force.
16.16 PROPELLER SHAFTS
Most modern engines, both reciprocating and turbine, have flanged propeller shafts.
Some of these flanges have integral internally threaded bushings that fit into
counterbores in the rear of the propeller hub around each bolt hole. Propellers with
these bushings are attached to the shaft with long bolts that pass through the
propeller. On others the flange has a ring of holes and bolts pass from the engine
side into threads in the propeller.
Dart propeller hub operating mechanism.
Fig 16.51

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Some flanges have index pins in the propeller flange so the propeller can be installed
in only one position relative to the shaft. See Figure 16.52. This is done for
synchronising and/or synchrophasing.
The most popular type of propeller shaft on the larger turboprop engines is the
splined shaft. The sizes of splined shafts are identified by an SAE (Society of
Automotive Engineers) number, SAE 20 splines are used on engines in the 200-
horsepowered range; SAE 30 splines are used in the 300- and 400-horsepowered
range, and SAE 40 in the 500- and 600-horsepowered range. SAE 50 in the 1,000-
horsepowered range and SAE 60 and 70 are used for larger engines.
Flanged propeller shaft with index pin
Fig 16.52

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Splines are longitudinal grooves cut in the periphery of the shaft. The grooves and
lands (the space between the grooves), as shown in Figure 16.53 are the same size,
and one groove is either missing or has a screw in it to form a master spline. The
purpose of the master spline is the same as the index pin.
The inside of the propeller hub is splined to match the shaft and the hub is centred on
the shaft with two cones (Figure 16.54). The rear cone is a single-piece split bronze
cone, and is considered to be part of the engine. The front is a two piece hardened
steel cone and is considered to be part of the propeller. The two halves are marked
with the same serial number to ensure that only a matched set is used. Prior to
attaching this type of propeller, a check is carried out to ensure correct contact of the
cones.
A splined shaft with a master spline
Fig 16.53
Propeller shaft centring cones
Fig 16.54

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Engineers blue is applied to the cones and the propeller is fitted and torque loaded.
The propeller is then removed and visually inspected to ensure that there is an even
contact of 80% as seen by the blue around the cone on the propeller. If 80% of
contact is not in evidence then the cone can be ‘stoned’ to fit, or replaced.
16.17 PROPELLER SPINNERS
All modern propeller-driven aircraft have spinners over their propeller hubs. These
spinners have the dual aerodynamic function of streamlining the engine installation
and directing cool air into the openings in the cowling. Figures 16.55a and 16.55b
show a typical spinner installation over a constant speed propeller.
The spinner bulkhead is installed on the propeller shaft flange and held in place by
attaching bolts. The propeller is then installed so that the dowel pins in the propeller
hub align with the holes in the flange. The propeller attaching nuts are installed and
tightened to the torque value specified in the aircraft maintenance manual. If a
spinner support is required, it is installed and the spinner is secured to the bulkhead
with the correct fixing screws.
The propeller spinner and bulkhead are critical components, and cracks in either one
can be repaired only if they do not exceed the allowable limits. Repairs can be
performed using the procedures in the aircraft maintenance manual, although special
care must be taken not to add weight where it could cause vibration.
Propeller spinner assemblies
Fig 16.55a

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16.18 PROPELLER PITCH CONTROL
The propeller blade roots can be rotated using a mechanism in the hub to vary the
blade angle about the pitch change axis by approximately 110°. Any movement of
the blade is controlled by a Propeller Control Unit (PCU) that sends hydraulic
pressure to turn the blade to one of the following positions (see Figure 16.56).
Propeller spinner assemblies
Fig 16.55b
Propeller blade positions
Fig 16.56

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16.18.1 REVERSE PITCH
Reverse pitch is used to obtain a negative thrust to provide a very efficient
aerodynamic brake on landing; and to reverse the aircraft during ground
manoeuvres. Due to a mechanical locking gate on the thrust levers, thrust reverse is
only available when the aircraft is on the ground.
16.18.2 GROUND FINE OR SUPERFINE PITCH
This position is used to off-load the engine during starting and taxiing, when power
available from the turbines is insufficient to drive the propeller efficiently (fixed shaft
engines).
When the propeller is in the ground fine pitch, it also acts as an effective brake
because the propeller discs in the airflow are producing drag. Selection of this blade
position is only available when the aircraft is on the ground.
16.18.3 FLIGHT FINE PITCH
This position is the minimum blade angle allowed in flight, and in this position the
angle of attack is small and so accelerates a smaller mass of air per revolution. This
allows the engine to turn at a higher speed, for example, take off RPM. So, although
the mass airflow is smaller due to the high RPM, the slip stream velocity is high and
with low forward aircraft speed the thrust is also high.
16.18.4 COARSE PITCH
Between the flight fine pitch and coarse pitch is the angle that the blades are
controlled by the PCU during flight. When coarse pitch is selected, the mass of air
accelerated is greater for a lower engine RPM, so saving fuel and engine wear in the
cruise phase of flight.
16.18.5 FEATHERING
If the engine fails in flight, the airflow will attempt to rotate (windmill) the propeller and
cause an increase in drag that makes a multi-engined aircraft yaw. The feathering
position allows the propeller blades’ leading and trailing edges to be positioned
parallel with the airflow, thus reducing drag. Protection devices are incorporated to
prevent more than one engine feathering at any one time.
16.18.6 ALPHA AND BETA MODES OF OPERATION
The 2 basic operating modes are alpha mode and beta mode. Alpha is the flight
mode, and it includes all operations from take off through to landing. Beta is the
ground operations mode and includes: engine start, taxi and reverse operations.
Control outside the normal flight range of any turboprop will be in the beta range,
particularly in the thrust reverse range. The transition point between normal (alpha)
control and beta control is usually a mechanical lock or gate on the thrust lever.
Various safety devices using air / ground sensors ensure that thrust reverse cannot
be selected unless the thrust lever is at idle and the aircraft is on the ground.

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16.19 GENERAL CLASSIFICATION OF PROPELLERS
Propellers can be classified as either tractors or pushers. Figure 16.57a shows an
example of an aircraft with both tractor and pusher propellers.
16.19.1 TRACTOR PROPELLERS
Tractor propellers are mounted on the front end of the engine structure. Most aircraft
are equipped with this type (or location) of propeller as in Figure 16.57b. A major
advantage of the tractor propeller is that relatively low stresses are induced in the
propeller as it rotates in relatively undisturbed air.
ATR 72 with tractor propeller
Fig 16.57b
Cessna 337 with tractor and pusher propellers
Fig 16.57a

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16.19.2 PUSHER PROPELLERS
Pusher propellers are mounted on the rear end of the engine behind the supporting
structure (Figure 16.57c). Seaplanes and amphibious aircraft use a greater
percentage of pusher propellers than other kinds of aircraft.
On land based aircraft, where the propeller-to-ground clearance is less than the
propeller-to-water difference of the seaplane, pusher propellers are subject to more
damage than tractor propellers. Rocks, gravel and small objects dislodged by the
wheels, may be thrown or drawn into a pusher propeller. Similarly, seaplanes with
pusher propellers are more likely to encounter propeller damage from water spray
thrown up by the hull during landing or takeoff. Consequently, the pusher propeller
quite often is mounted above and behind the wings to prevent such damage.
16.20 TYPES OF PROPELLER
In designing propellers, engineers try to obtain the maximum performance of an
aircraft from the horsepower delivered by the engine under all conditions of
operation, such as takeoff, cruise and high speed. An aircraft with a fixed-pitch
propeller is no more efficient than a car would be if it had only a single transmission
gear. It was only when propellers with controllable pitch were introduced that truly
efficient operation became possible.
16.20.1 FIXED PITCH
A fixed-pitch propeller is a rigidly constructed propeller on which the blade angles
may not be altered without bending or reworking the blades. When only fixed-blade
angle propellers were used on aircraft, the angle of the blade was chosen to fit the
principle purpose for which the aircraft was designed. The fixed-pitch propeller is
made in one piece with two blades that are generally made of wood, aluminium alloy
or steel, and are in wide use on small aircraft.
Beech Starship with tractor propeller
Fig 16.57c

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With a fixed blade-angle propeller, an increase in engine power causes increased
rotational speed, and this causes more thrust, but it also creates more drag from the
aerofoil and forces the propeller to absorb the additional engine power. In a similar
manner, a decrease in engine power causes a decrease in rotational speed and
consequently a decrease in both thrust and drag from the propeller.
When an aircraft with a fixed-blade angle propeller dives, the forward speed of the
aircraft increases. Since there is a change in the direction of the relative airflow, there
is a lower angle of attack, thus reducing both lift and drag and increasing the
rotational speed of the propeller. On the other hand, when the aircraft climbs, the
rotational speed of the propeller decreases, the change in the direction of the relative
airflow increases with the angle of attack, and there is more lift and drag and less
forward speed for the aircraft.
The propeller can absorb only a limited amount of excess power by increasing or
decreasing its rotational speed. Beyond this point, the engine will be damaged. For
this reason, as aircraft engine power and aircraft speeds increased, engineers found
it necessary to design propellers with blades that could rotate in their sockets into
different positions to permit changes in the blade-angle setting to compensate for
changes in the relative airflow brought on by the varying forward speed. This made it
possible for the propeller to absorb more or less engine power without damaging the
engine.
16.20.2 TWO-POSITION PROPELLERS
Ground-adjustable propellers were a step in the right direction, but with only minor
added weight and complexity, the propeller could be made far more efficient by
allowing the pilot to change the pitch of the blades in flight.
The first popular controllable-pitch propellers were hydraulically actuated by engine
lubricating oil supplied through a hollow crankshaft. A counterweight on an arm is
attached to each blade root so that the centrifugal force rotates the blade into a
higher pitch angle. A fixed piston in the end of the propeller shaft is covered by a
moveable cylinder attached through bearings to the counterweight arms. See Figure
16.58.

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Two-position controllable pitch propeller
Fig 16.58

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For takeoff, the two-position propeller control is placed in the LOW PITCH position
that directs engine oil into the cylinder and moves it forward over the piston. This
pulls the counterweights in and rotates the blades into their low pitch position.
When the aircraft is set up for cruise flight, the pitch control is moved to the HIGH
PITCH position. This opens an oil passage, allowing the oil in the propeller cylinder to
drain back into the engine sump. Centrifugal force on the counterweights moves
them outward into the plane of rotation, and rotates the blades into their high pitch
position.
This same configuration of propeller, when equipped with a flyweight governor to
control the oil into and out of the cylinder, is the popular constant speed propeller,
or Variable Pitch (VP) propeller.
16.20.3 AUTOMATIC PROPELLERS
At the end of World War II there was a tremendous boom in private aircraft, engine
and propeller development and manufacture.
One interesting development that became popular during that era was the Koppers
Aeromatic propeller. However, because its complexity was greater than its
advantages, it faded away. This propeller was fully automatic and used the balance
between the ATM and the CTM to maintain a relatively constant speed for any given
throttle setting.
The 2 forces were amplified by offsetting the blades from the hub with a pronounced
lag angle to increase the effect of the CTM trying to move the blades into a low pitch,
and by installing counterweights on the blade roots to help move the blades into high
pitch.
16.20.4 VARIABLE PITCH
Variable-pitch propellers consist of a number of separate blades mounted in a central
hub, and a mechanism to change the blade angle according to aircraft requirements.
The blades and hub are often aluminium alloy forgings, but the hub on a large
propeller may be constructed from steel forgings because of the high centrifugal
forces that it has to contain.
The blades are mounted in the hub in ball or tapered roller bearings, and the pitch
change mechanism is attached to the hub and connected to each blade through
rods, yokes or bevel gears. Operation and control of the pitch-change mechanism
varies considerably, and the main types are detailed in the following sections.

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16.21 SINGLE-ACTING PROPELLER SYSTEM
A single acting propeller is illustrated in Figure 16.59; it is a constant-speed,
feathering type and is typical of the propellers fitted to light and medium sized twin-
engined aircraft. A cylinder is bolted to the front of the hub, and contains a piston and
piston rod that move axially to alter the blade angle. On some propellers, oil under
pressure, fed through the hollow piston rod to the front of the piston, moves the
piston to the rear to turn the blades to a finer pitch; on other propellers the reverse
applies. When oil pressure is relieved, the counterweights and feathering spring
move the piston forward to turn the blades to a coarser pitch.
Counterweights produce a CTF but, because they are located at 90° to the chord
line, they tend to move the blades to a coarser pitch. Counterweights must be located
far enough from the blade axis, and must be heavy enough to overcome the natural
twisting moment of the blade, but since weight and space are limiting factors, they
are generally only used with blades of narrow chord.
Single-acting propeller
Fig 16.59

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16.22 DOUBLE-ACTING PROPELLER SYSTEM
This type of propeller is normally fitted to larger engines and, because of engine
requirements, is more complicated than the propellers fitted to smaller engines.
Construction is similar to that of a single-acting propeller, the hub supporting the
blades and the cylinder housing the operating piston. In this case however, the
cylinder is closed at both ends and the piston is moved in both directions by oil
pressure.
In the mechanism shown in Figure 16.60, links from the annular piston pass through
seals in the rear end of the cylinder, and are connected to a pin at the base of each
blade. In another type of mechanism, the piston is connected by means of pins and
rollers to a cam track and bevel gear, the bevel gear meshing with a bevel gear
segment at the base of each blade. Axial movement of the piston causes rotation of
the bevel gear and alteration of the blade angle. Operating oil is conveyed to the
propeller mechanism through concentric tubes in the bore of the engine reduction
gear shaft.
Double acting propeller
Fig 16.60

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16.22.1 MOVING PISTON
The illustration in Figure 16.61 shows a moving piston hydraulic pitch change
mechanism for a double acting propeller system. Linear movement of the piston
inside the cylinder is transmitted to the base of each blade by linkages, and
converted to rotary movement of the blades.
16.22.2 MOVING CYLINDER
The illustration in Figure 16.62 shows a moving cylinder hydraulic pitch change
mechanism for a double acting propeller system. Linear movement of the cylinder is
transmitted to the base of each blade by linkages, and converted to rotary movement
of the blades.
Moving cylinder system
Fig 16.62
Moving piston with blade links
Fig 16.61

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16.22.3 GEARED OR HYDROMATIC
The geared or hydromatic pitch change mechanism (Figure 16.63) utilises a piston
inside a stationary cylinder. The piston is connected to a pair of co-axial cylindrical
cams. The outer cam is fixed and the inner is free to turn. This carries a bevel gear
which meshes with bevel gear segments on the blade roots.
16.23 CONSTANT SPEED PROPELLERS
There are only 2 types of propellers installed on current production aircraft; fixed-
pitch propellers for the small and simple aeroplanes, and hydraulically actuated
constant-speed propellers for complex aeroplanes.
The tremendous advantage of being able to change pitch in flight opened new
possibilities for increased efficiency. Replacing the two-position valve with a
flyweight-controlled valve in a governor allows the blade pitch angle to be
continuously and automatically adjusted in flight to maintain a constant and efficient
engine speed.
16.23.1 PRINCIPLES OF OPERATION
The introduction of an engine-driven centrifugal governor, enabled the blade angle to
be altered automatically (within a pre-determined range), in order to maintain any
engine speed selected by the pilot, regardless of aircraft speed or altitude.
Geared hydromatic system
Fig 16.63

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A flyweight-type governor senses the engine speed and compares it with the speed
selected by the pilot. If an air load on the propeller causes it to slow down, the
governor senses this rpm decrease and directs oil into or out of the propeller to
decrease the blade pitch. The lowered pitch decreases the load, and the engine
returns to the desired speed. If the air load decreases, the RPM increases; the
governor senses the increase and directs the oil in the proper direction to increase
the pitch and cause the engine to slow down.
16.23.2 PROPELLER GOVERNOR
As the flight conditions are continually changing during a typical flight profile, the
engine RPM will fluctuate in response to the changing propeller torque. This is
undesirable for a turboprop aircraft, and to manually maintain a constant RPM would
be a full time occupation for the pilot.
The purpose of the propeller governor (shown in Figure 16.64) is to maintain the
RPM of the engine at the figure selected by the pilot, i.e. it is a range speed
governor. It is also used to limit the maximum RPM of the engine, i.e. it is a maximum
speed governor.
Basic constant – speed propeller governor
Fig 16.64

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This is achieved by controlling the pitch of the propeller blades and hence the load on
the engine. Propeller governors are sometimes known as Constant Speed Units
(CSUs) and Propeller Control Units (PCUs).
Almost all propeller governors use a pair of L-shaped flyweights, mounted on a
flyweight head and driven by the engine, to control the position of the pilot valve in
the oil passage between the engine and the propeller. A gear-type pump inside the
governor boosts engine oil pressure high enough for it to move the propeller piston
against the effect of the counterweights or the low pitch spring.
The governor pump and the flyweight head are driven by an accessory gear in the
engine. The speeder spring presses down on the toes of the flyweights and, in turn,
on the pilot valve plunger. The governor control lever rotates the adjusting worm,
which varies the compression of the speeder spring.
16.24 ROLLS-ROYCE DART ENGINE / FOKKER 27 AIRCRAFT
Single-lever Cockpit Control Operating a Dowty / Rotol 4-bladed Non-
counterweight Propeller.
16.24.1 GENERAL DESCRIPTION
The engine is a single-spool, fixed-shaft turboprop, consisting of a 2-stage centrifugal
compressor, connected to a 3-stage stage axial turbine. A reduction gear at the
forward end of the compressor / turbine shaft provides the drive for the 4-bladed
propeller. See Figure 16.65.
Rolls-Royce Dart
Fig 16.65

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16.24.2 GENERAL OPERATION
The power lever control system is mechanically operated by a power lever on the
pedestal quadrant on the flight deck. In principle, forward movement of the power lever
increase and changes governor settings. Provision is incorporated for the selection of
propeller ground fine by lifting and retarding the lever beyond the idle position.
The principle of operation of a simple propeller governor has already been outlined in
Section 6. This governor is now illustrated connected to the pitch change piston by oil
lines, and the piston to the blades by mechanical linkages (Figure 16.66). The
operation and control of governing and feathering is by electrical and hydraulic means,
and is now considered in more detail.
16.24.2.1 On-Speed Condition
When the propeller has fully absorbed the engine power, the governor flyweight force
equals that of the spring force. In this "on speed" condition the governor piston valve
blanks off the oil ports to the propeller pitch change piston, and high pressure oil from
the governor pump is by-passed through the main relief valve to the inlet side of the
pump (Figure 16.67).
Dart propeller system
Fig 16.66

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16.24.2.2 Over-Speed Condition
If the RPM rises above the selected speed, the governor flyweight force, being
greater than the spring force, raises the governor piston valve. The valve is raised to
a position where operating oil is directed to the front of the pitch change piston,
moving it rearwards to increase the pitch angle of the blades. This increases the
load on the engine. At the same time, displaced oil from the rear of the piston, is
directed by the governor piston valve, via drain, to the inlet side of the governor
pump. The increased blade pitch angle causes the RPM to fall until an equilibrium is
reached and the governor piston valve returns to the on speed condition (Figure
16.68).
Propeller on speed condition
Fig 16.67
Propeller over speed condition
Fig 16.68

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16.24.2.3 Under Speed
If the RPM falls below the selected speed, the spring force, being in excess of the
governor flyweight force, causes a downward movement of the governor piston valve. In
this position operating oil is directed to the rear of the propeller pitch change piston,
moving it forward and decreasing the pitch angle of the blades (i.e. decreasing the load
on the engine). At the same time, the oil displaced from the front of the piston is
returned, via drain, to the governor pump. This condition will apply until the selected
RPM is restored (Figure 16.69).
16.24.2.4 Feathering in Flight
The propeller blades may have to be set to "feather" in the event of an engine or
governor failure. In addition the requirement to feather may be as part of a Flight
Test. The pilot first stops the engine in the normal way; by setting the throttle to idle
followed by shutting down the engine using the HP Cock. This sequence of
operations is followed up by selecting "feather" by moving the HP Cock past the "Off"
position to the "Feather" position. This moves the feathering lever at the governor
which mechanically lifts the governor piston valve and opens the coarse oil line.
Remember the engine is stopped (propeller windmilling condition) so that full system
pressure is not available from the governor pump. The pilot has to operate a "Manual
Feather Switch" which activates the electric motor within the feathering unit. A
reserve supply of "feathering oil" is sucked from the oil tank and high pressure oil is
pumped to the pitch change mechanism via the governor.
Propeller under speed condition
Fig 16.69

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The pitch change piston is forced rearwards and the blades are thus set at the
feather position. Displaced oil is returned to drain via the governor. (Figure 16.70).
16.24.2.5 Unfeathering in Flight
Once a successful feathering operation has been carried out normal flight conditions
need to be restored. Before the engine is restarted the propeller blades need to be
moved towards the "Flight Range" position, and this will allow the negative torque
generated by the windmilling propeller to rotate the engine for starting.
Feathering in flight
Fig 16.70
Unfeathering in flight
Fig 16.71

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The pilot selects the HP Cock to the "Off" position. This moves the feathering lever at
the governor and the governor piston valve is lowered to the bottom of the unit under
spring pressure (Figure 16.71). The fine oil line is now open allowing oil from the front
of the pitch change mechanism to drain away as the pitch change piston moves
forward. The blades are moved towards fine pitch by operating the feather motor to
supply pressure oil to the pitch change mechanism
This will cause the propeller to windmill and the engine may now be restarted in the
normal way: i.e. by selecting the HP Cock to "Open" and pressing the re-light button.
As RPM increases the governor pump resumes operation and the selected "on
speed" condition is again controlled by the propeller governor.
16.24.2.6 Dead Throttle Movement
A fixed-shaft turbo-prop engine, as explained above, needs to be started with the
blades set at fine pitch. On the ground, when the start sequence is initiated, the
blades are at the "Ground Fine Pitch" angle so that the propeller torque is at a
minimum. This reduces the load on the electric starter motor, and prevents excessive
turbine temperature during the start.
Movement of the throttle lever from the "Ground Idle" (7000 rpm) position causes the
engine and propeller to accelerate to the "Minimum Cruise" position by the addition of
fuel to the engine. The minimum cruise condition is the point at which the governor
comes into effect, and is known as "Minimum Constant Speed" RPM.
Before minimum constant speed, which is determined by the loading of the governor
spring, the governor does not change the pitch of the blades. For the Dart engine
fitted with the Dowty Rotol propeller this is between 10 400 and 11000 rpm. The
"Dead Movement" is achieved by a sleeve fitted in the rack and pinion mechanism.
The first 35% of throttle lever movement only moves this sleeve via the RPM lever,
thereafter, the sleeve comes into contact with the governor spring. Movement of the
throttle lever beyond 35%, therefore, increases the governor spring loading thereby
causing an increase in rpm with propeller pitch progressively coarsening from ground
fine towards the flight fine range.
16.24.2.7 Pitch Range Selections
The Dowty Rotol propeller fitted to the Dart engine is a single stop propeller. This ‘stop’
enables the pilot to operate the propeller in the ‘flight range’, and automatically
prevents the propeller entering the ‘ground range’. Once the aircraft has landed the
pilot will need to select the propeller to the ground range.

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16.24.2.8 Stop Withdrawal to Achieve Ground Fine
The withdrawal of the stop to achieve ground range on the single stop Dowty Rotol
propeller, is performed from the flight deck by the pilot. The stop is removed when a
solenoid is energised, and allows pressure oil to flow from the governor pump to the
Lock Operating valve. This valve, also known as the Third Oil Line valve, opens a
feed from the governor pump to the pitch change mechanism, as illustrated in Figure
16.72.
This oil acts on the pitch lock piston, forcing it rearwards, which moves the lock
support rearwards allowing the spring collets to collapse as the pitch change piston
moves forwards. In addition to the hydraulic pressure in the chamber, the pitch
change piston is also tending to move forward as the blades move towards fine pitch
under CTM.
When the stop has been removed and the blades are operating in the ground range,
‘ground fine’ is achieved when the ground fine pitch stop on the pitch change piston
comes into contact with the machined face on the pitch change cylinder.
16.24.3 REVERSE PITCH
The pitch range of a propeller depends on the propeller type, but will always consist
of a ground range (beta mode) and a flight range (alpha mode). The ground range for
the Dart propeller described above does not incorporate reverse thrust.
Stop withdrawal to achieve ground fine
Fig 16.72

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The Hamilton – Standard propeller fitted to the ATR and Hercules aircraft engines
and the Dowty propeller fitted to the Fokker 50 engines, are just 2 examples of
aircraft / engine combinations where the ground range includes reverse thrust. The
reverse thrust range is selected and controlled by the pilot on the flight deck and
commands an additional range of movement in pitch change mechanism.
16.25 PRATT & WHITNEY 124 ENGINE / ATR AIRCRAFT
Two-Lever Cockpit Control for a Hamilton-Standard 4-bladed Non-
counterweight Propeller
The engine is a 2-spool turboprop, consisting of a first stage low pressure (LP)
centrifugal compressor and a second stage high pressure (HP) centrifugal
compressor. Each compressor is mounted on a separate concentric shaft
independently driven by a single stage axial turbine. See Figure 16.73.
A 2-stage free turbine located aft of the compressor turbines, drives the 4-bladed
propeller through a third concentric shaft that extends forward to the reduction
gearbox (RGB). The RGB has a ratio of approx. 16.7:1 and is situated at the front of
the engine. Because the free turbine drives the propeller, it is independent of the gas
generator RPM. The LP and HP shaft speed are referred to as NL and NH
respectively, and the free turbine shaft speed is designated NP.
Pratt & Whitney 124 engine
Fig 16.73

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This is a constant speed propeller because it operates throughout the operational
cycle at near 100% RPM. To hold the RPM constant, the fuel control adjusts the fuel
flow in relation to the engine load. When idling, the RPM remains high, but the
propeller angle is reduced until almost flat (approx. 0°), so that it produces very little
thrust and requires a minimum fuel flow.
When the engine is operating with a given propeller load and the power lever is
moved forward to increase the fuel flow, the RPM will try to increase. To prevent this,
the propeller governor increases the blade angle, which causes the RPM to remain
constant and the power produced by the engine to increase. When the power lever is
moved back the fuel flow is reduced, and the RPM begins to decrease. But the
propeller governor decreases the blade angle, which causes the RPM to remain
constant, and the power to decrease.
The maximum power this engine is allowed to develop may be limited either by the
amount of torque the airframe structure can safely accommodate, or by the maximum
temperature the turbine inlet guide vanes and first-stage turbine blades can
withstand. Turboprop engines are capable of producing more power, or torque, than
the airframe can accommodate and, therefore, are restricted in the maximum power
that they are able to produce.
16.25.3 FLIGHT DECK CONTROLS
This engine / aircraft combination uses 2 propeller control levers that are mounted on
the flight deck quadrant. These levers are referred to as the power lever and
condition (or speed) lever. See Figure 16.74.
The power lever relates to the throttle of a reciprocating engine, but it also gives the
pilot control over the propeller during ground operation.
Power and condition levers
Fig 16.74

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It effects the fuel flow, torque and exhaust gas temperature (EGT), and has 5
positions:
 Reverse
 Ground idle
 Flight idle
 Take off
 Maximum power
[Note: Power in the reverse mode is controlled on NP and in the forward mode on NH]
The condition (or speed) lever primarily controls the propeller RPM, and also acts as
a manual feather and fuel shut off lever. The condition lever has 4 positions:
 Fuel shut off
 On feather
 Low RPM (min NP)
 High RPM (max NP)
Figure 16.75 shows the various positions for both the power and condition levers.
Power Management
Fig 16.75

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16.25.4 PROPELLER PITCH CONTROL
Propeller pitch control is accomplished by using boosted engine oil pressure to obtain
linear movement of a 2-sided, differential area, pitch change piston. The hydraulically
operated differential piston slides in a domed cylinder that is secured to the front of
the propeller hub.
The piston is part of the pitch-change actuator that mechanically locates the propeller
blade trunions to provide a rotary movement of the blades from a linear movement of
the piston. Both front and rear piston chambers are supplied with oil via a sleeve that
is intergral with the dome. Windows in the sleeve are opened and blanked off by a 4-
way pitch change metering valve which slides in the sleeve.
The 4-way metering valve is connected to a pitch lock screw controlled by an oil tube
which runs through the propeller shaft. The tube enables transmission of the pitch
change mechanical signal from the PCU servo piston and the transfer of the high
pressure oil supply from the HP pump.
16.25.5 PCU PITCH CHANGE MECHANISM
Figure 16.76 shows the internal details of the pitch change mechanism.
Oil transfer tube:
 The oil transfer tube routes supply oil pressure to the pitch change valve and
to the pitch change actuator.
 The oil transfer tube connects the propeller pitch change mechanism to the
PCU pitch change mechanism.
 At the propeller end the tube is attached to the pitch change screw and
valve, at the PCU end the tube is spline into the ball screw.
Ball screw:
 The ball screw changes the axial movement of the servo piston into a
rotational movement of the oil transfer tube.
 The ball screw has right hand threads.
Servo piston:
 The servo piston has an area consisting of 2 chambers.
 Supply pressure is routed to the piston rear chamber which tend to move the
piston rearward (fixed pressure).
 Metered pressure is routed to the piston front chamber which tend to move the
piston forward (variable pressure).

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 The working area of the front chamber is twice the size of the rear chamber.
The servo piston movement stops (maintain blade angle) when the metered
pressure is half the supply pressure.
 The servo piston moves forward (decrease blade angle) when the metered
pressure is more than half the supply pressure.
 The servo piston moves rearward (increase blade angle) when the metered
pressure is less than half the supply pressure.
 Varying the metered pressure changes the blade angle.
16.25.6 GOVERNING MODE
Figure 16.77 shows the internal details of the PCU and pitch change mechanism in
governing mode.
Governor:
 The PCU pump provides the supply pressure (800 - 1000 psi).
 Through the metering valve, the governor meters the supply pressure going
to the servo piston.
 The governor is driven by the propeller shaft via a PCU drive coupling.
Pitch change mechanism
Fig 16.76

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 The metering valve is positioned by flyweights acting against speeder spring
tension set by the condition lever.
Condition lever (speed set cam):
 The speeder spring force is varied by a linkage attached to the condition
lever.
 Allows different propeller speeds to be selected.
Least selector:
 The least selector valve opens to the least of two pressures.
 During normal operation, the overspeed governor routes supply pressure to
the least selector valve, allowing the lower metered pressure from the
governor to flow through the least selector valve.
 The least selector is part of the propeller overspeed protection system.
Steady state:
 In steady state, the metered pressure is set to half of the supply pressure by
the metering valve.
 In steady state, the flyweight force acting on the metering valve is
counterbalanced by the speeder spring.
 The condition lever sets the speeder spring force.
Propeller Speed (Np) selection:
 The condition lever is used to select different propeller speeds.
 Pushing the condition lever towards maximum RPM increases the speeder
spring tension which overcome the flyweights force. The metering valve
moves towards the flyweights increasing the metered pressure. This will cause
the blade angle to decrease and the propeller to accelerate. As Np increases,
the governor flyweight force increases until an equilibrium is reached with the
speeder spring force (steady state).
 Pulling the condition lever towards minimum RPM causes the opposite
reaction. Blade angle increases, Np decreases until steady state condition is
reached.
Power change:
 During a power change, the PCU governor will vary the blade angle to
maintain Np.
 A power increase causes the blade angle to increase.
 A reduction of power causes the blade angle to decrease.

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16.25.7 BETAMODE
beta mode.
Purpose:
 In flight it ensures a minimum thrust (minimum blade angle) at low power.
 On ground it enables manual control of propeller blade angle with the power
lever.
Beta valve:
 The beta valve consists of two concentric sleeves.
 The outer sleeve is positioned by the servo piston via the beta rod.
 The inner sleeve is positioned by the power lever (Beta cam).
Governing mode
Fig 16.77

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Power lever (beta cam):
 In flight (governing mode) when power is reduced, the PCU governor
commands the blade angle to decrease to maintain Np. This causes the
servo piston and beta rod to move towards the beta valve. As the beta rod
pushes the beta valve outer sleeve, the valve opens, the metered pressure
drains out and prevents further decreases in blade angle (beta mode).
 Once in the beta mode, the blade angle is controlled directly by the power
lever from the point you entered beta mode (max beta) to full reverse. To
decrease blade angle, pull the power lever. This will rotate the power lever
beta cam, reposition the beta valve inner sleeve outwards, close the drain,
increase the metered pressure and decrease blade angle. As the blade
angle decreases, the servo piston beta rod pushes the outer sleeve, re-
opens the drain to stop the movement at the selected blade angle.
 The power lever also controls propeller rpm (Np) at low and reverse power
(Np fuel governing).
Low blade angle switch:
 The low blade angle switch ensures a minimum blade angle in the event the
blade angle decreases below the flight idle blade angle with the power lever
at or above flight idle.
 When triggered, the low blade angle switch activates the feather solenoid to
ensure a minimum blade angle.
 A micro switch on the power lever prevents the feather solenoid to be
activated by the low blade angle switch whenever the power lever is below
flight idle. This allows selection of lower blade angle on the ground.
Feather solenoid:
 Normally closed valve.
 It is activated via the low blade angle switch.
 When activated the feather solenoid drains the metered pressure to maintain
a minimum blade angle.
 In normal operation the feather solenoid can be activated by the condition
lever (micro switch), or the autofeather system to feather the propeller.

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16.25.8 REVERSE MODE
reverse mode.
Purpose:
 Allows to select negative blade angles in order to generate reverse thrust
and slow down the aircraft after landing.
Power lever beta cam:
 Repositions the beta valve inner sleeve outward to allow an increase of
metered pressure which will decrease the blade angle.
 Repositions the reverse valve inwards to prevent governor operation in
reverse.
Beta mode
Fig 16.78

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Reverse valve:
 Replaces the governor metered pressure by the supply pressure to ensure
uninterrupted oil supply to the servo piston.
 In the event of a propeller overspeed in reverse, the governor would try to
control the overspeed by increasing the blade angle (reduce metered
pressure), causing loss of negative pitch while reverse thrust is needed.
Low blade angle switch:
 It energises a cockpit light when the blade angle is below the flight idle blade
angle.
 A micro switch on the power lever prevents the feather solenoid to be
activated by the low blade angle switch whenever the power lever is below
flight idle. This allows selection of reverse blade angle on the ground.
 Schedule propeller speed (Np) as a function of the power lever angle and Np
fuel governing schedule.
Manual Control (EEC "Off"):
 There is no Np control in manual mode.
 Np will be limited to a maximum of 109% (1308 rpm) by the overspeed
governor pneumatic section.

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16.25.9 FEATHERING MODE
feather mode.
Condition lever:
 In feather position the condition lever cam opens the mechanical feather valve
and drains the metered oil pressure going to the servo piston, the blade angle
increases and the propeller feathers.
 To prevent an over torque while feathering the propeller, Np fuel governing
schedule is cancelled by a micro switch activated by the condition lever.
Feather solenoid:
 When the feather solenoid energises it drains the metered oil pressure going
to the servo piston allowing blade angle to increase and the propeller to
feather.
Reverse mode
Fig 16.78

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Auxiliary feathering pump:
 It provides an alternate oil pressure source to the PCU to feather the
propeller. It can also be used to feather or unfeather the propeller for
maintenance purposes.
 When activated the pump energises for a cycle of 15 to 30 seconds.
Autofeather:
 When an engine failure is detected during take-off, the autofeather system
commands the failed engine propeller to feather in order to minimise
propeller drag.
 Autofeather provides the following signals:
a. Uptrim power of opposite engine
b. Energises auxiliary feathering pump
c. Energises feather solenoid
d. Cancels Np fuel governing schedule
Feather mode
Fig 16.79

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16.25.10 ENGINE INDICATING
There are 7 instruments on the flight deck that are used to monitor the performance
of the engine:
Tachometer (NH) - Shows the RPM of the HP compressor in percentage of its
rated speed.
Tachometer (NL) - Shows the RPM of the LP compressor in percentage of its
rated speed.
Tachometer (NP) Shows the RPM of the propeller in percentage of its rated
speed.
Torquemeter Shows the torque, or shaft horsepower being developed.
EGT - Shows the temperature of the exhaust gases as they
leave the turbine.
Fuel Flow - Shows the mass of fuel per hour being delivered to the
engine.
Oil Pressure - Shows the operating pressure of the engine oil system
within a given range during engine running.
Engine Alerts
 Engine Over temperature
 Engine Out
 Engine Over torque
16.26 MCCAULEY AND HARTZELL NON-COUNTERWEIGHT CONSTANT
SPEED PROPELLER
Some McCauley and Hartzell constant-speed propellers do not use counterweights.
On this type of propeller, blade pitch is controlled using a combination of:
 Oil pressure - to increase the pitch
 Aerodynamic Turning Moment (ATM) - to increase the pitch
 Centrifugal Turning Moment (CTM) - to decrease the pitch
 Force from an internal spring - to decrease the pitch
Figure 16.80 shows the internal pitch change mechanism of the McCauley and
Hartzell non-counterweight constant speed propeller.

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When the propeller is operating in an under speed condition, the governor drains the
oil from the pitch change cylinder. The combination of CTM and the force of the
spring move the piston forward and the blades into a low pitch angle (Figure 16.81).
When the air load is low and the propeller tries to over speed, the governor sends oil
into the pitch change cylinder and moves the piston back, compressing the spring
and moving the blades into a high pitch angle. This increases the air load and returns
the engine to the desired RPM (Figure 16.82).
Non-counterweight constant speed propeller
Fig 16.80
Blades moving to a low pitch
Fig 16.81

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PROPULSION
SYSTEMS
When the engine is operating in the on speed condition, the governor blocks the oil
going into the propeller or draining from it, thus creating a hydraulic lock.
16.27 PRATT & WHITNEY 125 ENGINE / FOKKER 50 AIRCRAFT
Single-lever Cockpit Control for a Dowty 6-bladed Counterweight
Propeller
The engine is a 2-spool turboprop, consisting of a first stage low pressure (LP)
centrifugal compressor and a second stage high pressure (HP) centrifugal
compressor. Each compressor is mounted on a separate concentric shaft
independently driven by a single stage axial turbine. See Figure 16.83.
Pratt & Whitney 125 engine
Fig 16.83
Blades moving to high pitch
Fig 16.82

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engineering
JAR 66 CATEGORY B1
MODULE 15/17
PROPULSION
SYSTEMS
A 2-stage free turbine located aft of the compressor turbines, drives the 6-bladed
propeller through a third concentric shaft that extends forward to the reduction
gearbox (RGB), with a ratio of approx. 16.7:1, situated at the front of the engine.
Because the propeller is driven by the free turbine, it is independent of the gas
generator RPM. The LP and HP shaft speed are referred to as NL and NH
respectively, and the free turbine shaft speed is designated NP.
The construction of the propeller incorporates a counterweight clamped tightly
around each blade root, positioned so that as centrifugal force tries to move it into the
plane of rotation, it increases the blade pitch angle. Figure 16.84 shows an example
of blade counterweights.
Underspeed Condition When the Propeller Electronic Control (PEC) Unit
senses that the RPM is lower than that selected, engine oil, boosted in pressure
by a pump inside the overspeed governor, is sent through the hollow propeller
shaft into the propeller cylinder forcing the piston forward.
Pitch change mechanisms connecting the piston to the blade roots rotate the
blades to the lower pitch angle and the propeller speeds up to the desired RPM.
See Figure 16.85.
Blade counterweight
Fig 16.84

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engineering
JAR 66 CATEGORY B1
MODULE 15/17
PROPULSION
SYSTEMS
On-speed Condition When the engine is operating at exactly the RPM called
for by the pilot, the PEC closes the servo valve. This prevents oil from going to or
draining from the propeller.
Overspeed Condition If the nose of the aircraft momentarily drops, forward
speed increases, air load on the propeller decreases and the RPM increases.
The PEC opens a passage between the propeller shaft and the engine sump and
oil drains from the propeller. Centrifugal force acting on the counterweights
moves the blades into a higher pitch, the piston move rearwards, the blade angle
increases and the propeller slows down. See Figure 16.86.
Underspeed - blade moves to low pitch
Fig 16.85
Overspeed - blade moves to high pitch
Fig 16.86

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engineering
JAR 66 CATEGORY B1
MODULE 15/17
PROPULSION
SYSTEMS
16.27.3 CONTROL SYSTEM
The propeller operation has 3 control systems:
a. Hydromechanical control
b. Electronic control
c. Feathering system
Hydromechanical Control The hydromechanical control system adjusts the
propeller blade angle. The components within the system include:
a. An overspeed governor with an integral pump to supply high pressure oil,
and to prevent an overspeed of the propeller.
b. A servo valve to control oil pressure.
c. A pitch control unit (PCU) to control oil flow to the propeller cylinder.
d. A beta tube unit to transfer the oil between the PCU and the propeller
cylinder.
The power lever gives an input to control the propeller blade angle at low engine-
power conditions.
Electronic Control The electronic control system gives an output to the servo
valve on the PCU to adjust the oil pressure. The system has:
a. A Propeller Electronic Control (PEC) unit.
b. A Magnetic Pick Up (MPU) unit for actual NP information.
A push switch on the propeller panel permits the operation of the PEC. The engine
rating selection gives an input to the system for the NP demand.
Feathering System The feathering system controls the feathering and
unfeathering of the propeller. It has:
a. An autofeather unit (AFU) to control the automatic feathering of the propeller.
b. A feathering pump to make sure that the propeller goes to the full feathered
position.
c. For manual feathering, an input from the fuel lever controls the feathering
valve in the PCU.
Figure 16.87 shows a schematic of the Pratt & Whitney 125 / Fokker 50 propeller
control system.

uk
engineering
JAR 66 CATEGORY B1
MODULE 15/17
PROPULSION
SYSTEMS
Propeller control system
Fig 16.87

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