VARIABLE PITCH PROPELLER BRIEF

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VARIABLE PITCH PROPELLER BRIEF
The Firefly T67M Hoffman 2 bladed Variable
Pitch propeller
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Objectives

• To learn why a variable pitch propeller can
  assist aircraft performance.
• To understand how a Constant Speed Unit works.
• To learn how to change power with a variable
  pitch propeller.
• To efficiently and safely operate the Firefly
  propeller in all phases of flight.

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Q Why do we need a constant speed propeller?
A To cope with variations in

• Speed range
• Engine power

Lets see the variation in cruising speed range
of 2 different aircraft
Speed range
0 kt

• Low power (fixed pitch)
• e.g. Cessna 152

100 kt
Cruise speed range
0 kt
190 kt
High performance a/c
Cruise speed range
Q How can we make a propeller cope efficiently
with this greater speed range?
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• Firstly, let us take a simple aerofoil and push
  it forward through the air

An upward force is produced
The vertical component of this is LIFT
A propeller blade is just an aerofoil, angled on
the spinner so that when it is rotated, lift is
produced in a forward direction
Thrust
Direction of rotation
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Propeller Theory
Here is our fixed pitch propeller blade
Attached to a rotating hub
Producing forward lift thrust
Thrust
Direction of rotation
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Propeller Theory
In this case, the amount of Thrust produced by
the propeller equals the inflow drag and the
aircraft is in unaccelerated flight. If we now
increase the RPM, the prop produces more thrust
and the aircraft accelerates until the inflow
equals the thrust. It will maintain this higher
speed. We can continue increasing RPM and
therefore accelerating, but there will come a
speed where the maximum rpm is reached and the
aircraft cannot go any faster. If we were able
to increase the thrust produced by the propeller
at a given rpm, then the aircraft could go
faster We can do just this by turning the
propeller so that it has a greater angle of
incidence to the inflow by VARYING THE
PITCH So lets look at a variable pitch propeller

Fixed Pitch
Thrust
Thrust
Max Thrust
Inflow
Inflow
Max Inflow
Direction Speed of rotation
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Propeller Theory
Effective thrust can be increased or decreased by
varying the pitch of the propeller.
Variable Pitch
Let us now take a look at a graph of the relative
EFFICIENCY of a propeller.
Greater Inflow
Thrust
Greater Thrust
Direction of rotation
Pitch variable in between these limits
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Graph of Propeller Efficiency
Firstly, Fixed Pitch. As aircraft speed
increases, so does efficiency, up to a peak. At
faster speeds, efficiency then reduces.
Efficiency

• By varying the pitch, we can extend this maximum
  efficiency over a greater speed range.

Aircraft speed
To ensure that our engine is kept within
acceptable limits throughout this speed range, we
attach a mechanism which keeps rpm constant
Therefore, let us look in detail at such a
CONSTANT SPEED system, which is attached to the
propeller.
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First, take a propeller which can change its
pitch.
Connect it to a rod which causes the blades to
vary their pitch by moving up/down.
Attach the rod to a piston in a tube.
A spring pushes on one side of the piston, being
opposed by oil under pressure on the other.
A change of oil pressure will affect the position
of the spring and move the piston, which then
varies the pitch of the propeller.
Pressurised oil
Now, lets look at the unit which causes the
changes in oil pressure.
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A lever moves one part of a plate up or down.
Movement of the plate is restricted by a spring.
The spring is held by a bottom plate, which can
be varied up or down by the action of flyweights
rotating at engine speed.
The bottom of the plate is attached to an
operating rod, which in turn is connected to a
piston in a thin tube.
Movement of the piston allows or stops
pressurized oil entering a pipe, which is
connected to the coarse/fine unit described
before.
to coarse/fine unit
The oil is pressurized by an engine-driven pump.
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Constant Speed Unit Schematic ON SPEED
Let us see what happens in an OVERspeed situation.
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Flyweights speed up, centrifugal force increases
forcing the spring to contract, allowing the
piston to rise, allowing oil trapped in the
coarse/fine unit to exit, which pushes the
propeller to COARSE, so decreasing engine speed.
OVERSPEED
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Now you work out what happens in an UNDERspeed
situation.
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Flyweights slow down, centrifugal force reduces,
allowing the spring to expand, allowing the
piston to descend, allowing pressurized oil INTO
the coarse/fine unit, which pushes the propeller
to FINE, so increasing engine speed.
UNDERSPEED
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ON SPEED
Next, well look at operation of the RPM lever ,
going COARSE
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Constant Speed Unit Schematic ON SPEED
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Constant Speed Unit Schematic - Coarse
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Constant Speed Unit Schematic ON SPEED
Next - Fine
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Constant Speed Unit Schematic - FINE
Next Car Gearbox analogy
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The CAR GEARBOX Analogy
0 mph 20 mph 40 mph
faster!
2nd Gear
3rd Gear
4th Gear
1st Gear
0 kt 60kt 80kt
100 120
VARIABLE PITCH PROPELLER
Takeoff Climb
C r u i s e
Fine
Coarse
MAP inches 26 /2700rpm 25/2500rpm
24/2400rpm
Just as you would not move a car from a
standstill in 4th gear, dont take off with a
coarse prop
Next Effect of Altitude
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Effect of Altitude

• As we climb higher, the density of the air
  decreases (less molecules of air).
• This is why, for a constant True Air Speed the
  Indicated Air Speed reduces as altitude
  increases.
• This means less molecules of air hitting the
  prop, making it less efficient.
• So to maintain speed, bigger bites of air must be
  taken a coarser blade angle.

Next Effect of opening throttle
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• Q How does the pilot increase engine power?
• A By opening the throttle.
• Q What does this change?
• A Opens up the throttle plate, which allows
  greater air pressure into the inlet manifold. The
  pressure change is sensed by ECU which increases
  the amount of fuel injected into the cylinders.
• Q What effect does this have on the engine?
• A Injectors provide extra fuel, which the engine
  burns and wants to go faster,
• but the CSU keeps the prop speed (and
  therefore engine speed) constant,
• so
• Q what DOES change in the engine/prop
  combination?
• A This extra effort means that, in order to keep
  the prop at the same speed, the Constant Speed
  Unit will coarsen it off.
• The prop now bites bigger chunks of air, thus
  making good use of the extra effort provided by
  the engine, and the aircraft accelerates.

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Q What cockpit indication, apart from the speed
increase shown on the ASI, shows this change in
engine power?
A An INCREASE in the Inlet MANIFOLD PRESSURE So
let us examine Manifold Pressure a little more
closely
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Q Manifold Pressure What is it?

• A The actual pressure in the inlet manifold in
  inches of mercury (Hg), measured by a sensor
  downstream of the throttle plate.
• So manifold pressure is actually a measure of the
  LACK of pressure in the manifold, as measured
  against ambient.
• Q How is it shown in the Firefly?
• A On a Manifold Pressure Gauge (also known as a
  Boost gauge in older aircraft)
• Let us now see this gauge and examine its
  operation

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MANIFOLD PRESSURE GAUGE
Q With the engine running at 1100rpm, what will
the Manifold Pressure gauge read? A The engine
is idling the throttle plate is virtually
closed, so the engine has to suck hard to get any
air, causing the low MAP.
Q If the engine is stopped, Is this manifold
gauge working properly? A Yes - it is showing
29.92 Hg which is standard ambient pressure at
this airfield which is at sea level on a
standard ISA day. (Note at airfields higher
than sea level, the actual pressure is
shown. Ambient pressure reduces by 1hg per
1000ft (approx).
Q What will happen to MAP if we select FULL
Throttle? A MAP will increase to 26-28 Hg as the
engine is getting all the air it needs, the
throttle is wide open, allowing pressure to be
near to ambient. It will never fully reach
ambient when the engine is running even with the
throttle fully open as there are pressure
restrictions caused by filters, pipe bends etc.
Engine stopped
Engine running
Enough of this heavy stuff! Let us now look at
various levers on the centre console
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Lever Management
Pitch Control
Throttle
Mixture Control
Or, put simply Rev UP, Throttle Back
Using Pitch Throttle Go from Right to Left
To Increase Power
Go from Left to Right
To Decrease Power
Next Phases matrix
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Phases of Flight
IAS MAP (Throttle) Fuel
Consumption Mixture RPM Pitch Lever
Takeoff Climb Cruise Approach Go Around
0-60
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HIGH
2650-2700 FINE
RICH
70-80
26-28
BPM
2600 FINE
HIGH
90-110
Medium
18-2400 MED COARSE
BPM/BEM
20-24
1800-2400 MED or OUT of CSU range
70-80
18-22
LOW
RICH
60-80
- - - - - - - - As for takeoff - - - - - - - - -
- - - -
Next, CSU failure
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FAILURES

• Failure of the propeller oil pressure supply
• If the oil supply stops (inverted flying, oil
  leak?), then the CSU will default to the full
  COARSE. The engine slows down to approx 1300 rpm
  and is therefore protected from overspeeding.

Mechanical failure of the Constant Speed Unit The
CSU can stop working for a number of reasons
stuck, seized or leaking piston, broken spring,
bent operating rod etc. Depending on the problem,
the unit can remain in one position (seized
part), or go towards COARSE (bent rod) or go to
full COARSE (broken spring). It may be biased to
FINE if a leaking piston allows oil pressure past
it.
Solution In all cases, contain the RPM within
limits (1300-2700) by adjusting IAS and land the
aircraft as soon as practicable.
Next, Objectives
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Objectives
The objectives we set out to achieve were

• To learn why a variable pitch propeller can
  assist performance. Q Can you simply explain how
  varying the propeller pitch does this?
• To understand how a Constant Speed Unit works.
  Q Name the components of a constant speed unit -
  how does it work?
• To learn how to change power with a variable
  pitch propeller. Fill in the Blanks ???? up,
  ???? back
• To efficiently and safely operate the Firefly
  propeller in all phases of flight.
• Well, apart from the operation of the levers, we
  have not covered this yet, but a short flight
  dedicated to operation of the Variable pitch
  propeller in different phases of flight is all
  that is needed. This will be covered shortly.

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End of BriefAny Questions?
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• Click HERE to link to Variable Propeller Flight
  Brief