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

2. 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.

3. Q Why do we need a constant speed propeller?
A To cope with variations in:
Speed range
Engine power
Let’s 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?

4. An upward force is produced
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
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

5. Attached to a rotating hub
Propeller Theory
Here is our fixed pitch propeller blade
Attached to a rotating hub
Thrust
Producing forward lift = thrust
Direction
of rotation

6. Propeller Theory Fixed Pitch
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 let’s look at a variable pitch propeller …
Fixed Pitch
Thrust
Max Thrust
Thrust
Inflow
Inflow
Max Inflow
Add in the airflow due to forward speed
Direction & Speed
of rotation

7. Pitch variable in between these limits
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

8. 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.

9. 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, let’s look at the unit which causes the changes in oil pressure.

10. 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.

11. Constant Speed Unit Schematic – ON SPEED
Let us see what happens in an OVERspeed situation.

12. 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

13. Now you work out what happens in an UNDERspeed situation.

14. 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

15. ON SPEED
Next, we’ll look at operation of the RPM lever , going COARSE

16. Constant Speed Unit
Schematic – ON SPEED

17. Constant Speed Unit
Schematic - Coarse

18. Constant Speed Unit
Schematic – ON SPEED
Next - Fine

19. Next – Car Gearbox analogy
Constant Speed Unit
Schematic - FINE
Next – Car Gearbox analogy

20. The CAR GEARBOX Analogy
0 mph mph mph faster!
1st Gear
2nd Gear
3rd Gear
4th Gear
0 kt 60kt 80kt – 120+
VARIABLE PITCH PROPELLER
Takeoff Climb C r u i s e
Fine Coarse
MAP inches: 26” /2700rpm ”/2500rpm ”/2400rpm
Just as you would not move a car from a standstill in 4th gear, don’t take off with a coarse prop
Next – Effect of Altitude

21. Next – Effect of opening throttle
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

22. 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.

23. 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…

24. 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…

25. 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 1”hg 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…

26. Lever Management Pitch Control Throttle Mixture Control
FULL
IDLE
FINE
COARSE
Throttle
Pitch Control
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

27. Phases of Flight 0-60 28” HIGH RICH 2650-2700 + FINE 70-80 26-28” BPM
IAS MAP
(Throttle) Fuel Consumption Mixture RPM + Pitch Lever
Takeoff
Climb
Cruise
Approach
Go Around
0-60
28”
HIGH
RICH
FINE
70-80
26-28”
BPM
FINE
HIGH
90-110
20-24”
Medium
BPM/BEM
MED – COARSE
MED or
OUT of CSU range
70-80
18-22”
LOW
RICH
60-80
As for takeoff
Next, CSU failure

28. 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 ( ) by adjusting IAS and land the aircraft as soon as practicable.
Next, Objectives

29. 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.

30. End of Brief Any Questions?

31. Click HERE to link to Variable Propeller Flight Brief