VARIABLE PITCH PROPELLER BRIEF

The Firefly T67M

Hoffman 2 bladed

Variable Pitch propeller

Objectives

1. To learn why a variable pitch propeller can assist aircraft performance.

2. To understand how a Constant Speed Unit works.

3. To learn how to change power with a variable pitch propeller.

4. To efficiently and safely operate the Firefly propeller in all phases of flight.

Q Why do we need a constant speed propeller?

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

0 kt 100 kt

0 ktHigh performance a/c 190 kt

Speed range

A To cope with variations in: Speed range

Engine power

Cruise speed range

Cruise speed range

Let’s see the variation in cruising speed range of 2 different aircraft…

Q How can we make a propeller cope efficiently with this greater speed range?

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

LIFT

Direction of rotation

Thrust

Propeller Theory

Thrust

Direction

of rotation

Here is our fixed pitch propeller blade

Attached to a rotating hub

Producing forward lift = thrust

Fixed Pitch

Propeller Theory

Thrust

Inflow

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 …

Direction & Speed

of rotation

Inflow

ThrustMax Thrust

Max Inflow

Thrust

Propeller Theory

Greater Thrust

Greater Inflow

Variable Pitch

Pitch variable in

between these

limits

Effective thrust can be increased or decreased by varying the pitch of the propeller.

Direction

of rotation

Let us now take a look at a graph of the relative EFFICIENCY of a propeller.

Graph of Propeller Efficiency

Aircraft speed

Effi

cien

cy

Firstly, Fixed Pitch.

As aircraft speed increases, so does efficiency, up to a peak. At faster speeds, efficiency then reduces.

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

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.

Pum p

Re lie fVa lve

Flywe ig hts

O il sup p lyfro m e ng ine

O il re turnto e ng ine

C o a rse

Fine

RPM C o ntro lLe ve r

Fine

C o a rse

Pressurised oil

First, take a propeller which can change its pitch.

Attach the rod to a piston in a tube.

A change of oil pressure will affect the position of the spring and move the piston, which then varies the pitch of the propeller.

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

Connect it to a rod which causes the blades to vary their pitch by moving up/down.

A spring pushes on one side of the piston, being opposed by oil under pressure on the other.

Pum p

Re lie fVa lve

Flywe ig hts

O il sup p lyfro m e ng ine

O il re turnto e ng ine

C o a rse

Fine

RPM C o ntro lLe ve r

Fine

C o a rse

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.

The oil is pressurized by an engine-driven pump.

to coarse/fine unit

Pump

ReliefValve

Flyweights

Oil supplyfrom engine

Oil returnto engine

Coarse

Fine

RPM ControlLever

Fine

Coarse

Constant Speed UnitSchematic – ON SPEED

Let us see what happens in an OVERspeed situation.

Pum p

Re lie fVa lve

Flywe ig hts

O il sup p lyfro m e ng ine

O il re turnto e ng ine

C o a rse

Fine

RPM C o ntro lLe ve r

Fine

C o a rse

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

Pump

ReliefValve

Flyweights

Oil supplyfrom engine

Oil returnto engine

Coarse

Fine

RPM ControlLever

Fine

Coarse

Now you work out what happens in an UNDERspeed situation.

Pum p

Re lie fVa lve

Flywe ig hts

O il sup p lyfro m e ng ine

O il re turnto e ng ine

C o a rse

Fine

RPM C o ntro lLe ve r

Fine

C o a rse

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

Constant Speed UnitSchematic – ON SPEED

Pump

ReliefValve

Flyweights

Oil supplyfrom engine

Oil returnto engine

Coarse

Fine

RPM ControlLever

Fine

Coarse

Constant Speed UnitSchematic - Coarse

Pump

ReliefValve

Flyweights

Oil supplyfrom engine

Oil returnto engine

Coarse

Fine

RPM ControlLever

Fine

Coarse

Constant Speed UnitSchematic – ON SPEED

Next - Fine

Pump

ReliefValve

Flyweights

Oil supplyfrom engine

Oil returnto engine

Coarse

Fine

RPM ControlLever

Fine

Coarse

Constant Speed UnitSchematic - FINE

Next – Car Gearbox analogy

Pump

ReliefValve

Flyweights

RPM ControlLever

Coarse

Fine Oil supplyfrom engine

Oil returnto engine

Fin

e

Coarse

The CAR GEARBOX Analogy

1st Gear 2nd Gear 3rd Gear

0 mph 20 mph 40 mph faster!

Takeoff Climb C r u i s e

CAR

4th Gear

Fine Coarse

MAP inches: 26” /2700rpm 25”/2500rpm 24”/2400rpm

0 kt 60kt 80kt 100 – 120+

VARIABLE PITCH PROPELLER

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

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

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.

A An INCREASE in the Inlet MANIFOLD PRESSURE

So let us examine Manifold Pressure a little more closely…

Q What cockpit indication, apart from the speed increase shown on the ASI, shows this change in engine power?

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…

Q Manifold Pressure… What is it?

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.

Engine stoppedEngine running

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).

Enough of this heavy stuff! Let us now look at various levers on the centre console…

MANIFOLD PRESSURE GAUGE

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.

Lever Management

To Increase Power

To Decrease Power

Throttle

Mixture Control

Pitch Control

Using Pitch & Throttle

Go from Right to Left

Go from Left to Right

FINE

COARSE

Or, put simply:

Rev UP, Throttle Back

FULL

IDLE

Next – Phases matrix

Phases of Flight

Takeoff

Climb

Cruise

Approach

Go Around

IAS MAP

(Throttle) Fuel Consumption Mixture RPM + Pitch Lever

0-60 28” HIGH 2650-2700 + FINE

70-80 26-28” HIGH BPM 2600 + FINE

90-110 Medium 18-2400 + MED – COARSEBPM/BEM

70-80 18-22” LOW RICH 1800-2400 + MED or

OUT of CSU range

60-80 - - - - - - - - As for takeoff - - - - - - - - - - - - -

20-24”

RICH

Next, CSU failure

Failure of the propeller oil pressure supplyIf 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.

Next, Objectives

Mechanical failure of the Constant Speed UnitThe 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.

SolutionIn all cases, contain the RPM within limits (1300-2700) by adjusting IAS and land the aircraft as soon as practicable.

FAILURES

Objectives

1. To learn why a variable pitch propeller can assist performance. Q Can you simply explain how varying the propeller pitch does this?

2. To understand how a Constant Speed Unit works. Q Name the components of a constant speed unit - how does it work?

3. To learn how to change power with a variable pitch propeller.Fill in the Blanks ???? up, ???? back

4. 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.

The objectives we set out to achieve were…

End of Brief

Any Questions?

Click HERE to link to Variable Propeller Flight Brief