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Edited: May 15, 2020

`Constant Speed Prop

Propeller, an rotating airfoil convert engine torque to lift (forward lift ).

Propeller rotates to create lift (thrust); if aircraft remains stationary, the relative wind of the blade shall be in direction of the rotation plane. However, as the aircraft moves forward, the relative wind will be displaced (or tilted forward) due to the forward airspeed. Hence, the Angle of Attack become smaller; and forward lift(thrust) reduced.

The faster the aircraft travel, the larger the forward velocity, the less effective of the propeller.

Usually, fixed pitch prop is designed as Climb Prop. It generates best performance during low speed. It will lose efficiency once forward velocity increases.

In order to target this problem, eliminate the deficiency of the AOA reduction, aircraft designer introduce the constant speed(variable pitch) propeller . The purpose of constant speed propeller is, increase the effectiveness of the blade, by increasing the blade angle during cruise phase.

I will utilize McCauley Prop on the C172RG for explanation. Keep in mind, not all aircraft design the same way; but you can still learn the basic concept with this. It breaks down to Propeller and Propeller Governor.

It utilize oil pressure into/out from the propeller hub, to against the pre-set force on the spring.

When spring tension > oil pressure in the hub -the propeller will lower the pitch of the blade, and achieve a fine pitch (High RPM)

(If lack of oil pressure/ oil leak, the spring will force the blade to full fine and act as a fixed pitch climb prop.)

When oil pressure is available, it overcomes the spring tension to increase pitch angle to coarse. (Low RPM)

You can consider that's fight between the pre-set speeder spring tension and oil pressure.

(Fine VS Coarse)

We control the blade pitch by controlling oil pressure going into the hub.

Of course, we, pilots are lazy; we can't continuously adjusting the oil flow in or out to the prop hub thru out the flight.

Therefore, designer introduce propeller governor to regulate the oil pressure flow in/out.

But HOW??

Thanks to the Propeller Governor, it saved up a lot of trouble.

Before I explain further, lets have a look on the key component inside the propeller governor.

Pilot Valve in green (control the oil flow)

Speeder Spring in BLUE (Pushes the Pilot Valve Down) Flyweight in Orange (Pushes the Pilot Valve Up)

Pilot moves levers to adjust the speeder spring tension thru the threaded shaft. And as the flyweight assembly driven and rotate by the gears.(Prop speed)

Assume: Pilot sets prop at 2300RPM.

When in normal condition (On-Speed), (target 2300RPM = actual prop speed 2300RPM)

Centrifugal force of the flyweight balances the pre-set Speeder spring tension, means the the pilot valve remains and no oil flowing in or out towards the propeller hub.

If propeller rotates faster than the set-speed (e.g. actual 2350RPM) , Centrifugal force of the flyweight overpowered the Speeder spring tension; the flyweight will pivot outward and lift the pilot valve up, which allow the engine oil flow to the prop hub. Increase the blade angle to slow down to target speed. Vice Versa.

For application, before takeoff, we need to put the prop control to full fine. With full fine pitch, which act as fixed pitch prop, it has a lower pitch angle (reduce aerodynamic load). When applied full power, it can provide max rotation speed and achieve max available thrust.

As aircraft airborne, forward velocity picks up, it loses its efficiency. In order to recover the lose efficiency, pilot will pull back on the prop control; to command the propeller governor, thus the propeller to increase the blade angle.

That's the purpose of Constant Speed Propeller.

Hope you guys can pick up some knowledge here~

Leave some comment and thots below~

6 comments

6 Comments

Ma Lok chun jeff

May 16, 2020

to Billy,

From my understanding,

constant speed unit indeed means constant prop speed.
if u push the throttle, the csu will increase the propeller pitch. The excess energy has now been used to turn the propeller blade with a larger aoa.
the manifold pressure gauge?

Billy LAI

May 17, 2020

Replying to

@Ma Lok chun jeff

Thanks for your reply!

Billy LAI

May 15, 2020

Thanks for yr explanation! I have a few questions:

1) So the word "speed" in constant speed prop means prop rotation speed (rpm), but not the IAS?

2) Given that I set the desired rpm (using the prop lever), if I move the throttle forward, the rpm won't increase, right? Then where does the excess energy I increase by moving the throttle forward, goes to? Does it go to IAS and I will fly faster?

3) In fixed-pitch propeller, I can observe the effect of pushing the throttle forward (increase thrust) in the rpm display (increase rpm). In constant speed prop, as the rpm remain the same as long as I set it by the prop control, when I increase thrust, what instrument can show the effect?

Many thanks!!

ATVpilot

May 16, 2020

Replying to

Thanks for yr question~ @billy I very mich appreciate when u guys comment and clarify with me. 1) that's is a great question. Constant Speed prop means the prop will rotate at a constant pre set (rotational) speed, regardless of yr IAS. 2) @Ma Lok chun jeff answered yr 2nd question below, however, i will like to add some on his answer. Of coz, u can advance the power lever to increase the power, while maintain constant rpm with higher pitch, but u also need to consider the stress on the prop n engine. It might cause detonation or prop failure in extreme cases. 3) again, jeff has the precise answer, manifold pressure. You can consider it as power indicator. (how much throttle opened? How much fuel going into the engine?) On fixed pitch prop, the power is proportional to RPM, more throttle more rpm. However, in constant speed prop, the rpm is constant; in order to indicate the power setting, we use MAP manifold absolute pressure. Here is the question for you. What is MAP? And is it possible to get more than 29.92 in" on MAP? And why? Enjoy on your research~ Btw, anyone is welcome to join the discuss~comment below

Billy LAI

May 19, 2020

Replying to

@ATVpilot Thanks for your explanation!

I have summaries my understanding on MAP in my own wordings after browsing some online resources. Could you please correct me if some of my concepts or logic is wrong? Thank you!!

“MAP measure the vacuum pressure experienced by the fuel-air mixture inside intake manifold as caused by the intake stroke in the cylinder. When the engine is in shut-off position, MAP will read ambient air pressure (atmospheric pressure) due to lack of vacuum (normal surrounding pressure). One the engine is started (at idle), MAP drop significantly. The intake stoke of the engine wish to suck mixture into cylinder, but this action is restricted by the butterfly valve. Thus, creating a vacuum state (low pressure) inside intake manifold. As the restriction (or vacuum) is largest at idle engine (engine most starving for mixture), MAP will read the lowest when engine run idling. Once throttle setting increase, the vacuum inside intake manifold decrease (i.e., higher pressure), because the engine is able to get more mixture it wants. Thus, MAP increase when throttle setting increase. At maximum throttle input, engine get all the mixture it wants without obstruction by the butterfly valve. Thus, MAP read the highest (or equal to the outside atmospheric pressure) at maximum throttle input. Note that MAP cannot exceed outside air pressure, which is 22.92 Hg in position of MSL (as stated by ISA) according to the mechanism of MAP. “

ATVpilot

May 20, 2020

Replying to

@Billy LAI

In general, your concept is right~

“MAP measure the vacuum pressure experienced by the fuel-air mixture inside intake manifold as caused by the intake stroke in the cylinder. When the engine is in shut-off position, MAP will read ambient air pressure (atmospheric pressure) due to lack of vacuum (normal surrounding pressure). Once the engine is started (at idle), MAP drop significantly. The intake stoke of the engine wish to suck mixture into cylinder, but this action is restricted by the butterfly valve. Thus, creating a vacuum state (low pressure) inside intake manifold. As the restriction (or vacuum) is largest at idle engine (engine most starving for mixture), MAP will read the lowest when engine run idling. Once throttle setting increase, the vacuum inside intake manifold decrease (i.e., higher pressure), because the engine is able to get more mixture it wants. Thus, MAP increase when throttle setting increase. At maximum throttle input, engine get all the mixture it wants without obstruction by the butterfly valve. Thus, MAP read the highest (or equal to the outside atmospheric pressure) at maximum throttle input. Note that MAP cannot exceed outside air pressure, which is 29.92 Hg in position of MSL (as stated by ISA) according to the mechanism of MAP. “

As altitude increases, the atmospheric pressure decrease; engine can't maintain its rated power. Therefore, that's why some aircraft introduce engine augmentation, by supercharger and turbocharger.

With those Augmentation, Max Manifold pressure can reach up to 42" of Hg (depends on engine design). But as it produce more power, it takes more airmenship to handle to engine; esp. on overboosting the engine and Shock cooling on the engine or turbo.

https://www.hkcockpit.com/forum/aviation-gen-knowledge/engine-augmentation