Propellers driving me True-ly nuts, any help?

[BenFI]

Hi all,

So, a question: I watched a YT video on the Propellers. As I hear, all props are constant speed, some have a selectable pitch. Correct?

If yes, an aircraft with a variable pitch, (constant speed) propeller, set to fine pitch, accelerates down the runway, the

1. RPM would stay constant, right?
2. What about the blade pitch angle? Would that increase/ decrease/ stay the same?
3. What about the Angle of Attack? As I understand, due to the forward speed, the angle of attack should decrease, right?

I'd be grateful for any help on this!

Regards!

 

[SebIp]

Eh what? Most common GA is a Cessna 172 isn’t it and most of them are fixed pitch.

Constant speed props have governors that change the pitch angle to try and maintain a prop speed constantly. It will have limits so at some point when it’s full fine pitch it’ll slow as you decrease power more.

Angle of attack for a prop does depends on pitch and relative airflow. It’s more complicated than just forward speed although that can be used as a simplified approach to the idea.

Not sure what YouTube you are watching. Maybe should pickup a book instead?

 

[BenFI]

Eh what? Most common GA is a Cessna 172 isn’t it and most of them are fixed pitch.

Constant speed props have governors that change the pitch angle to try and maintain a prop speed constantly. It will have limits so at some point when it’s full fine pitch it’ll slow as you decrease power more.

Not sure what YouTube you are watching. Maybe should pickup a book instead?

I have the books, just looking for additional clarifications, as the books and the GS material from four separate sources aren't covering every aspect. Hence the YT video, this one.

So, from your reply, I get that some of them are fixed pitch, no governors in them.

So, the specific question is this and my comments in blue italics:

Aircraft with variable pitch prop (constant speed), set to fine pitch, accelerates down the runway. What are the changes?

– Would the RPM stay constant? Change? (This the answer would be Constant, no issues)

– What about the blade pitch angle? (I am not sure how to answer this question. IF the pitch is set to fine, then it should not change on its own. BUT then again, with the flywheels in the prop, it should adjust the angle so the RPM does not get too high)
– What about the blade pitch angle of attack? (Again: I am not sure how to answer this question. Because of the forward acceleration the relative wind angle changes, so the relative airflow in relation to the chord line should decrease)

BUT apparently, the right answer would be
- engine RPM stays constant (OK)
- Blade pitch angle increases (not sure what the assumption is, but I can understand based on what I say above, i.e. flywheels)
- Angle of attack stays constant > this I am not sure at all.

Any CFI's here?

 

[BenFI]

OK, for others too! Now answering my own question, and this video did it.

In short: with the constant speed,

- you are setting the RPM, i.e. the desired RPM of the prop
- the governor sets the pitch using the flywheels

Now it makes sense.

So,
- the engine RPM stays constant,
- the [governor adjust the] blade pitch angle (it will increase the pitch to more coarse to maintain the RPM)
- the angle of attack will stay constant because the blade pitch angle will be adjusted to the relative airflow, i.e. the relative will be more angled compared to the plane of rotation, but the pitch will move along with it.

I hope I am getting this right!

 

[Pinecone]

The pitch isn’t going to change. A lot of pilots don’t even touch the pitch until at cruise, skipping second gear. Nobody repitches during takeoff, even though in theory, the blade would benefit a bit.

The angle of attack is trickier. The angle of attack at the hub does not change. But as the airplane accelerates & lifts the nose, all lifting bodies will change their angle of attack.

The pilot does NOT control the pitch. The pilot controls the desired RPM. So while the pilot does not touch the prop control on takeoff, this does NOT mean that the prop pitch is not changing.

For a properly set up engine/prop, at full throttle at the start of the takeoff roll, the prop should be about on the fine pitch stop. As the plane accelerates, the AOA of the prop blades change due to the change in relative wind, so the prop may increase pitch to maintain the proper RPM setting.

 

[Dana]

For the sake of completeness, it should be mentioned that there are variable pitch props that are not constant speed; i.e. the pilot sets the pitch directly. They're not common any more, and some are just two position (fine or coarse pitch), but they do exist. At any particular pitch setting, they behave like a fixed pitch prop.

 

[Roger Wilco]

Hello BenF,

I'm pretty slow on the keyboard, so several others have answered your questions while I was typing this...and making coffee. Nonetheless, I'll post anyway. Propellers and their aerodynamic properties are complicated and often misunderstood. Fortunately it's not necessary to be an expert in order to operate the airplane safely and efficiently, but a solid grasp of the basics is certainly helpful in the real world, not to mention the written test and the examiner's inquisition.

So, a question: I watched a YT video on the Propellers. As I hear, all props are constant speed, some have a selectable pitch. Correct?

It is not correct to say that all props are constant speed. Here is a brief overview of different propeller types found on general aviation airplanes:

Fixed-pitch: Propeller blade angle cannot be changed.

Ground Adjustable: Blade angle can be adjusted by hand (or with a tool). Obviously accomplished with the aircraft on the ground, and much less dramatic with the engine stopped.

"Aeromatic" variable pitch: a vintage design rarely encountered today, so perhaps the less said here the better.

Variable pitch: Pitch can be changed while the engine is running, but there is no governor to maintain a constant RPM. Well, actually there is a governor: the pilot's hand on the propeller control.

Constant speed: Blade pitch can be changed while the engine is running. A governor in the pitch control system maintains the RPM set by the pilot through a wide range of power settings and airspeeds.

If yes, an aircraft with a variable pitch, (constant speed) propeller, set to fine pitch, accelerates down the runway, the

1. RPM would stay constant, right?
2. What about the blade pitch angle? Would that increase/ decrease/ stay the same?
3. What about the Angle of Attack? As I understand, due to the forward speed, the angle of attack should decrease, right?

1. Theoretically true, perhaps, but not necessarily in practice. RPM at the beginning of the takeoff roll (static RPM) is generally determined by the propeller system's low pitch (fine pitch) stop and the actual power being produced by the engine. RPM may be less than the maximum limit at the beginning of the takeoff roll, but should increase to the limit as airspeed increases.

2. During takeoff roll with the propeller control set to maximum RPM, blade angle will remain at its low pitch stop until RPM reaches the limit, then increase as necessary to maintain maximum RPM as airspeed continues to increase.

3. Your understanding is correct, insofar as it applies to a fixed-pitch propeller. As airspeed increases, the angle of attack (AoA) of the propeller blades decreases. With a constant speed propeller system, as airspeed increases the propeller governor will act to increase the pitch of the blades as necessary to maintain the RPM set by the propeller control. Conversely, when airspeed decreases, the governor will decrease pitch to maintain RPM. What the governor is really doing is balancing the engine power against the aerodynamic drag on the propeller blades. When engine power is constant and airspeed changes, the governor changes blade pitch so as to maintain a constant blade AoA, and thus constant aerodynamic drag on the propeller, which results in a constant RPM. If engine power increases or decreases, the governor increases or decreases blade pitch, therefore increasing or decreasing blade AoA which results in a corresponding increase or decrease in drag as necessary to maintain a constant RPM.

– What about the blade pitch angle of attack? (Again: I am not sure how to answer this question. Because of the forward acceleration the relative wind angle changes, so the relative airflow in relation to the chord line should decrease)

Based on the explanation above, with a constant speed propeller the propeller control sets desired RPM, not blade angle. The governor then adjusts blade angle to maintain the desired RPM. There are no flywheels involved: propeller governors have flyweights (different from a flywheel) which act against a spring to maintain desired RPM. During takeoff, the propeller control is set for maximum RPM. As airspeed increases, the governor will maintain maximum RPM by adjusting blade angle as necessary.

 

[Dan Thomas]

The pitch isn’t going to change. A lot of pilots don’t even touch the pitch until at cruise, skipping second gear. Nobody repitches during takeoff, even though in theory, the blade would benefit a bit.

The governor does indeed control that prop all the time. The whole point of a constant-speed prop is to get the most out of the engine and airplane. A fixed-pitch prop is like being stuck in second gear in your car. (How many here have even ever driven a manual transmission??) The constant-speed prop lets the engine run up to redline RPM for takeoff and initial climb, and redline is where that engine is developing its designed maximum horsepower. It's even better than first gear in your car, since even first gear doesn't let the engine come to max RPM right as you start moving. The CS propeller does give you max RMP and therefore max thrust. In cruise, it gives you maximum thrust, and therefore speed, for whatever cruise setting you choose.

Read the stuff from the people who make them: https://mccauley.txtav.com/-/media/mccauley/files/mccauleytechguide.ashx

Good training material from the FAA: https://www.faa.gov/sites/faa.gov/files/09_amtp_ch7.pdf

From the aircraft systems course I used to teach, we learn about angle of attack on a fixed-pitch prop. This is a pretty big prop, so the RPMs are lower.


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And while we're at it, we can show why propeller blades have that twist in them. It's all about trigonometry:

 

[rhkennerly]

The pilot does NOT control the pitch. The pilot controls the desired RPM. So while the pilot does not touch the prop control on takeoff, this does NOT mean that the prop pitch is not changing.

For a properly set up engine/prop, at full throttle at the start of the takeoff roll, the prop should be about on the fine pitch stop. As the plane accelerates, the AOA of the prop blades change due to the change in relative wind, so the prop may increase pitch to maintain the proper RPM setting.

TNX. Learn something new every day.

 

[BenFI]

Hello BenF,

I'm pretty slow on the keyboard, so several others have answered your questions while I was typing this...and making coffee. Nonetheless, I'll post anyway. Propellers and their aerodynamic properties are complicated and often misunderstood. Fortunately it's not necessary to be an expert in order to operate the airplane safely and efficiently, but a solid grasp of the basics is certainly helpful in the real world, not to mention the written test and the examiner's inquisition.

It is not correct to say that all props are constant speed. Here is a brief overview of different propeller types found on general aviation airplanes:

Fixed-pitch: Propeller blade angle cannot be changed.

Ground Adjustable: Blade angle can be adjusted by hand (or with a tool). Obviously accomplished with the aircraft on the ground, and much less dramatic with the engine stopped.

"Aeromatic" variable pitch: a vintage design rarely encountered today, so perhaps the less said here the better.

Variable pitch: Pitch can be changed while the engine is running, but there is no governor to maintain a constant RPM. Well, actually there is a governor: the pilot's hand on the propeller control.

Constant speed: Blade pitch can be changed while the engine is running. A governor in the pitch control system maintains the RPM set by the pilot through a wide range of power settings and airspeeds.

1. Theoretically true, perhaps, but not necessarily in practice. RPM at the beginning of the takeoff roll (static RPM) is generally determined by the propeller system's low pitch (fine pitch) stop and the actual power being produced by the engine. RPM may be less than the maximum limit at the beginning of the takeoff roll, but should increase to the limit as airspeed increases.

2. During takeoff roll with the propeller control set to maximum RPM, blade angle will remain at its low pitch stop until RPM reaches the limit, then increase as necessary to maintain maximum RPM as airspeed continues to increase.

3. Your understanding is correct, insofar as it applies to a fixed-pitch propeller. As airspeed increases, the angle of attack (AoA) of the propeller blades decreases. With a constant speed propeller system, as airspeed increases the propeller governor will act to increase the pitch of the blades as necessary to maintain the RPM set by the propeller control. Conversely, when airspeed decreases, the governor will decrease pitch to maintain RPM. What the governor is really doing is balancing the engine power against the aerodynamic drag on the propeller blades. When engine power is constant and airspeed changes, the governor changes blade pitch so as to maintain a constant blade AoA, and thus constant aerodynamic drag on the propeller, which results in a constant RPM. If engine power increases or decreases, the governor increases or decreases blade pitch, therefore increasing or decreasing blade AoA which results in a corresponding increase or decrease in drag as necessary to maintain a constant RPM.

Based on the explanation above, with a constant speed propeller the propeller control sets desired RPM, not blade angle. The governor then adjusts blade angle to maintain the desired RPM. There are no flywheels involved: propeller governors have flyweights (different from a flywheel) which act against a spring to maintain desired RPM. During takeoff, the propeller control is set for maximum RPM. As airspeed increases, the governor will maintain maximum RPM by adjusting blade angle as necessary.

Big thanks Roger Wilco!

 

[Roger Wilco]

Big thanks Roger Wilco!

Big "you're welcome" BenFI.

I wish training focused more on how to get the most out of our propellers by teaching us the way in which the engine and propeller function as a single propulsive unit, instead of dwelling on the invisible monkey motion that occurs inside the governor and hub.

 

[Clip4]

Hi all,

So, a question: I watched a YT video on the Propellers. As I hear, all props are constant speed, some have a selectable pitch. Correct?

If yes, an aircraft with a variable pitch, (constant speed) propeller, set to fine pitch, accelerates down the runway, the

1. RPM would stay constant, right?
2. What about the blade pitch angle? Would that increase/ decrease/ stay the same?
3. What about the Angle of Attack? As I understand, due to the forward speed, the angle of attack should decrease, right?

I'd be grateful for any help on this!

Regards!

Simple stuff, a fixed pitch prop is a fan blade. Only efficient at 1 rpm and one speed traveling thru the air. A constant speed prop varies the pith to remain at the RPM selected by the pilot and efficient at many RPM and airspeed selections.

 

[Dan Thomas]

A constant speed prop varies the pith to remain at the RPM selected by the pilot and efficient at many RPM and airspeed selections.

Almost. The twist in the CS propeller blade is also only efficient at one particular pitch setting and airspeed. The entire blade rotates to change pitch, and as pitch increases, it moves the load toward the tip, while the root should be increasing a lot more, but can't. Trigonometry, again. The ideal prop would be able to rotate its root much more than the tip, but that's an engineering nightmare, considering the enormous forces on the prop.

 

[fast99]

If fixed pitch is a compromise between high and low speed. What speed or RPM are they are most efficient, somewhere in the middle?

 

[MRC01]

If fixed pitch is a compromise between high and low speed. What speed or RPM are they are most efficient, somewhere in the middle?

Most fixed pitch props are pitched to optimize cruise rather than climb. That means relatively more (coarser) blade pitch. You sacrifice takeoff roll and climb performance for cruise speed & efficiency. But you can get a "climb prop" having less (finer, flatter) blade pitch optimized for takeoff & climb performance at the expense of cruise speed & efficiency.

 

[MRC01]

Another aspect of constant speed props is power output. Power moves the airplane, and power is torque * RPM. And power is also thrust * speed. Consider a typical engine with fixed pitch prop that has relatively flat torque vs. RPM and redlines at 2700 PRM. Static RPM full throttle sea level will be about 2300 RPM. That's 85% of the 2700 RPM redline, so the engine is making only 85% of its rated power (assuming flat torque vs. RPM).

Now if that same airplane & engine gets a constant speed prop, no other changes, it makes 2700 RPM which means full power. That's an 18% increase in power (1/.85) which means an 18% increase in thrust at any given airspeed. This is a SIGNIFICANT improvement resulting in shorter takeoff roll, faster climb rate and higher payload.

The above is an oversimplification, as the engine torque curve isn't flat and prop efficiency is not the same at all pitch settings. But it illustrates the basic physics.

 

[Dan Thomas]

Consider a typical engine with fixed pitch prop that has relatively flat torque vs. RPM and redlines at 2700 PRM. Static RPM full throttle sea level will be about 2300 RPM. That's 85% of the 2700 RPM redline, so the engine is making only 85% of its rated power (assuming flat torque vs. RPM).

The above is an oversimplification, as the engine torque curve isn't flat and prop efficiency is not the same at all pitch settings. But it illustrates the basic physics.

Way oversimplified. The power chart for an O-320-A or -E model, which makes 150 HP at sea level at 2700 RPM as in your example:



At 2300 RPM it's making about 92 horsepower, or 61.3% power. Torque is nowhere near level. These engines usually produce max torque at or near their redline.

 

[Dan Thomas]

Fixed-pitch propellers are normally pitched so that the engine reaches redline at full throttle at sea level, in level flight. A "climb" prop is going to result in a considerably lower cruise speed, as it will quickly run past redline once you level off, at normal throttle settings.