Propellers

Terry

Terry

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Terry
Can someone recommend a good book on constant speed propellers?

I am still having trouble on the purpose of the throttle and the manifold pressure.

What I don't understand is how the pitch and manifold pressure are related.

Also, if it is truly a constant speed, why not set the throttle and just adjust pitch only?

Do they make an "automatic" constant speed propeller?

Thanks,
Terry


p.s.> Does a crop duster adjust the MP every time he starts down the field and during climb out? Is he constantly adjusting the MP?
 
Pitch and manifold pressure are pretty straightforward. For a selected RPM, as manifold pressure increases, pitch increases to maintain constant RPM. On manifold pressure reduction, the pitch decreases.

For a selected engine speed (RPM) manifold pressure determines engine power (ignoring fuel flow/mixture setting - that's another topic). High manifold pressure (thing high airflow volume) results in high engine power output. The higher engine power output, the more blade pitch required to maintain a give engine speed (RPM).

Edit: Pitch can't be directly selected because the throttle plate has to be adjusted to provide the airflow required for engine power.
 
All the gauge is doing is measuring atmospheric pressure in inches of mercury. On a non running engine it will display local atmospheric pressure (30 inches +/-).

On a normally aspirated engine, if the engine is at full power, wide open throttle the gauge will read slightly less due to a vacuum created by restrictions in flow from the intake system. As you close the throttle to reduce power, the vacuum will increase in the intake manifold, and the gauge will read correspondingly less.
 
Terry said:
Can someone recommend a good book on constant speed propellers?

I am still having trouble on the purpose of the throttle and the manifold pressure.

What I don't understand is how the pitch and manifold pressure are related.

Also, if it is truly a constant speed, why not set the throttle and just adjust pitch only?

Do they make an "automatic" constant speed propeller?

Thanks,
Terry


p.s.> Does a crop duster adjust the MP every time he starts down the field and during climb out? Is he constantly adjusting the MP?
Click to expand...


With a constant speed propeller, you have a governor which sends & releases oil to the propeller and changes the pitch of the propellor to change the load on the engine so it can turn the correct RPM for the amount of fuel you are sending it.

The throttle controls the fuel and air going to the engine at a ratio of 14.7:1 by weight. With 520 cubic inches turning 2500 RPM, that is a lot of air trying to be sucked into the engine. Now we regulate the amount of air pulled into the engine to regulate the amount of power we make in an engine (this in regulates the fuel flow through either vacuum or mechanical linkage to the metering system). We do this with a throttle plate that restricts the air flow. When the pistons pull down their intake stroke, they pull a suction in the intake tubes that we measure as manifold pressure. MP is indicated in absolute atmospheric pressure (perfect vaccum being 0) so anything below the altimeter setting (At Sea Level, on the ground for me for awhile now lol) is a vacuum reading. With the throttle closed, there is a lot of resistance so the vacuum increases resulting manifold pressure decreasing and having and less power due to less air and fuel. When we put the throttle forward the plate is as straight as it gets and will flow as much air as it can so as more air is sucked into the engine. Since the restriction is removed, the pistons aren't sucking so hard to move air so it deos and fuel flow increases as does engine speed up to the value the governor is set to. at this point a set of spinning flyweights in the Governor pull open a valve and run oil into the prop to increase pitch, which as you know will increase lift and drag, therefor load. That load is what keeps the RPM from increasing any further as it limits the amount of air that can flow. The engine can only make so much power flowing so much air so the RPM remains constant through maintaining a continuous load through various demand cycles of the airframe (climbing & diving for example).

As you see, all Constant Speed props are already automatic. Cirrus sets it up so the plane chooses for you though if that's what you mean. Single power lever does it all? No problem. Variable Pitch props are rare as hens teeth, but they exist. Those are manual.

PS, hell no. Spraying a field I'll adjust power to maintain delivery rate as my load decreases. As for operational maneuvers if I have to move the power, I screwed it up. Turbine Air Tractors get flaps in every turn though.
 
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Beg or rent a ride in a plane with a CS prop... Have the pilot show you how it works... 60 seconds in the plane is worth more than ten thousand words in a book...

denny-o
 
Dr. O said:
Beg or rent a ride in a plane with a CS prop... Have the pilot show you how it works... 60 seconds in the plane is worth more than ten thousand words in a book...

denny-o
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:rofl::rofl::rofl: True words there.
 
Let's go one step at a time. MP is a bit of misnomer, IMO, because in a non-turbo, there never is higher pressure in the intake manifold than atmospheric. So the MP is measuring atmospheric pressure when the engine is off and can be used as a rough altimeter even (1" per 1000' so would read 25" at 5000' pressure altitude). Once you start the engine, the engine acts as a vacuum pump against the throttle valve and generates partial vacuum in the manifold. If there were a perfect vacuum the MP would read 0" but there is not so it reads, what, 10" at idle? As you add power, you open the throttle and kill the vacuum more and more, causing the MP to rise. Under full open throttle, the MP would again approach the atmospheric pressure of 30" Hg.

So MP is an indication of how far open the throttle valve is and the load on the engine.
 
RPM is a function of the pitch of the prop. As you increase the pitch on the prop, it has to work harder. It generates more forward thrust at higher pitch so increasing the pitch is like shifting to a higher gear in your car. We take off at the lowest pitch (1st gear), a bit higher pitch (lower RPM) in climb (2nd gear) but we cruise at the highest pitch (3rd gear).
 
A few more items. To achieve the fastest airspeeds, the knob will again be pushed full forward, or perhaps slightly back. Where this differs from a road vehicle, is that will prop won't be at it's finest pitch, as it was for takeoff, even though the blue knob is full forward again.

It sometimes sounds confusing, but is really simple in general operation. For instance, on takeoff, I go full throttle & the blue knob full forward. Since my plane has some reserve power (RV6 with 180HP), I can pull the prop back a hundred rpm or so, for noise, and still easily climb.

I then have choices as I level out. I'll pull the prop back to 2300-2350 while throttle remains at full, if I want the speed. My engine prop combo has a limitation of short durations between 2000 & 2250 rpms. I don't like running below 2000, so I'll just keep it above 2250. If I want to fly slower, to stay with my slower friends, I'll just pull the power back, while leaving the prop at 2350 rpms. Should I want to climb, I'll slightly pull the stick back, to begin loading the engine, while pushing the prop knob forward (low pitch/high rpm)........then, I'll add power (MP).

To transition from cruise to landing, I can leave the prop knob at 2350. Shove it in too soon, and it's like shoving a car or motorcycle into low gear at cruise speed. The engine will over-rev, and you'll be thrown against the seat belts. I can actually enter the pattern at high speed, and the prop acts as a real good speed brake, even still set at 2350. I can even land that way.

However, the reason you push the knob full forward on final approach, is that you'll find a great loss of power...........should you do a touch & go. Hopefully, you'll think "woops", and shove the knob full foward. Like I said, shove it full forward too soon, and it's like shoving it into low gear at high speeds. That's why you wait, until the airplane has slowed down on final.

Some pilots actually leave the throttle (MP) at full, and use the rpms for airspeed. I never have preferred that method.

L.Adamson
 
Dr. O said:
Beg or rent a ride in a plane with a CS prop... Have the pilot show you how it works... 60 seconds in the plane is worth more than ten thousand words in a book...

denny-o
Click to expand...
:no: But you and Henning are truly made for each other in your bad advice.

Other books which describe this well include the Airplane Flying Handbook and Bob Gardner's Complete Advanced Pilot. After that, some training with w real CFI is appropriate.
 
Ron Levy said:
:no: But you and Henning are truly made for each other in your bad advice.

Other books which describe this well include the Airplane Flying Handbook and Bob Gardner's Complete Advanced Pilot. After that, some training with w real CFI is appropriate.
Click to expand...

Bad advice? Hardly. When I was instructing and students couldn't get the concept of MP and the constant-speed affair, I showed them how it worked in the airplane. Bingo: they got it.

Dan
 
Dan Thomas said:
Bad advice? Hardly. When I was instructing and students couldn't get the concept of MP and the constant-speed affair, I showed them how it worked in the airplane. Bingo: they got it.

Dan
Click to expand...


Exactly, everything I wrote I can demonstrate very clearly in a plane in under a minute with an extra minute pointing things out on preflight. Makes everything obvious when you watch the reactions happen in the plane. The caveat is that the person you beg or rent the ride from actually knows anything is only about 20% regardless if they are CFIs.
 
Unlike Ron, I think it'll click for you quickly when you see it in the airplane. In a fixed pitch you're looking at the tach mostly for power. You'll find a constant speed prop is actually easier to operate because you set a RPM and it holds it (with some exceptions) from there you'll be looking at manifold pressure as that is what will mostly be changing when you're moving the throttle.

A fixed pitch is annoying as the speed of the prop keeps accelerating once you level off causing you to have to tweak it as things balance out. CP is more of a set it and forget it operation.

At, oh say, 2500 RPM you can move the throttle around causing instantianous changes in manifold pressure and power output while the prop speed doesn't change what so ever. At some point though the CS prop won't hold 2500 rpm as ou really back the throttle out and will begin to slow. In most of the airplanes we fly it will hold 2500rpm all the way up to full power.

There are some combinations of MP and RPM settings that are hard on the engine. Refer to the POH.
 
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Then some azz is gonna show you how to reduce the power using the mixture handle without changing MP or RPM...:rofl::rofl::rofl:
 
Ron Levy said:
Start with John Deakin's excellent treatise on AvWeb.
http://www.avweb.com/news/pelican/182081-1.html on MP
http://www.avweb.com/news/pelican/182082-1.html on props
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Ron,

Thanks for the read. It will take me a couple of times to pick up everything in this article.

As an older pilot, I respond better to the written/illustration approach.

I have my High Performance endorsement and a B.S. in Mechanical Engineering. That doesn't mean I will just "automatically" understand CSP's by turning the knobs.

Again, thanks for the article links.

Terry
 
Terry said:
Ron,

Thanks for the read. It will take me a couple of times to pick up everything in this article.

As an older pilot, I respond better to the written/illustration approach.

I have my High Performance endorsement and a B.S. in Mechanical Engineering. That doesn't mean I will just "automatically" understand CSP's by turning the knobs.

Again, thanks for the article links.

Terry
Click to expand...

That's kinda scary.
 
Henning said:
That's kinda scary.
Click to expand...

Yes, I am a scary kind of guy. You should see me land. :rofl:

Henning,

I will explain.

Several years ago I checked out in an 182. I spent 3 days and three hours and was signed off with my HP endorsement.

I tried to rent the airplane a week later and it had been sold. I have not flown a HP airplane as PIC since then. I knew the procedure, what to do, but I did not totally understand CSP.

I am working toward getting my commercial and was wanting to read up on CSP's BEFORE I started my flying practice.

In all honesty, you guys need to lighten up and relax. :D

Terry
 
And are you old enough to remember the old "fuel economy" gauges?

gauge.jpg


Do you see the relationship?
 
alfadog said:
Seriously. :yesnod:

So are you clear now on what the MP gauge is measuring?
Click to expand...

Thanks for the support. :D

Yes, I understand the MP. However, at this time I am not able to explain or draw out the diagram of the CSP specifics.

I will master this in the next few weeks. By master, I mean try and be able to explain CSP's to an examiner where I can handle his questions.

Thanks,
Terry
 
Terry said:
Thanks for the support. :D

Yes, I understand the MP. However, at this time I am not able to explain or draw out the diagram of the CSP specifics.

I will master this in the next few weeks. By master, I mean try and be able to explain CSP's to an examiner where I can handle his questions.

Thanks,
Terry
Click to expand...

The prop lever sets and holds a specific RPM for the engine/prop. How does it do this? By varying the pitch until the given RPM is achieved. By varying the load on the engine. More pitch, more load = less RPM for a given throttle setting. That is why MP changes when you change RPM. Every time you change RPM, you are changing the load on the engine.

Remember always this is 1930's technology. It is mechanical. The governor is set and allows more/less engine oil (hydraulic fluid) into the pitch-changing mechanism until the governor is at the required speed.
 
alfadog said:
And are you old enough to remember the old "fuel economy" gauges?

gauge.jpg


Do you see the relationship?
Click to expand...

Yes, very informative.

I have only had my PPL since 2002. I am IR and 65 years old.

I am a refrigeration engineer by trade and handle large grocery stores. Our manifold pressure and vacuums, while basically the same, are worlds apart, in function and useability.

When using MP and vacuums to handle propeller pitch and RPM, this is all new to me.

On refrigeration principles, running compressors in a vacuum is bad. It is something we avoid. To change our performance we electronically change our cubic displacement. In all honesty, the higher pressure we can operate at the more efficiency we achieve.

Terry
 
Terry said:
Thanks for the support. :D

Yes, I understand the MP. However, at this time I am not able to explain or draw out the diagram of the CSP specifics.

I will master this in the next few weeks. By master, I mean try and be able to explain CSP's to an examiner where I can handle his questions.

Thanks,
Terry
Click to expand...

The governor is the exact same thing that Watt designed for throttling steam engines. You have a set of adjustable tension spring loaded fly weights that actuate a 2 directional spool valve. When the engine goes too fast, the weights go out further opening the pressure side of the spool valve pushing oil into the hub of the prop which has a piston. The propeller blades have a block at the circumference of the root the notches into the piston. As oil pushes the piston, it rotates the blade in the hub coarsening the pitch and adding load until the engine slows back down and the flyweights com back in to close the valve. As engine speed reduces, the fly weights are retracted by the spring and it opens the other side of the spool valve releasing oil pressure. When this happens, there is either a spring in the hub or counterweights on the prop blade that will drive the blades to fin/flat pitch until the engine speeds back up and the flyweights hit balance with the spring tension.

What the Blue Knob does is adjust the spring tension in the governor.

That clear you up?
 
Terry said:
Thanks for the support. :D

Yes, I understand the MP. However, at this time I am not able to explain or draw out the diagram of the CSP specifics.

I will master this in the next few weeks. By master, I mean try and be able to explain CSP's to an examiner where I can handle his questions.

Thanks,
Terry
Click to expand...

OK, you are an ME. Without looking, I would guess that the governor is connected to a pressure regulating valve that controls the engine oil pressure that is allowed to reach the prop pitch mechanism. If the governor is underspeed, it decreases the pressure, pitch decreases, the RPMs rise; if overspeed, the opposite.
 
alfadog said:
OK, you are an ME. Without looking, I would guess that the governor is connected to a pressure regulating valve that controls the engine oil pressure that is allowed to reach the prop pitch mechanism. If the governor is underspeed, it decreases the pressure, pitch decreases, the RPMs rise; if overspeed, the opposite.
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That isn't mastery though, that is primmer level.
 
Terry said:
When using MP and vacuums to handle propeller pitch and RPM, this is all new to me.
Click to expand...

MP is simply an indication of how much power the engine is producing. Do not mix it up too much with RPM. They are different. Yes, you use them in tandem just like you use the throttle and transmission of your car to regulate your speed but they are not connected systems. They do influence one another and you should understand how.

For example, you set 2400 RPM and 24" MP, what happens to RPM as you gradually reduce power (i.e. MP) to idle? Why?
 
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Terry. If you have one cylinder volume of air-fuel mix at 30" MP, and one cylinder volume of air-fuel mix at 15" MP, are there not twice as many molecules about to be combusted in the first situation?

So is not the power output going to be significantly higher in siutation #1, as in (not quite but near) twofold?

View the MP as a measure of "how much input" you are pushing through the engine. measure. (Remember PV=nRT?)
 
bbchien said:
Terry. If you have one cylinder volume of air-fuel mix at 30" MP, and one cylinder volume of air-fuel mix at 15" MP, are there not twice as many molecules about to be combusted in the first situation?

So is not the power output going to be significantly higher in siutation #1, as in (not quite but near) twofold?

View the MP as a measure of "how much input" you are pushing through the engine. measure. (Remember PV=nRT?)
Click to expand...

Yes and no. If you have FADEC it's accurate.
 
jesse said:
A fixed pitch is annoying as the speed of the prop keeps accelerating once you level off causing you to have to tweak it as things balance out.
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That's caused by a training deficiency. A pilot should be taught the A-P-T technique. Attitude-Power-Trim: Attitude first (level off to cruise attitude), leave the throttle alone until cruise speed is reached, then set the throttle to cruise RPM (while holding the controls so the nose stays put), then set the trim to remove the pressure. If this is done in the right sequence and isn't rushed, the airplane won't need throttle or trim adjustments after that.

Chapters 6, 7 and 8 of the Canadian Flight Instructor Guide:
http://www.tc.gc.ca/eng/civilaviation/publications/tp975-partii-menu-5329.htm

I imagine the FAA has something similar.

Violating the APT (or PAT for descents) is one of the most common deficiencies in the flight training world.

Dan
 
alfadog said:
And are you old enough to remember the old "fuel economy" gauges?

gauge.jpg


Do you see the relationship?
Click to expand...

That is, however a vacuum gauge, it measures the reverse of absolute manifold pressure. High numbers = high vacuum = low manifold pressure.

It's also a bit misleading, it just assumes low power = good economy which is generally true for a carbureted automobile, but all piston engine are most efficient at wide open throttle. In airplane terms, a high MP coupled with a low fuel flow (lean of peak condition) is far more efficient than the converse for the same power output.
 
Sac Arrow said:
That is, however a vacuum gauge, it measures the reverse of absolute manifold pressure. High numbers = high vacuum = low manifold pressure.

It's also a bit misleading, it just assumes low power = good economy which is generally true for a carbureted automobile, but all piston engine are most efficient at wide open throttle. In airplane terms, a high MP coupled with a low fuel flow (lean of peak condition) is far more efficient than the converse for the same power output.
Click to expand...

That's also the reservation I had with Bruce's statement. There is another handle on most planes that throws a monkey wrench into things. Airspeed is actually the best indicator of power being made, second is fuel flow. MP is just one piece of information.
 
Henning said:
That's also the reservation I had with Bruce's statement. There is another handle on most planes that throws a monkey wrench into things. Airspeed is actually the best indicator of power being made, second is fuel flow. MP is just one piece of information.
Click to expand...

Agreed. Power charts for CS engines (% power for various RPM and manifold pressure combinations) assume a specific leaning condition, such as best power or best ROP economy.
 
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