Constant Speed Propellors
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Constant Speed Propellors
Can anyone explain the intricacies of variable pitch and constant speed propellers to me?
I am only used to the simplicities of a fixed pitch propeller, where thrust is linked to RPM via the throttle - pretty easy to follow .
I can just about grasp the idea of a variable pitch propeller (increased angle of attack gives greater "forward lift" (=thrust), and presumably implies a reduced RPM, which decreases the fuel flow (i.e. like a high gear in a car?).
Now, if we have a constant speed propeller, what is keeping it at constant speed, and how do the throttle and propeller pitch come into the game?
In my simple mind, pitch X RPM = thrust, so for a given thrust we can either play with the pitch or the RPM. If the propeller speed is constant then we can only play with the pitch, so what is the throttle doing?
I am only used to the simplicities of a fixed pitch propeller, where thrust is linked to RPM via the throttle - pretty easy to follow .
I can just about grasp the idea of a variable pitch propeller (increased angle of attack gives greater "forward lift" (=thrust), and presumably implies a reduced RPM, which decreases the fuel flow (i.e. like a high gear in a car?).
Now, if we have a constant speed propeller, what is keeping it at constant speed, and how do the throttle and propeller pitch come into the game?
In my simple mind, pitch X RPM = thrust, so for a given thrust we can either play with the pitch or the RPM. If the propeller speed is constant then we can only play with the pitch, so what is the throttle doing?
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Hi there GroundBound,
First the caveats: I converted from Piper Warrior to VP version of it, the Piper Dakota (PA28-236) earlier this year, so I think I can answer you in parctical terms - albeit with only recent experience. No doubt someone more experienced than me can make a better technical explanation, but here is how I see it:
The VP mechanism is a governor, usually driven by engine oil pressure, which basically varies the pitch of the prop and senses the resultant speed, aiming to keep it constant. Its an analogue feedback loop, in engineering terms.
The throttle works the same as before - more power = go faster! But because the governor is keepin the prop at a constant speed, the RPM indication doesn't change thereffore doesn't help if you want to set a precise setting. Thats why we have the additional Manifold Pressure gauge in VP aircraft.
On a VP aircraft, the MP gauge provides an indication of "how much throttle" you have selected. So, 24" MP might be the cruise setting, 20" might be selected for decent, and 17" for the circuit say. If it were fitted to fixed pitch prop a/.c it would do the same, but is unnecessary.
You have the prop lever adjustment as well in order to set various prop. speed settings. The a/c manual (POH) will have a table (also printed behind the sun visor on the Dakota) with recommended combinations of MP/RPM settings for various presure altitudes and speeds to set a given % power. Some settings you commit to memory for the type you are flying, e.g. maybe "24/24" for the cruise - which would be 24" MP and 2400 RPM. On the Dak at our club, we use "24/22", i.e. 24"MP and 2200 RPM, as it gives a bit less noise for the neighbours and prolongs engine life a bit.
Its not as difficult as it sounds. I do remember finding it awfully confusing when reading about it - but once flown using VP, it all becomes clear and is not really difficult. I recommend you get some conversion training with a friendly instructor on a VP type, and then you'll see for yourself.
Most of all, what I like about VP props is that the a/c performs better - faster ROC, faster cruise
Hope this helps,
Andy
First the caveats: I converted from Piper Warrior to VP version of it, the Piper Dakota (PA28-236) earlier this year, so I think I can answer you in parctical terms - albeit with only recent experience. No doubt someone more experienced than me can make a better technical explanation, but here is how I see it:
The VP mechanism is a governor, usually driven by engine oil pressure, which basically varies the pitch of the prop and senses the resultant speed, aiming to keep it constant. Its an analogue feedback loop, in engineering terms.
The throttle works the same as before - more power = go faster! But because the governor is keepin the prop at a constant speed, the RPM indication doesn't change thereffore doesn't help if you want to set a precise setting. Thats why we have the additional Manifold Pressure gauge in VP aircraft.
On a VP aircraft, the MP gauge provides an indication of "how much throttle" you have selected. So, 24" MP might be the cruise setting, 20" might be selected for decent, and 17" for the circuit say. If it were fitted to fixed pitch prop a/.c it would do the same, but is unnecessary.
You have the prop lever adjustment as well in order to set various prop. speed settings. The a/c manual (POH) will have a table (also printed behind the sun visor on the Dakota) with recommended combinations of MP/RPM settings for various presure altitudes and speeds to set a given % power. Some settings you commit to memory for the type you are flying, e.g. maybe "24/24" for the cruise - which would be 24" MP and 2400 RPM. On the Dak at our club, we use "24/22", i.e. 24"MP and 2200 RPM, as it gives a bit less noise for the neighbours and prolongs engine life a bit.
Its not as difficult as it sounds. I do remember finding it awfully confusing when reading about it - but once flown using VP, it all becomes clear and is not really difficult. I recommend you get some conversion training with a friendly instructor on a VP type, and then you'll see for yourself.
Most of all, what I like about VP props is that the a/c performs better - faster ROC, faster cruise
Hope this helps,
Andy
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AA
Many thanks - that helps (a bit)
If I follow your explanation, increasing the throttle increases the power. However, the RPM is constrained to a constant speed by the governor - which does so by varying the propellor pitch? That seems reasonable. Errr.... what's the pitch lever for then?
I am just trying to build up some post PPL hours and haven't yet had a chance to "move up a gear" - perhaps in a few months (or so)
Thanks again.
Many thanks - that helps (a bit)
If I follow your explanation, increasing the throttle increases the power. However, the RPM is constrained to a constant speed by the governor - which does so by varying the propellor pitch? That seems reasonable. Errr.... what's the pitch lever for then?
I am just trying to build up some post PPL hours and haven't yet had a chance to "move up a gear" - perhaps in a few months (or so)
Thanks again.
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Hi GroundBound
The pitch lever is used to set the RPM you want. Example: you are crusing at 24" MP/2400 RPM and you decide that you want to reduce power to 20" MP/2000 RPM, then you would:
There is a maxim to remember: "Rev Up, Throttle Back" i.e. when reducing power, the throttle goes back first. Conversely, when increasing power, the picth lever goes up first. So, to increase power from "20/2000" back to "24/2400" you'd:
Obviously, this is just an example and you should always check the POH!
Hope this helps,
Andy
Errr.... what's the pitch lever for then?
- Use the throttle to reduce power so that MP indication reduces from 24" to 20"
- Now use the pitch lever to reduce the RPM from 2400 to 2000... if you don't, it will stay at 2400 RPM all by itself...
There is a maxim to remember: "Rev Up, Throttle Back" i.e. when reducing power, the throttle goes back first. Conversely, when increasing power, the picth lever goes up first. So, to increase power from "20/2000" back to "24/2400" you'd:
- Use pitch lever to increase RPM to 2400...
- Then use throttle to increase power back to 24" MP
Obviously, this is just an example and you should always check the POH!
Hope this helps,
Andy
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Err, I think I hear the sound of a fresh can of worms being opened here.
Please don't think of VP props as 'like gears on a car' because they are not.
The reason for having a VP prop (not all are constant speed) is to use the power produced by an engine as efficiently as possible; ergo, the RPM lever (for a constant speed prop) is a 'command lever' for the engine speed.
If high thrust is required then a high RPM is selected to realise that thrust and the govenor will regulate the pitch to achieve that RPM. Statically, this will result in the prop being quite close to it's fine pitch stops but will progressively move courser as the aircraft accelerates.
An engine will only develop it's rated horsepower when turning at Redline RPM, therefore in a fixed pitch machine at take off the amount of power available will typically be less than full HP. Static thrust is equivalent to the cube of the RPM therefore you will be missing out on a great deal more thrust than just plain HP.
When cruising the POH will provide values for RPM and manifold pressure (MP is NOT a measure of throttle opening but is roughly equivalent to 'load') which will provide a good balance between engine efficiency and propellor efficiency. Since drag increases to the square of the speed but thrust reduces in a cubic relationship the aerofoil of the prop must reside at a happy medium between thrashing around creating too much drag and turning so slowly as to produce too little power.
The constant speed bit is exactly as described - a governor for making life easier and more comfortable. There are two main arrangements. One uses counterweights and the other uses a dual acting oleopiston in the propellor hub. Both essentially do the same thing by 'sensing' an over or underspeed condition as it relates to the lever position and correcting the blade angle to compensate.
It's all a bit long winded but I am available for full CS/VP type conversions if interested.
I use the maxim 'Fine First' - i.e. a power setting requiring an increased RPM should start with the RPM lever being advanced first and vv.
Cheers all
Please don't think of VP props as 'like gears on a car' because they are not.
The reason for having a VP prop (not all are constant speed) is to use the power produced by an engine as efficiently as possible; ergo, the RPM lever (for a constant speed prop) is a 'command lever' for the engine speed.
If high thrust is required then a high RPM is selected to realise that thrust and the govenor will regulate the pitch to achieve that RPM. Statically, this will result in the prop being quite close to it's fine pitch stops but will progressively move courser as the aircraft accelerates.
An engine will only develop it's rated horsepower when turning at Redline RPM, therefore in a fixed pitch machine at take off the amount of power available will typically be less than full HP. Static thrust is equivalent to the cube of the RPM therefore you will be missing out on a great deal more thrust than just plain HP.
When cruising the POH will provide values for RPM and manifold pressure (MP is NOT a measure of throttle opening but is roughly equivalent to 'load') which will provide a good balance between engine efficiency and propellor efficiency. Since drag increases to the square of the speed but thrust reduces in a cubic relationship the aerofoil of the prop must reside at a happy medium between thrashing around creating too much drag and turning so slowly as to produce too little power.
The constant speed bit is exactly as described - a governor for making life easier and more comfortable. There are two main arrangements. One uses counterweights and the other uses a dual acting oleopiston in the propellor hub. Both essentially do the same thing by 'sensing' an over or underspeed condition as it relates to the lever position and correcting the blade angle to compensate.
It's all a bit long winded but I am available for full CS/VP type conversions if interested.
I use the maxim 'Fine First' - i.e. a power setting requiring an increased RPM should start with the RPM lever being advanced first and vv.
Cheers all
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M14P: I thought someone with more experience would know more about it than me I bow to your technical explanation - hopefully my description of what I think when I actually fly is helpful too 
Andy
I bow to your technical explanation - hopefully my description of what I think when I actually fly is helpful too Andy
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For the intracacies of how the governer works check out an article by John Deakin on AVWEB on those marvelous props
He;s also got a good explanation of what the MP gauge is actually measuring
-me
He;s also got a good explanation of what the MP gauge is actually measuring
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No worries AA - There's actually quite a lot of old balls talked about CS/VP installations like the 'oversquare' myth and all sorts of bizarre wives tales.
As I keep preaching, read and inwardly digest the POH or, failing that, corner the local mechanic in the bar and ask him some questions to find out how the thing works. Each type is different and each is worthy of a read up to avoid poor operating practise or - in extremis - damage to the engine / prop.
Didn't mean to get all snappy n'all!
M14P
As I keep preaching, read and inwardly digest the POH or, failing that, corner the local mechanic in the bar and ask him some questions to find out how the thing works. Each type is different and each is worthy of a read up to avoid poor operating practise or - in extremis - damage to the engine / prop.
Didn't mean to get all snappy n'all!
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M14P
Glad you bought that up cause the old saying of never go over square i.e. 24” / 2400rpm is a load of bollocks!
I have an aircraft powered by an IO540 and at 2000’ 2200 rpm and 26” is a book value, which I like to use as the engine is turning over at a slower rate and not wearing out as quick!
So the long and short is to look at the book, it will tell you what to run!
NAP
Glad you bought that up cause the old saying of never go over square i.e. 24” / 2400rpm is a load of bollocks!
I have an aircraft powered by an IO540 and at 2000’ 2200 rpm and 26” is a book value, which I like to use as the engine is turning over at a slower rate and not wearing out as quick!
So the long and short is to look at the book, it will tell you what to run!
NAP
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Couple of points about CS props in an attempt to simplify things, excuse me if I repeat something already said.
The reason that CS props give better efficiency than fixed pitch or variable speed props is that they allow the blades to work at their optimum angle of attack (about 4 degrees) over a wide airspeed range.
The angle of attack of a fixed-pitch prop is of course greatly affected by the forward airspeed of the aircraft and its pitch angle is by definition a compromise of design. A fine pitch angle is needed for low IAS and a coarse pitch for high IAS. A fine pitch prop will give good initial acceleration and climb but is no good for economical high speed cruise. Conversely, a coarse pitch prop is no good for acceleration and climb but gives a good cruise performance.
This is reputedly one reason why the early Schneider (high speed) Trophy aircraft were seaplanes. This was before the days of the CS prop. These aircraft needed a very coarse prop setting to get to the high IASs required to win the prize which meant at low speeds the prop was very inefficient. The aircraft accelerated poorly from rest and consequently needed a very long takeoff run. As airfields were quite small in those days, a water takeoff offered a greater distance.
The reason for the rule "rev up / throttle back" is to prevent mixture detonation caused by so-called "overboosting", or too much MAP (too much mixture charge allowed into the cylinders) at too small an engine RPM. Detonation will of course cause piston damage and possible engine mechanical failure in a short time. How much MAP the engine can stand at any given RPM depends on its individual characteristics so the POH certainly is the only authoritative advice to follow.
Hope this helps.
The reason that CS props give better efficiency than fixed pitch or variable speed props is that they allow the blades to work at their optimum angle of attack (about 4 degrees) over a wide airspeed range.
The angle of attack of a fixed-pitch prop is of course greatly affected by the forward airspeed of the aircraft and its pitch angle is by definition a compromise of design. A fine pitch angle is needed for low IAS and a coarse pitch for high IAS. A fine pitch prop will give good initial acceleration and climb but is no good for economical high speed cruise. Conversely, a coarse pitch prop is no good for acceleration and climb but gives a good cruise performance.
This is reputedly one reason why the early Schneider (high speed) Trophy aircraft were seaplanes. This was before the days of the CS prop. These aircraft needed a very coarse prop setting to get to the high IASs required to win the prize which meant at low speeds the prop was very inefficient. The aircraft accelerated poorly from rest and consequently needed a very long takeoff run. As airfields were quite small in those days, a water takeoff offered a greater distance.
The reason for the rule "rev up / throttle back" is to prevent mixture detonation caused by so-called "overboosting", or too much MAP (too much mixture charge allowed into the cylinders) at too small an engine RPM. Detonation will of course cause piston damage and possible engine mechanical failure in a short time. How much MAP the engine can stand at any given RPM depends on its individual characteristics so the POH certainly is the only authoritative advice to follow.
Hope this helps.
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Yes you are correct. But that was because the long runway was so wet.
