CS principle, please explain
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CS principle, please explain
Hello guys
I am an instructor but did some reading and it kind of made me uncertain as to what is true and what really happens when flying with a CS prop.
Tried to post my question in a different forum, but no one really gave it a shot so I decided to take it to a higher level = You guys
Ace the Pilot Techical Interview's definition, and CS in general:
Regarding CS/VP propeller he gives the following statement, of course taken from a larger context:
"A variable-pitch propeller maximizes the propeller's efficiency through a large speed range by maintaining a constant blade angle of attack that thereby produces a constant thrust value."
In a sentence above this one, he states the complete opposite using almost the same words.
What are your thoughts on that statement?
- My own would be that a CS prop changes its propeller PITCH constantly to maintain a given revoultion setting, correcting either for increased/decreased airspeed or MP.
- I just started wondering if the statement was correct in the sense that when the factors airspeed or MP are changed, in effect the propeller AOA also changes, and to maintain a given RPM setting it changes it's pitch to maintain that AOA creating the same amount of propeller drag giving the same resistance to maintain the set number of revs.
I am an instructor but did some reading and it kind of made me uncertain as to what is true and what really happens when flying with a CS prop.
Tried to post my question in a different forum, but no one really gave it a shot so I decided to take it to a higher level = You guys
Ace the Pilot Techical Interview's definition, and CS in general:
Regarding CS/VP propeller he gives the following statement, of course taken from a larger context:
"A variable-pitch propeller maximizes the propeller's efficiency through a large speed range by maintaining a constant blade angle of attack that thereby produces a constant thrust value."
In a sentence above this one, he states the complete opposite using almost the same words.
What are your thoughts on that statement?
- My own would be that a CS prop changes its propeller PITCH constantly to maintain a given revoultion setting, correcting either for increased/decreased airspeed or MP.
- I just started wondering if the statement was correct in the sense that when the factors airspeed or MP are changed, in effect the propeller AOA also changes, and to maintain a given RPM setting it changes it's pitch to maintain that AOA creating the same amount of propeller drag giving the same resistance to maintain the set number of revs.
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Think of a fixed pitch prop. As airspeed increases the AOA of the blades to the relative airflow increases. Consequently, you will have a particular airspeed whereby the optimum AoA is achieved (just like a wing). If you are above/below this airspeed the blade AoA is not at the optimum value.
By adjusting the pitch (in essence selecting a constant RPM) you are ensuring the a constant AoA to the relative airflow and therefore best Lift/Drag ratio for the propellor.
By adjusting the pitch (in essence selecting a constant RPM) you are ensuring the a constant AoA to the relative airflow and therefore best Lift/Drag ratio for the propellor.
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Ok, so my though about constant AOA was generally correct?
But increased airspeed would, the way I see it, decrease - not increase prop AOA if uncorrected by govenor?
What about increased/decreased MP? Wouldn't an increase in MP in an essence be an attempt to increase propeller revolutions - which is then maintained constant by govenpor changing pitch/maintaining AOA?
If I increase prop revoultion I increase AOA of propeller if uncorrected by govenor, correct? So in this case the govenor would change to a finer pitch to maintain same AOA?
But increased airspeed would, the way I see it, decrease - not increase prop AOA if uncorrected by govenor?
What about increased/decreased MP? Wouldn't an increase in MP in an essence be an attempt to increase propeller revolutions - which is then maintained constant by govenpor changing pitch/maintaining AOA?
If I increase prop revoultion I increase AOA of propeller if uncorrected by govenor, correct? So in this case the govenor would change to a finer pitch to maintain same AOA?
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Ok, thats how I understand this:
Pitch angle or Blade angle = [helix angle (TAS vs Rotational Speed)]+AoA
Helix angle increases if TAS increases or RS decreases.
Helix angle decreases if TAS decreases or RS increases.
"My own would be that a CS prop changes its propeller PITCH constantly to maintain a given revoultion setting, correcting either for increased/decreased airspeed or MP". TRUE.
CSP tries to maintain the RPM, it is done by means of increase or decrease the Torque (Which is the cool name for Prop Drag, and Thrust is Lift in that analogy...), Torque increases with AoA.
....
"What about increased/decreased MP? Wouldn't an increase in MP in an essence be an attempt to increase propeller revolutions - which is then maintained constant by govenpor changing pitch/maintaining AOA?"
MP= Manifold Pressure? I suppose.
True Again.
More MP means more Power then the force leading the prop will be increased as a result an acceletarion will be there ( Unbalanced force means acc), to avoid this acc the governor increases the AoA (Setting a "Coarser" pitch)increasing the Torque (Drag) hence RPMs are maintained.
My two cents.
Pitch angle or Blade angle = [helix angle (TAS vs Rotational Speed)]+AoA
Helix angle increases if TAS increases or RS decreases.
Helix angle decreases if TAS decreases or RS increases.
"My own would be that a CS prop changes its propeller PITCH constantly to maintain a given revoultion setting, correcting either for increased/decreased airspeed or MP". TRUE.
CSP tries to maintain the RPM, it is done by means of increase or decrease the Torque (Which is the cool name for Prop Drag, and Thrust is Lift in that analogy...), Torque increases with AoA.
....
"What about increased/decreased MP? Wouldn't an increase in MP in an essence be an attempt to increase propeller revolutions - which is then maintained constant by govenpor changing pitch/maintaining AOA?"
MP= Manifold Pressure? I suppose.
True Again.
More MP means more Power then the force leading the prop will be increased as a result an acceletarion will be there ( Unbalanced force means acc), to avoid this acc the governor increases the AoA (Setting a "Coarser" pitch)increasing the Torque (Drag) hence RPMs are maintained.
My two cents.
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The book mentioned by the OP is the most awful load of rubbish imaginable, incompetently written and full of awful screaming errors. Sadly, the editors didn't bother to get the thing proof-read, and now pilots around the world find themselves wasting money on it, and filling their heads with rubbish. Some have the insight to get confused and post questions like the one above...
Someone's planning a book-burning tomorrow. Perhaps we could organise a separate day on which everyone can put their copies of this shocking waste of paper to the flame?
Someone's planning a book-burning tomorrow. Perhaps we could organise a separate day on which everyone can put their copies of this shocking waste of paper to the flame?
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Regarding CS/VP propeller he gives the following statement, of course taken from a larger context:
"A variable-pitch propeller maximizes the propeller's efficiency through a large speed range by maintaining a constant blade angle of attack that thereby produces a constant thrust value."
In a sentence above this one, he states the complete opposite using almost the same words.
What are your thoughts on that statement?
