Propeller theory
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Propeller theory
I would be grateful if someone could clarify some terminology used with propellers. I have found some conflicting descriptions. All the references that I have found concur that the flat side of the propeller (facing the engine) is the “Blade face” and the cambered section (facing the airflow in flight) is the “Blade back”
My problem is that some references talk about a “pressure surface” being the high pressure area acting on the “blade face” – and referred in 2 cases as the “thrust surface” Not one of these referred to it as the “thrust face”.
Other references talk about the “thrust face” as being the cambered surface of the blade, unfortunately these references did not mention anything about the “flat side”
Is this just an interpretation problem “pressure surface” and “thrust surface” as compared to the “thrust face” or is the “blade face” (flat side) called the “pressure surface / thrust surface” and the “blade back” (cambered side) called the “thrust face”
If anyone can help me I would be most grateful if you give me an authoritative reference. I have even contacted four propeller manufacturers with negative results.
My problem is that some references talk about a “pressure surface” being the high pressure area acting on the “blade face” – and referred in 2 cases as the “thrust surface” Not one of these referred to it as the “thrust face”.
Other references talk about the “thrust face” as being the cambered surface of the blade, unfortunately these references did not mention anything about the “flat side”
Is this just an interpretation problem “pressure surface” and “thrust surface” as compared to the “thrust face” or is the “blade face” (flat side) called the “pressure surface / thrust surface” and the “blade back” (cambered side) called the “thrust face”
If anyone can help me I would be most grateful if you give me an authoritative reference. I have even contacted four propeller manufacturers with negative results.
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Hi,
I have found it much more clear to refer to the propellor with regard to it's 'blade angle', (at it's reference point), which is usually pretty close to zero, mostly a little negative, because of the blade twist, when the prop is stopped, prior to ground starting.
That's assuming the prop is gear driven, not free turbine driven.
And is at 90 deg, or pretty close to it, when it's feathered.
That would make the cambered side normally forward facing!
Hope I haven't confused you (or me).
Cheers.
I have found it much more clear to refer to the propellor with regard to it's 'blade angle', (at it's reference point), which is usually pretty close to zero, mostly a little negative, because of the blade twist, when the prop is stopped, prior to ground starting.
That's assuming the prop is gear driven, not free turbine driven.
And is at 90 deg, or pretty close to it, when it's feathered.
That would make the cambered side normally forward facing!
Hope I haven't confused you (or me).
Cheers.
I'll try another way and hope it doesn't get too far afield.
In the gas turbines that use giant fan blades (ala propellors with many blades) they are both highly cambered and highly twisted to accomodate the significant variations in surface speed beween their root or hub mounting location and their tip or duct case location.
Where the blade surface speed operates near supersonic at the tip the blade is mostly flat faced on either side. If you look real close you can make out an airfoil like surface, however it's thicker toward the trailing edge than at the leading edge.
When you get to the inner hub or attachment location, the blade has a high degree of camber (curvature) and its cross section is thicker towards the leading edge vs the trailing edge.
The camber or curvature has a convex (outer side) towards the low pressure or entrance air and a concave or inner side towards the high pressure or thrust side..
With propellors that reverse by changing the twist angle, I would suspect that you would want less camber to effectively work in either drection.
Anybody else feel free to jump in.
In the gas turbines that use giant fan blades (ala propellors with many blades) they are both highly cambered and highly twisted to accomodate the significant variations in surface speed beween their root or hub mounting location and their tip or duct case location.
Where the blade surface speed operates near supersonic at the tip the blade is mostly flat faced on either side. If you look real close you can make out an airfoil like surface, however it's thicker toward the trailing edge than at the leading edge.
When you get to the inner hub or attachment location, the blade has a high degree of camber (curvature) and its cross section is thicker towards the leading edge vs the trailing edge.
The camber or curvature has a convex (outer side) towards the low pressure or entrance air and a concave or inner side towards the high pressure or thrust side..
With propellors that reverse by changing the twist angle, I would suspect that you would want less camber to effectively work in either drection.
Anybody else feel free to jump in.
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Yes, got to agree with the previous post.
Look, take my advice, go into the kitchen open a fresh pack of mince pies, pour yourself a large brandy, put on a paper christmas hat and watch wizard of oz (again)
ITS CHRISTMAS!!! worry about propeller theory next week.
Look, take my advice, go into the kitchen open a fresh pack of mince pies, pour yourself a large brandy, put on a paper christmas hat and watch wizard of oz (again)
ITS CHRISTMAS!!! worry about propeller theory next week.
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For the interest of those who require...Please click the following link.
Shame on those who cannot be sensible in this informative forum.
http://www.thaitechnics.com/propeller/prop_intro.html
Smooth skies,
Dan
Shame on those who cannot be sensible in this informative forum.
http://www.thaitechnics.com/propeller/prop_intro.html
Smooth skies,
Dan
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valid question!!!
i am preparing for ATPL principles of flight and performance first week of Jan and the practice tests asked that question.
problem is Oxford says one thing and Bristol says just the opposite.
i emailed Bristol and they said they will get back with me on that one. they saw the conflict as well.
can't help you at all, but i am there waiting with you. i tried the thai technic but it wouldn't fly for me.
chairs
problem is Oxford says one thing and Bristol says just the opposite.
i emailed Bristol and they said they will get back with me on that one. they saw the conflict as well.
can't help you at all, but i am there waiting with you. i tried the thai technic but it wouldn't fly for me.
chairs
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Thanks to those that have tried to help. I can not comment on whether it is important or not, but as it appears both in the CAA aircraft technical and ATPL syllabi, it would be convenient to have a technically correct answer.
I have found two references that agree with Bristol’s version, one of them being Thai technics, but again are they authorative? It really does seem to be in the wording at this point, with the “pressure face” being the “back” or flat side of the propeller and the “thrust face” being the “front” or cambered side.
Still searching !!!!
I have found two references that agree with Bristol’s version, one of them being Thai technics, but again are they authorative? It really does seem to be in the wording at this point, with the “pressure face” being the “back” or flat side of the propeller and the “thrust face” being the “front” or cambered side.
Still searching !!!!
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Answer
Take this as the absolute answer, this is directly from many text books over here in Australia (including those from Aviation Theory Centre, Bob Tait, other various books, and even on the internet...)
It's a pity I can't draw a diagram here for you, I get this question from my students all the time.
Put simply...
The Blade Back is the most curved part of the propellors aerofoil that you can't see when sitting in the cockpit of a single engined aircraft.
the Blade Face is the flate side that Faces towards you.
Now heres a real hard question about propellor theory...
What is the Experimental Pitch of a propellor?
It's a pity I can't draw a diagram here for you, I get this question from my students all the time.
Put simply...
The Blade Back is the most curved part of the propellors aerofoil that you can't see when sitting in the cockpit of a single engined aircraft.
the Blade Face is the flate side that Faces towards you.
Now heres a real hard question about propellor theory...
What is the Experimental Pitch of a propellor?
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Experimental pitch is the pitch (rough enough) measured to the zero-lift line of the airfoil. Propellers with the same experimental pitch will behave the same with respect to pitch, irrespective of the airfoil shape. This is because all airfoils produce about the same amount of lift for each increment in angle of attack: Hence if you have more experimental pitch, that propeller will want to go faster.
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Thanks again. I concur with the definitions of the “Blade face” and “Blade back” But I’m still struggling with the “Thrust face” against the “Pressure face”
While “Geometric pitch” is defined as the theoretical distance that a propeller should move forward in one revolution without slip under conditions of 0° angle of attack, “Experimental pitch” is the theoretical distance that a propeller should move forward in one revolution without slip under conditions of zero lift.
While “Geometric pitch” is defined as the theoretical distance that a propeller should move forward in one revolution without slip under conditions of 0° angle of attack, “Experimental pitch” is the theoretical distance that a propeller should move forward in one revolution without slip under conditions of zero lift.
