Propeller blade angles
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Propeller blade angles
Greetings! May be all the prop masters can give me a feedback on this one.
I am trying to understand the Propeller rather gas turbine or piston - Constant speed propeller positions relative to the aircraft's phases.
I have just drawn a diagram (my illustrator skills rock!) with the position of the propeller blade in different phases of the aircraft.
please refer the image shown or
pls click http://flightlevel350.tumblr.com/image/36592655596 if not displayed
Simply as at a glance this looks like an alien spider from Andromeda, let me give a small insight!
The Blade airfoil shows 5 positions of the propeller. it is simply straight fwd as they are labeled. POR - Plane of Rotation and the propeller blade is moving down shown by the arrow under the airfoil.
The red arc is the Throttle positions
The Blue arc is the relative airflow
Now going one-by-one as numbered...
No1. Now, if I start the engines, the initial position of the blade is parallel to the airflow of rotation of the blade which directly meet the leading edge from the ground arrow at the bottom. Since there is no AOA, it does not produce any thrust.
No2. Now when taxiing, you set the pitch little more up (Ground Fine) so that the little increase AOA produce a little thrust fwd. Now at the take off run since because the aircraft is now moving fwd the resultant airflow will move towards up-left along the blue arc. So you move the blade angle further more to compensate the best AOA.
No.3 Now at the cruise aircraft at it's cruise speed and meet the maximum continuous airflow from the front hence the resultant relative airflow will move further up along the blue arc. To compensate this the blade angle is increased more to Coarse.
No.4 Now, boom! during in-flight engine shutdown! when the engine is shutdown the props stop rotating so there is no prop rotation induced airflow from the bottom, so only airflow is from the front, so to avoid windmilling the blade is set parallel to the incoming airflow, no AOA, hence no thrust no drag (well less drag!)
No.5 Reverse thrust!
Is this diagram perfect enough to understand the operation of a Constant speed propeller at a glance? or any suggestions or corrections?
Thanks and sorry for the lengthy post!
Cheers
Code0
I am trying to understand the Propeller rather gas turbine or piston - Constant speed propeller positions relative to the aircraft's phases.
I have just drawn a diagram (my illustrator skills rock!) with the position of the propeller blade in different phases of the aircraft.
please refer the image shown or
pls click http://flightlevel350.tumblr.com/image/36592655596 if not displayed
Simply as at a glance this looks like an alien spider from Andromeda, let me give a small insight!
The Blade airfoil shows 5 positions of the propeller. it is simply straight fwd as they are labeled. POR - Plane of Rotation and the propeller blade is moving down shown by the arrow under the airfoil.
The red arc is the Throttle positions
The Blue arc is the relative airflow
Now going one-by-one as numbered...
No1. Now, if I start the engines, the initial position of the blade is parallel to the airflow of rotation of the blade which directly meet the leading edge from the ground arrow at the bottom. Since there is no AOA, it does not produce any thrust.
No2. Now when taxiing, you set the pitch little more up (Ground Fine) so that the little increase AOA produce a little thrust fwd. Now at the take off run since because the aircraft is now moving fwd the resultant airflow will move towards up-left along the blue arc. So you move the blade angle further more to compensate the best AOA.
No.3 Now at the cruise aircraft at it's cruise speed and meet the maximum continuous airflow from the front hence the resultant relative airflow will move further up along the blue arc. To compensate this the blade angle is increased more to Coarse.
No.4 Now, boom! during in-flight engine shutdown! when the engine is shutdown the props stop rotating so there is no prop rotation induced airflow from the bottom, so only airflow is from the front, so to avoid windmilling the blade is set parallel to the incoming airflow, no AOA, hence no thrust no drag (well less drag!)
No.5 Reverse thrust!
Is this diagram perfect enough to understand the operation of a Constant speed propeller at a glance? or any suggestions or corrections?
Thanks and sorry for the lengthy post!
Cheers
Code0
Last edited by code0; 26th Nov 2012 at 16:11.
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May I suggest that the overall diagram be labeled for a single-shaft turboprop (T56, 250, TPE331 etc.) as opposed to a free-turbine two-shaft machine (PT6, T64, CT7, PW100 series...)
The chief difference will be at shutdown, where the prop on a free-turbine engine will slew to feathered, and not flat-pitch, condition.
(Notable exception: for a floatplane/seaplane installation, the free-turbine prop may be held in flat pitch, to prevent it from cycling through the forward-thrust range during startup, which would present handling issues...)
The chief difference will be at shutdown, where the prop on a free-turbine engine will slew to feathered, and not flat-pitch, condition.
(Notable exception: for a floatplane/seaplane installation, the free-turbine prop may be held in flat pitch, to prevent it from cycling through the forward-thrust range during startup, which would present handling issues...)
Last edited by barit1; 26th Nov 2012 at 15:52.
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Thanks Barit. I thought when shutdown the blade goes to stop windmilling when moored. and when engine starting the blade goes to full flat which produces no thrust. isn;t this common to all constant speed propellers?
thanks for the reply!
Code0
thanks for the reply!
Code0
I'd question if there is a zero pitch stop on most props. You"re on the ground fine stop (why does 11 degrees ring a bell Dowty Rotol?) making some thrust, just not enough to move the aircraft. So I don't think condition one exists. The prop stays on the ground fine stop until the engine turns it fast enough to reach the governing range where the pitch increases to keep the speed under control.
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code0:
When shutdown in flight, you always want the prop to feather to a minimum-drag angle. With many free-turbine installations this is automatic, and happens as the fuel lever is moved to the shutdown (or stopcock) condition.
But with a single-shaft machine, some separate action (another handle or button or ...) is required to move the prop to feather, since a standard ground shutdown would place it in flat pitch. Maybe someone else can step forward with specific examples of how this is controlled in the cockpit.
BTW - there is a minor variation on the feathered blade position. The Dowty prop (e.g. Saab 340/CT7) goes to a nearly-feathered position called "autocoarsen"; instead of stopping rotation completely, the prop slowly windmills. This is allegedly a lower-drag condition than fully feathered. This is merely a footnote to your blade-angle diagram, and I wouldn't necessarily make a big deal of it.
When shutdown in flight, you always want the prop to feather to a minimum-drag angle. With many free-turbine installations this is automatic, and happens as the fuel lever is moved to the shutdown (or stopcock) condition.
But with a single-shaft machine, some separate action (another handle or button or ...) is required to move the prop to feather, since a standard ground shutdown would place it in flat pitch. Maybe someone else can step forward with specific examples of how this is controlled in the cockpit.
BTW - there is a minor variation on the feathered blade position. The Dowty prop (e.g. Saab 340/CT7) goes to a nearly-feathered position called "autocoarsen"; instead of stopping rotation completely, the prop slowly windmills. This is allegedly a lower-drag condition than fully feathered. This is merely a footnote to your blade-angle diagram, and I wouldn't necessarily make a big deal of it.
...And also note that the term "Constant Speed Propeller" is usually applied to PISTON Engines, which do not have a "Beta" range and rarely have reverse thrust- ground operation is usually at the Fine Pitch stop with thrust controlled by the Throttle.
Dowty props, at least the ones on the RR Dart, the blade angle at feather was 87 degrees a certain percentage of blade span, 75%?? The slipstream turned the prop at about 100 rpm when feathered in order to supply oil pressure to the engine bearings as well as apply a positive pressure on the air seals in the engine.
MarkerInbound you are correct. Ground fine pitch was 11 degrees. (The flight fine lock was 16 or 18 degrees depending on the installation and the cruise pitch lock was at either 30 or 32 degrees, again depending on installation. Least that's how I remember it.)
MarkerInbound you are correct. Ground fine pitch was 11 degrees. (The flight fine lock was 16 or 18 degrees depending on the installation and the cruise pitch lock was at either 30 or 32 degrees, again depending on installation. Least that's how I remember it.)
Last edited by pigboat; 29th Nov 2012 at 01:29.
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code0
I am sad that you did not consider my suggestions. I was trying to lead you gently into the concept of twist and from there why twist is valid for only one pitch setting.
Mike the Oldie
I am sad that you did not consider my suggestions. I was trying to lead you gently into the concept of twist and from there why twist is valid for only one pitch setting.
Mike the Oldie
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hey Mike! not that I didnt consider, didnt have much time past few days to put into this. am gona read each carefully in near future when i get a time to light up a cigarette over a coffee! . . so be ready for few more questions on the way 
thanks!
Code 0
thanks!
Code 0
