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falstaff
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How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 2:12 pm

Last night I was watching a neat old film with Arthur Godfrey flying a Eastern Super Connie. He mentioned something about changing the pitch of the props. I have seen this in movies too. I understand why you would want to change the pitch, but I don't get the how. How does the system work? Is it hydraulic, like oil pressure changing cam timing in a automotive engine? With the prop spinning I can't see there being any electrical wiring or hydraulic tubing.
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Starlionblue
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RE: How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 3:29 pm

Oil pressure changes and counterweights are two methods used. I don't really know a lot about this.
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oly720man
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RE: How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 3:47 pm

One example here.

http://www.thaitechnics.com/propeller/prop_control_3.html

There's a gear in the prop hub that drives against gear teeth at the prop blade root, rotating the prop blade about its axis. The gear at the prop root is rotated by a peg and slot drive system on a piston that moves backwards and forwards.
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zanl188
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RE: How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 3:58 pm



Quoting Falstaff (Thread starter):
With the prop spinning I can't see there being any electrical wiring or hydraulic tubing

They don't use wire or tubes. For electric they could use a set of contact rings with conductive rollers or bearings. For fluids you just design the hub in such a way as to pass fluids while rotating. I believe it's possible to use a mechanical means as well, take a look at a helicopter rotor hub to see what I mean...


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ALexeu
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RE: How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 6:43 pm

Is it economical to change prop pitch? What's the purpous (apart from obvious thrust reverse)?
 
zanl188
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RE: How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 7:18 pm



Quoting AlexEU (Reply 5):
Is it economical to change prop pitch?

Yes

Quoting AlexEU (Reply 5):
What's the purpous (apart from obvious thrust reverse)?

- Feathering the prop. Reduces drag in case of engine failure...
- Changing the pitch so it's appropriate for the power being produced and aircraft attitude. In much the same way that an automobile transmission changes gears when going uphill
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avt007
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RE: How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 7:19 pm

The Dash 8, ATR, etc, uses oil pressure transmitted through a transfer tube from the PCU (propellor control unit) on the engine to the prop actuator in the hub. The pressure is ported to one side or the other of a piston, so it moves back and forth. Attached to the actuator is a slotted ring. Each of the four blades has an offset pin on it's base, with a bearing. These pins fit in the slot, and as the piston moves the blades rotate. Of course the base of the blades are mounted in large bearings with big ball bearings. HTH
 
zanl188
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RE: How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 7:51 pm



Quoting Falstaff (Thread starter):
Last night I was watching a neat old film with Arthur Godfrey flying a Eastern Super Connie.

I found a similiar film here if anyone is interested...

http://www.archive.org/details/Flyingwi1953
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Starlionblue
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RE: How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 11:23 pm



Quoting ZANL188 (Reply 6):
- Changing the pitch so it's appropriate for the power being produced and aircraft attitude. In much the same way that an automobile transmission changes gears when going uphill

Also there are constant speed props. That way the engine moves at optimal RPM but puts out different power levels with different pitch.
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N231YE
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RE: How Do Props Change Pitch While Turning?

Sun Jan 27, 2008 11:56 pm



Quoting AlexEU (Reply 5):
Is it economical to change prop pitch? What's the purpous (apart from obvious thrust reverse)?



Quoting ZANL188 (Reply 6):
- Feathering the prop. Reduces drag in case of engine failure...
- Changing the pitch so it's appropriate for the power being produced and aircraft attitude. In much the same way that an automobile transmission changes gears when going uphill

 checkmark  Think of a propeller as a transmission on a car, or the gears on a multi-speed bicycle.

Quoting Starlionblue (Reply 9):
Also there are constant speed props. That way the engine moves at optimal RPM but puts out different power levels with different pitch.

I hazard a guess that variable pitch propellers (not ground adjustable props) and constant speed propellers are one in the same anymore. I could be wrong, though.
 
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Jetlagged
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RE: How Do Props Change Pitch While Turning?

Mon Jan 28, 2008 1:34 am

Constant speed props are variable pitch. Variable pitch props are not necessarily constant speed. However, I doubt there are many, if any, v-p props in use today that aren't constant speed. It's a lot of extra work for the pilot to adjust pitch every time power or airspeed changes.
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David L
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RE: How Do Props Change Pitch While Turning?

Mon Jan 28, 2008 1:36 am

So it's not necessarily as simple as some kind of swash plate, as found on helicopters, but with "collective" only and no "cyclic" action?

Quoting N231YE (Reply 10):
I hazard a guess that variable pitch propellers (not ground adjustable props) and constant speed propellers are one in the same anymore

Aren't constant speed propellers just propellers where the pitch is automatically adjusted by a some kind of governor arrangement?
 
redflyer
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RE: How Do Props Change Pitch While Turning?

Mon Jan 28, 2008 2:01 am



Quoting Oly720man (Reply 2):
One example here.

http://www.thaitechnics.com/propeller/prop_control_3.html

There's a gear in the prop hub that drives against gear teeth at the prop blade root, rotating the prop blade about its axis. The gear at the prop root is rotated by a peg and slot drive system on a piston that moves backwards and forwards.

Why does slot that the peg moves through have a sudden angle change? Shouldn't it be a standard curve all the way through the movement with perhaps a sudden angle change at the forward end where the prop would go into a "no-thrust" position?
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Buzz
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RE: How Do Props Change Pitch While Turning?

Mon Jan 28, 2008 10:37 am

Hi Red Flyer, Buzz here. I'll have to re-read in an old Hamilton Standard Hydromatic Prop book on the shelf. One version of the prop was called "quick feathering", so the "normal" part of the slot provided the normal constant speed operation.
When the Feather button was depressed, the prop would go into high pitch, then move around the bend and reach the feathered position a little faster. A prop that doesn't feather doesn't have that curve in the slot.

B.T.W, (some of it's coming back to me) those "slots" are called "cams", and it's a roller that runs along the track. 2 cams that pivot in opposite directions for greater mechanical advantage... if you pick one up (they are rather heavy steel parts) be careful to -not- stick your fingers in between the moving cams... it'll try and scissor your fingers off.

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N231YE
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RE: How Do Props Change Pitch While Turning?

Mon Jan 28, 2008 7:21 pm



Quoting David L (Reply 12):
Aren't constant speed propellers just propellers where the pitch is automatically adjusted by a some kind of governor arrangement?

I am not too sure of how it works, but the propeller does automatically change pitch in order to keep the propeller rotating at the same speed, even when engine power settings are changed (reasonably).
 
CWUPilot
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RE: How Do Props Change Pitch While Turning?

Mon Jan 28, 2008 8:00 pm

Constant Speed Props are available on many single engine aircraft and any modern multi engine aircraft I can think of.

-The pilot sets a speed for the prop (2500 RPM for example) using a propeller control in the cockpit. The prop governor then changes the prop's pitch as power is increased or decreased to keep the blades turning at the same speed.

-Pilots will select a higher speed for a climb and a lower speed for cruise flight. To keep this simple, think of using a lower gear on a bike as you pedal uphill and a higher gear on flat ground.

-When a higher speed is selected, the blades will go to a lower pitch, moving less air per revolution and allowing them to spin at the faster speed.

I and any other CFI who wants to hop on this discussion could type pages of info on how the governor works to regulate the prop pitch, but in general:

-Oil Pressure from the engine or boost pump enter the propeller hub to move the blades to a higher pitch on a single-engine a/c and a lower pitch on a multi-engine a/c. Other forces inherent in the prop and a spring counter act the oil pressure to make the prop move the opposite way.

There's a start. More to come if anyone wants it.

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redflyer
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RE: How Do Props Change Pitch While Turning?

Mon Jan 28, 2008 8:16 pm



Quoting Buzz (Reply 14):
When the Feather button was depressed, the prop would go into high pitch, then move around the bend and reach the feathered position a little faster. A prop that doesn't feather doesn't have that curve in the slot.

 checkmark 
Thanks for that great explanation! Makes sense.
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