Constant speed prop blade angle

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MrManH

Pre-takeoff checklist
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MrManH
Hey guys,

As I'm preparing for my commercial oral I'm going over the constant speed prop.
Do you agree with this illustration (found online) of ground idle and take off blade angles?

I was expecting the take off blade angle to basically look like the ground idle.

My reasoning is that when taking off, the propeller control is pushed all the way in which results in the smallest blade angle allowing for max RPM. If so when pulling back the propeller control to idle, I would expect a much flatter blade angle. Instead this illustration shows a vertical blade (small angle) for ground idle.

Word for word the PIM says:
"When oil pressure is relieved, centrifugal force, assisted by an internal spring twists the blades toward low pitch (high RPM).

propbladeangle.jpg
 
Last edited:
MrManH said:
If so when pulling back the propeller control to idle, I would expect a much flatter blade angle.
Click to expand...

I think that's where you're going wrong.

The prop control is not pulled back to idle* - typically it is all the way forward when the throttle is at idle. And the diagram seeks to show the shallow angle the blades would take with no load on them.



*In fact, that makes no sense - idle is a term used for the throttle, or the engine in general - not the prop.
 
Those blade angles seem exaggerated, it looks like they are meant to be an example of a turboprop which uses much larger blades and lower RPM than a propeller on a piston engine would.

The Piper Arrow IV pitch stops are 12.5º to 27.5º. That said, the governor will increase the blade angle slightly during takeoff because as the aircraft accelerates the air resistance on the propeller decreases.

If so when pulling back the propeller control to idle
Click to expand...
That's not what it's called, there's no "idle" position on a propeller control. It's high rpm (fine pitch) or low rpm (course pitch). The prop control is normally all the way forward when the engine is idling on the ground.
 
Unless you have the old beech electric prop, you arent setting blade angle with the prop control. You are setting desired rpm. The governor then sets the blade angle to try and achieve that rpm. At idle with the prop control full forward, the engine is at a lower rpm than redline so the governor flattens the blades to try and increase speed. The governor is blind to power, MAP, etc, all it can do is change blade angle in response to rpm.
 
Yup... Looks like a turbo prop. I'm not aware of any recip that has reverse.
 
JHW said:
Unless you have the old beech electric prop, you arent setting blade angle with the prop control. You are setting desired rpm. The governor then sets the blade angle to try and achieve that rpm. At idle with the prop control full forward, the engine is at a lower rpm than redline so the governor flattens the blades to try and increase speed. The governor is blind to power, MAP, etc, all it can do is change blade angle in response to rpm.
Click to expand...
This is key.
 
Dowty Rotol has the best terminology, when a prop goes to high pitch they say it "coarsens up". ( coarse thread) When the prop goes to a low blade angle, they say it "fine's off " (as in fine thread) When that is understood, it makes the British's prop manuals almost readable.
 
JHW said:
Unless you have the old beech electric prop, you arent setting blade angle with the prop control. You are setting desired rpm. The governor then sets the blade angle to try and achieve that rpm. At idle with the prop control full forward, the engine is at a lower rpm than redline so the governor flattens the blades to try and increase speed. The governor is blind to power, MAP, etc, all it can do is change blade angle in response to rpm.
Click to expand...
There are only three speeds in prop governor, "over speed", "under speed" and "on speed". the only thing a prop control does is change the setting in the speeder spring. every thing else is reactive to that setting.
 
Says right below the title that they are approximates.
 
Kritchlow said:
Yup... Looks like a turbo prop. I'm not aware of any recip that has reverse.
Click to expand...

There are some recip seaplanes that have reversing propellers. My neighbor has one on his Seabee.
 
To answer the fundamental question, at idle, the blades will normally be on the low pitch stops, since the prop will be turning well below the commanded RPM. At takeoff power, they will come off the low pitch stops in order to limit the prop to the commanded maximum limit RPM. If the blades were still on the low pitch stops at full throttle/full forward prop control, then either the engine would overspeed or it would be developing way too little power. You can see this as you advance the throttle -- the tach will reach max RPM before the throttle gets to full forward. From that point on, the blades are coming further off the low pitch stops to limit RPM to redline. Also, if you pull the throttle back after takeoff with the prop control still full forward, the throttle will come back considerably before the RPM comes back off the redline, which required that the blades move to lower pitch in order to keep the RPM up there as power is reduced.
 
MrManH said:
Hey guys,

As I'm preparing for my commercial oral I'm going over the constant speed prop.
Do you agree with this illustration (found online) of ground idle and take off blade angles?

I was expecting the take off blade angle to basically look like the ground idle.

My reasoning is that when taking off, the propeller control is pushed all the way in which results in the smallest blade angle allowing for max RPM. If so when pulling back the propeller control to idle, I would expect a much flatter blade angle. Instead this illustration shows a vertical blade (small angle) for ground idle.

Word for word the PIM says:
"When oil pressure is relieved, centrifugal force, assisted by an internal spring twists the blades toward low pitch (high RPM).
Click to expand...

The blade angle is regulated by RPM. At T/O power you are making way more power than at flight idle, so it needs to increase angle to increase resistance to keep it from over speeding.
 
FastEddieB said:
I think that's where you're going wrong.

The prop control is not pulled back to idle* - typically it is all the way forward when the throttle is at idle. And the diagram seeks to show the shallow angle the blades would take with no load on them.



*In fact, that makes no sense - idle is a term used for the throttle, or the engine in general - not the prop.
Click to expand...

My bad I didn't mean to say idle on the propeller control. I meant idle as in propeller full in (high RPM) and throttle idle. I'm looking through the answers and links posted, thanks everyone :)
 
Are these blade angles even close (in the drawing)?

The other day when I was playing with my prop (ok now), I was really surprised just how little change in angle is made when increasing the pitch - which would result in normal cruise RPM. (I have the beech electric prop.) I can't imagine getting to 20degreese, then again I'm only turning 185 HP max..
 
MrManH said:
Word for word the PIM says:
"When oil pressure is relieved, centrifugal force, assisted by an internal spring twists the blades toward low pitch (high RPM).
Click to expand...


That applies to props on most piston singles. On twins, the opposite occurs. Oil pressure forces the blades into low pitch, and counterweights or springs or gas pressure forces the blades to a higher pitch. That way, if the engine quits in flight, the oil pressure falls while the prop is windmilling and the blades will go to feather. Blade locks prevent feathering when shutting down on the ground at low RPM.

SOme props use oil pressure to work the blade both ways. Double-acting props.

Dan
 
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