Constant speed prop question

JCranford

JCranford

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JCranford
I should probably know this but I’ll ask it anyway...

So a CS prop stays at the set RPM’s regardless of MP setting. But that’s not true, is it? During runup you increase MP until the RPM’s reach whatever. On takeoff you increase MP to full and the RPM’s increase to max, then you settle into climb and cruise and set your MP and prop and the RPM’s remain constant. Until what? At what MP does it react to a lower MP and the RPM’s start dropping with MP?
 
When the prop hits the mechanical stop and can’t go any finer. Then it starts to decrease RPM.
 
Sluggo63 said:
When the prop hits the mechanical stop and can’t go any finer. Then it starts to decrease RPM.
Click to expand...
Unless, of course, you have an MT electrically controlled CS prop.
Set the controller to 2500 RPM and you can decrease MAP to 17 inches and the RPM does stay at 2500.
The fine pitch is said to act somewhat as a speed brake on landing at these settings.
 
wrbix said:
Unless, of course, you have an MT electrically controlled CS prop.
Set the controller to 2500 RPM and you can decrease MAP to 17 inches and the RPM does stay at 2500.
The fine pitch is said to act somewhat as a speed brake on landing at these settings.
Click to expand...
I’m guessing that the throttle will reduce below 17 inches, and he prop will eventually slow below 2500 RPM because it’s on the low pitch stop.
 
Isn’t that almost exactly how any CS prop works electric or hydraulic?
 
MauleSkinner said:
I’m guessing that the throttle will reduce below 17 inches, and he prop will eventually slow below 2500 RPM because it’s on the low pitch stop.
Click to expand...
Dunno, will have to push those limits at some point....suspect so. At ground idle MAP RPM does of course decrease to 800 or so. Can’t say, right off hand, what throttle setting will push RPM to 2500 initially. Stand by.
 
wrbix said:
Dunno, will have to push those limits at some point....suspect so. At ground idle MAP RPM does of course decrease to 800 or so. Can’t say, right off hand, what throttle setting will push RPM to 2500 initially. Stand by.
Click to expand...
So like all other constant speed props, when it gets to the low pitch stop, it can’t maintain the selected rpm anymore. The numbers may change, but the operation is the same.
 
Last edited:
wrbix said:
Dunno, will have to push those limits at some point....suspect so. At ground idle MAP RPM does of course decrease to 800 or so. Can’t say, right off hand, what throttle setting will push RPM to 2500 initially. Stand by.
Click to expand...

Many airplanes have the prop low-pitch stops set so that the engine doesn't reach redline until it's doing 30 MPH or whatever in the takeoff roll. It will come close in the initial full-throttle application, but not quite redline. Once it passes that speed, the RPM will want to rise and the governor will increase the blade pitch. Others will reach redline when the throttle is almost wide open and the governor will kick in to keep it at redline with any further throttle opening. It all depends on what the airframe manufacturer's engineers specified on the design drawings and engine/propeller combination.

For example, we have this note in the the propeller section of the typical Cessna maintenance manual:

NOTE
It is possible for either the propeller low pitch (high rpm) stop or the governor high rpm stop to be the high rpm limiting factor. It is desirable for the governor stop to limit-the high rpm at the maximum rated rpm for a particular airplane. Due to climatic conditions, field elevation, low pitch blade angle, and other considerations, an engine may not reach rated rpm on the ground. It may be necessary to readjust the governor stop after test flying to obtain maximum rated rpm when airborne.
 
Jeff767 said:
Isn’t that almost exactly how any CS prop works electric or hydraulic?
Click to expand...

The (only) difference with an MT electric prop is that when it hits the fine pitch stop a green light is lit on the controller face plate. That said, the certified fine pitch stop setting is quite flat on my MT electric prop (static rpm is redline) so it has a large braking effect.
 
wrbix said:
Unless, of course, you have an MT electrically controlled CS prop.
Set the controller to 2500 RPM and you can decrease MAP to 17 inches and the RPM does stay at 2500.
The fine pitch is said to act somewhat as a speed brake on landing at these settings.
Click to expand...

"Somewhat" is hardly the case. See Figure 12-3 in the Airplane Flying Manual to see the drag coefficient with a windmilling prop.

Bob
 
For the non-cool kids, what does "MT" mean in "MT electrically controlled CS prop"?
 
JCranford said:
I should probably know this but I’ll ask it anyway...

So a CS prop stays at the set RPM’s regardless of MP setting. But that’s not true, is it? During runup you increase MP until the RPM’s reach whatever. On takeoff you increase MP to full and the RPM’s increase to max, then you settle into climb and cruise and set your MP and prop and the RPM’s remain constant. Until what? At what MP does it react to a lower MP and the RPM’s start dropping with MP?
Click to expand...

AggieMike88 said:
@JCranford .... If you haven't read them already, John Deakin's "Pelican's Perch" articles on AvWeb cover the CS Prop in a well written series of articles.

Heck, all of his stuff is good.

https://www.avweb.com/flight-safety/pelicans-perch-index/
Click to expand...

This! Start with Manifold Pressure Sucks and then go to Those Marvelous Props. This is BY FAR the best material available anywhere on the operation of constant speed props and the engines attached to them. And the FAA's materials on the subject are woefully awful...
 
There is some great information on measured drag from windmilling propellers in Horners Fluid Dynamics Drag. It’s long out of print but a local engineering university might have a copy.
 
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