Understanding the Constant Speed Prop

[Ksaunders]

I'm new to the constant speed prop.  I am very careful to keep all the numbers right on the button as the POH and previous owner has instructed me to during my orientation.  I use Full in during takeoff (27") then I back off during cruise to 2600 RPM - 26" MP with the prop.  It's really not that much of an adjustment but you can hear the pitch change when you back down the MP.  When landing, the instruction is to have the prop full in (27" MP) - Again, not very much adjusting going on with the prop.  

I'm just wondering from a real world view when (or why) one would ever back off the prop to below 26" (?).  What would be the result if RPM was in the 2600 range and the prop was backed down to below the recommended 26"  (?).  If I'm really not adjusting the prop very much at all and fail to see why the range would be so great if you really don't make too much of an adjustment? 

  

[Parker_Woodruff]

Hold on a second.

You directly control Manifold Pressure with the throttle.

You directly control RPM with the prop lever.

You are not bringing the propeller to 26" or 27".  You are setting an RPM.

I recommend Deakin's articles; here's a good one. http://www.avweb.com/news/pelican/186778-1.html

[Ksaunders]

Yep, got my RPM's and MP's backwards.  Sorry.   

[201er]

Have you looked at the POH power setting table??

[danb35]

First, on climb-out, there's no reason to adjust either MP or RPM--leave both controls full forward until cruise.  At that point, if you choose, pull the prop back a bit--I usually cruise at around 2500 RPM.  I second the recommendation for Deakin's articles, as they're a much better explanation of how the things work than any of us will give here.

To directly answer your question, if you pull the prop control back further (say, to 2500 RPM), the governor will call for a coarser pitch on your prop, which will slow down the engine, and result in it producing less power.

[PTK]

Quote: danb35

First, on climb-out, there's no reason to adjust either MP or RPM--leave both controls full forward until cruise.  At that point, if you choose, pull the prop back a bit--I usually cruise at around 2500 RPM.  I second the recommendation for Deakin's articles, as they're a much better explanation of how the things work than any of us will give here.

To directly answer your question, if you pull the prop control back further (say, to 2500 RPM), the governor will call for a coarser pitch on your prop, which will slow down the engine, and result in it producing less power.

[drapo]

Personnally, I do 2700 rpm on take off till about 2000' than reduce to 2500 rpm, full throttle till at cruise altitude. I then adjust MP to around 24 in. , depending on altitude and the use of the Powerboost, before adjusting mixture.

On the descent, I keep 2500 rpm and speed around 170mph in the yellow arc, so I get around 500fpm. If I need a more rapid descent, I'll reduce MP to around 20 in, IAS in the 140's, close to 1000 fpm.

I've always asked myself the question about keeping rpm and speed high in the descent, since there are two school of thought on this ( As is other things like LOP and ROP.... OOPS!  ). But my instructor told me that it's better to adjust MP since there are restrictions on the prop's RPM.

 

 

[jlunseth]

I am a turbo.  I use 100% HP and 2700 (max) RPM's for all climbs.  I use 2450 for cruise below about 17 or 18, because the engine is very smooth at that speed.  I use 2500-2650 for cruise in the teens and high flight levels because the prop starts to become inefficient at lower speeds.  Although 2700 is recommended in the POH for cruise at the highest altitudes, I generally don't do that.  I usually pull the prop back a little, to 2650, just to keep it from overspeeding.

[orangemtl]

Beyond slowing down a bit, might I ask what benefit I'd get from reducing prop RPMs that I cannot produce with decreased MP and fuel mixtures?

I've backed it off a few times more for curiosity than anything (2400 or so); a bit quieter, but with Bose headsets, it's a moot point. If I wanted to slow down while maintaining higher MPs I could achieve it in this fashion, but: I cannot think of a scenario in my typical flight needs where I'd want to do so.

Educate me, any and all. Thanks.

 

[carusoam]

Michael,

 

For your engine and prop combination, you probably won't find much value in lower engine RPM.   Improved glide distance...

 

The four cylinder O-360 engines have a much bigger vibration variation with RPM.  Lower RPMs will have lower noise and vibration.

 

Initial training in my Ovation was simple, leave the engine at 2,500 RPM from take-off to landing.

 

Best regards,

 

-a-

[201er]

Gas, gas, gas.... the more you pull the blue knob back the less money you're blowing out the exhaust. Mooney's are just as fuel efficient as they are fast. Local sightseeing, logging flight time, holding patterns, etc.... no need to hurry to go nowhere so you may as well reduce rpm (and possibly throttle but not necessarily at higher altitude) to reduce power and fuel flow. Don't forget to properly set the mixture.

[Jeff_S]

Quote: orangemtl

Beyond slowing down a bit, might I ask what benefit I'd get from reducing prop RPMs that I cannot produce with decreased MP and fuel mixtures?

I've backed it off a few times more for curiosity than anything (2400 or so); a bit quieter, but with Bose headsets, it's a moot point. If I wanted to slow down while maintaining higher MPs I could achieve it in this fashion, but: I cannot think of a scenario in my typical flight needs where I'd want to do so.

Educate me, any and all. Thanks.

 

[jetdriven]

Wouldnt a sufficiently LOP setting also have lower TITs as well?  May have to run a little leaner than 10 LOP, but shoud be doable, right?