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  #1  
Old 05-12-2019, 06:21 PM
blaplante blaplante is offline
 
Join Date: Jan 2010
Location: Southern California
Posts: 102
Default fixed pitch rpm vs. altitude?

I know that as we climb, the air gets thinner, causing 1) less drag, 2) less HP from the engine and 3) less thrust from a prop per rev. The effects more (or less) equal each other out so that a FP prop that is well matched to the plane isn't suddenly way too fine or way to coarse at altitude.

But does anyone know (for a fixed pitch prop) - as you climb above 7500' will the maximum rpm attainable go up or down or stay the same? I've seen some incidental info that implies it will go down somewhat (maybe 25 rpm per 1000').

My curiosity comes from having a prop that will exceed 2700 rpm at 7500'. I'm wondering if it would do the same at 12500', or would I be able to go WOT and stay under 2700? I suppose I could go burn some fuel and find out but it takes a while to climb and I suspect someone has this info.

Thanks!
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  #2  
Old 05-12-2019, 07:20 PM
Kyle Boatright Kyle Boatright is offline
 
Join Date: Aug 2005
Location: Atlanta, GA
Posts: 3,833
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My experience is that it goes down. By 10,000', I'm 150-200 RPM down vs low altitude.
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  #3  
Old 05-12-2019, 07:40 PM
brad walton brad walton is offline
 
Join Date: Jul 2007
Location: Cypress, TX
Posts: 510
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More altitude will give you less WOT RPM.
I get WOT and 2700 RPM at about 16,000 density altitude
What I find interesting is that I get the same true airspeed at a specific RPM, regardless of altitude. But for the same TAS and RPM, my fuel consumption decreases with altitude.
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  #4  
Old 05-12-2019, 10:33 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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as you mentioned, as you climb your engine produces less power. The fact that your max RPM reduces cannot be solely attributed to the prop. It is a combination of the factors mentioned in your post.

As I climb in the higher altitudes with an FP, I see roughly the same TAS for a given RPM. I deduce from this that the reduction in prop thrust is proportional to the reduction in induced drag. I also see fuel flow reducing for that given RPM as I climb, telling me that it is requiring less power to turn that RPM, supporting the assumed reduction in thrust generated.

Larry
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Last edited by lr172 : 05-12-2019 at 10:35 PM.
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  #5  
Old 05-12-2019, 11:24 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by blaplante View Post
I know that as we climb, the air gets thinner, causing 1) less drag, 2) less HP from the engine and 3) less thrust from a prop per rev. The effects more (or less) equal each other out so that a FP prop that is well matched to the plane isn't suddenly way too fine or way to coarse at altitude.

But does anyone know (for a fixed pitch prop) - as you climb above 7500' will the maximum rpm attainable go up or down or stay the same? I've seen some incidental info that implies it will go down somewhat (maybe 25 rpm per 1000').

My curiosity comes from having a prop that will exceed 2700 rpm at 7500'. I'm wondering if it would do the same at 12500', or would I be able to go WOT and stay under 2700? I suppose I could go burn some fuel and find out but it takes a while to climb and I suspect someone has this info.

Thanks!
As you climb the RPM will reduce.

That is why a prop that most people would consider a climb prop is also a good cruise prop at higher altitudes (11K-12K range).

Unless you plan to never go above about 7500', a prop that requires you climb to 10K plus before you can run WOT and not exceed 2700RPM will give you the best overall performance that you can get with a fixed pitch prop.
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  #6  
Old 05-13-2019, 11:29 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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Quote:
Originally Posted by rvbuilder2002 View Post
As you climb the RPM will reduce.

That is why a prop that most people would consider a climb prop is also a good cruise prop at higher altitudes (11K-12K range).

Unless you plan to never go above about 7500', a prop that requires you climb to 10K plus before you can run WOT and not exceed 2700RPM will give you the best overall performance that you can get with a fixed pitch prop.
+1

My prop will still turn 2700 at 13K. Down at 8K, I routinely turn 2740 at 8.1 GPH. While I rarely have the desire to burn more than this, It is a versatile prop for me. Others will want to run ROP and go faster at 8K or unwilling to go over 2700 and this is not a good prop for them. I will turn 2800 on my 320 on the rare occassion that I want to burn more gas at 8K.

I am not in the camp that believes an engine will self-destruct at 2800 RPM and my prop is rated for 3000 RPM.

It is a series of compromises based upon what is most important to you.

Larry
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Last edited by lr172 : 05-13-2019 at 11:33 AM.
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  #7  
Old 05-13-2019, 02:48 PM
plehrke's Avatar
plehrke plehrke is offline
 
Join Date: Sep 2006
Location: Defiance, MO
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Quote:
Originally Posted by blaplante View Post
But does anyone know (for a fixed pitch prop) - as you climb above 7500' will the maximum rpm attainable go up or down or stay the same? I've seen some incidental info that implies it will go down somewhat (maybe 25 rpm per 1000').
I posted my flight data 8 years ago in this post. I get >2800 rpm down low at WOT and only 2700 rpm at 10,000 ft
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  #8  
Old 05-13-2019, 04:46 PM
jrs14855 jrs14855 is offline
 
Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,266
Default R/M

1000 hours on a 0 360 in a Pitts turning as high as 3600 r/m for 300 hours then 3300 for 700 hours on every aerobatic flight. About 80% of that time was aerobatics. Engine was just as strong at 1000 hours as when newly overhauled. 25 hour oil changes, no filter.
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