Cruise RPM??

[GeneralT001]

I was scanning the POH and came across the following:

"A good rule of thumb for choosing an economical cruise power
setting, is to cruise at the same RPM that you get during full-throttle
static run-up before takeoff."

Can someone explain this too me? I was kinda under the impression that the RPM would decrease with altitude (less air less power). But maybe with a fixed pitch prop there is less air so less friction for the prop and it will increase RPM with altitude?? Mildly confused.

 

Thanks

 

Tony

[Kent Ashton]

That statement may be true but first define "economical cruise power".  With a fixed pitch prop and no wind or a good tailwind you can cruise at low RPM, leaned, and get tremendous range and low fuel flow.  OTOH, "economical cruise" (no wind) might mean cruising at peak power, leaned, above 8500' up to say, 18,000'.  The higher the better.   If I am only going a short distance and in no hurry, I might use less than peak power (say 2200RPM) which will lower my fuel flow and increase range.  A lot depends on headwinds/tailwinds.  With a headwind I would fly faster--less time sitting in the headwind.

Setting the throttle wide open reduces pumping losses but increases fuel flow.  That is offset by getting up high and leaning to either peak power or lower.  My usual practice is to fly high--around 12-13000', WOT, leaned to peak power seeing 8-8.5 GPH or to stooge around at 3000-5000' at less than 75% power leaned out, seeing 4 gph or less. 

RPM at altitude will be higher than static because the engine is not loaded down at altitude.  My engine (normally aspirated O-360) will not overspeed at static or at takeoff but it will overspeed by about 50-70 rpm at altitude which I control by leaning.

I judge prop performance by getting up to 8500' (where the engine cannot produce more than 75% power), WOT, leaned to peak power, (i.e. peak rpm).  If the prop will not allow 2700 rpm it is likely pitched too high for me, if more than 2700 it could use more pitch.  It is just a consistent way to evaluate a fixed pitch prop.  Then I accept whatever the prop gives me for takeoff or cruise at high altitude.  If I was flying out of short fields I might want a prop that gives maximum power (max rpm) at takeoff.

Then there is a concept called "Carson speed".

I hope I haven't bloviated on stuff you already know.  🙂

 

[GeneralT001]

10 minutes ago, Kent Ashton said:

That statement may be true but first define "economical cruise power".  With a fixed pitch prop and no wind or a good tailwind you can cruise at low RPM, leaned, and get tremendous range and low fuel flow.  OTOH, "economical cruise" (no wind) might mean cruising at peak power, leaned, above 8500' up to say, 18,000'.  The higher the better.   If I am only going a short distance and in no hurry, I might use less than peak power (say 2200RPM) which will lower my fuel flow and increase range.  A lot depends on headwinds/tailwinds.  With a headwind I would fly faster--less time sitting in the headwind.

Setting the throttle wide open reduces pumping losses but increases fuel flow.  That is offset by getting up high and leaning to either peak power or lower.  My usual practice is to fly high--around 12-13000', WOT, leaned to peak power seeing 8-8.5 GPH or to stooge around at 3000-5000' at less than 75% power leaned out, seeing 4 gph or less. 

RPM at altitude will be higher than static because the engine is not loaded down at altitude.  My engine (normally aspirated O-360) will not overspeed at static or at takeoff but it will overspeed by about 50-70 rpm at altitude which I control by leaning.

I judge prop performance by getting up to 8500' (where the engine cannot produce more than 75% power), WOT, leaned to peak power, (i.e. peak rpm).  If the prop will not allow 2700 rpm it is likely pitched too high for me, if more than 2700 it could use more pitch.  It is just a consistent way to evaluate a fixed pitch prop.  Then I accept whatever the prop gives me for takeoff or cruise at high altitude.  If I was flying out of short fields I might want a prop that gives maximum power (max rpm) at takeoff.

Then there is a concept called "Carson speed".

I hope I haven't bloviated on stuff you already know.  🙂

 

Thanks Kent. All good info to know. It seems to be the norm to run these engines pretty much wide open with a fixed pitch prop?

 

Tony

[Kent Ashton]

14 minutes ago, GeneralT001 said:

Thanks Kent. All good info to know. It seems to be the norm to run these engines pretty much wide open with a fixed pitch prop?

Yep, Lycomings were built to run flat out.  There is no need to baby them but I suppose you would get a little more hours out of them by running at reduced RPM.  I guess the biggest thing to know is to lean them whenever possible (when <75% power).  It seems that lots of instructors do not teach students how and why to lean.  I got 30 hours in the T-41 (CE-172) in A.F. training and I don't recall my instructor discussing leaning although I saw him do it once coming back from the training area.  I guess it just wasn't important to the curriculum and I did not appreciate it until I had my own airplane.  These days, I lean as much as possible right after engine start.  Go full rich for takeoff up to 3500-4000' where I lean to peak power during climb, then usually very lean again after level off.  I might richen just a bit in the pattern but still pretty lean then full rich again for a touch-and-go.

John Deakin wrote a lot about operating engines.  Check out his articles   https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-index/

[GeneralT001]

2 hours ago, Kent Ashton said:

Yep, Lycomings were built to run flat out.  There is no need to baby them but I suppose you would get a little more hours out of them by running at reduced RPM.  I guess the biggest thing to know is to lean them whenever possible (when <75% power).  It seems that lots of instructors do not teach students how and why to lean.  I got 30 hours in the T-41 (CE-172) in A.F. training and I don't recall my instructor discussing leaning although I saw him do it once coming back from the training area.  I guess it just wasn't important to the curriculum and I did not appreciate it until I had my own airplane.  These days, I lean as much as possible right after engine start.  Go full rich for takeoff up to 3500-4000' where I lean to peak power during climb, then usually very lean again after level off.  I might richen just a bit in the pattern but still pretty lean then full rich again for a touch-and-go.

John Deakin wrote a lot about operating engines.  Check out his articles   https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-index/

Thanks. I don't have a lot of experience with leaning but will learn