RPM Setting for cuise with an O300

RPM Setting for cuise with an O300

I'd like to know what people use for a cruise setting on their 172's.  I have a '56 model and used to cruise at 2300rpm, I bumped it up to 2450 and picked up 7 - 10 mph.  At 2600 I pick almost 10 more mph...  2700 is red line, I'd like to know what the 75%, 65% and 55% RPM settings are...

    Dave

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Re: RPM Setting for cuise with an O300

I just did a search and ran across this thread that talkes about this subject.  I feel like such a nitwit, I even participated in it smile

<http://www.cessnaowner.org/cgi-bin/phorum-3.4.1/read.php?f=1&i=6262&t=6219#reply_6262>

I still don't know if that totally answered my question but from what I could get out of it from re-reading it is that under 3000' 2400 - 2500 rpm, over 3000' go up to 2600 rpm.

I guess I don't understand that the altitude has to do with anything.  Anyone care to explain it where a simple minded computer guy can understand it?

     Dave

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Re: RPM Setting for cuise with an O300

In the POH for my 172A it has a cruise performance chart. It gives all the data your asking about. It's listed in Section VI-Operational Data. I would think your POH would have the same chart.

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Re: RPM Setting for cuise with an O300

Dave,
The altitude is a factor in determining what percent of full power you are producing at any given altitude.  Most folks don't want to run any higher than 75% power in cruise, and as the altitude increases it takes a higher RPM setting to attain 75%.  Conversely, at lower alts. 75% is achieved at lower RPM's.  A POH should list the power produced at different alts. and RPM settings so that you will know what to expect.  Also, you might want to calibrate your tach with some sort of electronic gizmo, like Prop-Tach.  Tach errors can account for major misconceptions when it comes to figuring power output.
Bob

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Re: RPM Setting for cuise with an O300

Dave,
I think these will be simple words...
As you go UP, the air gets "thinner".  It makes breathing less effecient for you, me and our planes.  So..at a given power "setting", the power will reduce as you climb.  If we go high enough, we won't even  be able to get 75%.  All things being equal....Less Air = Less Power.
At, least that's the way I understand it.
I think a turbo might change that, but not for me and my budget  ;p
Michael

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Re: RPM Setting for cuise with an O300

I get it.  Thanks guys. 

Jay; I don't think my manual has this information.  I am at work now but I will check it again when I get home.

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Re: RPM Setting for cuise with an O300

Jay; I looked in my manual and it is there on page 42.  I don't know many times I looked in there, looked right at the chart and thought "why is the information I want not here"...

    Dave

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Re: RPM Setting for cuise with an O300

I have found that at lower altitudes (below 5000) my 172 runs well at 2450 rpm. It's not fast but I only burn 7.5 gph and for me that makes pretty economical flying. And I'm usually not in any hurry to go any place. If that were important I wouldn't have a 172.

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Re: RPM Setting for cuise with an O300

I'm not usually in a hurry to get where I am going either, but there are times when it is important.  Not important enough to buy a 182,  I looked into that last winter and decided I didn;t want the extra expense in insurance, annuals, manintenance and operating costs smile

    Dave

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Re: RPM Setting for cuise with an O300

Dave,

I'm in violent agreement with you.  I look at it this way, if you fly for 4 hours in a C172 instead of 3 hours in a C-182, you earn approx $200 for flying that last hour.

But it would be nice to have one!

Barry

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Re: RPM Setting for cuise with an O300

The answer to the original question has been danced around but not specifically answered.

The reason is that mass airflow is the determining factor in HP output when rich of peak EGT (as we fly 172s).  When at low altitudes, the air is dense and it takes fewer pumping cycles (rpm) to get enough air into the engine to produce 75%.  While at higher altitudes it takes more pumping cycles (rpm) to move enough mass air flow through the engine to produce 75% power since as was mentioned earlier, the air is less dense. 

Mass air flow is the answer.

When you move the throttle in a 172, you are changing the MP just like in a 182.  The only difference is that the rpm is your measure of the power output since you have no MP guage.  When I fly a 172, I leave the throttle full in until I reach cruise altitude, then adjust as necessary--IF any adjustment is necessary.  If I'm above 7500 feet, I can't get 75% power even with the throttle wide open.

Walter Atkinson
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Re: RPM Setting for cuise with an O300

Hey Walter, if you get a second would you please go to TheCFI.com. There is a discussion in the forums that is right up your alley, and I would like to get your take on it! It's under the heading about sea level and 59F.

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Re: RPM Setting for cuise with an O300

Walt,
I want to save my engine and only turn a maximum of 2550 RPM at anything after the climb to my desired cruise altitude.  I understand that turning the reduced RPM equals less power, but I feel that less RPMs also equals longer engine life.  What are your thoughts on this?  Do you have data that you would like to share regarding the wear factor at higher altitudes/higher engine RPM?  I appreciate any and all you have to offer.

Thanks,
Rick

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Re: RPM Setting for cuise with an O300

Rick:

We've always heard that lower rpms means fewer cycles and less wear and intuitively that MUST be true.  However, by all indications this is such a small effect as to be insignificant in the list of stresses an engine must tolerate to make it to TBO.

I have come to the opinion that lowering the internal cylinder pressures are far more important than reducing the rpm cycles.
Now, all that said, in a C-172, I don't think it really mounts up to ANY measurable difference at all.  The data suggests that it doesn't make much difference whether you reduce the rpm in climb or not as far as TBO and wear is concerned.

The real issue is that the other stresses are lower and you get to altitude quicker with full throttle.  Those a good things. Is it something to argue over?  I don't think so.

Walter Atkinson
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Re: RPM Setting for cuise with an O300

Matt:

<TheCFI.com>

Well, I wasted some time over there reading that thread!  Whew!

There is some good info and some erroneous info there.  What in particular were you interested in?  What, specifically, is your question?

Walter Atkinson
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Re: RPM Setting for cuise with an O300

Well the biggest thing that was getting me, was the full rich at 20,000 ft.  I am working on getting my A&P, but apparently, I still don't know that much about carbs or fuel injection. I was running some numbers today, and let me know if I'm way off, but I guess if the temps at 20,000 feet were about 200 C below standard temp(which would be about -225 C) than you could run full rich. Right? All of this non-turbo of course!

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Re: RPM Setting for cuise with an O300

Walter; Thanks for the response.  I do have a manifold pressure gague in my plane.  Not sure what I am supposed to do with it, but it's therre smile

    Dave

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Re: RPM Setting for cuise with an O300

Matt...

I dont think full rich at -225º C  <-373ºF> is going to make much difference that full lean.  At that temp AvGas is going to have the consistancy of a window pane.

Aviation Gasoline: This is motor spirit prepared especially for aviation piston engines, with an octane number suited to the engine, a freezing point of -60oC and a distillation range usually within the limits of 30oC and 180oC.

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Re: RPM Setting for cuise with an O300

*Well the biggest thing that was getting me, was the full rich at 20,000 ft.*

Well, that was one of the things that got me, too! <g>

Walter

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Re: RPM Setting for cuise with an O300

I don't think anything would actually work at that temp, but hypothetically(sp?), would my math work? The colder the air, the more dense, so if it actually could get that cold, at that altitude, could you then run full rich? I know this is way out there, but I just thought I'd try!

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Re: RPM Setting for cuise with an O300

Quote:
>If I'm above 7500 feet, I can't get 75% power even with the >throttle wide open.

I thought that I understood this - but now I seem to be getting more and more confused (Confusing me is not difficult smile

If I am over 7500 ft in my 172H with climb prop, and I go throttle wide open, then my RPM is above redline.

Here is my confusion - is it OK to be above redline at altitude?

I majored in arts - I never did quite grasp the sciences smile

Thanks in advance

Tony

Cessna 172H C-GICE

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Re: RPM Setting for cuise with an O300

Tony:

A climb prop is flatter in pitch than a cruise prop.  It takes a smaller bite of the air.  As the air thins, it offers less resistance and the prop can turn faster than it would if it had more pitch. This makes a climb prop very inefficient in cruise.  That's why a variable pitch prop is so much more efficient than a fixed pitch prop.  It can be altered for the condition at hand.

Engine redline rpms are set for several reasons.  In some cases, it's for the rotating masses inside the engine.  Sometimes it's a prop issue.  I would try not to exceed the rpm redline.

Unless there is a COMPELLING reason to have a climb prop on a 172, I'd much prefer the cruise prop.

Walter Atkinson
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Re: RPM Setting for cuise with an O300

Hi Walter

Thanks for your response - I appreciate it.
The reason I swithched to a climb prop was to let me get in and out of shorter fields, loaded, when the temperature is between 35C - 40C. I had a scare this summer when I had to go around, from a short field and I had 40 degrees of flaps out and trees in front of me. I have lost a couple of knots in cruise but my takeoff is much improved. I went from a pitch of 73 to a pitch of 71.

Best regards

Tony
Cessna 172H  C-GICE

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Re: RPM Setting for cuise with an O300

*The reason I swithched to a climb prop was to let me get in and out of shorter fields, loaded, when the temperature is between 35C - 40C.*

Well, I'd call that a compelling reason, to say the least! <g>

Walter

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Re: RPM Setting for cuise with an O300

this is a very dated message but wondering how much improvemnt you recieved from a climb prop. i have a cruise prop and am thinking of repitching too. thanks cessna 172a

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