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

<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

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

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.

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.

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

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.

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
Advanced Pilot Seminars

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!

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.

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!

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
Advanced Pilot Seminars

*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