What RPM do you cruise at in your 172?

What RPM do you cruise at in your 172?

For the sake of argument, lets assume my Tach (and yours) are accurate and in agreement (save the suggestions for digital tachs, calibration, etc. for the next posting).  I fly a 1981 172P and usually cruise at 2450 RPM max under 3,000 ft. msl, 2500 max at 3000-5000 ft., and 2550 max over 5000 ft.
I see folks who talk about crusing at full throttle or just below redline all the time, and that this is OK.  I somehow feel guilty whenever I exceed the max.'s that I listed above.   Am I just babying my engine, which was designed to run "flat out" ?  Or am I doing it a real favor by being conservative?
 
I am also interested in others' "overall" performance numbers (again, save the comments re. differences in gauges, OAT's, etc. for another posting.)   Speaking "in general", at 2450rpm @ 3000 ft my TAS is usually right at 105 kts.  At 2500rpm @ 5,000 ft. it's 112; at 2550rpm @ 7500 ft.  it's 115.  How does this stack up against other similar 172's?

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Re: What RPM do you cruise at in your 172?

I have a 1956 C172.  I run mine between 2400 - 2500 and see 91 knots or 105 MPH on a regular basis.

    Dave

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Re: What RPM do you cruise at in your 172?

I fly my 172M as you do.  I feel that the closer I fly to redline, the closer I am to helping the engine have a reduced life.  Our speeds are pretty similar as well.  My recommendation is to keep flying as you are comfortable!  And as always--FLY SAFE!!

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Re: What RPM do you cruise at in your 172?

I generally cruise at close to 75% turbulence permitting no matter what the altitude.  TAS are usually ~110 knots. (105 knots at low altitudes).  Generally, I don't believe in exceeding redline or manufacturer's recommendations.  What's the point? 

I heard from one pilot where an engine overhauler told him that those Lycomings like to run hard.  I wondered if that concept was generated from self-interest!  What you think?  I can't conceive of an Overhauler keeping records of how people run their engine and then correlate that information to TBO.  And if he did know, what would that recommendation be...  Do oil companies really want you to conserve gas!

Before I realized my tach was only indicating 150 rpm low, I beat all the performance figures.  So I got my Tru Tach - but save that for another posting.

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Re: What RPM do you cruise at in your 172?

Barry, I'd like to hear about your tru-tach.  Cost, installation (or is it portable?), accuracy, etc.

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Re: What RPM do you cruise at in your 172?

I fly at my 172K usually at about 2500 rpm (digital) especially at or below 3000 ft - often and at higher altitudes I hold it at 2600 rpm - that is 75 % up there, where I usually hold about 118 kn - I've stated that I can hold 120 kn or so, but usually it varies between 115 and 120 ( on occasion it only holds about 110  kn ) accounting for wind and reading groundspeed by GPS - going both ways into and with the wind.
in preflight I use 115 and it comes out very well with distance and wind calcs. You've may have read my statements about powerflow exhaust - and I know that helps. "Firewalled" at alt is still running the engine only at 75 % but it does eat alot of gas.   
A Cessna  salesman was showing me a New 172 and stated that I could have 180 HP and cruise at 115 or 120 kn - I told him that I allready do that with what I have.
I picked up a knot or two when I removed the big main tires and went with the "originals" but I changed back - I like the big mains more than the couple of knots.
I don't always firewall it but occasionally I like to when time is more of a factor or I want to put more distance between me and some weather. I don't want to misuse the aircraft or engine but I like to put it to good use, and the other options would to buy a 182 or something which would cost more and use more gas ALL the time, so I enjoy my 172 just the way it is.
I do have a digital tach and like it alot - along with the fact that it'll even give you a warning light on a "hot" mag - it monitors both and gives the average or you can select either one anytime you wish.
I was unknowingly running closer to real redline ( higher RPM at lower altitudes ) than I realized with the old tach - which is still installed - because it was reading 80 RPM slow, but many of the mechanical tachs are innacurate and most are reading on the low side.
I like the digital tach but I like to see the dial of the mechanical - and it's a backup so I've kept it in.
The digital tach has also made it easier to hold altitude - I hadn't expected it but with the digital reacting so quickly ( sometime a little too quick ) I noticed that as it increased or decreased even very small amounts I can make very small changes in pitch before alt drifts much - results in holding alt much closer and easier.

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Ken Wanagas

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Re: What RPM do you cruise at in your 172?

My digital tach is a Horizon Inst - You can find their advertisement in COO magazine - and I can recomend highly it to anyone, even if only to detect a "hot" mag ( or inflight mag failure) .

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Ken Wanagas

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Re: What RPM do you cruise at in your 172?

I have been considering a digi. tach--now I'm even more interested.  I can see how even tiny power changes would show up immediately on a digi. and enable tighter control of altitude and cruise settings.

But, there are a couple of things I don't understand.  If 2600 RPM's at 7500' is not excessive (according to Cessna) why is it excessive at 3000'?  I know that the engine's measured output, for a given RPM, IS higher at lower altitudes, but it seems like RPM's would be the REAL culprit in excess engine wear. 
I also don't understand how, if the O-320 is rated at 160 HP @ 2700 RPM's, how could it be producing only 75% power at 2600 (or 70% @ 2450 at lower alts.) according to the charts in the Cessna owner's manual?  75% of 2700 is only 2025. 

Probably a couple of dumb questions, but, as I admitted...I don't understand! 
Where's George Horn when you need him!

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Re: What RPM do you cruise at in your 172?

Bob,  RE: Tru Tach

I thought you weren't interested in this?

Use the Search feature on the Cessna Owner Forum to find out other possibilitites.  I finally got it from the manufacturer after Aircraft Spruce was out of it.  Try Aircraft Spruce again it seems it might be in now (see the website and part number below).  If not, call and ask Aircraft Spruce for the phone number of the manufacturer.  My wife, says you will have to beg the guy to send it to you.  They don't usually do it.  Get some "sweetthing" to do the talking.

<http://www.aircraft-spruce.com/nsearch.php?s=10-24745+>

Part number 10-24745    TRUTACH HANDHELD TACHOMETER    $79.950    

Aircraft Spruce, (877/477-7823)

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Re: What RPM do you cruise at in your 172?

Barry,
As usual my posting started off on a tangent and it's probably my fault.  The Tru-tach is a portable device, I see, and the mfgr. just decided not to make them any longer, from what I've heard.  I have never had to hire someone to sweet talk a parts mfgr. to sell me something, but for $79.95, what have I got to lose?  smile  Does this one have the LCD that points upwards when pointed at the prop, so it's difficult to read?

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Re: What RPM do you cruise at in your 172?

Yes, but you can turn it sideways and see the indicator.

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Re: What RPM do you cruise at in your 172?

Would it get a good reading just sitting on it's side on the top edge of the glareshield--could you mount it there and it work properly?

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Re: What RPM do you cruise at in your 172?

Power vs RPM settings - Max

In an auto engine - or one like - Your RPMs cause you grief at max or redline because damage can be caused by the pistons trying to swallow the valves at high RPM (roughly twice the RPMs of aircraft engines) - the valves don't have enough time to be retracted by the springs when things are moving to fast - Engines meant for High RPMs have heftier valve springs or inner springs (and other stuff like notched pistons, etc.)  to retract the valves faster which  help achieve higher RPM safer - but your Power is also maxed and that overstresses everything , specifically - wrist pins, rods, bearings and mains - higher power engines have more bolts on the main bearings because they take such a beating at high power output.

Aircraft engines develope Power at lower RPMs (mostly because of the cam and valves - lift, timing, and duration), but at altitude there is less air (oxygen) to be had (with a nornally aspirated engine) so even at max throttle the engines don't  get to the high power problem as easy and RPM's are low enough the pistons don't want to swallow the valves yet.

Of course you can turbo boost and overpower your aircraft engine and overstress the rods, wristpins, mains etc., and blow up your engine at any altitude. 

Just a couple of the differences between auto and aircraft and Power redline vs RPM redline - either way the redline is the point where you exceed your engines normal design capabilities - so after that expect things to come unglued - for different reasons.

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Ken Wanagas

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Re: What RPM do you cruise at in your 172?

Ken--thanks--very informative post.  I learned a few things I had not thought about.   So...I am assuming that RPM red-line is basically a "do not exceed" number due to some of the things you've mentioned, as opposed to being the number at which the rated horsepower is produced.  However, I guess the rated horsepower and red-line numbers could also be the same, as with the 0-320 @ 2700 RPM ? 
I am still wondering how, if it produces 160 BHP @ 2700, that it could produce 75% of rated BHP @ 2550 or other similar RPM's, since 75% of 2700 is 2025 RPM's (or 120 BHP)?  I guess it is not a linear progression of numbers, assuming the mfgrs. know what they're talking about...and I'm sure they do!  Thanks again for sharing your knowledge.

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Re: What RPM do you cruise at in your 172?

Bob,

The reason an engine does not produce 75% power at 75%(at a constant DA) of it's maximum RPM is because it is not a linear function, would be nice if it was. When discussing hp and torque of a given engine it is usually refered to as HP and torque curves, thus the non-linear nature.

The reason for this was contained a bit in what Ken was summing up about the internal components of any engine. I raced stockcars for 10 years and built my own engines resulting in many hours of of dyno testing. During this time I learned that  specs on the cam(lift & duration), cam timing, valve train, compression, ignition timing, carburator tuning(fuel curve), exhaust system design, fuel and even weight of the lubricating oil all affected the rpm my engine made the most torque and hp. There is even more but that is really getting off the subject. The bottom line here is that each engine is taylored to yield the desired results for a given application.

In the case of aircraft one limiting factor is propellor RPM. Propellors have a max speed they can turn before the tips approach the speed of sound which is not a good speed to be at. I am not a prop expert so I cannot explain much about them. Given the limits one must stay within(due to the prop and other factors), aircraft engines are adjusted accordingly. ie where in the rpm range they produce the most power, useful rpm range etc.

Also, when talking engines most people misunderstand the terminology. Torque is what is actually measured on an engine dyno and is the amount of turning force an engine can put out. Horsepower is the rate(work done over time) at which the engine can produce torque calculated from torque and rpm.

Just my 2 cents on engines.

Vin

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Re: What RPM do you cruise at in your 172?

The required HP to turn a propeller at a certain speed depends on, among other factors, air density.  At low altitudes, air density is higher so you must develop more horsepower to turn a propeller for each RPM setting.  You can see that in your performance charts.
   
The % HP is not the same thing as % RPM, as Ken and Vin said, this is not linear function.

I am a mechanical engineer and have over 28 years of experience, but not in aviation.  So what I am going to say is based on my experience in the nuclear power industry, which has very stringent design standards. I presume the aviation industry also has very similar standards.  So here is my opinion regarding why you should operate at 75% cruise in stead of full throttle when full throttle exceeds 75% power.  The manufacture has a design point at which all limits meet required margins of safety.  Also, the most information is known at the design point.  So operating at the design point is usually safe.  Operating above the design point for long periods of time can be detrimental to your health. 

Another consideration is age of the aircraft and engine parts.  When stringent design requirements are enforce, the design is based on an assumed operating life. That is, there would be assumptions about operating at the design point for X duration and certain number of cycles.  At the end of life all the design margins are still met.  After the end of life is reached, all bets are off.  What is the design life of an engine?  I don't know!  You need to get this information from the engine manufacturer. What is the weak link after the design life is met?  How does the design life vary based on how an aircraft is actually operated versus what is assumed in by the designer in establishing a design?  Is the weak link still the propeller speed?  What about a new propeller on an old engine? 

I don’t know, so I tend to be conservative about flying or I think I am.  Unfortunately, at this point in my aviation career, I don't even know what I don't know, but I’m getting there!  In fact, I'll bet there are volumes that could be written in this category.  So I try not to “push the envelope” very far the known.

my 2 cents

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Re: What RPM do you cruise at in your 172?

At the risk of beating a dead horse, regarding the HP requirment increasing at lower altitudes.  The propeller generally moves a fixed volume flow rate of air at a given speed setting regardless of air density.  As the density of the air being moved increases more work is required to move it.  As someonelse said, since HP is rate at which work is done, the required horsepower must increase to turn the propeller.

In my last post, I thought I answered the question by discribing the result rather than answering Bob's question.

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Re: What RPM do you cruise at in your 172?

If I missed the answer, sorry about asking again.  If my engine red lines at 2700rpm, how do I calulate what 75% power is?  Or say 65% or 55% for that matter....

   Dave

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Re: What RPM do you cruise at in your 172?

Dave--after reading the last few posts, I think the answer to your question is "read the manual" !  smile  Sounds like a lot of rocket science went into determining redlines, etc., so I am following the good advice of Cessna.

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Re: What RPM do you cruise at in your 172?

The only way to find 75% or any other % is by the specific manual for that engine and the charts - there is almost always some degree of interpolation as it's usually a progressing curve ( not with one radius ) - I shouldn't get into all the types of curves either.

The redline may be the max RATED, and also the max DO NOT EXCEED.

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Thanks

Ken Wanagas

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Re: What RPM do you cruise at in your 172?

All the neat engineering aside, I agree, get into the POH and you will see the information you are looking for.

I did that with my 150 and have found an RPM for all occasions! Seriously, I run 2600rpm right up to 8000ft. At 3000 it works out to 78% and at 6000 it is 75%. I thought this was a good comprimise between speed and engine health. I also lean as Cessna/Continental recommends.

Very informative discussion, thanks to all that contributed.

Vin

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Re: What RPM do you cruise at in your 172?

Bob; Read which manual?  The flight manual for my '56 plane is only a few pages and this is not mentioned...

    Dave

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Re: What RPM do you cruise at in your 172?

Dave--good question!  I wasn't thinking about the early models....mine is an '81.   Maybe the engine mfgr. has some specs., or maybe the tach at least has a red line that you can stay under. I have a feeling that Cessna has the rpm's and power percentages that you need, somewhere, in some publication.  Call their customer support dept.  Good luck!

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