Cruising RPM - Engine welfare

[Robert Eilers]

Interesting discussion under Passenger Briefing regarding cruise RPM. I have always thought (not sure why) that it was best for the engine to operate at lower RPMs, i.e., 2450 vs. 2600. The suggestion that it might be OK to operate at higher RPMs and achieve better ground speeds is appealing to me for one good reason. My wife is only good for about 3 hours max in the airplane - after that she is a basket case. Therefore, the farther I can get in 3 hours the better (I'm willing to pay for the fuel). So, what is the general concensus out there - is cruising at 2600 RPM acceptable engine management (assuming leaning is not aggressive) - watch engine temps and go for it? Is it actually better for the engine to run at high RPMs (as was suggested) or is it better to operate at more conservative RPM settings?

[bsdunek]

I know that the original owners manual for my 170A says maximum cruise rpm is 2450. I know that this is below 75% power at altitude, but they don't say anything about higher rpm at altitude to attain 75% power. In 1950, those things didn't seem to be considered in owners manuals, and I don't know where to find better information.
I can see your dilemma, but can't offer any better advice. Hopefully, someone on this forum will have some better information.

[flyguy]

I can only repeat what I posted over in that other discussion. Bruce and I share much the same idea about what the C145 likes.

I can't understand why the "Maximum" cruise would be any different in the POH for the '49 - '51 170A with the C145 than for the later '52 version. The "recommended" cruise of 2450 RPM is in "Section III, in the first part of the '52 POH. In Section IV, pp 39, the "cruise performance" chart details the RPMs that will produce different results.

As I said in that discussion, I have always run my engines around 2650 - 2700 without problems, except the first one. On that first engine, I did have one valve stem fracture and the valve head embed itself in the top of the piston. I was not aggressive on leaning at that time and this was about the time that 80 octane phased out in our area. The diet of only 100LL with poor leaning technique may have been the cause of my problem. The apparent culprit was considerable pitting and corrosion under the carbon/lead deposit on the stem and radius of all six valves. The plane sat idle for much of the six years I was in Saudi Arabia and the failure occured shortly after we returned and started flying a lot. I choose to believe those deposits attracted moisture and caused the pitting of the stems just behind the valve head, and thus the failure. The engine had 1760 hours when it failed.

I never had a stuck valve on the next two that I ran to TBO without any problems, nor (NOK ON WOOD ) on the current engine on the plane now. The much more aggressive "leaning" and running higher RPMs seems to have solved that problem. On overhaul of "Engine #2, I didn't notice excessive wear on internal components or excessive build up on the valve stems. Except for being an erratic "OIL" burner, this current engine gives me no reason to "BABY" the old C145s.

So MFWIW says if the slightly higher gas cost isn't a factor, go for the SPEED!

I also have a C175 with the GO300 that turns 3200 RPM at cruise. Much the same "block and pistons" but a little more sophisticated oil cooling system. According to several mechanics I have talked to over the years, many operators of the C175s had problems because, in their thinking, the O300 should not be subjected to such violent treatment and they moped around behind the power curve and caused the GO300 to prematurely fail. Although it does come with a greatly reduced TBO, I think that was a perception that higher operating RPMs would wear out the engine sooner. This one I have has already gone way past the recommended TBO and shows good numbers. That leads me to think there is little wear of critical parts.

[GAHorn]

Robert, if you view the Cruise performance charts in your Owner's Manual you can see that at 7500 ASL, 2600 RPM is only 66% power. In other words, IF you can achieve that RPM (and with a standard prop it's unlikely except in a descent), you are not anywhere near to straining this engine. Even at Sea Level, 2500 RPM is only 75% power, and THAT is certainly not a strain either.
Looking at a median altitude of, say 5,000 feet ASL.... the red line of 2700 is only 75% power!

Are you getting the idea that RPM does not hurt this engine? (And also, isn't it fairly obvious that high RPM is not normally attainable in level flight above 5,000'?)

2450 is the recommended at low altitudes, however, and will result in a LEANED fuel burn of about 8 gph.

[Robert Eilers]

Yes - I did review the performance charts for the 170 and noticed that the fuel burn and BHP figures are based on leaning to best RPM. I just had difficulty over coming the bashed in cautions over the years about operating at high RPMs - "you know RPM kills engines" I was once told by someone I thought should know.

[GAHorn]

Yeah, I know what you mean. But standard-sayings aren't always applicable in every circumstance.
It might be safer to say "Too MUCH RPM kills engines." Of course, according to the charts, this engine's normal operating range is all the way up to 2700 so anything up to that is not considered harmful.

(I'm reminded of the famous sayings like "look before you leap". ... and how they are almost always contradicted by those like "he who hesitates is lost". )

[flyguy]

George, does BHP have an application in rating performance figures for airplane engines with a fixed pitched prop? We know that our "long in the tooth" engines probably don't produce rated HP, but what about engines just out of the factory door? What about less restrictive exhaust systems like those on the Swift?

The description of BHP I posted on the other thread came from Wikipedia's offering when searching the item. I didn't edit out the "drive wheel" part that completes the whole description. Could a researcher use the "propeller in the place of the "offending" drive wheel?

On the subject of MP. Here is an article on MP, but figures described are obtained while using a CS Prop. Some of the properties will also remain valid with the fixed pitch prop on a normally aspirated engine.

http://www.warmkessel.com/jr/flying/td/jd/15.jsp

[GAHorn]

George, does BHP have an application in rating performance figures for airplane engines with a fixed pitched prop? We know that our "long in the tooth" engines probably don't produce rated HP, but what about engines just out of the factory door? What about less restrictive exhaust systems like those on the Swift?

The description of BHP I posted on the other thread came from Wikipedia's offering when searching the item. I didn't edit out the "drive wheel" part that completes the whole description. Could a researcher use the "propeller in the place of the "offending" drive wheel?

On the subject of MP. Here is an article on MP, but figures described are obtained while using a CS Prop. Some of the properties will also remain valid with the fixed pitch prop on a normally aspirated engine.

http://www.warmkessel.com/jr/flying/td/jd/15.jsp

This is a mixture of two discussion threads which may be confusing to the casual reader or to anyone just joining this discussion, but.... I'll attempt it anyway.

The definition of "brake horsepower" does indeed apply to our aircraft engines, but in a difficult-to-ascertain way due to "slippage" of propellers. Propeller design efficiency is a "black art" that leaves a lot to be desired when attempting to measure output or efficiency in hard engineering terms, (and I'm no engineer anyways.) There are just too many variables between individual props and atmospheric conditions for the average user to make hard-case claims.
TCM offers a "prop-loading" graph in their engine operator's manual, which is available and can be downloaded at our Members Only page, but it's not very friendly as far as graph's go. We basically must accept the engine mfr's claims with regard to HP/Output.
In the case of our engines, they only make "rated" HP of 145 at 2700 RPM. How much of that HP is wasted is a matter of prop design efficiency. (For example, the fact that a single bladed prop is more efficient than a two-bladed prop which is more effiecient than a 3-bladed prop, but vibration becomes out-of-control with single-bladed props, and then there's the variables of blade-shape/form efficiencies versus atmospherics and actual RPM versus speed-of-sound... and you get the idea.
But in a "bottom-line" sort of conculsion, our C145/O-300 engines only produce about 118 HP at 2300 RPM (where the standard prop operates at static takeoff power), and that means we are not achieving full 145 HP for takeoff, so we need more runway. If one wishes to make more horsepower one needs more RPM and that means prop with a finer pitch that will allow the engine to speed up against the resistance of the air, (say to the 2700 maximum allowed.)
The problem with that is, once airborne, the prop will overspeed the engine's red-line of 2700 RPM after takeoff when the aircraft has accelerated, so.... the pilot must reduce throttle (power). Thus, in-turn, the contraption now needs a coarser-pitched propeller in order to regain efficiency after forward motion is achieved.
The standard 7653 prop is a "compromise" between takeoff power and cruise power efficiencies.

This situation is exactly why constant speed propellers are useful. They can be kept at a constant RPM regardless of throttle settings (within limits, of course.) So full 2700 can be had at takeoff, and a fuel-and-engine-saving lower RPM can be had during cruise or let-down. What's the trade-off/compromise? Weight and complexity and cost.

Our fixed-pitch props are simple, durable, and give the most bang-for-the-buck/weight, for most operators.

[SteveF]

This is a great discussion but I am unsure about the leaning with the increased RPM.

I have a 53B with an O-300-B using a 7653 pitch prop. Thanks to Ron, Bruce, George, and the association the engine has a nice new STC, 337, and log book entry. I am interested in running the engine at the best conditions for long engine life regardless of fuel economy or best mileage / range. I also use TCP in the fuel.

I have an oil temp gage and an EGT single probe. The forum says to watch the temp at the 2600 rpm setting. Should I continue to lean to max egt and back off 50 degrees, lean to max RPM and back off 50 rpm and arrive at 2600 rpm, or lean a lot less. I can work the throttle - it is setting the mixture that I need to understand better.

SteveF

[Bruce Fenstermacher]

I have a DM 7654 prop. Yes I had it twisted specifically to this pitch and measured twice. For those that don't know this would be between a standard and a cruise prop pitch for our airplanes.

At takeoff my MP is at least 25 inches and my RPM I believe about 2200 on the roll. On climb out RPM increases. I tend to climb pretty flat and at higher airspeed than most. At or under 3000 ft msl in level flight I set the MP at 22.5 inches which produces 2600 RPM. Full throttle produces 2700 and higher MP but I rarely do this.

Returning from Branson my throttle technique was to go full throttle full rich for take off and climb from the 1000 ft msl airport to 7500 ft msl. About 5000 ft I leaned the engine a bit. At 7500 ft I leveled off and leaned a pinch more. I never reduced throttle.

I took 3 readings throughout my 7.4 hour flight after each level off at 7500 ft msl and got the following results. MP 19.5 inches 2580 RPM, MP 18.5 inches 2600 RPM, MP 19 inches 2600 RPM. The RPM was taken using an electronic tack I had on board and is known to be accurate. BTW my aircraft tach was just overhauled at Keystone Instruments in Lock Haven and was with in 20 RPM at 2600.

So George, it is possible at least to 7500 ft msl to achieve the book rpm of 2600 or at least my engine can and with a higher pitch prop than standard. I did climb a few times to 8500 ft msl to clear cloud tops and noted that my RPM and MP where less but never recorded what I saw. I would suspect that if I had a 7653 pitch prop I might be able to maintain 2600 RPM to at least 8000 ft and George with your 7655 cruise prop I'm not surprised you can't get 2600 RPM at 7500ft msl.

I run my engine at 2600 RPM and will continue to do so. I do not believe I'm doing the engine any harm and do not feel running at a lower RPM will significantly change the life of the engine for the better. I should point out I have no long term experience like running 4 engines to TBO to back this up either way.

[Bruce Fenstermacher]

Steve,

If you want to run your engine at 2600 rpm then you would set your throttle at a position that after leaning using what ever technique you want to use, the rpm is 2600.

Personally I could never see much rise in RPM probably because I never had a tach that didn't bounce all over till a week ago. I lean by pulling the mixture about an inch and noting a rise in egt and cht and I leave it. From experience I know I'm not leaning to or past peak which is why I say I only lean conservatively.

I also don't worry to much if I'm running 2580 or 2630. I actually use my MP to set power. I set it at 22.5 inches and what ever the RPM is is fine unless it it at or near 2700 then I reduce the MP a little. I'm not afraid to run 2700 I just don't want to exceed it.

[SteveF]

Thanks Bruce,

I don’t have a manifold gage so I have to go by RPM and EGT. Will lean to peak RPM or EGT and then back off so as to be on cool side of peak and close to 2600 rpm.

As an aside when I was coming back from Petit Jean I was at 9500 ft and 2450 RPM leaned as usual and the throttle was about an inch short of full throttle so I think the 2600 RPM can usually be maintained to a fairly high altitude like the 7000 ft you saw.

I will try it tomorrow and let you know.

[GAHorn]

I have a DM 7654 prop. ...I took 3 readings throughout my 7.4 hour flight after each level off at 7500 ft msl and got the following results. MP 19.5 inches 2580 RPM, MP 18.5 inches 2600 RPM, MP 19 inches 2600 RPM. ...So George, it is possible at least to 7500 ft msl to achieve the book rpm of 2600 or at least my engine can and with a higher pitch prop than standard. .... ...

The problem with these sort of discussions is our inability to compare apples to apples. As you noted, my EM7655 prop is not the same as a DM7654 prop. It's also not the same as a DM or MDM7653 (the "standards".)
Additionally, none of us are using PRESSURE altitude and compensating for TEMPERATURE which would be mandatory in order to achieve meaningful comparisons. (In fact, we'd also ALL have to be using the SAME set of gauges, or at least a calibrated MASTER set of gauges for correction purposes.) The barometric pressure from Branson to Pennsylvania was doubtless quite different than from Branson to Texas, and the temperatures were likely different as well. I'm sure everyone sees the point.

Yep, found at least one error and my burn rate is now down to 10.7 per hour.....

(And my prop with the leaning technique I follow uses 3 gallons per hour less than Bruce consumes. I consistently use 7.8 gph at 2450 rpm and true 104 kts/120 mph)

The leaning technique that the Owner's Manual suggests is the one I use, and the one I recommend to everyone. It is not "best rich power" in the traditional sense... it is "best lean power". Basically it says to set the throttle, then lean until max rpm is achieved...continue to lean until the first indication of rpm reduction, then enrichen just back to the max rpm positon. (Note: This is slightly different than what the TCM Operator's Manual suggests: At any cruising altitude mixture control for best rich power by moving toward "lean" position to obtain maximum r.p.m. with a fixed throttle, then return toward full rich until r.p.m. drops just perceptibly."
The reason I recommend best lean power is since the factory did their recommendation using 80 octane, and we are stuck with higher lead content 100LL, it considerably reduces lead contamination/fouling (which contributes to valve sticking and spark plug fouling) as well as reduces fuel consumption, and at virtually any useful cruising altitude above 5,000 it will not cause engine damage due to the low power settings achievable at those altitudes.

...I should point out I have no long term experience like running 4 engines to TBO to back this up either way.

It should also be noted that neither has Ol' Gar. (His comment re: 4 engines included the present mystery-plane which hasn't run in years and may not actually exist.)

[flyguy]

WHOSA MATHEOMATIKS IN MESSIN UP NOW? SUMBODIES REEDIN SKILLS IS TU LAKIN AKRACIE I SED - - THE FIRST ENGINE FAILED AT 1750! I STARTED OUT WITH AROUND 500 "SINCE NEW" HOURS ON THE REPLACEMENT THAT CAME OFF A '57 172 AND RAN IT TO 1860 WHEN I CHOSE TO OVERHAUL. REPLACE THE 6 CYLS WITH OEM, AND THE AI AND I LOOKED CAREFULLY AT THE INTERNALS AND FELT WE DIDN'T HAVE TO GO INTO THE LOWER END - JUST ROD BEARINGS AND SEALS. TOTAL TIME ON THE ENGINE REMOVED FOR THE CURRENT ONE WAS AROUND 3650. THIS CURRENT ENGINE CAME OFF A '48 SWIFT THAT HAD ABOUT 200 HOURS SINCE NEW BUT HASN'T BEEN MY FAVORITE. PLENTY OF POWER BUT FREQUENT ERRATIC OIL CONSUMPTION, ALWAYS WITHIN CONTINENTAL LIMITS BUT TROUBLESOME. THAT AIN'T QUITE 4 ENGINES TO TBO BUT CLOSE

PROPS AND RPMS - - IF I REMEMBER CORRECTLY I BOUGHT THE CAL TWIST FROM FRANK SNAREY IN 1976 WHILE HE WORKED FOR TORRENCE AIR PARTS. THAT PROP IS A 7655 SOMETHING. I ALSO BOUGHT TWO NEW BENDIX MAGS AND PUT THEM ON AT THE SAME TIME AS THE PROP. I HAVE CONSISTENTLY BEEN ABLE TO SPIN UP TO 2700RPM IF I CHOSE TO. DON'T CARE MUCH FOR RUNNING QUITE THAT HIGH ALL THE TIME BUT ON THE DOZENS OF FLIGHTS BETWEEN HERE AND OUR HOME IN NORTH MISSOURI I COULD DO THOSE RPMS WITH BOTH OF THE TWO ENGINES. FOR MOST OF THOSE YEARS SINCE I PUT THE CAL TWIST ON I NEVER HAD A GPS SO ONLY THE LAST 5 YEARS COULD I MEASURE GROUND SPEED VS INDICATED AIR SPEED. MY CROSS COUNTRIES WERE FROM HERE TO MANY A GRAND TOTAL OF 7 MILES AT 100 AGL! DIDN'T WORRY TOO MUCH ABOUT "TIME TO CLIMB" OR FUEL BURN. JUST SCARE THE HECK OUT OF THE CORMORANTS ROOSTING IN THE DEAD SNAGS STICKING UP OUT OF THE LAKE! ON A COUPLE OF TRIPS OVER TO CONROE AND A COUPLE HOUSTON AREA, MY INDICATED WOULD BE AROUND 135 AND GROUND SPEEDS VERY CLOSE FACTORING IN THE WINDS.

[SteveF]

Today was a fantastic day in Massachusetts, clear and smooth.

I went out and flew at 2600 rpm instead of 2450. Did not do any fuel consumption checks but ran at 3 and 5 thousand feet at both rpm for about 20 minutes each. I leaned as I normally do to peak egt and back off about 50 degrees. At the end of the leaning I was at 2450 or 2600 rpm. I wound up pulling the mixture lever out about the same amount for both rpm.

My oil temp does not have temp on it only a large green arch. At 2600 the temp was a small amount higher than at 2450. On the gage it usually runs on the right edge of the letter O in OIL (Temp) and at 2600 it was between the I and L. Still left of center on the gage so just fine as far as I was concerned. Outside air temp at 3K was about 80 and at 5K was about 75.

Speed wise I picked up about 9 knots at 2600 rpm. At 2450 I was indicating a tad better than 110 mph and the gps said 89 kts and at 2600 I was indicating 120 mph and 98 kts. At 5 thousand it was 112 mph 90 kts and at 2600 at 5k it was 122mph 100 kts.

Long and short of it the engine sounded better at the 2600 rpm, the plane was more responsive, and you could sense the extra speed. No adverse oil temp rise and it leaned fine. When I got down I pulled the prop through and maybe it was me but it seemed there was more compression or resistance.

I am going to go with the higher rpm for a while and see what the exhaust stains look like and fuel consumption and range turn out to be.

SteveF