Hi all, i've been viewing now for about 8 months now. Great site , TONS of info. I've only been flying for 4 years now. (52yo got started late) I currently own a glastar with an O-360 cs prop.I do alot of $100 dollar hamburger flights to nowhere. My question is, i like to slow the prop down to something like about 19.5 inches MP 1920 rpm.Fuel flow drops down to around 6 gph leaned to around 1400 egt..Still around 127 mph,plenty fast, as i would rather be in the air than on the ground.Factory new engine with about 300 hours .Am i doing harm to the engine at these settings?Cylinder head temps are around 350. Any info would be appreciated.

Do you have a cylinder heating problem if you try to maintain 2500 RPM in cruise or full throttle in a climb? Have a friend with an O-360 in a Glastar. In cruise his fuel burm is similar to mine..I have an O-360 in a Citabria.
HC

It will go up to about 410 in a hard climbout in the summer. I just lower the nose a little and it will drop.According to my lyc. manual it is safe to 435.It's usually around 380 on normal cruise.24-24 around 10 gph.But if i pull it back to about 19.5 square my fuel flow goes down to about 6.2 gph and lose about 12-15 mph.

Depends on the type of prop you have and the placard for it. With an MT you can cruise there all day. Just keep in under 5" out of square.

I'd run a little higher manifold pressure, but 1900 rpm and 21 inches works just fine, and they'll run there all week.
MTV

mtv wrote:I'd run a little higher manifold pressure, but 1900 rpm and 21 inches works just fine, and they'll run there all week.
MTV

I agree and the fuel flow will not be significantly different.

I run a 0-540 (basically a 6 cyl 0-360) at 19 squared based on a recomendation from the engine builder. It really likes that setting and runs easily there all day long.

Many answers you will get on this thread are anecdotal; The techniques others use with seemingly good results.

What I'd like to hear is what anyone thinks the theoretical negative effects are on engine health in the long run of running lower RPM, if all other adjustable parameters are kept the same: EGT, CHT, ratio of MP to RPM, etc. Aircraft engines are detuned already to run direct-drive in an RPM range optimized for the propeller, which in turn also gives them longer life. The GO-300 for instance (same engine as O-300), put out 180hp at much higher RPM than the O-300's 2700 rpm redline, but was geared down so the prop could run at a "normal" RPM. TBO was shortened as a result.

Oil distribution will be the same. The only possible thing I can think of is balance. Like dawgdriver said, the RPM range is generally tied to the prop. In some cases there is an RPM range that a particular prop will not balance well at, and extended period in that range will reduce longevity of bearings, journals, anything that has a force exerted upon it. If all is smooth, I can think of no ill effects of lower RPM in cruise aside from speed.

Lowering fuel flow is an objective, but the engine only responds to mixture, right? If CHT and EGT are within normal operating range at a favorable fuel flow, all seems good.

Effect on valves and valve seats? Proper CHT and EGT, and maybe cleansing additives are the only answer to valve health as far as I know.

Interesting topic, because I have read on the 170 forum that "babying" the O-300 would do no good, and possibly was a poor technique for caring for the engine. What the theory is behind that, I would like to read.

Zane,

This is not an O300, and it is equipped with a constant speed propeller. Pretty different beast, in that you can adjust the propeller pitch to make the propeller much more efficient.

Note that on the big turboprops, for example, the props typically turn somewhere around 1800 rpm, rarely much faster. Props that turn slow are much more efficient at turning horsepower into thrust, which is the name of the game.

I've asked Lycoming reps what they think of running these engines at low rpm and oversquare. Their response: Don't do so during break-in of the engine. Once the engine is broken in, have at it.

MTV

mtv wrote:Note that on the big turboprops, for example, the props typically turn somewhere around 1800 rpm, rarely much faster. Props that turn slow are much more efficient at turning horsepower into thrust, which is the name of the game.
MTV

Excellent discussion. Let me throw in the counter argument and say that props are more efficient at higher rpms, provided the tips are kept out of the trans-sonic range. Turboprops turn 1800 because the props are long and they have to turn that slow to keep the tips from howling. Take a King Air for example. The props are max rpm (decreased pitch) at takeoff and then get pulled back (increased pitch) for cruise. Max rpm = max thrust for takeoff. Slow the props down or increase the pitch for better cruise performance.

The PT6 on the King Air is not a direct drive system. The prop is not physically connected to the engine, it uses the high speed gases exiting the engine to drive a set of blades that turn the prop. The Garrett 331 is direct drive, the gearbox that turn the 40 or 50,000 RPM (whatever it is) engine RPM's into 1900 prop RPM's would make your high school auto shop teacher go weak at the knees.
Turboprops are a whole different ballgame.
Interesting thread for me since I haven't flown a piston engine for a while. Looks like everyday operating practices have been updated a bit, not to mention a lot better and more accurate EGT and CHT information. I guess the old practice of lean it until it runs rough then richen it up half a turn is a relic of the past.

crazyivan,

Your argument is actually supporting my point. What I was discussing and what the thread started talking about was CRUISE power settings. You're absolutely correct in that takeoff power settings are a very different scenario. And, you're also correct that turboprops run slower rpms because of the longer blades on those propellers. Nevertheless, the slower you can run those propellers (within practical limits) the more EFFICIENT the propeller is (efficiency is not necessarily what we're looking for at takeoff power, but it is key in cruise), which is why you can achieve lower fuel burns at lower POWER settings by running the prop at low rpm. You may still be running 65 % power at that much lower prop rpm, but you'll burn less fuel, because the propeller is actually converting that power into thrust, which is what we really need for efficiency.

I'll present an example from the Husky, and forgive me if my numbers are a little off, I don't have a power chart handy. As I recall, a power setting of about 2300 rpm and 21 inches of manifold pressure provided a certain percentage brake horsepower, say 65%. At about 1900 and 22 inches, you might also be running the engine at 65 % power, but you'll be burning almost one gallon an hour less fuel, while going the same speed or maybe a tad faster. Those numbers are from my foggy memory banks, so they aren't precise, but they illustrate the point.

And, this is distinctly NOT new technique, folks. Consider that while Charles Lindbergh traveled throughout the Southwest Pacific during WWII working for Chance Vought as a technical advisor on the Corsair he taught the AAF P-38 pilots how to extract some 30 to 40 % more range from their fuel loads.....by running low rpm/high manifold pressures--the very technique that allowed him to cross the Atlantic.

The speed of the turbine has nothing to do with this discussion, actually. The speed of the propeller is the key. Take a look at the Wright Brothers 1903 Flyer. They had a VERY low horsepower engine, but VERY long propellers, being driven at very low rpm. It turns out that the brothers pretty much hit a home run on propeller efficiency--those propellers were just about as efficient as any propeller made to date. Turning at very low rpm.

MTV

I use the JPI with fuel flow coupled to the GPS. This allows me to see the Knot MPG for each power setting. Continental Big bore 6s. Surprisingly at the low cruise speeds RPM / MAP changes work out to .1-.2 mpg changes. I have found that higher RPMs lower MAP I get the best MPG for a specific IAS. Our aircraft engines have fixed timing that really limits us. I use and tell my pilots for x-country 8K - 10K cruise alt , WOT into the wind 2550 RPM LOP, with the wind WOT 2450 rpm LOP. Each airframe, Engine design, prop combi changes the optimum settings. I built a O-235 Long Eze test bed with Electronic ignition and EFI. Incredible performance from our engines with modern accessories. Get a good engine instrument and hook it up to the GPS. Some even have vibration analysis that tell you more than the designers had when they wrote the books.

I just read an old 180/185 Club newsletter where a guy claimed the oil consumption of his 470J went down by running a couple inches oversquare at low cruise power instead of squared. He thought more engine pressure being put on the rings made them seal better.
I've only owned fixed-pitch prop airplanes, all of which were pitched pretty flat (medium-to-climb). Judging from what I've seen on the MP gauge in a couple of fixed-pitch airplanes, it seems like you run pretty undersquare with that set-up. I know the fuel burn seems pretty high-- for example, I burn close to 8.5gph with my O-320 at 2500 rpm turning a 74-56 prop. I think that's around 65-70% power, & I see about 125 IAS. I think with a c/s prop I could cruise at the same speed at the same or lower power, on at least a gallon an hour less fuel.

I usually fly reduced power settings with an O-520/C66-90-2. Not in any hurry, less fuel I burn=more time I get to fly . 16-17"s MP, 2300 RPM. I have the idea that the engine is working easier at these lower settings by turning a little higher RPM and less tendancy for lugging/detonating.

I was out flying yesterday and playing with this a litle. I do see a little gain in speed and reduction in fuel burn by dropping the rpms down around 2100, temps seem fine, engine is running smooth. Is this fine, any risk to the engine? Can I safely run lower rpm's in the 15-17" MP range? The bottom of the green on my tach is 2200.

Thanks Gary

Shortfielder,

You SHOULD have a book that provides you with power charts for your engine. If not an engine manufacturer's book, then refer to the airframe manufacturer's POH guidelines for power settings. If all else fails, contact the engine manufacturer's technical representatives and chat with them about power settings. They are the real experts. For Continental engines, Loren Lemen is a GREAT resource, and he knows more about those engines than anyone, and he'll shoot you a straight story.

You aren't going to damage that engine by running low rpm. That's not what causes detonation or pre-ignition. Again, though, follow the engine manufacturer's guidelines.

Skydive,

You are absolutely correct in noting that every engine is different. I have always run IO 520s pretty hard, and they seem to like that. We started switching our IO 520s to IO 550s. Both big Continentals, and some of our pilots didn't bother to read the engine manufacturer's power charts for the bigger engine. Continental prohibits leaning the IO 550 above 70 % power, and we ruined a few engines before we got ahead of that.

I agree that running these big engines lean of peak offers the BEST fuel economy, no doubt. Unfortunately, carbureted engines are a LOT harder to get to run LOP.

MTV

shortfielder wrote:I usually fly reduced power settings with an O-520/C66-90-2. Not in any hurry, less fuel I burn=more time I get to fly . 16-17"s MP, 2300 RPM. I have the idea that the engine is working easier at these lower settings by turning a little higher RPM and less tendancy for lugging/detonating.

I was out flying yesterday and playing with this a litle. I do see a little gain in speed and reduction in fuel burn by dropping the rpms down around 2100, temps seem fine, engine is running smooth. Is this fine, any risk to the engine? Can I safely run lower rpm's in the 15-17" MP range? The bottom of the green on my tach is 2200.

Thanks Gary

I have the 520 on my Bo. Around the local area I usually run a little less than 45% which is 19"/2100 and then lean it back to 8.5 GPH. This allows me to be only slightly faster than my 182 buddies if they run wide open so we all get to the restaurant about the same time. If I need to go a little faster I can bump up the RPM's to 2300 at 10.5 GPH. When going cross country I always go wide open.

My concern(and there may be others that I am not aware of) is the possability of running a lower power setting15-17", and trying to take too big a bite with the prop1800-2200 rpm, and the possability of causing harm to my engine(pponk O-470-50).

Perhaps to illustrate how engine-dependent this stuff is, there's this SB from Continental about not cruising below 2300 RPM on a bunch of models of the 470 and 520:

http://www.tcmlink.com/pdf2/CSB09-11.pdf

My engine isn't on the list (O-470K), but it gave me some pause when I read it.

shortfielder wrote: possability of causing harm to my engine(pponk O-470-50).

I want to know what this "harm" is? We always speak of "harm" in sort of a mysterious, unspecified, vague bad consequences sort of way. I am not disagreeing that "harm" may occur, I just would like to know what the actual consequences are.

Low rpm + high load resistance? High torque required to spin the prop but the power isn't being generated. How are the forces on the crankshaft, camshaft, piston wrist pins, rods, etc increased? I would like to know where the stress is increasing and why. Where can strain finally occur?

I try to make the analogy of lugging a car with a manual transmission. We know for certain that can be damaging, but why are the forces actually increased on engine internals? Momentum and inertia carrying the crank into the next stroke?

There has to be a graph somewhere.