Efficient low altitude cruise

[photofly]

...in a normally aspirated 260HP (injected) 182. 27.5" MP, 2050rpm, and leaned back to 11gph. Who says you mustn't cruise "over square"? At least three or four knots faster than a more traditional setting for the same fuel flow.

image.jpeg (125.86 KiB) Viewed 5302 times

Note: analogue fuel flow gauge under-reads.

[CpnCrunch]

I notice you're above the green MP range, so presumably that isn't one of the recommended cruise settings in the POH. If it's ok to cruise at those settings, how come they aren't listed in the POH?

[switchflicker]

Also, the RPM is below the green arc which makes me think that it's not an authorized cruise setting. Might be faster and more economical in the short term but how about in the longer scheme of things. Bear in mind I'm not any kind of engineer (design or fixing type).

[Big Pistons Forever]

I googled Continental IO 470 Operators Manual and found a 2011 edition on line. It lists the numbers for every dash number of this model engine.

There is a power chart for each one and they all have a "recommended" cruise section that shows the recommended possible combinations of MP and RPM. The maximum recommended MP at 2050 is around 22 inches and the maximum recommended cruise MP for any RPM is 24 or 24.5 inches. The power setting shown by the original poster is far outside the manufacturers recommended practice so I personally would not use such a power setting

As an aside both Lycoming and Continental publish operator manuals for all of their engines. I highly encourage airplane owners to get one as you get far more detailed information than is found in a POH.

[Glasnost]

I'm not sure of your engine model, but here is some reading related to minimum cruise rpm.

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

[photofly]

"Not recommended" isn't the same as "prohibited". And something explicitly given as "not recommended" is further distinguishable from something that is merely absent from a list of recommendations.

However, to answer your question, this isn't the stock engine, it's injected, with balanced nozzles, so runs smoothly lean of peak. The "lean of peak is bad" argument was fought and lost ten or more years ago no, but LOP operation was not a realistic option when the POH was written. What surprised me today was how slow the engine would turn and still run smoothly.

I'm absolutely not violating any maximum or minimum limits given in section 2 ("limitations") of the Aircraft POH, either the original or the flight manual supplement for the engine replacement STC.

[photofly]

I'm not sure of your engine model, but here is some reading related to minimum cruise rpm.

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

That's extremely helpful. This engine is not one of the affected models.

The power setting shown by the original poster is far outside the manufacturers recommended practice so I personally would not use such a power setting

The power setting is actually about 55% of maximum rated BHP. It's not possible to get more with such a low fuel flow. CHT's were stable at 310F approximately, across the board, well below even my extremely conservative personal limits of 350.

The maximum recommended MP at 2050 is around 22 inches and the maximum recommended cruise MP for any RPM is 24 or 24.5 inches.

Which is the usual nonsense about limiting power by controlling manifold pressure. The oxygen flow through the engine at 2050RPM and 27.5" of manifold pressure is identical to that at 23.5" pressure and 2400rpm, which would be more traditional. Lean of peak at 11gph, in both cases approximately the same power will be developed. But at 27" and 2050rpm you are wasting much less power against frictional losses in the engine and suction losses through the induction system. You also push the peak intracylinder pressure closer to TDC because of the slower rotation, which improves efficiency. Too close? I don't think so - there are no CHT issues.

I don't know if there are any counterweight resonance issues with low rpm operations but TCM haven't flagged this model.

[CpnCrunch]

Which is the usual nonsense about limiting power by controlling manifold pressure. The oxygen flow through the engine at 2050RPM and 27.5" of manifold pressure is identical to that at 23.5" pressure and 2400rpm, which would be more traditional. Lean of peak at 11gph, in both cases approximately the same power will be developed. But at 27" and 2050rpm you are wasting much less power against frictional losses in the engine and suction losses through the induction system. You also push the peak intracylinder pressure closer to TDC because of the slower rotation, which improves efficiency. Too close? I don't think so - there are no CHT issues.

I don't know if there are any counterweight resonance issues with low rpm operations but TCM haven't flagged this model.

Surely the CHT is just an average value, and doesn't tell you anything about peak cylinder pressure. As I understand it, CHT will be roughly the same for a given power setting, so if you increase MP and reduce RPM there should be no difference in CHT. What will increase is peak cylinder pressure, mean cylinder pressure, and torque. The increased cylinder pressures will cause increased wear to the cylinders, and the increased torque will cause increased stress on the engine components.

By saying your engine is within its CHT range all you're saying is that you're not doing any heat damage to the engine.

[photofly]

The torque is up by a factor of 2300/2050 or 12% over the same power at 2300rpm. It's the same torque at 2300rpm and about 65% BHP, which is very unexceptional.

Engine wear isn't caused by intracylinder pressure. The wear should be lower at 2050rpm vs 2300 because there are fewer power strokes.

I don't believe the peak pressure at that low power setting is anywhere close to damaging, but I'll have to look for corroboration.

[Big Pistons Forever]

Just out of curiosity what model of IO 470 are you running ? I ask because the only C 182 conversions I have ever seen use the IO 470 N version.

[goingnowherefast]

The engine and prop were designed to operate at certain RPM/load ranges. In your case, you are running a really high load at a low RPM. Putting excessive torque on the crank and bearings. Normally a manifold pressure that high is only used for a short period of time, and at a high RPM (take-off). This leads to less flexing of internal engine parts. Drop the RPM way down, and keep the high manifold pressure and the internals of the engine are experiencing higher amounts of flexing for a much longer period of time (cruise).

If it were my airplane, I'd set the RPM at the bottom of the green arc and the MP at the top of the green arc. They paint those lines on there for a reason, and I don't think it's to keep the paint shop busy.

This chart is from Lycoming for their 180hp engines, but the point is still the same.

[photofly]

I'm absolutely not running a really high load. I'm running at 55% of the engine rated BHP.

I don't think you know what manifold pressure actually means. When is the manifold pressure absolutely the highest and the RPM the lowest, in a normally aspirated engine? Power is not about MP, it's about volume of air pumped and fuel flow.

The red box is interesting but you're not interpreting it correctly. It's a "best power mixture" setting graph, and I'm not using a "best power" mixture. For instance, scaling up by horsepower, it suggests that operating my engine at 27.5" and 2000rpm would draw 14gph. That would be peak power at that rpm/mp and give 70% BHP. I'm not burning 14gph, Im way leaner than that burning 11, reducing the power, the peak pressure, the torque and the stress.

I am sensitive to resonance issues with the crankshaft balance, but I don't think there's a loading issue here.

Incidentally, the OAT was +15 degrees C. If the OAT was -15 degrees the engine (it's an IO470F Bonanza engine) would probably refuse to run at {2050RPM, 27.5", 11gph} - can you see why?

[CpnCrunch]

Power = torque * rpm. MP is essentially the torque, so I think there may be an error in your logic when you say "same torque at 2300rpm and about 65% BHP", but I don't have time to figure it out now.

Essentially you're pushing the engine 10% above it's maximum recommended cruise MP, which means it's also 10% above it's maximum recommended cruise torque. While that may or may not be an issue, I'm not sure it's worth risking an expensive overhaul just to save a little fuel.

[DonutHole]

how do you know your analogue fuel flow gauge is under reading?

think maybe you should fix it?

[B208]

volume of air pumped and

Power is related to the mass of air through the engine. The mass of air is a function of volume, temperature and (manifold) pressure.

[Big Pistons Forever]

how do you know your analogue fuel flow gauge is under reading?

think maybe you should fix it?

The fuel flow gauge shown is not really a fuel flow gauge it is a fuel pressure gauge. Fuel pressure is proportional to fuel flow so the scale is presented as gals/hr, but it is not an exact relationship and these gauges will be less accurate at low fuel flow settings.

The only way to get really accurate fuel flow information is to have a true fuel flow transducer which measures fuel flow directly, like what you find in a shadin system. You should never lean a fuel injected engine by only looking at the analog fuel flow, you need good EGT and direct reading fuel flow to properly lean the engine, especially if you are anywhere near or below peak EGT.

As an aside the most valuable information that an analog fuel gauge will give you is full power fuel flow. It is very important that the gauge shows top redline fuel flow or even a little bit over on takeoff. A low fuel flow at takeoff power will quickly damage the engine.

[jschnurr]

Back several years ago, a certain flight school in Southern Ontario thought that to save fuel, they would run their fleet of Bonanzas at 2200/22" instead of 2300/23". It worked fine for a while, but after several hundred cumulative hours across the fleet and 3 catastrophic engine failures later, it was determined that the lower power setting was setting up harmonics within the engine that caused microscopic cracks to grow until failure.

I don't have the book on hand to see if 2200/22 was actually outside the Operators manual specifications, but the lesson is that often short term gain causes long term pain. Operating outside the recommended parameters (while it may not be specifically prohibited) can still be indeterminably detrimental down the road.

[Glasnost]

One thing the engine manufacturer counts on when considering power settings is the ability for the piston to move on the power stroke at a rate that will allow the expansion of the ignited fuel air mixture. If the piston is moving to slowly, the pressure will build up and cause detonation. This is the same reason why a spark that is too far advanced will cause detonation, because the piston isn't moving out of the way fast enough by the time the flame front reaches it. At a low MP, no big deal, the peak pressures aren't anything to be concerned about. If the engine is allowed to increase RPM with higher MP, again, no big deal. But if you look at the POH or engine operating handbook, they don't include every combination of MP and RPM that will give you 55 or 65 BHP for cruise settings. Typically, they will recommend lower MPs with lower RPMs.

Spark advance and CHTs is a specific limitation, and power settings are only a recommendation. But I'm smart enough to defer to those who know a lot more about engines than I do. I've also seen more than a few spun bearings and separated heads and broken big ends and thru studs in my career which could certainly be caused by operations outside of recommended parameters.

[SuperchargedRS]

Not sure about your engine, but on my 520 continental came out with a bulletin saying not to cruise below 2300, some words like "catastrophic" and "failure" were used.

[DonutHole]

how do you know your analogue fuel flow gauge is under reading?

think maybe you should fix it?

The fuel flow gauge shown is not really a fuel flow gauge it is a fuel pressure gauge. Fuel pressure is proportional to fuel flow so the scale is presented as gals/hr, but it is not an exact relationship and these gauges will be less accurate at low fuel flow settings.

The only way to get really accurate fuel flow information is to have a true fuel flow transducer which measures fuel flow directly, like what you find in a shadin system. You should never lean a fuel injected engine by only looking at the analog fuel flow, you need good EGT and direct reading fuel flow to properly lean the engine, especially if you are anywhere near or below peak EGT.

As an aside the most valuable information that an analog fuel gauge will give you is full power fuel flow. It is very important that the gauge shows top redline fuel flow or even a little bit over on takeoff. A low fuel flow at takeoff power will quickly damage the engine.

n

Thanks, I know

If the orifice on one if your injectors is plugged you can under read but in fact be pissing straight non atomized fuel into your cylinder and be running rich

[photofly]

MP is essentially the torque,

MP tells you nothing about torque. The MP at this precise moment is about 29.94" and the torque is ... zero. Go figure.

How do I know the torque? Because I know the power: 11GPH is close to 55% power when LOP, so you can work out how the torque varies with RPM. If the RPM goes down the torque goes up in mathematical proportion. Hell, that's how the constant speed unit reduces the RPM, by twisting the blades to absorb more torque so the engine slows down.

Someone asked about how do I know what the fuel flow is... there's a calibrated transducer and digital display, aside from the analogue pressure gage.

Not sure about your engine, but on my 520 continental came out with a bulletin saying not to cruise below 2300, some words like "catastrophic" and "failure" were used.

Someone posted a link to it earlier in this thread. This engine isn't in it.

Back several years ago, a certain flight school in Southern Ontario thought that to save fuel, they would run their fleet of Bonanzas at 2200/22" instead of 2300/23". It worked fine for a while, but after several hundred cumulative hours across the fleet and 3 catastrophic engine failures later, it was determined that the lower power setting was setting up harmonics within the engine that caused microscopic cracks to grow until failure..

Maybe they should have tried 2050/27.5" then. 450 hours past TBO on this one so far, and still going. I'll let you know what happens.

volume of air pumped and

Power is related to the mass of air through the engine. The mass of air is a function of volume, temperature and (manifold) pressure.

Yes, my whoops, sorry.

[photofly]

One thing the engine manufacturer counts on when considering power settings is the ability for the piston to move on the power stroke at a rate that will allow the expansion of the ignited fuel air mixture. If the piston is moving to slowly, the pressure will build up and cause detonation. This is the same reason why a spark that is too far advanced will cause detonation, because the piston isn't moving out of the way fast enough by the time the flame front reaches it.

This is very interesting, and the relative advance in the peak pressure back towards TDC with low rpm (because the piston is moving more slowly) is one part of why the efficiency increases, but also why you shouldn't operate high power settings with low RPM. However one of the features of very lean mixtures like the one I was using is the flame speed is low, so peak pressure is delayed, allowing lower RPM to be used. You normally see the result of low flame speed as a high EGT.

[Big Pistons Forever]

Not sure about your engine, but on my 520 continental came out with a bulletin saying not to cruise below 2300, some words like "catastrophic" and "failure" were used.

Someone posted a link to it earlier in this thread. This engine isn't in it.

.

This engine isn't in it.

So far........

[photofly]

That's a fair comment. There aren't thousands upon thousands of hours operating this engine with those parameters like there are at 2300/23". Does that mean nobody should? I'm not smart enough to answer that one.

[Changes in Latitudes]

Is this your bird or someone else's?