Efficient low altitude cruise

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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?

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I think if you fly someone else's aircraft you should fly it the way they want. This one is mine.

I see in the TCM engine manual that 55% power is approved at 2050RPM. The data is for peak power mixture so the MP given is lower, but I have no doubts about the safety of adding more air for the same fuel flow making the mixture leaner, cooler and longer burning, and delaying the pressure peak past where it would be at the "approved" mixture for this rpm/power.

[CpnCrunch]

I think if you fly someone else's aircraft you should fly it the way they want. This one is mine.

I see in the TCM engine manual that 55% power is approved at 2050RPM. The data is for peak power mixture so the MP given is lower, but I have no doubts about the safety of adding more air for the same fuel flow making the mixture leaner, cooler and longer burning, and delaying the pressure peak past where it would be at the "approved" mixture for this rpm/power.

I think I see your point. MP is basically telling you how much air mass is entering the cylinder during each cycle. If your mixture is very lean then that means you don't actually have a high torque, so it probably isn't going to overstress the engine.

Even so, if it was my plane I'd still prefer to stick to the POH than second-guess the designers with my own calculations.

Out of curiosity, is this a 182 with an Air Plains or Texas Skyways conversion? What kind of cruise speed do you get out of it?

[photofly]

Peterson conversion. It's a 260SE minus the canards.
Typically 130KTAS on 10.5 - 11 gph.

[CpnCrunch]

Peterson conversion. It's a 260SE minus the canards.
Typically 130KTAS on 10.5 - 11 gph.

What's max cruise speed?

[photofly]

I don't know; I've never tried to find out.

[CpnCrunch]

I don't know; I've never tried to find out.

How come?

[Big Pistons Forever]

Photo

55 $/hr errrr........ I mean 10.5 -11 gals hr at 130kts is doing pretty good. When I had my 180 with the stock 230 hp O 470, a realistic min cruise fuel flow was around 12 gal/hr at 65 % power.

A question:

What fuel flow do you get at, for lack of a better word, a conventional power setting that gives the same percentage power; like 23 in MP and 2300 RPM leaned to the same LOP value ?

Finally the whole "lean of Peak" thing is hardly new. When I flew the DC 6 we always cruised lean of peak, in this case set by referencing a drop in the BMEP gauge. This was the approved procedure in the manual which was written in 1951. As an aside you would see some eye popping numbers with the big Radials. Takeoff was up to 59 in MP at 2700 RPM but there was a strict 2 min limit on this. Also there was a chart that showed max allowable MP for every RPM setting. At low RPM's max MP was severely restricted. So for instance say running 59 in at 2000 RPM, a combination far outside the permitted range, would probably result in an uncommanded power reduction....to zero, in fairly short order

Lean of Peak is not a free lunch however. I occasionally fly a Cessna 210 with GAMI injectors and the latest G4 engine monitor. The difference between 75 Rich of Peak and 75 Lean of peak is a reduction of about 3.5 Gal/hr, but you also lose 12 kts of true airspeed. Most days you are better off but if there is a very strong head wind there is little fuel savings with LOP and the trip time will be increased. The one exception is on very hot days. It is hard to keep CHT's down running ROP unless you trail the cowl flaps. LOP however you will see up to a 40 deg C reduction on CHT's and the airplane easily runs cool with the cowl flaps fully closed.

I also occasionally fly a C 206 on Amphib floats. Because the aircraft is so slow there is very little advantage running LOP because the speed reduction almost negates the fuel savings.

[photofly]

A question:

What fuel flow do you get at, for lack of a better word, a conventional power setting that gives the same percentage power; like 23 in MP and 2300 RPM leaned to the same LOP value ? .

It's such a long time since I've cruised at 2300rpm I can't remember. I'll check it out tomorrow, and let you know.

Down low I I pick a fuel flow (10.5 or 11gph) and a cruise RPM (2150 perhaps) and open the throttle to adjust MP to get the best airspeed. (Clearly you need readjust the mixture with each throttle adjustment to bring the fuel flow back to the required value.) MP usually ends up at 23 to 25" in summer but only 21-23 in winter. I know you all laugh when I moan about trying to set power by picking a manifold pressure value, but the difference in MP for the same RPM/fuel flow/power setting between winter and summer temperatures is really obvious this way.

Yesterday, at 3000'ASL and 18°C, I thought I'd have a laugh and see what MP worked best for a really really low (for me) RPM - 2050 - with the same fuel flow. Turns out it was with nearly wide open throttle - and the TAS was a few knots faster than usual.

[Big Pistons Forever]

Thanks it will be interesting to see the difference.

My fearless prediction:

I bet that you can achieve pretty much the same results, 130 kts 10.5-11 GPH leaned to your usual LOP value at a 23 squared settings as you are getting at very oversquare power settings.

It sure is easy to be brave on an anonymous internet forum


Anyway back to the topic at hand. The never fly over square thing is obviously wrong. Your engine is not going to blow up if you set 23.5 inches and 2300 RPM. However saying no over square power settings is a myth is not the same thing as saying you can set any combination of MP and RPM that strikes your fancy.

For instance the Lycoming provided "Operator Manuals" for pretty much all of their engines does have a chart of allowable MP vs RPM. For instance the IO 540 allows any amount of MP at 2400 RPM or higher. Below 2400 RPM however lower MP limits apply. So 28.5 inches and 2400 RPM is permitted, but 26.5 and 2350 is not.

Continental Manuals do not have equivalent guidance, but clearly there is an issue with low RPM cruise settings in most of the 6 Cylinder Models, or they would not have issued such a strongly worded Service Alert.

Like everything else in aviation before trying something new, it is important to check all of the official references, POH, Engine manufacturers Operating Manuals, Service bulletins etc, to make sure you have all the information to make an informed decision.

[photofly]

That service alert included a very specific list of engines, including one model of IO-470. But not this one. That speaks volumes to me.

Let me ask you a question: in the Operator's manual for this engine, 55% power at 2050 rpm is inside the approved cruise power box. Why would it be any more stressful to achieve that at 27" vs. 25", or 23"? The fuel flow is way too low to cause excessive CHT or detonation. Other than that, and resonance problems (which are likely to depend on RPM and power - already approved by the manufacturer) - what are you scared of?

You appear to be succumbing to the temptation to view MP as an all-encompassing mysterious proxy for engine power, stress, or damage, or something. It's not - it's just the air pressure inside the inlet manifold; to the engine it has no further significance.

[Big Pistons Forever]

You appear to be succumbing to the temptation to view MP as an all-encompassing mysterious proxy for engine power, stress, or damage, or something. It's not - it's just the air pressure inside the inlet manifold; to the engine it has no further significance.

Actually I am simply using MP as one of the factors that Continental uses to define "recommended cruise" power settings. So according to the chart in the Continental Motors Operators Manual "Sea Level Altitude Performance Cruise IO 470F" the highest MP for 2050 RPM that is within the "recommended Cruise" area of the chart is 23.4 inches MP.

My personal preference is to follow manufactures or other approved data sources for operating aircraft. However you are quite correct that your 27.5 in 2050 RPM settings is not specifically prohibited. So ultimately the choice is up to the pilot. The good news is that these kinds of discussions are IMO, one of the strengths of Avcanada. We may not agree on this subject but it has been a good discussion. The only bad practice is to mindlessly use some power setting because your instructor told you so without any understanding of why, something I find discouragingly common.

Finally I find that the 6 cylinder Continentals seem to like the 2200 to 2300 RPM range for smoothest operation, which is one of the reasons I tend to stay in this band. However every engine seems to have a sweet spot for the smoothest operation so I will hunt around for the "best" RPM which is hardly ever a round number.

[photofly]

So according to the chart in the Continental Motors Operators Manual "Sea Level Altitude Performance Cruise IO 470F" the highest MP for 2050 RPM that is within the "recommended Cruise" area of the chart is 23.4 inches MP.

And, according to the same graph, when you follow TCM's recommendation for setting the mixture at that MP and RPM what is the power developed by the engine? What is the fuel flow? Is that a rich-of-peak mixture?

[co-joe]

I read an interesting book with a terrible name years back by; Lieutenant Richard C. Kirkland who flew Lockheed P-38 Lightnings in New Guinea during World War II, called "War Pilot". Good read. I remember one story in the book where Lockheed engineers came up with a weird combination of extremely high MP and low RPM that allowed the P38 to extend its range something quite large. I donated the book years ago so I can't give you exact numbers but it was basically doing something that the flyers had been specifically trained not to do in setting very low RPM and greatly increasing range which in the south pacific made a land based aircraft much more effective.

[Big Pistons Forever]

So according to the chart in the Continental Motors Operators Manual "Sea Level Altitude Performance Cruise IO 470F" the highest MP for 2050 RPM that is within the "recommended Cruise" area of the chart is 23.4 inches MP.

And, according to the same graph, when you follow TCM's recommendation for setting the mixture at that MP and RPM what is the power developed by the engine? What is the fuel flow? Is that a rich-of-peak mixture?

You are right there are countless combinations of all the factors that determine what power the engine produces. Specifically with respect to Manifold Pressure it seems to me that there has to be differences in the instantaneous peak cylinder pressures, flame front propagation rates, wrist pin peak loads etc etc between say 21 in and 2300 RPM and 27 in and 2050 RPM even though they generate the same percentage of power. To me that is a convincing argument to colour inside the lines of the manufactures power chart. I frankly don't know enough about the subject to want to experiment with unorthodox power settings.

I guess we will just have to agree to disagree on this one.

I still am interested in the quantitative difference in TAS and fuel flow between a very oversquare power setting and a more traditional matched setting like 23 in and 2300 RPM. I am looking forward to your report.

[photofly]

I did some tests yesterday. DIfferences to the day before are:
- aircraft 300lbs heavier (10% of max gross)
- OAT 10°C yesterday, 20°C the day before

10.5gph fuel flow

2300RPM/23"MP 124 KTAS
2200RPM/24"MP 127 KTAS
2100RPM/25"MP 124 KTAS


11.0gph fuel flow
2300RPM/23"MP 126KTAS
2200RPM/24"MP 127KTAS
2080RPM/25"MP 129KTAS

I couldn't get smooth operation at 2050/27" like the day before, I put this down to the air density difference due to the different temperature. The air yesterday was (273+20)/(273+10) = 3.5% denser, which accounts for about a 1" lower MP.