Piper Arrow 180 Help
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The way to hurt engines is actually the opposite. By that I mean operate with very high RPM and very low Manifold pressures
It was required that I actually cause detonation in these engines at the outset of the test, to prove that I would detect it during the testing of the subject fuel. What I found for both of these engines, was that detonation could be caused on 80/87 Avgas, with a power setting of 2000 RPM, and 25" MP, while the engine was running very hot. I could repeat the detonation, with these conditions, at will.
Detonation takes time to occur. That time becomes available when the engine is turning more slowly. Increasing MP makes the engine work harder and hotter, which increases the risk of detonation. The faster you turn it, the less time is available for detonation during the compression stroke. Faster at a lower MP will give you the power of slower and high MP, but cooler. We downshift while towing a trailer up hill don't we? Same rationale, speed the engine up, lower the MP.
Interestingly, once the detonation I caused began, it was very hard to stop. Reducing power did not stop it right away, and the engine actually ran hotter for a minute or so, when I reduced the power. Though I did not damage the engine during these tests, I certainly cleaned off the piston tops well, indicating that detonation had occurred at a low severity. The pistons were replaced during the inspection which followed the testing.
Having reviewed a Cessna Cardinal RG Flight Manual I have, I agree that the IO 360 is approved to run over square. Obviously Lycoming has proven this is operation with adequate margin. They are the experts, so I will accept that - but I still don't do it. Watching the indication of detonation during my testing, and the temps running away on their own, has left me very cautious about running any engine hard, other than square or less. Maybe I'm being too cautious, and wasting some gas, but other than stuck exhaust valves, I have never experienced an aircraft engine drive train failure.
Sorry for the thread drift...
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I have a SAB Bulldog reference card in front of me which includes 1800 RPM, MAP of 22.5 (which gives a nice 4.8 GPH - cruising around 90 KIAS) with a maximum setting of 1800RPM / 25Hg MAP. I only mention it as it has a Lycoming IO-360 engine. Probably best to stick to the handbook / manuals rather than using rules of thumb like "never fly oversquare" unless you don't have the manual handy...
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Pilot DAR, no thread drift, very good post. Any private pilots transitioning to CSU would do well to digest your post above.
I have a best range question, still in line with the first post. I once read that Charles Lindburgh was consulted during WWII in order to get more range out of single seat fighters at the time in and around the Pacific theatre. I'm sure he advised, and flew to prove it, that he applied very high continuous MAP (somewhere around combat settings) but with exceedingly low RPM; in other words very oversquare settings. If I remember what I read correctly, he achieved ranges not achievable before thereby opening up greater usability of such aircraft; but there was a penalty, and this is where my memory fades. One can easily speculate of course, but does anyone have knowledge on this?
I have a best range question, still in line with the first post. I once read that Charles Lindburgh was consulted during WWII in order to get more range out of single seat fighters at the time in and around the Pacific theatre. I'm sure he advised, and flew to prove it, that he applied very high continuous MAP (somewhere around combat settings) but with exceedingly low RPM; in other words very oversquare settings. If I remember what I read correctly, he achieved ranges not achievable before thereby opening up greater usability of such aircraft; but there was a penalty, and this is where my memory fades. One can easily speculate of course, but does anyone have knowledge on this?
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I don't think WW2 comparisons can really be made with the present. The technique might work, but then engine longevity was not seen in the same way it is today!
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'just' lean of peak?
Dependent upon temps/altitude etc, its best to fly off the EGT just lean of peak.
Unless you have some spare exhaust valves with you I would suggest RoP operation unless you have the aforementioned GAMIs and a JPI.
Our Arrow 1 performs very similar to the figures provided in the previous weblink (we usually operate 24"/2400) and on a trip a few years back to the Canaries achieved the best endurance figures also.
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Even by running with an individual cylinder at peak EGT, you wouldn't be hurting your engine, provided the power is somewhere in range of 65% or below. Of course, 24"/2400 rpm gets you quite a lot of power (140HP in your case to be precise, +- few HP due to temperature deviation), which is actually over 75% and thus not even recommended by Lycoming for cruise power setting. You actually can't state that running at 24"/2400 rpm power setting (cca. 77%) gets you best endurance/economy/range/you name it. The best efficiency in normal-aspirated aircraft piston engine is achieved by running it WOT (wide-open throttle) and leaned somewhere to peak BSFC - depends on the engine, but usually somewhere from peak EGT to 50-70°F LOP. If you need maximum endurance, you would need to reduce RPM to some quite low value (2000, perhaps even 1900 RPM - if prop allows it), with throttle fully open. But if you need maximum range, you would fly pretty high, due to higher true airspeed, ability to achieve normal cruise power (60-70%) with WOT. Surely this is usually at least twice the altitude at which you will get 24" with WOT.
The problem with ROP operation is that it's actually not safer than LOP. If you look at the now famous (thanks to John Deakin) EGT/CHT/% power/BSFC/FF chart, you will see that the power and CHT curve (which are almost the same) have a smaller gradient on the rich side, which means that if you want to play it safe and get out of the critical area (50°F ROP), you need to push the mixture far in/forward, thus giving the engine much more extra fuel to put the EGT at 100-150°F ROP. But on the other side, if you want to play it safe on the lean side of the EGT curve, you only need to lean it a bit more (in range of 10-20°F), and the CHT will drop significantly, while the decrease in power output will be barely noticeable. So basically, you are giving the engine a little less fuel and getting the same (or lower) CHT than you would by pushing an extra gallon or two per hour through injectors.
On the topic of burnt valves: you can burn your valves even by running full-rich, since the combustion slows down on both sides of peak EGT (stoichiometric burn), the only difference is you'll be using a whole bucket of fuel while burning them if running ROP. Besides, usually the main reason for "burned" exhaust valves is running the engine to some obscene CHT - some people think that if Lycoming states that the CHT limit is 500°F this is to be used for normal operations.
The problem with ROP operation is that it's actually not safer than LOP. If you look at the now famous (thanks to John Deakin) EGT/CHT/% power/BSFC/FF chart, you will see that the power and CHT curve (which are almost the same) have a smaller gradient on the rich side, which means that if you want to play it safe and get out of the critical area (50°F ROP), you need to push the mixture far in/forward, thus giving the engine much more extra fuel to put the EGT at 100-150°F ROP. But on the other side, if you want to play it safe on the lean side of the EGT curve, you only need to lean it a bit more (in range of 10-20°F), and the CHT will drop significantly, while the decrease in power output will be barely noticeable. So basically, you are giving the engine a little less fuel and getting the same (or lower) CHT than you would by pushing an extra gallon or two per hour through injectors.
On the topic of burnt valves: you can burn your valves even by running full-rich, since the combustion slows down on both sides of peak EGT (stoichiometric burn), the only difference is you'll be using a whole bucket of fuel while burning them if running ROP. Besides, usually the main reason for "burned" exhaust valves is running the engine to some obscene CHT - some people think that if Lycoming states that the CHT limit is 500°F this is to be used for normal operations.
