David-aviator said:
I sense there is a certain frustration with Lycoming among the engineers who are tuned in here.
Why not spell out exactly what some of those frustrations are? Reading Lycoming's document SSP700A I got the 'sense' that Lycoming thinks the average pilot is probably too slow and clueless to operate a pointy stick, let alone a big red knob. I also got the 'sense' that the author kicks his dog and doesn't eat enough fiber, but so what?
This isn't an EST class, we are simply talking about Engine 101. Yes, I'm an engineer but this is all stuff you can get from a good engine tuner. Furthermore, when we put aside all the touchy feely human emotion nonsense, there is pretty much nothing I am going to say about an aircraft engine that you can't see in Lycoming's own charts and data.
Frankly, I find even the idea of technical folks at Lycoming being digusted with their users galling. Remember, it is 2006 and the number one cause of premature failure on a normally aspirated Lycoming engine is manufacturing flaws. Forget crankshaft problems, were talking stuff like chambers out of round, valve guides mounted off center... *Manufacturing* 101 type stuff, at least today.
I'm not upset with you. I just wish that folks at Lycoming would accept that some of us want to fully understand what we are going in the cockpit. Telling us not to worry our pointy little heads instead of giving sound technical explanations can grate on those of us who fall in this camp.
David-aviator said:
However, to operate it required a bit of common sense.
Are we talking about 'common sense' or 'tribal lore'? Newly minted pilots learn leaning from newly minted CFIs, who were themselves once student pilots... At some point far back in the chain the ju-ju magic of some alpha male (or female) was mimiced, but any caveats or meaningful explanations are always long forgotten.
Think about it, the engineers at Lycoming set a fuel flow for the engine's full rich setting. And you'd be hard pressed to find official recommendations to overide that for climbs (Service Instruction 1094D repeatedly warns against it). But you meantioned leaning until EGT comes alive for climbing.
Is that 'sense', as in Age of Reason? Did you put thought into detonation margins or the expected variations in air cooling? Or is it 'lore'? As in, someone told you that you will use less fuel and climb faster, but it won't hurt the engine. There is nothing wrong with using tried and true procedures even without complete understanding. But we need to be careful about equating the procedures to 'good sense' when underlying conditions change.
David-aviator said:
I had an EGT guage hooked up to one cylinder on the 0235 and 0360. How much meaninful information does that provide? Not much really.
Well, EGT was enough to get Lindy across the Atlantic. It also cut the number of P-38 pilots dying in post mission fuel starvation accidents over the Pacific down to nearly zero in WW-II. It also was the foundation of emissions control science for a long time and the foundation for automative tuning until just a few years ago.
Because of my engineer's enthusiasm for wideband sensing people might get the idea that I pooh pooh EGT. Not so. It is a simple way to get a real glimpse into what is occuring in the engine. It has its limits and can even be misleading, but just looking at EGT, throttle position, and RPM can give you amazing insight into an engine's health and performance. In other words, the information is there, but you have to learn the language.
With a rudimentary float carb, information specifically useful for leaning might be another story (another place Lycoming and I agree, but I'll get to that below).
David-aviator said:
I tend to believe it was lack of fuel not flow that caused it. Why else would it happen when flyinging into colder air - to make it run smooth.
What we believe isn't always what is so. I once used Venturi the Flatulent Fairy to explain the odd thermal behavior of a particular updraft float carb. All and all, a pretty convincing spiel, but he wasn't there when we finally took the thing apart.
However, in this case both Lycoming Service Instruction 1094D and I agree with you. The engine is rough because of misfires and the misfires occur because some cyls are too lean. The problem is not general air/fuel ratio, but terrible mix distribution, especially as fuel flow decreases. In 1094D, Lycoming goes so far as to contend that with their float type carbs distribution is so bad that EGT leaning may not be practical because the probe cannot be placed in the leanest cyl (each cyl varies widely).
With Lycoming fuel injected engines we know that fuel distribution with smaller flows is still a problem because many stock engines cannot be run LOP without roughness, but the same engine can be run at stoichiometric ratios above 1.0 when injectors are replaced or the distribution otherwise leveled.
Here is a case were 'lore' can break down. A carb driven Skyhawk is going to always run rough ROP. A 172R, with fuel injection - you really don't know. Just rich of 'rough' may well put the engine exactly where you don't want to be. Hence EGT and fuel flow gauges are standard on R models.
David-aviator said:
It is not very scientific but then, neither is the engine.
Actually, that is unfair. A huge amount of R&D went into aircraft engines in the first half of the 20th century. That research isn't just the foundation of the modern understanding of combustion, we're talking about core research in cooling, ignition, detonation, the first significant study of mixture... You name it.
You cannot pick up a research paper at a conference on engine design *today* without still finding citations to this work, some of it now 60 years old. Remember, aviation represented a significant challenge in power, weight, and efficiency. All modern auto engines stem from this work and it is a real testament to the scientific approach and enormous investment that some 50 year old engine designs remain difficult to top for their application today.
Just because, as an applied science, we are talking about something that Gomer can fly and Goober can service with a big hammer right there in Mayberry doesn't mean that the engines are not major technological achievements.
David-aviator said:
If you guys really want to get wound up on the science of an engine, get into some the deep secrets of todays auto engines. They are incredible.
Tell me about it, as I recently commented in a PM the oil cooling in some Porche engines, engines which are achieving Best Power at mixes as lean as .9 Lambda, are amazing. But they are also complex. I'm not all that sure I would want to be running a GA single behind one. Especially if it was a rental and I didn't know whose 'ju-ju' had been last used on it!
-jjf