OK, half the day is gone, and I am just getting to this thread! Let me see if I can help answer a few questions for you guys. Just remember I have no vested interest in saying this……Sign up for the next APS class, it will open your eyes to not just knowing something, but better than that, understanding a whole range of things. Knowing a thing often applies to one thing specifically but understanding a whole range of things allows you to work out for yourself any specific thing.
That would be about fair, closer to 70% we would suggest more like 125 or so ROP, but lets remind ourselves these are rubbery numbers and conceptual, there is no knife edge. What was alarming was some statements earlier in this thread where running up around 27" and just rich of peak.
Nothing like that is going to kill your engine any time soon, but if pressure and temperature are the enemies of strength and longevity, then one has to assume that avoiding that while still getting the job done means you will keep doing the job longer and with less stress on the engine.
Tom,
You are most welcome. 50dF ROP is possibly best described as the least optimal place to run, so lets look at why and do so at various power settings. At full power this is where detonation is most likely to occur. Not easily achieved I might add with a NA engine, but with a Turbo it is. Run the CHT/IAT/Oil temps up and or some lower grade fuel, and away you go. But nobody seriously sets out to do that.
In the cruise what do we get? Well it is not quite Best power, that despite what your book says is around 75dF but we are splitting hairs a little here. I am at a loss as to why it got the name best power, it suggests best as in the perfect place, or most optimal, but what it really means is the point where peak power is obtained for that given MP&RPM.
The reason APS recommends the big pul is that many folk would sit there watching the LEAN FIND function on their EMS and slowly watch each cylinder peak. This is sub optimal on a number of fronts and produces negative results, and high CHT's especially in TN/TC engines at high power. It was not because you would crater your engine in a minute or less. So do not read too much into this. A BMP is just a much better way to do things. Once there let things cool off even more and if you want to accurately set a give XXdF LOP sneak up to peak on the lean side. This way you will find out where peak EGT is without running the CHT's up and reducing volumetric efficiency.
So what bad things are going to happen at 50dF ROP. Well none if the power is low enough. but it is neither the most power and assuming at 70+% power nor is it rich enough to keep the ICP and CHT at a level we think is optimal for long service life. If you want better efficiency a LOP power setting will be better. If you are at 60% power, and you want the most power possible 75dF is a good place to be.
For a given power, look at the following and see which has the best outcome.
And when you consider how that peak pressure works with CHT from the next graph
………it all starts making sense.
So if we are to keep the Internal Cylinder Pressures under control and within the designers desired operating parameters, we can do this at varying powers by either running Rich of Peak, and sufficiently so, or by running lean of peak, and again sufficiently so.
Just note that 1/BSFC is dependent on the power produced, that brown line moves depending on the % power made. So at 80+% it is peaking in the 60-80dF LOP range and when at very low powers such as 60-65% it is more around 10-20dF LOP. Again…fuzzy edges here, no knife edges.
Have a play around with this link here http://www.advancedpilot.com/redbox.html This was posted up after one of the students down here in Australia asked how best to demonstrate this to the young commercial pilots that worked in her charter company. It is a great tool and one we use in class. On the lower graphic, grab the knob with the mouse pointer and set the power setting.
Last of all, as you can see from the BLACK HP curve if you start out at say 75% power on the rich side of peak and then lean out past peak, the power falls away and by the time you get to an optimal LOP setting you have lost about 10% power, or in other words you will be around 65-67%. So when you start at 75% (ROP settings) and you think that means 40dF LOP, well you should anticipate being about 65% so thus you actually wanted about 10-20dF LOP. This explains why some people end up being so far LOP that they suffer big speed losses. The other is from volumetric efficiency loss as mentioned above…..but lets save that for another day.
So while you respectfully asked and not wanting a lesson for free, and I appreciate that (too many freeloaders out there) the reason we stress doing the course is that a 2.5 day laid out in perfect teaching order course is hard to wrap up in one post on the internet. It just can't be done. But get along to the next one, I might be back over there again if the planets align.
Hope that helps!
