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#21
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Mixture--science vs witchcraft?
Douglas Paterson wrote:
: "BillJ" wrote in message : ... : Maybe flying with an instructor who knows about this critical question : would help? : Indeed!! Even--or perhaps "especially"--some ground school on the subject, : and I have been actively pursuing (not finding) just that. Know where I can : find such a CFI?!? Every instructor I've approached on the subject has one : of two answers: "if the POH says full rich, put it there," or "lean it, oh, : about *that* much...." ARGHH!! : Speaking as a big-iron guy w/ military procedures drilled deeply into his : skull from day one, I find the lack of information on this disturbing.... As a avid "Deakin-reader" and a cheap-******* pilot/owner, I can try to paraphrase as follows: - It's impossible to hurt the engine at or below approx 70% power no matter what you do with the mixture, PROVIDED you keep the CHT below 350-380 as measured at the bayonette lugs. - For normally-aspirated engines, it's impossible to exceed 75% power below about 6000-8000' DA. - EGT will remain roughly the same for a given fuel/air ratio.... SO .... find out what your full-rich EGT is at a sea-level takeoff and lean to that as DA increases.... whether it's at a high DA takeoff or as you do a long climbout. It's generally about 200-250 rich of peak. e.g.... On my plane, I see about 1400 EGT on a sea-level, standard-day full-rich takeoff. During a long climb I'll lean to the same EGT as I climb. On a hot summer day (2100' MSL field, 4500' DA takeoff) I'll see 1300 full rich and doesn't make full power. Leaning to the 1400 before takeoff I get full power, and still have good cooling. In cruise below 70%, I'll lean until it wheezes so long as CHT stays below 375. -Cory -- ************************************************** *********************** * Cory Papenfuss, Ph.D., PPSEL-IA * * Electrical Engineering * * Virginia Polytechnic Institute and State University * ************************************************** *********************** |
#22
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Mixture--science vs witchcraft?
wrote)
For normally-aspirated engines, it's impossible to exceed 75% power below about 6000-8000' DA. "...impossible to exceed 75% power below about 6000-8000' DA." Below or Above? Paul-Mont |
#23
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Mixture--science vs witchcraft?
Here is what I would suggest. Fly at the field elevation in level
flight (obviously somewhere where the ground is lower). Lean for maximum RPM. At that altitude (7000+'), you don't have to worry about overleaning. Just lean to peak rpm. Now go and land at THAT setting. Now leave it at THAT setting and go do a runup AT THAT SETTING. Turn the mixture knob some both ways to see what the effect on peak rpm is at your runup rpm. NOW USE THAT mixture/rpm behavior to lean. It will probably be right at peak, but if it slightly below then lean to that. Hope this makes sense. Basically what I am advocating is leaning to peak in the air at full throttle, then go on the ground and learn how to recognize THAT mixture setting based on runup rpm behavior. Good luck! If all else fails, just runup at full rpm and lean to peak rpm. That will work also. (A bit risky with the prop blast at full rpm, but if you watch where you do it and are careful can be done). |
#24
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Mixture--science vs witchcraft?
Montblack wrote:
: wrote) : For normally-aspirated engines, it's impossible to exceed 75% power below : about 6000-8000' DA. : "...impossible to exceed 75% power below about 6000-8000' DA." : Below or Above? Above, of course. The "rule of thumb" that I've heard is 8000', but I think that's at best-power mixture. The best-economy mixture setting is more like 6500' I think. -Cory -- ************************************************** *********************** * Cory Papenfuss, Ph.D., PPSEL-IA * * Electrical Engineering * * Virginia Polytechnic Institute and State University * ************************************************** *********************** |
#25
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Mixture--science vs witchcraft?
On Aug 18, 3:43 pm, "Douglas Paterson"
wrote: Still getting to know my new baby (1990 TB-20, normally aspirated 250hp Lycoming IO-540). I imagine my question here must apply to most non-turbo, non-FADEC pistons (though I gather there's some sort of altitude compensator on some airplanes' engines?). I live in Colorado, which means routine high-elevation airport ops. The people who manufacture that engine, and who have lots to lose if it quits on you, have a procedure he http://www.lycoming.textron.com/supp...fs/SI1094D.pdf Dan |
#26
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Mixture--science vs witchcraft?
That ol black magic has me in it's spell
That ol black magic that we know so well Those quivering needles up and down my spine That same ol back fire when it hits red line lots more snipped So darlin down and down we go Round and round we go in a spin wonderin the spin were in Starin at that ol EGT against the pin the devil made me do it - honest! |
#27
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Mixture--science vs witchcraft?
Excellent!!!!!!!!!!!!
Ο "Denny" έγραψε στο μήνυμα oups.com... That ol black magic has me in it's spell That ol black magic that we know so well Those quivering needles up and down my spine That same ol back fire when it hits red line lots more snipped So darlin down and down we go Round and round we go in a spin wonderin the spin were in Starin at that ol EGT against the pin the devil made me do it - honest! |
#28
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Mixture--science vs witchcraft?
On Aug 19, 10:58 am, "Douglas Paterson"
wrote: Maybe flying with an instructor who knows about this critical question would help? Indeed!! Even--or perhaps "especially"--some ground school on the subject, and I have been actively pursuing (not finding) just that. Know where I can find such a CFI?!? Yes. Walk around the hangars, and find someone who owns an airplane, flies it a lot, works on it himself (preferably a lot), has a day job (preferably outside aviation), and teaches on the side. There are some out there. For that matter, a CFI ticket isn't really important. They're not very hard to get and don't assure much in the way of knowledge or instructional ability. Just find an owner who flies a plane in the same performance class as yours (a lot) and works on it himself. Buy him lunch (or better yet, help with the maintenance) and you will get all the ground school you need. As for your specific situation, since your engine is instrumented with CHT/EGT/fuel flow, I can give you a prescriptive system for determining and applying the proper procedure, as well as the reasons that underlie it. Here goes. Aviation engines at takeoff are normally operated 200+ deg F ROP (rich of peak). Contrary to popular belief, this is neither maximum power (which occurs about 25-50 ROP) nor primarily for cooling (although cooling can be a factor). The primary reason we do not lean for peak power for takeoff is engine timing. The spark advance on aviation engines these days is fixed some number of degrees (of crankshaft rotation, not temperature) before (typically 15-25 degrees before) the piston hits top dead center (TDC). Since it takes some time for the flame front to propagate from the ignition point (spark plug) to the piston surface, by the time the flame front hits the piston surface and transfers the energy to the piston (yes, I am simplifying tremendously here, but go with it) the piston is already well on the way down, meaning past TDC. This is critically important - transferring energy to the piston before that is terrible - it can put excessive stresses on the crankshaft, rods, etc. It can damage the engine, much like pre-ignition and detonation. The time the flame front will take to travel through the combustible gas is primarily a function of two things that are somewhat under your control - charge density (meaning basically how much air is there) and mixture. The denser the charge (meaning the denser the outside air and the more open the throttle) the faster the flame front moves. The closer the mixture is to stoichiometry (meaning just enough air to burn all the fuel and no more) the faster the flame front moves. At sea level on a standard day at full throttle, you can actually damage the engine by leaning - because the flame front will arrive too early. At a density altitude of 10,000 ft this is impossible unless something is really wrong (for example, the bolts securing your magnetos are loose and the magnetos have moved increasing the spark advance - it is actually true that you set the spark advance on these engines by loosening those bolts, moving the magneto by hand until the points open the number of degrees before TDC you want, then securing the bolts again). Now I have neglected the cooling aspects somewhat here, but they are important. You really don't want to weaken your cylinders by letting them get above 400F. 380F is better. 350F is VERY conservative. This is because the aluminum your jugs are made of does start to lose tensile strength appreciably above these temperatures. So having laid out the background, here's what you do. Pull into position, hold the brakes, full power, lean until EGT's peak, then enrich (for how much - see below), and take off. No need to do this every time - pretty soon, you will know where peak EGT is for your engine at a given density altitude, so you will just pull the mixture back to that number. You will also soon learn the position of the mixture control that is appropriate, so you will be able to do it quickly. Do your initial practice at strips with plenty of room for error. Once you get to where you can set what you need quickly, you are ready for the challenging ones. So how far rick of peak should you be? Al less than 5000 DA, leave it alone. At 8000 ft DA, probably about 100-200F. That's because you can still hurt your engine by overleaning. By the time you hit 10,000 DA maybe 50-100. Above that 25-50. Even peak EGT can't hurt above 10,000 DA or so, so as long as you haven't leaned enough to lose power, anything you do at 10,000+ is OK. Interpolate between the given points however you want to - the error in the method of interpolation will be less than the error of setting the mixture, which is why there is some room for error in the numbers I give you. Now this isn't going to be enough for long term cooling - but you're not going to overheat your cylinders in two minutes by doing this - and after that point, you should be solidly climbing, absolute best performance won't be critical anymore, and you can monitor CHT's and increase fuel flow and/or airspeed as needed. Is that sufficiently scientific for your tastes? Michael CFI, ATP, A&P, PhD, and other good alphabet soup |
#29
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Mixture--science vs witchcraft?
Michael wrote: [...] The time the flame front will take to travel through the combustible gas is primarily a function of two things that are somewhat under your control - charge density (meaning basically how much air is there) and mixture. [...] It may be interesting that the vulnerable time between the spark and TDC is also to some extent under one's control, with the blue knob. Thus here's another reason for using maximum RPM for takeoff. - FChE |
#30
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Mixture--science vs witchcraft?
On Aug 23, 9:24 pm, (Frank Ch. Eigler) wrote:
It may be interesting that the vulnerable time between the spark and TDC is also to some extent under one's control, with the blue knob. Good point, and you are of course correct. This takes us into a whole 'nother discussion - not can you run oversquare safely (because of course you can, and virtually every flat-four, six, and eight out there does) but how much oversquare you can run and under what circumstances. This is precisely the factor that limits how much oversquare you can run in most circumstances. I left that part of it out mainly because I've never heard anyone advocating making high density altitude takeoffs at reduced RPM, and the post was already longer than most people will read. Michael |
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