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#1
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Mixture--science vs witchcraft?
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. I'm looking for guidance on proper mixture setting for takeoff & landing at high-elevation (with correspondingly high DA) fields. What's the best way to achieve maximum power in these conditions? The "book" answer, per the POH, of full rich for takeoff and landing is clearly wrong--indeed, I stalled the engine on my first landing roll-out back here (I was lean of full rich, but, obviously, not enough!). I'm looking for some "science" to put behind this, instead of "mmm, about *there*".... I've been tweaking the mixture for highest rpm during the run-up (2,000 rpm), then looking for a couple of gph above the book's climb fuel flow for the existing DA on takeoff roll. That seems to work OK for takeoff, but, of course, I'm somewhat back to guessing for landing (especially at a different field or if the DA has significantly changed). Any suggestions or comments? FYI, the field I'm basing from is 7,030' elevation, with 9 - 10K' DAs typical; and we've been to Leadville (LXV)--elevation 9,927', North America's highest municipal airport & highest paved runway, DA of 11,700' when we visited. This is far more than just an academic discussion for me!! -- Doug "Where am I to go/Now that I've gone too far?" -- Golden Earring, "Twilight Zone" (my email is spam-proofed; read the address and make the appropriate change to contact me) |
#2
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Mixture--science vs witchcraft?
"Douglas Paterson" wrote in message . .. 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. I'm looking for guidance on proper mixture setting for takeoff & landing at high-elevation (with correspondingly high DA) fields. What's the best way to achieve maximum power in these conditions? The "book" answer, per the POH, of full rich for takeoff and landing is clearly wrong--indeed, I stalled the engine on my first landing roll-out back here (I was lean of full rich, but, obviously, not enough!). I'm looking for some "science" to put behind this, instead of "mmm, about *there*".... I've been tweaking the mixture for highest rpm during the run-up (2,000 rpm), then looking for a couple of gph above the book's climb fuel flow for the existing DA on takeoff roll. That seems to work OK for takeoff, but, of course, I'm somewhat back to guessing for landing (especially at a different field or if the DA has significantly changed). Any suggestions or comments? FYI, the field I'm basing from is 7,030' elevation, with 9 - 10K' DAs typical; and we've been to Leadville (LXV)--elevation 9,927', North America's highest municipal airport & highest paved runway, DA of 11,700' when we visited. This is far more than just an academic discussion for me!! http://www.avweb.com/news/pelican/list.html (Start with #18 and go through the entire "Engine-Related Columns" series) -- Matt Barrow Performance Homes, LLC. Cheyenne, WY |
#3
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Mixture--science vs witchcraft?
"Matt Barrow" wrote in message
... http://www.avweb.com/news/pelican/list.html (Start with #18 and go through the entire "Engine-Related Columns" series) Matt, that was a great series, thanks for the pointer. I found #63, "Where Should I Run My Engine? (Part 1)," to most closely address my question. http://www.avweb.com/news/pelican/182179-1.html However, I'm still disappointed. Deakin's advice still falls on the "witchcraft" side of the equation, in my book, boiling down to "put the mixture where it feels right. From that article: The books are full of various techniques for this, but I find the simplest and most effective is just add full throttle, full RPM, then grab the mixture knob and move it aggressively from full rich to whatever feels like "more power" on the takeoff roll. You can't hurt the engine with momentary mixture settings like this on normally aspirated engines! Saw that mixture knob back and forth, and feel the power change in the seat of your pants! At some point as you pull the mixture out from full rich, you'll feel the power first increase, then for a large part of the movement you'll feel no power change at all, because the "best power" mixture setting is very flat in that area. (In other words, "best power" occurs over a fairly wide range of rich settings, but not at full rich.) Go ahead, pull it a bit too far, and you'll feel the power drop off from being not rich enough. Push it back in to the point where you first felt the best power, and forget it. It's quick, simple, and very effective, and pinpoint accuracy is not necessary. However well this may work (??), it hardly qualifies as "science." Anyone have any suggestions on what "books" he may be referring to when he says "the books are full of various techniques"? A big take-away from these articles is that I'm probably wasting my time tweaking for max power (rpm) during the runup. Also, reading between the lines seems to indicate that using the climb fuel flow chart from the POH is probably a decent starting point--though I'm not yet really convinced of that. Thanks again for the discussion--I appreciate any & all insights! -- Doug "Where am I to go/Now that I've gone too far?" -- Golden Earring, "Twilight Zone" (my email is spam-proofed; read the address and make the appropriate change to contact me) |
#4
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Mixture--science vs witchcraft?
"Douglas Paterson" wrote in message ... Deakin's advice still falls on the "witchcraft" side of the equation, in my book, boiling down to "put the mixture where it feels right. I believe that the best "science based" information about this topic is available from the GAMI folks in ADA, Oklahoma http://www.gami.com/frames.htm They have perhaps the best instrumented piston airplane engine anywhere and have collected lots of repeatable data that are the basis for their recommendations. They have seminars that answer the kinds of questions you're asking and now offer them online, too. Lots of graphs and tables. I haven't attended yet, but I plan to. In another message, you asked about the mixture setting for maximum power. I think max power is in the vicinity of 25 - 50 degrees rich of peak EGT. In cruise at altitude, pull the mixture out until the EGT starts decreasing, push it back in until it peaks, then push it in some more to get 25 - 50 degrees less than peak. That procedure doesn't work on takeoff; the method suggested by David Paterson in another post is a good one for takeoff. Running full rich for takeoff at very high density altitude will put you way too rich and you'll give up lots of horsepower when you need it the most. Running full rich for low density altitude takeoffs is important because the engine is producing at or close to maximum power, where cylinder cooling is needed. The richer mixture effectively retards the timing, which causes the peak cylinder pressure to occur later in the combustion cycle, which results in cooler CHT. Dennis |
#5
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Mixture--science vs witchcraft?
"Dennis Johnson" wrote in message . .. "Douglas Paterson" wrote in message ... Deakin's advice still falls on the "witchcraft" side of the equation, in my book, boiling down to "put the mixture where it feels right. I believe that the best "science based" information about this topic is available from the GAMI folks in ADA, Oklahoma http://www.gami.com/frames.htm Agreed!!!! Also, their sister company, http://www.advancedpilot.com seminar is about the best thousand bucks you'll ever spend. It's paid itself back many times over in terms of 100LL $$$ and maintenance (when I traded my 1800 hr TN Bonanza 36, there was virtually zero lead buildup in the engine and compressions were all still well into the 70's). |
#6
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Mixture--science vs witchcraft?
"Douglas Paterson" wrote in message ... "Matt Barrow" wrote in message ... http://www.avweb.com/news/pelican/list.html (Start with #18 and go through the entire "Engine-Related Columns" series) Matt, that was a great series, thanks for the pointer. I found #63, "Where Should I Run My Engine? (Part 1)," to most closely address my question. http://www.avweb.com/news/pelican/182179-1.html However, I'm still disappointed. Deakin's advice still falls on the "witchcraft" side of the equation, in my book, boiling down to "put the mixture where it feels right. From that article: The books are full of various techniques for this, but I find the simplest and most effective is just add full throttle, full RPM, then grab the mixture knob and move it aggressively from full rich to whatever feels like "more power" on the takeoff roll. You can't hurt the engine with momentary mixture settings like this on normally aspirated engines! Saw that mixture knob back and forth, and feel the power change in the seat of your pants! At some point as you pull the mixture out from full rich, you'll feel the power first increase, then for a large part of the movement you'll feel no power change at all, because the "best power" mixture setting is very flat in that area. (In other words, "best power" occurs over a fairly wide range of rich settings, but not at full rich.) Go ahead, pull it a bit too far, and you'll feel the power drop off from being not rich enough. Push it back in to the point where you first felt the best power, and forget it. It's quick, simple, and very effective, and pinpoint accuracy is not necessary. However well this may work (??), it hardly qualifies as "science." Anyone have any suggestions on what "books" he may be referring to when he says "the books are full of various techniques"? The old P&W series from the 40's and 50s. I think, though, you're confusing his method of getting from rich to lean. Given his copious charts and lengthy explanations of each, and discussion of the "Danger Zone", lingering in the peak areas is, his points are in fact, well documented. The part you mention above is, IIRC, how to do it without proper instrumentation. A big take-away from these articles is that I'm probably wasting my time tweaking for max power (rpm) during the runup. Also, reading between the lines seems to indicate that using the climb fuel flow chart from the POH is probably a decent starting point--though I'm not yet really convinced of that. Thanks again for the discussion--I appreciate any & all insights! Read his articles about the lunacy in most POHs. You might find it helpful to print out his entire engine series and read them slowly, trying to integrate all that he has to say. There is certainly a load of information there and it's more difficult to grasp in that so much of it goes contrary to what most all of us have been taught over the past couple generations. (Think of the Enlightenment and Renaissance after the Inquisition). Good luck with your new bird!! -- Matt Barrow Performance Homes, LLC. Cheyenne, WY |
#7
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Mixture--science vs witchcraft?
250 HP needs about 20-22 gph for takeoff. After takeoff and throughout
the climb set mixture to achieve 1250-1300 EGT, any altitude. For landing leave the mixture where it was for cruise. If you need to do a go around then make sure to advance the mixture a little as you ease the power in. 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. I'm looking for guidance on proper mixture setting for takeoff & landing at high-elevation (with correspondingly high DA) fields. What's the best way to achieve maximum power in these conditions? The "book" answer, per the POH, of full rich for takeoff and landing is clearly wrong--indeed, I stalled the engine on my first landing roll-out back here (I was lean of full rich, but, obviously, not enough!). I'm looking for some "science" to put behind this, instead of "mmm, about *there*".... I've been tweaking the mixture for highest rpm during the run-up (2,000 rpm), then looking for a couple of gph above the book's climb fuel flow for the existing DA on takeoff roll. That seems to work OK for takeoff, but, of course, I'm somewhat back to guessing for landing (especially at a different field or if the DA has significantly changed). Any suggestions or comments? FYI, the field I'm basing from is 7,030' elevation, with 9 - 10K' DAs typical; and we've been to Leadville (LXV)--elevation 9,927', North America's highest municipal airport & highest paved runway, DA of 11,700' when we visited. This is far more than just an academic discussion for me!! |
#8
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Mixture--science vs witchcraft?
-----Original Message----- From: Newps ] Posted At: Saturday, August 18, 2007 8:21 PM Posted To: rec.aviation.owning Conversation: Mixture--science vs witchcraft? Subject: Mixture--science vs witchcraft? 250 HP needs about 20-22 gph for takeoff. After takeoff and throughout the climb set mixture to achieve 1250-1300 EGT, any altitude. For landing leave the mixture where it was for cruise. If you need to do a go around then make sure to advance the mixture a little as you ease the power in. The engine won't generate 250 HP at altitude will it? Isn't that a sea level rating based on the pressure of the air available at sea level? So the 20-22 gph would be for sea level wouldn't it? At FL180 the pressure is 1/2 of sea level so we can interpolate that at 9000' the pressure would be only 3/4 of sea level, then factor in the temperature for the density altitude the engine will really breathe. Doesn't this mean that the normally aspirated engine in the OPs question will produce significantly less than 250 HP? I'm not going to do the math because I'm sure to get it wrong and there are many others on here more qualified than I, so I'm only guessing that we might see 200 HP. If that's the case then instead of 20-22 gph wouldn't we be looking for around 16 gph? |
#9
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Mixture--science vs witchcraft?
Jim,
Doesn't this mean that the normally aspirated engine in the OPs question will produce significantly less than 250 HP? I'm not going to do the math because I'm sure to get it wrong and there are many others on here more qualified than I, so I'm only guessing that we might see 200 HP. If that's the case then instead of 20-22 gph wouldn't we be looking for around 16 gph? You're dead on. -- Thomas Borchert (EDDH) |
#10
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Mixture--science vs witchcraft?
Jim Carter wrote: The engine won't generate 250 HP at altitude will it? Isn't that a sea level rating based on the pressure of the air available at sea level? So the 20-22 gph would be for sea level wouldn't it? You need that for proper cooling to make up for the little airflow you're getting. |
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