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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?
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!! |
#4
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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? |
#5
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Mixture--science vs witchcraft?
Any suggestions or comments?
Do you have an engine analyzer on board, Doug? If so, this tool (we have the JPI EDM-700) lets you lean with confidence that you're not harming anything... -- Jay Honeck Iowa City, IA Pathfinder N56993 www.AlexisParkInn.com "Your Aviation Destination" |
#6
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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) |
#7
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Mixture--science vs witchcraft?
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!! Maybe flying with an instructor who knows about this critical question would help? |
#8
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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. |
#9
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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) |
#10
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Mixture--science vs witchcraft?
"Jim Carter" wrote in message
news:000d01c7e206$d059b690$4b01a8c0@omnibook6100.. . 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? That is precisely what's at the root of this issue. 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. I don't think the relationship is that linear, is it? (going from SL to 1,000' is a bigger change than going from 9,000' to 10,000', AIUI) But I agree with you in general. 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? And, as it happens, that 16 gph is pretty much right in the ballpark of what I've been using. The climb chart tells me I should be seeing around 14 gph in a climb through 9,000' DA, so including the 2 gph "enrichening factor," 16 is what I'm seeing (numbers from memory, I do not have the chart in front of me). This sounds a LOT closer to the "science" I'm looking for here!! What's this math that you don't want to do in public? If there's some equation I can plug the variables into & come out with the right answer, I'll be a happy camper! Is there "someone more qualified" than Jim (your words! to show me the math? Thanks! -- 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) |
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