Will.iam Posted April 27, 2022 Report Posted April 27, 2022 (edited) So I was watching a video from Mike Busch about maximizing fuel economy by flying over square he stated that you really needed to get the power charts from continental as they show lower RPMs that the engine can safely run at compared to what’s in our Mooney POH manual, and the lower the RPM’s are better for giving the engine time to extract that power due to slower flame front when running LOP before the exhaust valve opens. I found a few charts but maybe I don’t understand as some charts seem to conflict with others on what is the lowest RPM I can run. For example: This chart shows I can run at 2000rpm at 29”MP giving me 110HP and at 2200rpm money’s lowest rpm chart in poh I can run 30”MP(I think the Continental chart has a typo as 32 is listed twice) giving about 136HP. But this second chart: shows rpm at 2200 I can go as high as 34”MP so is 30.1” to 34” considered only temporary and not for cruise? also here is that same #2 chart but for 2700 rpm and of course it can go to 36”MP which is WOT but look at the HP! Especially at 14,000 to 16,000ft it says 220HP at first I thought they put the encore’s chart by mistake but WOT for encore is 39”MP not 36”MP as shown in this chart, so what gives? How can my engine produce more HP at 14-16k compared to sea level? Is it because the air is thinner for less drag on the prop? curious minds want to know. And finally this last chart I have no idea how to read it except it is for sea level only but can I run at 2000rpm at full throttle since the chart line goes all the way to 2000rpm on the chart? I see where 2250 is best sbfc but that is the bottom line is that like min throttle? Is the top line max throttle? any help would be appreciated. Edited April 27, 2022 by Will.iam Forgot to say while running LOP Quote
carusoam Posted April 27, 2022 Report Posted April 27, 2022 Looks like a fast pitch up the middle for somebody like @kortopates to take a swing at…. For anything M20K and leaning… my favorite MSer to read his experiences is @jlunseth (Oddly quiet for months?) an example can be found below… Best regards, -a- Quote
A64Pilot Posted April 28, 2022 Report Posted April 28, 2022 Be careful with too much oversquare, the usual explanations given as to why low RPM is more efficient is lower engine friction which is true and greater propellor efficiency, which is also true. There is also some truth to lower RPM will extract more power from the expanded air as there is a longer time to push on the piston, but as there are fewer combustion events that is pretty much a wash. The increase in efficiency which is real, often isn’t as much as we would want it to be, its there, but is say a half gallon an hour worth pushing or exceeding POH limits? Toyota’s Prius for example will often run at full throttle and with its variable transmission ratio will pull the RPM down until output matches desired, also full throttle decreases engine pumping losses. But the Prius can vary compression ratio with variable valve timing and has a knock sensor to prevent detonation, aircraft have neither, but the Prius does things for tiny improvements, they are happy with a couple of percent here and there. Bottom line running too much oversquare is exactly pulling the hill in your loaded truck without downshifting, except the hill never ends, and we all accept that’s not a good thing especially when the truck starts “pinging” I have taken a calibrated instrumented aircraft out to measure the differences in low RPM / high MP at the same speed as higher RPM / lower MP and as long as Airspeed was identical couldn’t really measure much if any difference, so I decided to run an RPM that the engine ran the smoothest, which coincidentally or maybe not, was pretty much in the middle of the green That was a NA motor not a boosted one. For goodness sakes only use the POH charts, there are installation differences from one airplane to another and the POH charts take those differences into account, due to cooling and exhaust, induction system and other differences an engine will have different capabilities in different airframes. Quote
Will.iam Posted April 28, 2022 Author Report Posted April 28, 2022 What i have noticed is all my charts are for peak TIT not to exceed 1650F this was back in the day. Now we know you can get longer turbo life if you don’t run it so hot. I try for 1550 or 100 degrees below redline which consequently where my jpi yellow warning light turns on. What i have noticed is that flying LOP increases my TIT temperature by about 10 to 15 degrees. I can lean out further LOP to offset that temperature rise but then power really drops off or reducing RPM has a dramatic effect of lowering TIT and maybe that is because less of the flame is not being dumped out the exhaust? So i would rather keep close to the same power but run cooler TIT but it would push my oversquare higher or live with running a hotter TIT. I think mooney stopped at 2200 rpm because BSFC bottoms out at 2250, i. E. Going to 2100 or 2000 even though the engine can do it, mooney found the efficiency got progressively worse below 2200. And mooney is all about efficiency even if it might be at the expense of longevity of parts due to heat. Re charting 50 ROP in the past. I would give up some efficiency for longevity especially if I’m just flying around the pattern or doing approaches. Quote
carusoam Posted April 28, 2022 Report Posted April 28, 2022 Some summary points… 1) The engine doesn’t know it is over square… 2) It is similar to selecting a higher gear in your firebird… 3) Mixing a higher gear with a high throttle position or MP can be taxing to the hardware 4) Slowing the rpm increases the ICP that occurs before TDC… 5) As ICPs increase… so does the temperature and rate of reaction… exponentially… 6) If something is going to go wrong… pre-ignition May start to occur as ICPs increase 7) We don’t have the luxury of an ICP sensor… but the guys who wrote the POH did… 8) Oddly some POHs did a good job of outlining safe practices for ROP… 9) Oddly some POHs completely left LOP to the imagination… 10) Running a Continental engine LOP is a nice way to burn all of the fuel… convert all the fuel to power… 11) With a TC’d Continental… it can use extra compressed air for CHT and TIT control…. 12) Compressing extra air doesn’t add to the efficiency… so it has its limit as well… Sooooo… for those reasons running over square, beyond POH limits, doesn’t get a lot of traction… Running LOP with a TC takes experience… knowing something about the big pull is pretty helpful… When discussing things that occur inside the cylinder… I like to invite @Shadrach to the thread… Best regards, -a- PP thoughts only, not a combustion engineer… Quote
Will.iam Posted April 28, 2022 Author Report Posted April 28, 2022 2 hours ago, carusoam said: 4) Slowing the rpm increases the ICP that occurs before TDC… 5) As ICPs increase… so does the temperature and rate of reaction… exponentially… 6) If something is going to go wrong… pre-ignition May start to occur as ICPs increase 7) We don’t have the luxury of an ICP sensor… but the guys who wrote the POH did… 8) Oddly some POHs did a good job of outlining safe practices for ROP… Number 4 is true but LOP slows the flame front and the further LOP you go the slower to the point of the flame still traveling when the exhaust valve opens thus continuing to burn in the exhaust. Hince I’m using slower RPM’s in a way to advance the timing since our timing is fixed and LOP is pushing the ICP further past TDC, if i slow the piston travel i effectively put the ICP back closer to the 18 degrees TDC again thus making better use of the expansion forces of the flame. number 5 I’m offsetting with LOP. number 6 The test stand at Ada Oklahoma they prove that as long as you stay out of the red box i.e. 65% power less on 100 low lead fuel it was impossible to get detonation no matter where the mixture is set. 2 Quote
Shadrach Posted April 28, 2022 Report Posted April 28, 2022 5 hours ago, JohnZ said: Busch is undoubtedly an accomplished A&P. But seriously, ignore the POH and operate your engine at experimental power settings to save like $2 per hour in gas? It’s your airplane… Personal (and probably an unpopular) opinion is that the whole “red box”/LoP/Busch cult thing is overrated. Just run the airplane at normal power settings… like it has probably been doing successfully for decades. I think it’s advisable for people that have the bear minimum of understanding regarding a subject to keep there operations as simple as possible. What’s interesting is when they feel the need to use pejoratives to describe individuals that have likely forgotten more on the subject in question than they’ve ever learned… I’m reminded of a flight instructor that I met years ago in a group of other pilots. He was saying not so flattering things about the gents from APS. It was clear his understanding of engine ops could be summarized in a single section of a 40 year old POH. After smugly jabbing at APS and LOP ops and how the fuel savings was all from the power/speed reduction, he declared “I’m just not smart enough to run LOP”. I smiled warmly and said “I completely agree”. 2 Quote
A64Pilot Posted April 28, 2022 Report Posted April 28, 2022 (edited) Do this, go out and set as exact an indicated airspeed as you can and let things settle down, takes a few minutes the way you normally cruise, then reduce the RPM to the bottom of the green and increase MP to recover the speed exactly, may have to relearn. Then report back I’d like to see if what you get correlates to what I got out of an IO-540, I don’t think airframes matter as what we are doing by matching airspeeds as close as we can is establishing the same thrust, which should correlate to close to the same HP, the lower RPM motor should require slightly less HP due to prop efficiency. In test flying I’ve often run into things that can be definitely shown on paper, but in an airplane they are in the noise layer, that’s to say the difference is so slight it’s tough to quantify. Don't bust POH numbers but run as low an RPM and as high a MP as allowed, the run in the middle and or choose a smooth RPM. Write down fuel consumption difference, I’d be interested. Be sure to relean though as mixture will likely change Edited April 28, 2022 by A64Pilot Quote
Cruiser Posted April 28, 2022 Report Posted April 28, 2022 28 minutes ago, JohnZ said: @Shadrach It’s a piston engine… not rocket science. Sorry that you got upset that I said LoP is overrated. Perhaps I just haven’t seen the light yet..? I wouldn’t advise anyone I was flying with to run the engine outside of the safe values listed in the POH. I don’t think that is indicative of lack of intelligence or understanding. perhaps you are being mislead by the terms...... the POH list are recommended values proven by actual flight test. They do not necessarily represent 'safe' values (or limits unless in the limitations section of the POH) as you are suggesting. 1 Quote
carusoam Posted April 28, 2022 Report Posted April 28, 2022 43 minutes ago, JohnZ said: @Shadrach It’s a piston engine… not rocket science. Sorry that you got upset that I said LoP is overrated. Perhaps I just haven’t seen the light yet..? I wouldn’t advise anyone I was flying with to run the engine outside of the safe values listed in the POH. I don’t think that is indicative of lack of intelligence or understanding. Somewhere along the way… You will realize… What you say matters… And how you say it matters as well… Welcome to MS…! MS is on the internet… But it isn’t the internet… MS is a real community of real people… I get it… you don’t like LOP… Many people on MS use LOP on every flight… LOP didn’t make it into many Lycoming POHs… And many Lycoming POHs include using 50°F ROP Some people like 100°F LOP… The Ovation1 POH has both LOP AND operations on turf runways… Some POHs lack certain chunks of data… Following POH data to the word can be challenging after half a century… There are certain things that can be added to the POH… some are only 100 pages long… The Ovation has 300 or so pages… There are probably an equivalent of 200 pages missing from the M20J POH by comparison… How does one handle following the POH when it recommends CHTs and TITs that are expensive to maintain? Or use a grass strip when the newer POHs have left that section out….? A64 is still getting to know my writing style… expect some interesting feedback on My take on POHs and their missing data… My writing skills aren’t very good, and can be easily misinterpreted… Summary… Try to be thoughtful to the other MSers… we want everyone coming back… Best regards, -a- Quote
A64Pilot Posted April 28, 2022 Report Posted April 28, 2022 (edited) There is nothing new about LOP, my Father would climb to 10 or 12 thousand and lean his 210 way back on long trips, 60 years ago, wasn’t called LOP, just “lean it out” most everyone with experience and not in a big hurry did, those in a hurry ran richer, usually about 100 ROP or more. Then I don’t know about you guys but I was taught way back when to pull the mix knob until it got a little rough then slowly push it back in until it smoothed out. Guess where that puts you in an engine capable of LOP? Yep pretty far into LOP. But as we were taught not to lean below 5,000 it didn’t hurt anything. Lycoming doesn’t recommended LOP likely because it turns the engine into a dog and they don’t want that rep. Mooney likely didn’t either for the same reason, knocks off a bunch of speed at normal cruising altitudes when the throttle is fully open, and Mooney was selling speed. Personally I run deep LOP most of the time, because I’m not in a hurry, but I do recognize the majority of my fuel savings comes from going slow, not from any huge increase in BSFC. Edited April 28, 2022 by A64Pilot Quote
carusoam Posted April 28, 2022 Report Posted April 28, 2022 We have a few types of pilots around here… 1) Those that like speed…. 2) Those that like efficiency… 3) Those people that like to blend speed with efficiency… And my favorite…. 4) Those that like to operate in flaming dragon mode… None are incorrect… Some have a larger operations budget than others… Go Mooney! Peace, -a- Quote
A64Pilot Posted April 28, 2022 Report Posted April 28, 2022 My take on the POH, is if you follow it, it’s safe, don’t and you are a test pilot, now I had that job from 1989 until 2017 or so, but I’m retired now, no more test pilot. Its real hard to justify operating an aircraft not IAW the POH to an accident investigator. 2 Quote
Shadrach Posted April 29, 2022 Report Posted April 29, 2022 (edited) On 4/28/2022 at 6:49 PM, JohnZ said: @Shadrach It’s a piston engine… not rocket science. Sorry that you got upset that I said LoP is overrated. Perhaps I just haven’t seen the light yet..? I wouldn’t advise anyone I was flying with to run the engine outside of the safe values listed in the POH. I don’t think that is indicative of lack of intelligence or understanding. You’re right, it’s literally combustion science. And I am guessing if you had any actual understanding, you’d not disparage the folks who’ve helped loads of pilots become better, more educated and safer operators as a “cult”. I don’t care how anyone chooses to operate their engine as long as they have a firm understanding of what they’re doing. When my POH was written, it would have been perfectly acceptable for an AME to conduct a flight physical with a lit cigarette in the exam room. Things change. My airplane came from the factory with a single EGT and CHT…Things change. In 1967 pilots had limited access to sophisticated training materials. Knowledge came from flight instructors, technical publications or was passed down through the tribe (a lot of it incorrect)… things change. You want to party like it’s 1959, have at it. Using the term “cult” to describe those that’ve done the work to advance beyond the old days of “cookbook operations” to actually understanding what’s happening under cowl is obnoxious. It may not demonstrate a lack of intelligence but certainly suggests that you don’t know what you don’t know. Most of us find ourselves in that situation from time to time. It takes wisdom not to showcase it. Edited May 1, 2022 by Shadrach 3 Quote
carusoam Posted April 29, 2022 Report Posted April 29, 2022 18 minutes ago, JohnZ said: . Not disparaging anyone who runs their engines lean. I really couldn’t care less. Chill out guy. John, You probably got mis-understood… In your own words… “Personal (and probably an unpopular) opinion is that the whole “red box”/LoP/Busch cult thing is overrated.” That could be mis-understood as disparaging…. I mis-understood what you meant by the word cult… Keep in mind… MS covers many Continents, and English isn’t always a primary language…. Ages run from about 20 to just over 90…. Some work 24/7… others are comfortably retired… Some people don’t realize they are making disparaging remarks… If I ran LOP and read Mr. Bush’s books… I would probably think you were poking fun at me… Sometimes when I mis-write something… It gets a follow-up… Oops… My bad. I fixed it… Or this is what I meant by that… Not everybody is an award winning writer around here… It helps to know the audience some… Best regards, -a- 2 Quote
Will.iam Posted April 29, 2022 Author Report Posted April 29, 2022 1 hour ago, A64Pilot said: Do this, go out and set as exact an indicated airspeed as you can and let things settle down, takes a few minutes the way you normally cruise, then reduce the RPM to the bottom of the green and increase MP to recover the speed exactly, may have to relearn. Then report back I’d like to see if what you get correlates to what I got out of an IO-540, I don’t think airframes matter as what we are doing by matching airspeeds as close as we can is establishing the same thrust, which should correlate to close to the same HP, the lower RPM motor should require slightly less HP due to prop efficiency. In test flying I’ve often run into things that can be definitely shown on paper, but in an airplane they are in the noise layer, that’s to say the difference is so slight it’s tough to quantify. Don't bust POH numbers but run as low an RPM and as high a MP as allowed, the run in the middle and or choose a smooth RPM. Write down fuel consumption difference, I’d be interested. Be sure to relean though as mixture will likely change I have to a certain extent, i lower the RPM and the MP rises but the fuel flow decreases. Speed drops off. But what i think i need to do is get some preset MP RPM and FF combinations that all equal the same calculated HP and then see what combinations are coolest on the TIT. Theoretically if the HP is the same the speed should be too but will have to wait until i can test in the plane which is down waiting on a fuel pump overhaul. 1 Quote
carusoam Posted April 29, 2022 Report Posted April 29, 2022 It takes some practice… Very few are good at writing to this wide of an audience… It is worth the extra effort… Then the weekend arrives… Wine starts pouring… and plenty of goofy things get written… Don’t drink and write… Best regards, -a- Quote
Shadrach Posted April 29, 2022 Report Posted April 29, 2022 30 minutes ago, JohnZ said: @Shadrach haha come on man all I said is that my opinion of it is that it’s overrated. It’s just an opinion. Let’s not take this into paragraphs-long iterations about how stupid the guy you’ve never met is who said LoP is overrated on MooneySpace. I read his book ‘On Engines’. I just found some of it hard to follow. Like if you don’t do the “big pull” then you’re abusing your engine in the red fin of doom. Not disparaging anyone who runs their engines lean. I really couldn’t care less. Chill out guy. No worries. I have plenty of chill. Just took a moment to articulate my opinion. Quote
Shadrach Posted April 29, 2022 Report Posted April 29, 2022 12 minutes ago, Will.iam said: I have to a certain extent, i lower the RPM and the MP rises but the fuel flow decreases. Speed drops off. But what i think i need to do is get some preset MP RPM and FF combinations that all equal the same calculated HP and then see what combinations are coolest on the TIT. Theoretically if the HP is the same the speed should be too but will have to wait until i can test in the plane which is down waiting on a fuel pump overhaul. This is exactly what I would do. I would not be overly concerned about trivial differences in TIT as long as you’re within operating range. Quote
A64Pilot Posted April 29, 2022 Report Posted April 29, 2022 (edited) 9 hours ago, Will.iam said: I have to a certain extent, i lower the RPM and the MP rises but the fuel flow decreases. Speed drops off. But what i think i need to do is get some preset MP RPM and FF combinations that all equal the same calculated HP and then see what combinations are coolest on the TIT. Theoretically if the HP is the same the speed should be too but will have to wait until i can test in the plane which is down waiting on a fuel pump overhaul. Even if you find charts showing you what you want, there are differences in aircraft and engines. If you match speed as close as you can you remove most variables. Do the low RPM test first, then the higher one and use the A/S you got from the low RPM test as of course you can go faster with higher RPM. First test point use the bottom of the green, second pick the smoothest RPM, if there is a significant difference your on to something, if it’s small, then maybe it’s not worth it. Have to have smooth air. I’m of the opinion that smooth is easier on the airframe, the avionics and less tiring on the people and probably easier on the engine too. It helps if you have altitude hold, smooth air of course and hit different test points on the same flight to remove any differences in atmosphere, you can correct for DA but why bother, it doesn’t take long, but be as precise as you can, let speed level off. Relean to a set point like say 25 LOP don’t use a set temp as mixture will change with RPM if you do. I did several test points on my IO-540W1A5D in my Maule comparing 25 LOP to Lycoming’s recommended 50 ROP. I had Gami injectors and fine wire plugs, newly overhauled Mag, new ignition harness and engine 200 SMOH with new Millenium cylinders. Try as I could but the 540 just wouldn’t go leaner than 25 LOP and stay smooth. I did the testing between 10 an 12 thousand as the Maule was fastest between those altitudes at 135 true, ROP, full throttle, RPM in middle of the green at 2200 Anyway what I came up with was two fold, first I lost more than 10 kts LOP, I don’t remember the exact number it was years ago, but secondly there was very little difference in fuel consumption if speed was identical. of course I was full throttle LOP but pulled back ROP. Now there are other reasons to run LOP, I almost always cruise my Mooney LOP unless I’m in a hurry and maybe the Lyc 360 works better LOP than the 540, it certainly will run smooth much, much deeper LOP than the 540, but I suspect the majority of fuel savings from LOP comes from speed reduction. If low ITT is your goal, I believe you’ll find the lower ITT will occur at higher RPM and lower manifold pressure, but I don’t fly a turbo so that’s a guess. If you conduct the testing please report back, whatever the results they will be interesting. Edited April 29, 2022 by A64Pilot Quote
Will.iam Posted April 29, 2022 Author Report Posted April 29, 2022 I will. The reason i sent off my fuel pump throttle body and spider to be overhauled as i would get some surging at lower power setting and could not lean out the idle cutoff to get 50 rpm rise at shutdown. Hopefully when returned my surging issue will go away, but i wish i had a jpi 830 or 930 as my jpi 700 doesn’t record or display MP i still have an analog display that is so small on the scale +- .5 MP is about the best i can read. Same with my OAT gage being analog. I have a digital one but waiting until annual to put it in. At least staying at the same altitude will minimize the temp especially if i stay in the same area. I do have a digital tach and tested against a photosensor tach it was +- 10rpm which i think is it’s resolution limit for 1000’s of rpm. All that to say my results will only be a precise as my input and i wish i had better display/control of my input. Else any performance gain will be lost in the noise. One of my goals is if i can keep the same airspeed but on cooler TIT i would be happy. Not asking for a lot of difference. Currently my TIT is at 1575 to 1600 LOP where as 1480 to 1510 ROP at 2500 RPM. When i go to 2200 RPM the LOP TIT comes back to 1550 but so does the speed. Quote
A64Pilot Posted April 29, 2022 Report Posted April 29, 2022 (edited) Having a digital display does not mean it’s any more accurate, sometimes is, but not always, source of the signal is more important Besides your not trying to develop charts, your just trying to see how much difference in fuel consumption there is, so what if your instrumentation is off a little your after does low RPM / high MP burn significantly less fuel than higher RPM /lower MP if I understand your question. The answer is it should burn less but the question in my mind is by how much, enough to be significant? if you keep the other variables the same, altitude, airspeed and same LOP point, just be sure to relean at each each test point so it’s apples to apples, once leaned the only thing you will be changing is throttle to match airspeeds, lean when your close but as mix changes power you will have to make a power adjustment. Close is good enough To quote my fixed wing test pilot mentor “perfection is the enemy of good enough” His point was you can test fly forever trying for perfection, when good enough is all you need. My assumption is if there is a significant difference, then this idea is worth chasing, but if not, maybe not. But at least you will know, you did it yourself I’ve tried it in a turbine too as in keeping it at TO RPM or reducing to bottom of the green, of course all that’s changing on a turbine is RPM / Torque, but HP and Ng speed and ITT and fuel flow remain the same, so all that changes is propellor RPM and efficiency. I saw no difference in speed, I thought it would speed up a couple of kts at low RPM, but it made no difference, all that changed was noise and vibration. Most run all the MP they can and middle range RPM as an RPM reduction means slowing down as you can’t increase MP, so it’s sort of a mute question. It’s similar to the LOP argument of simply increase MP to regain power lost from being LOP, but if your at max to start with, how do you do that? US NA guys are usually WOT, you turbo guys are I assume at max MP, or WOT if above critical altitude? Edited April 29, 2022 by A64Pilot Quote
Will.iam Posted April 30, 2022 Author Report Posted April 30, 2022 3 hours ago, A64Pilot said: Having a digital display does not mean it’s any more accurate, sometimes is, but not always, source of the signal is more important Besides your not trying to develop charts, your just trying to see how much difference in fuel consumption there is, so what if your instrumentation is off a little your after does low RPM / high MP burn significantly less fuel than higher RPM /lower MP if I understand your question. The answer is it should burn less but the question in my mind is by how much, enough to be significant? if you keep the other variables the same, altitude, airspeed and same LOP point, just be sure to relean at each each test point so it’s apples to apples, once leaned the only thing you will be changing is throttle to match airspeeds, lean when your close but as mix changes power you will have to make a power adjustment. Close is good enough To quote my fixed wing test pilot mentor “perfection is the enemy of good enough” His point was you can test fly forever trying for perfection, when good enough is all you need. My assumption is if there is a significant difference, then this idea is worth chasing, but if not, maybe not. But at least you will know, you did it yourself I’ve tried it in a turbine too as in keeping it at TO RPM or reducing to bottom of the green, of course all that’s changing on a turbine is RPM / Torque, but HP and Ng speed and ITT and fuel flow remain the same, so all that changes is propellor RPM and efficiency. I saw no difference in speed, I thought it would speed up a couple of kts at low RPM, but it made no difference, all that changed was noise and vibration. Most run all the MP they can and middle range RPM as an RPM reduction means slowing down as you can’t increase MP, so it’s sort of a mute question. It’s similar to the LOP argument of simply increase MP to regain power lost from being LOP, but if your at max to start with, how do you do that? US NA guys are usually WOT, you turbo guys are I assume at max MP, or WOT if above critical altitude? Actually no very far from WOT actually because Of my over square question / fear originally stated. For example, 25”mp 2500rpm and 9.5 puts me at 25 F LOP. Now if i increase MP to say 28” then LOP goes to something like 50 LOP and pushing to 31” MP and things get rough as I’m now down to something like 75 or 80 degrees LOP since i kept the fuel flow at 9.5 and rpm at 2500 for each MP change. To the engine adding air is the same as removing fuel you are leaning out the mixture which slows down the flame front speed alot. Since that piston is still goverened to 2500rpm at some point you get flames in the exhaust which heat up my tit again. Mike Busch has a good article on a cirrus 22T pilot that had this crazy backfire issue. After numerous parts swaps under warranty the dealership and continental were at a loss as to why this was happening. No one thought to ask the pilot how he was flying the plane but when he downloaded the data savvy saw he was trying to run LOP at 2700rpm and the flame was exploding out the exhaust valve into the exhaust. Once he told the pilot to pull back on the interlinked throttle, he could slow the RPM’s to 2500 before any throttle reduction would occur and the backfiring stopped. i think my limit is 9.5ff which is right at 1550tit so slowing the prop to add more power is the last option i have to keep the tit in check and maybe it’s not an option and i have to live with this as max performance with tit under 1550 LOP or fly ROP or live with overhauling my turbo sooner with pushing higher temps in higher power lop setting. as -A- says flaming dragon mode. 1 Quote
carusoam Posted April 30, 2022 Report Posted April 30, 2022 Other things to consider for this exercise… 1) Prop design…. The TopProp has a peak of efficiency at 2550 rpm…. (Other props may have a best rpm to run at as well) 2) at slower rpm…. The number of power strokes per minute is about 18% less than at higher rpms… more time in each stroke to burn fuel towards completion… 3) Peak EGT… is the point all fuel is used to produce power…. None used for cooling purposes… 4) Vz… the best aerodynamic efficiency for the plane… Karsen’s speed (spell check on his name?)… Slow, close to best glide and Vy… 5) Engine thermodynamic efficiency… This is where your low CR and turbo make sense… So…. We have a lot of different efficiencies to take into account… 1) Engine efficiency…. Set by CR…. 2) Prop efficiency…. Check with your prop manufacturer… 3) Mode of operation efficiency… how close to peak you can operate… 4) Airframe efficiency… How high can you fly?… thinner air provides less drag…. Climbing to that altitude has to make sense as well… Yay TC! 5) Cruising at Karsen’s speed is what you do while trying to cross the country non-stop…. (Find the MSer that did that in his M20E, San Diego to Georgia?) 6) Weather efficiency… selecting the day of the year that has the best tailwinds is a seasonal thing… modern day sailors like cruising above 200kts GS…. 7) once past peak… power equals FF… in an NA airplane you can watch the FF drop off going more LOP than peak….. and air speed will drop as the FF does…. No magic here…. In a TC airplane you can watch the FF stay set as airflow is increased…. A nice way to maintain power and go further LOP…. This takes knowing your engine really well… 8) ITT limits are a metallurgical consideration… set by the manufacturer… which are often higher than an owner wants to operate at… 9) Oddly, we don’t have a turbo speed sensor, or airflow sensor, or exhaust flow sensor…. 10) It gets more challenging to control the engine as ITT gets cooler… 11) There is probably a turbo rpm that is most efficient as well…. (Close to as hot as possible before the blades contact the turbo case) So… which efficiencies are most important to you? Often NMPG are important…. Read up on Mr. Karsen…. Find Norman around here… he wrote a nice Master’s Thesis on the subject… Fuzzy PP memories not an aerodynamicist… Best regards, -a- Quote
A64Pilot Posted April 30, 2022 Report Posted April 30, 2022 (edited) Why do you think your max fuel flow is 9.5? My NA 360 burns right at 20 on takeoff, and honestly more than 9.5 in normal cruise If the engine is capable of 36” continuously, then it’s capable of 36” continuously, but obviously you don’t have to run the snot out of it if you don’t want to, but some feel the need. We all operate oversquare at takeoff, fixed pitch guys even moreso, my little 140 is at 2300 and 30” at takeoff, R-1340 I cruised at 30” and 2000 RPM which is considered gentle, it’s max RPM from memory is 2250 and if memory is right it’s max MP at sea level is 36 Oversquare with a boosted engine within POH is normal and actually probably more ideal, it after all is sort of the point of boosting. Your Cirrus story is a good example of why to operate an engine IAW the POH, if he had, then he wouldn’t have had the problem. I know this never having seen a Cirrus POH. The POH limits have been checked, outside of POH hasn’t, Listen to who built the airplane and not all the internet experts, if the POH values were wrong they would have been corrected long ago. Or you can believe as so many do that all the manufactures are stupid and that aircraft have been operated safely for half a Century by pure luck Edited April 30, 2022 by A64Pilot Quote
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