Over Square?
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cjam,
I've recently started flying 206s on ATOs and was curious as to the glide speeds you used as the person who checked me out didn't really give me a definitive answer. Cheers.
I've recently started flying 206s on ATOs and was curious as to the glide speeds you used as the person who checked me out didn't really give me a definitive answer. Cheers.
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Dogcharlietree. Should be no problem with the old radials - we invariably flew oversquare. Example: DC3 take off 48 inches manifold pressure and 2700 rpm for take off - 30.5 MP and 2050 rpm cruise - all ops normal. On base and final DC3 15MP and 2050 rpm. GA myths are like cockroaches - hard to kill and keep coming back.
Just need to be careful with the older engine gauges, some of the only have a single EGT probe which may not be the most critical cylinder, you might have one that is optimum, but unknowingly running one far too lean. Jamair has the right idea, probes for each cylinder..
Far too lean
The risk is not running 'far too lean'. Far too lean will just mean the fire going out in a cylinder and no power being produced. The vibration alone will indicate something is not right.
The risk with uneven fuel distribution is running around peak EGT at high power settings, which cause the highest peak cylinder pressures and temperatures. This is the worst place to run an engine.
To quote John Deakin. This was about the large flat sixes, such as the IO-520/550 and the IO-540, normally aspirated.
The Dangerous Red Box
Just where is that "red box" I keep talking about? Some rough numbers, good (that is to say, BAD) for most of these engines -- these are "no fly zones," DO NOT set the mixture between them:
--------------------------------------------------------------------------------
Red Box = No Fly Zone
At and below about 60% power, there is no red box. Put the mixture wherever you want it.
At about 65% power or so, 100ºF ROP to Peak.
At about 70%, 125ºF ROP to 25ºF LOP.
At about 75%, 180ºF ROP to 40ºF LOP.
At about 80%, 200ºF ROP to 60ºF LOP.
--------------------------------------------------------------------------------
All those numbers are approximate! Please don't start splitting hairs, here!
You probably don't want to run your engine between those mixture settings. If you do, you are running very high peak pressures inside the combustion chambers, and that peak pressure is occurring too close to top dead center.
There's a chance you read too fast, and missed this very important point, so let me put it another way:
--------------------------------------------------------------------------------
Outside the Box
At 65% power, use richer than 100 ROP, or leaner than peak EGT.
At 70%, use richer than 125ºF ROP, or leaner than 25ºF LOP.
At 75%, use richer than 180ºF ROP, or leaner than 40ºF LOP.
At 80%, use richer than 200ºF ROP, or leaner than 60ºF LOP.
--------------------------------------------------------------------------------
(On most of these engines, with a properly set mixture at full rich, at sea level, full power, the EGT ends up at about 250ºF ROP, with some as high as 300ºF ROP.)
Bevan..
The risk with uneven fuel distribution is running around peak EGT at high power settings, which cause the highest peak cylinder pressures and temperatures. This is the worst place to run an engine.
To quote John Deakin. This was about the large flat sixes, such as the IO-520/550 and the IO-540, normally aspirated.
The Dangerous Red Box
Just where is that "red box" I keep talking about? Some rough numbers, good (that is to say, BAD) for most of these engines -- these are "no fly zones," DO NOT set the mixture between them:
--------------------------------------------------------------------------------
Red Box = No Fly Zone
At and below about 60% power, there is no red box. Put the mixture wherever you want it.
At about 65% power or so, 100ºF ROP to Peak.
At about 70%, 125ºF ROP to 25ºF LOP.
At about 75%, 180ºF ROP to 40ºF LOP.
At about 80%, 200ºF ROP to 60ºF LOP.
--------------------------------------------------------------------------------
All those numbers are approximate! Please don't start splitting hairs, here!
You probably don't want to run your engine between those mixture settings. If you do, you are running very high peak pressures inside the combustion chambers, and that peak pressure is occurring too close to top dead center.
There's a chance you read too fast, and missed this very important point, so let me put it another way:
--------------------------------------------------------------------------------
Outside the Box
At 65% power, use richer than 100 ROP, or leaner than peak EGT.
At 70%, use richer than 125ºF ROP, or leaner than 25ºF LOP.
At 75%, use richer than 180ºF ROP, or leaner than 40ºF LOP.
At 80%, use richer than 200ºF ROP, or leaner than 60ºF LOP.
--------------------------------------------------------------------------------
(On most of these engines, with a properly set mixture at full rich, at sea level, full power, the EGT ends up at about 250ºF ROP, with some as high as 300ºF ROP.)
Bevan..
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Bevan,
what would you do in my situation, POH and company SOP's saying do one thing, and a few seemingly intelligent and experienced people on the net saying that that is the worst place to run my engine.
I'm going to stick to the POH and SOP's until I can learn more but it is a little disconcerting hearing that the most dangerous place to run it is exactly where the POH says to.
How do you know you don't have mild detonation occuring when you are running lean, purely due to a lean mixture being more susceptable to detonation? Do those extra gauges you were talking about let you know?
Cheers, cjam
what would you do in my situation, POH and company SOP's saying do one thing, and a few seemingly intelligent and experienced people on the net saying that that is the worst place to run my engine.
I'm going to stick to the POH and SOP's until I can learn more but it is a little disconcerting hearing that the most dangerous place to run it is exactly where the POH says to.
How do you know you don't have mild detonation occuring when you are running lean, purely due to a lean mixture being more susceptable to detonation? Do those extra gauges you were talking about let you know?
Cheers, cjam
cjam,
If you are not paying for the fuel or the maintenance, operate the engine how the SOPs/POH says.
If you are paying for the engines and if the instrumentation in the aircraft does not allow LOP operations, then enrichen the mixture to 75-100 ROP not 50 ROP (This is how I operate the aircraft I fly (A36 Bonanza - IO-520BB) as it only has the old style single EGT and does not have GAMI injectors.
If you use the guidelines above, your engine will run cooler, have greater detonation margins and last a lot longer.
I wish everyone would get away from Lean == Bad == Detonation. Lean == Cooler Engine == Happy Engine.
Detonation is caused by high cylinder temperatures and high pressures (which are found near peak EGT at high power settings). Stay away from these zones at high power settings and wrong mixture settings (around peak EGT) and you'll have more than adequate detonation margins. (Remember there are some margins built into the engine design to protect against dumb pilots, on the larger engines, this margin is not so great, as they have been designed to produce more power than smaller engines.
How do you know how cool your engine is running - the CHT guage. I endevour to keep mine below 175 degrees C (350F) and at those temps detonation should not be an issue.
I believe John Deakin also wrote an article in there about detonation.. give that a read.
Bevan..
If you are not paying for the fuel or the maintenance, operate the engine how the SOPs/POH says.
If you are paying for the engines and if the instrumentation in the aircraft does not allow LOP operations, then enrichen the mixture to 75-100 ROP not 50 ROP (This is how I operate the aircraft I fly (A36 Bonanza - IO-520BB) as it only has the old style single EGT and does not have GAMI injectors.
If you use the guidelines above, your engine will run cooler, have greater detonation margins and last a lot longer.
I wish everyone would get away from Lean == Bad == Detonation. Lean == Cooler Engine == Happy Engine.
Detonation is caused by high cylinder temperatures and high pressures (which are found near peak EGT at high power settings). Stay away from these zones at high power settings and wrong mixture settings (around peak EGT) and you'll have more than adequate detonation margins. (Remember there are some margins built into the engine design to protect against dumb pilots, on the larger engines, this margin is not so great, as they have been designed to produce more power than smaller engines.
How do you know how cool your engine is running - the CHT guage. I endevour to keep mine below 175 degrees C (350F) and at those temps detonation should not be an issue.
I believe John Deakin also wrote an article in there about detonation.. give that a read.
Bevan..
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sweet, thats what I'll do then, I'll run at 75 ROP instead of 50, that will keep everyone happy, hopefully the plane too.
cheers.
PS Is a Gami injector similar to a Gami leg?
cheers.
PS Is a Gami injector similar to a Gami leg?
Hudson. Yes, this is what I was saying about some of us were taught oversquare all the time on radials.
In fact when I was taught (yes I know - tin hat's on and in the trenches ;-) we had our wrist broken if we ever came close to not being oversquare.
From base position on, we could bring the MAP back a little if required.
And pertaining to another thread re pitch full fine on finals, No we never did this. Not good for the engines or pax confidence.
Procedures for missed approach were;
Throttles - 30"
Undercarriage - UP
Pitch - Full Fine
Throttles - 48"
Retract Flaps in stages.
When gear is up and 100Kts - METO power.
Ah, after that, I'm just gonna go and put my "Round Sounds" CD on full volume............Ah, I miss a proper (round) engine!
In fact when I was taught (yes I know - tin hat's on and in the trenches ;-) we had our wrist broken if we ever came close to not being oversquare.
From base position on, we could bring the MAP back a little if required.
And pertaining to another thread re pitch full fine on finals, No we never did this. Not good for the engines or pax confidence.
Procedures for missed approach were;
Throttles - 30"
Undercarriage - UP
Pitch - Full Fine
Throttles - 48"
Retract Flaps in stages.
When gear is up and 100Kts - METO power.
Ah, after that, I'm just gonna go and put my "Round Sounds" CD on full volume............Ah, I miss a proper (round) engine!
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Gday all,
Splat,
It is an old wives tale also that engines fail more often on the first power reduction. Thats just when most instructors simulate the failure. Unless you brutally yank the pitch levers back and fling a counterweight of cutting your engine in half, you can safely reduce your power when you or the boss see fit.
Lycoming and Continental both have data to prove more donks go to pieces in the cruise.
The over square myth has been sorted by previous posts.
Cheers,
I'm gone!
Splat,
It is an old wives tale also that engines fail more often on the first power reduction. Thats just when most instructors simulate the failure. Unless you brutally yank the pitch levers back and fling a counterweight of cutting your engine in half, you can safely reduce your power when you or the boss see fit.
Lycoming and Continental both have data to prove more donks go to pieces in the cruise.
The over square myth has been sorted by previous posts.
Cheers,
I'm gone!
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Gday,
My first boss( a LAME ) on his first flight with me,(after watching me set flying school power in one of his C182's) said to me " you won't hurt the old sh!ts by gettin up 'em!" and promptly put on a couple more inches!
Cheers,
I'm gone!
My first boss( a LAME ) on his first flight with me,(after watching me set flying school power in one of his C182's) said to me " you won't hurt the old sh!ts by gettin up 'em!" and promptly put on a couple more inches!
Cheers,
I'm gone!
On radials and over-square. Most (all?) radials are supercharged and so they are capable of quite high MAPs. Eg we used to do aeros in our Harvard at 30" and 2000 RPM and cruise at 26"/1600.
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Just read John Deakin's comments.
Its all explained very well here...
http://www.avweb.com/news/columns/182146-1.html
G
Its all explained very well here...
http://www.avweb.com/news/columns/182146-1.html
G
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Dont use over square power settings?
What a load of bull! The instructor who told you this needs to go and have a chat with their local LAME.
Dont forget, that the stuff about engine operations in your POH is approved by the engine manufacturer before its incorporated in to the POH when the aircraft is being certified.
As for reducing power after T/O. I havent seen 1 POH that says to reduce power after T/O, unless the aircraft has a time limit on MCP listed in the POH. If you reduce power at 300ft after T/O on most light twins they wont outclimb even the most average looking hills, with a full load in summer. If the book says you can use full power for an en-route climb USE IT! If it says you can use over square power settings USE IT!
I have been using full power for all en-route climbs in all of the twins i have been flying on charters over the years. I have never had an engine overheat or misbehave in any sort of way - even in the middle of summer with the OAT showing 40 degrees plus outside. If you operate the aircraft according to the book, you will get the right performance and you wont hurt it in anyway.
I also teach my students not to reduce power after T/O, and to use over square settings on the company Partenavia and Arrow if they are required to be used and the book says they can be used.
What a load of bull! The instructor who told you this needs to go and have a chat with their local LAME.
Dont forget, that the stuff about engine operations in your POH is approved by the engine manufacturer before its incorporated in to the POH when the aircraft is being certified.
As for reducing power after T/O. I havent seen 1 POH that says to reduce power after T/O, unless the aircraft has a time limit on MCP listed in the POH. If you reduce power at 300ft after T/O on most light twins they wont outclimb even the most average looking hills, with a full load in summer. If the book says you can use full power for an en-route climb USE IT! If it says you can use over square power settings USE IT!
I have been using full power for all en-route climbs in all of the twins i have been flying on charters over the years. I have never had an engine overheat or misbehave in any sort of way - even in the middle of summer with the OAT showing 40 degrees plus outside. If you operate the aircraft according to the book, you will get the right performance and you wont hurt it in anyway.
I also teach my students not to reduce power after T/O, and to use over square settings on the company Partenavia and Arrow if they are required to be used and the book says they can be used.
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Interesting topic.
Was taught in normally asperated engine "if book figures are not available don't oversquare"
big fan of lower rpm.
work it out. 200 rpm = 12 000 extra turns per hour, 1.2 million extra every 100 hours and a staggering 20.4 million extra revalutions of the engind over a 1700 hr lifetime. Thats alot of wear and tear to save by running a lower RPM.
Was taught in normally asperated engine "if book figures are not available don't oversquare"
big fan of lower rpm.
work it out. 200 rpm = 12 000 extra turns per hour, 1.2 million extra every 100 hours and a staggering 20.4 million extra revalutions of the engind over a 1700 hr lifetime. Thats alot of wear and tear to save by running a lower RPM.
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What I don't get, is why two aircraft with the same engine and prop can have differing power settings for certain conditions despite having the same engine and prop. Example, a Bonanza I used to fly used 23"/2400RPM for the cruise. A Baron I've been flying recently uses 24"/2300RPM for cruise. Both figures as per the respective POH and both aircraft have same engine and prop.
Why so?
520.
Why so?
520.
Grandpa Aerotart
Conti 520...and others,
The big problem with talking power settings is people don't understand what the terms MP/RPM/Mix really mean.
24/23 or 23/24 are NOT POWER SETTINGS.
Manifold pressure is simply an indication of how much air is available for combustion....my Bonanza is sitting in a hangar at Redcliffe, the MP gauge says 29.5 odd inches...is it developing full power?
The throttle controls how much air the pistons can suck in on the intake stroke.
RPM is certainly the speed that the prop is turning but it, in a direct drive engine like most aero engines, it is also an 'indication' of how fast the pistons are travelling up and down, and therefore how much air they are sucking into the combustion chambers, and to a lesser extent how fast the engine driven fuel pump is turning.
What RPM is not is an indication of power/thrust. If I climb overhead YRED and pull the mixture control to cutoff and lower the nose enough the RPM will remain at whatever it was....but it's not producing any thrust/power.
Mixture controls the fuel side of the fuel/air ration while MP & RPM control the air side. Fuel will only ignite and do 'work' over a reasonably small range of fuel/air ratios. Too rich and it wont burn...too lean and the fire is snuffed out as well.
At a specific ratio all the fuel and all the air is burnt and the maximum work is done..cant remember the word (stociometric?) off the top of my head. Imagine that fuel/air ratio plotted on a graph of fuel/air ratios....it's the pinacle of a parabola...leaner down one side/richer down the other (you can see a depiction in JD's articles)
That point is essentially, (although not exactly because it's almost impossible in the real world to get perfect burning), 100% power but at ANY mixture setting either leaner or richer you can get any other power setting between 0 & 100%.
What has always stopped LOP in the past has been crap injectors. Balanced injectors tailored to specific cylinders balance the fuel air ratios across the cylinders and the engine then runs smooth at almost any mixture setting therefore opening up the other half of the mixture spectrum that has been almost impossible to achieve since flat engines were invented...on radials LOP was normal but EGTs were not invented so it was not called LOP.
So to 'oversquare'.
On the vast majority of piston engines there is absolutely NO limitation on MP/RPM...MCP being usually full throttle/2700rpm.
Closing the throttle is materially exactly the same as flying with a blocked air filter....how many of us do that knowingly?
The correct RPM is whatever RPM you want and that only varies with what you want to do...high/low/fast slow etc.
Changing RPM can be likened to changing gears in a manual car.
The red mixture knob/lever is the only control that can vary power from o% to 100%.
With standard ****e injectors you can only make that variation on the rich side of peak 'cause the engine will run too rough because the fuel/air ratios will be so out of whack that you have essentiall 4 or 6 single cylinder engines all running at different power outputs...no wonder the thing shakes like a dog passing a peach seed.
On the rich side of peak and at full throttle you have lots of both fuel and air so you must run very rich to control CHTs or reduce the air so that you can reduce the fuel flow to reach a new 'power' setting that doesn't cook the engine. Other than a small influence on the speed of the engine driven pump RPM is immaterial.
Bottom line is you're cooling the engine, partly, with extra fuel.
On the lean side of peak you are cooling the engine with excess air but the same range of power setting is available (at MSL/ISA-- turbonormalising fixes that nicely)
I get 75% power in my Bonanza at about 27in/2200rpm and 50 odd degrees LOP EGT (roughly 50 liters/hr)....or I could get it at about 23in/2400 and 80 odd ROP EGT and around 65 liters an hour.
The first way gives me cooler CHTs, slows down the rate of combustion thereby widening the detonation margins, lowers the peak pressures in the cylinder and saves nearly $20 bucks an hour in fuel.
If you aren't taking into account the mixture you are not talking power settings...you can get essentially any power setting at any combination of MP/RPM...what you do with the mixture is the important bit.
Oversquare being bad, like setting 25/25 right after takeoff is flying school horse****e!!!
Chuck.
The big problem with talking power settings is people don't understand what the terms MP/RPM/Mix really mean.
24/23 or 23/24 are NOT POWER SETTINGS.
Manifold pressure is simply an indication of how much air is available for combustion....my Bonanza is sitting in a hangar at Redcliffe, the MP gauge says 29.5 odd inches...is it developing full power?
The throttle controls how much air the pistons can suck in on the intake stroke.
RPM is certainly the speed that the prop is turning but it, in a direct drive engine like most aero engines, it is also an 'indication' of how fast the pistons are travelling up and down, and therefore how much air they are sucking into the combustion chambers, and to a lesser extent how fast the engine driven fuel pump is turning.
What RPM is not is an indication of power/thrust. If I climb overhead YRED and pull the mixture control to cutoff and lower the nose enough the RPM will remain at whatever it was....but it's not producing any thrust/power.
Mixture controls the fuel side of the fuel/air ration while MP & RPM control the air side. Fuel will only ignite and do 'work' over a reasonably small range of fuel/air ratios. Too rich and it wont burn...too lean and the fire is snuffed out as well.
At a specific ratio all the fuel and all the air is burnt and the maximum work is done..cant remember the word (stociometric?) off the top of my head. Imagine that fuel/air ratio plotted on a graph of fuel/air ratios....it's the pinacle of a parabola...leaner down one side/richer down the other (you can see a depiction in JD's articles)
That point is essentially, (although not exactly because it's almost impossible in the real world to get perfect burning), 100% power but at ANY mixture setting either leaner or richer you can get any other power setting between 0 & 100%.
What has always stopped LOP in the past has been crap injectors. Balanced injectors tailored to specific cylinders balance the fuel air ratios across the cylinders and the engine then runs smooth at almost any mixture setting therefore opening up the other half of the mixture spectrum that has been almost impossible to achieve since flat engines were invented...on radials LOP was normal but EGTs were not invented so it was not called LOP.
So to 'oversquare'.
On the vast majority of piston engines there is absolutely NO limitation on MP/RPM...MCP being usually full throttle/2700rpm.
Closing the throttle is materially exactly the same as flying with a blocked air filter....how many of us do that knowingly?
The correct RPM is whatever RPM you want and that only varies with what you want to do...high/low/fast slow etc.
Changing RPM can be likened to changing gears in a manual car.
The red mixture knob/lever is the only control that can vary power from o% to 100%.
With standard ****e injectors you can only make that variation on the rich side of peak 'cause the engine will run too rough because the fuel/air ratios will be so out of whack that you have essentiall 4 or 6 single cylinder engines all running at different power outputs...no wonder the thing shakes like a dog passing a peach seed.
On the rich side of peak and at full throttle you have lots of both fuel and air so you must run very rich to control CHTs or reduce the air so that you can reduce the fuel flow to reach a new 'power' setting that doesn't cook the engine. Other than a small influence on the speed of the engine driven pump RPM is immaterial.
Bottom line is you're cooling the engine, partly, with extra fuel.
On the lean side of peak you are cooling the engine with excess air but the same range of power setting is available (at MSL/ISA-- turbonormalising fixes that nicely)
I get 75% power in my Bonanza at about 27in/2200rpm and 50 odd degrees LOP EGT (roughly 50 liters/hr)....or I could get it at about 23in/2400 and 80 odd ROP EGT and around 65 liters an hour.
The first way gives me cooler CHTs, slows down the rate of combustion thereby widening the detonation margins, lowers the peak pressures in the cylinder and saves nearly $20 bucks an hour in fuel.
If you aren't taking into account the mixture you are not talking power settings...you can get essentially any power setting at any combination of MP/RPM...what you do with the mixture is the important bit.
Oversquare being bad, like setting 25/25 right after takeoff is flying school horse****e!!!
Chuck.
Last edited by Chimbu chuckles; 9th Mar 2004 at 00:21.