I would like to dispel a myth here.
09-06-2005, 02:39 PM
#29
RE: I would like to dispel a myth here.
I'm gonna try one more time to make my point in a clear, succinct manner. Irrespective of other factors, no normally aspirated, carbureted, internal combustion piston engine will lean out "at altitude". Any increase in altitude will result in a richer mixture due to decreased air density. Period.
This is not an opinion. This is a fact.
It will lean out "at speed" when it unloads, or in a nose high attitude (is that you, Dan?
), and in other situations, so running rich-of-peak is a good thing. But not because of leaning "at altitude".This was fun though, wasn't it?[:@]
09-06-2005, 02:48 PM
#32
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RE: I would like to dispel a myth here.
I think when people say that the plane leans at altitude, I don't think they mean as in barometric altitude..... (at least I don't)....
I think they just mean once the plane is up and in the air... as in the prop is unloading and leaning out......
as in it has reached its flying altitude and the plane is leaning out (not do to barometric/atmospheric pressure, but just due to .... it ain't on the bench no more)
I think they just mean once the plane is up and in the air... as in the prop is unloading and leaning out......
as in it has reached its flying altitude and the plane is leaning out (not do to barometric/atmospheric pressure, but just due to .... it ain't on the bench no more
)
09-06-2005, 02:51 PM
#34
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RE: I would like to dispel a myth here.
I have a question though [a real one, not a "question" meant as sarcasm!] - as the air density decreases, and since our motors rely on suction to draw fuel, why does the mixture richen? I would have thought that the pressure drop through the carb would decrease [as would the pressure differential between atmosphere and the cylinder] and result in less fuel draw as well as reduced mass of air. The amount of fuel drawn in per intake stroke is dependent on the needle setting (akin to jet size) and the pressure drop through the venturi. I believe it does change, but not sure why one influence outweighs the other.
Mike D.
Mike D.
09-06-2005, 03:04 PM
#36
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RE: I would like to dispel a myth here.
Boy, Evil Homer: You sure started sumpin. Wow.
There is no comparison to leaning a full scale engine as you gain altitude and leaning out a model that may at best be flying at 500'.
When we richen a model engine, we call this the "sweet spot" and you will find, if you have the telemetry to analyize it, that the typical model engine will gain back any loss of RPM at ground level. I usually, and most flyer do, richen the mixture so that the engine loses perhaps 200 RPM on the ground and then when the model gets up off the ground and gains speed, picks this lost RPM back up.
If you fly the way you propose everyone to do so, you will burn your engine up the first time you take off on the lean side.
3dbob
There is no comparison to leaning a full scale engine as you gain altitude and leaning out a model that may at best be flying at 500'.
When we richen a model engine, we call this the "sweet spot" and you will find, if you have the telemetry to analyize it, that the typical model engine will gain back any loss of RPM at ground level. I usually, and most flyer do, richen the mixture so that the engine loses perhaps 200 RPM on the ground and then when the model gets up off the ground and gains speed, picks this lost RPM back up.
If you fly the way you propose everyone to do so, you will burn your engine up the first time you take off on the lean side.
3dbob
09-06-2005, 03:07 PM
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RE: I would like to dispel a myth here.
It's because you need so many molecules of fuel to react with so many molecules of oxidant (in air) to get a clean burn. If there is less air - at significant altitude I'd better add - it will require less fuel to maintain the correct fuel/air ratio and produce less power. Since you cannot normally reduce the fuel at altitude to compensate, then the engine would run rich. Suction is not the issue.
09-06-2005, 03:11 PM
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RE: I would like to dispel a myth here.
I think what MJD was trying to say is there are less air molecules pulling the fuel molecules in, so why doesn't it just balance out.
09-06-2005, 03:13 PM
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RE: I would like to dispel a myth here.
[quote]ORIGINAL: flycfii
All I know is that I have to lean the engine on the Cherokee when I hit a decent cruise altitude.
I thought that was more for fuel economy and to avoid fouling plugs, and rich mixture was for power, throttle response and reliability on takeoff, landing etc. Then again, I did ground school 1-1/2 times but only have about three hours stick time full scale, and that goes back a while! So what do I know.
Mike D.
All I know is that I have to lean the engine on the Cherokee when I hit a decent cruise altitude.
I thought that was more for fuel economy and to avoid fouling plugs, and rich mixture was for power, throttle response and reliability on takeoff, landing etc. Then again, I did ground school 1-1/2 times but only have about three hours stick time full scale, and that goes back a while! So what do I know.
Mike D.
09-06-2005, 03:14 PM
#40
RE: I would like to dispel a myth here.
MJD- I think I have an answer for you, but I wil lhave to do some more reading to be sure.
Its like Bodge said. As you gain altitude, the ratio of gases remains the same, and the VOLUME of air ingested remains the same, but its "thinner". So you have to reduce your fuel ratio to achieve an "ideal" mix. Not that you ever regain the power you get at sea level- that is why airplanes have ceilings. That is where the power produced can no longer compensate for the other forces, and you are at an equilibrium.
Its like Bodge said. As you gain altitude, the ratio of gases remains the same, and the VOLUME of air ingested remains the same, but its "thinner". So you have to reduce your fuel ratio to achieve an "ideal" mix. Not that you ever regain the power you get at sea level- that is why airplanes have ceilings. That is where the power produced can no longer compensate for the other forces, and you are at an equilibrium.
09-06-2005, 03:19 PM
#41
RE: I would like to dispel a myth here.
[quote]ORIGINAL: MJD
ORIGINAL: flycfii
I thought that was more for fuel economy and to avoid fouling plugs, and rich mixture was for power, throttle response and reliability on takeoff, landing etc. Then again, I did ground school 1-1/2 times but only have about three hours stick time full scale, and that goes back a while! So what do I know.
Mike D.
I thought that was more for fuel economy and to avoid fouling plugs, and rich mixture was for power, throttle response and reliability on takeoff, landing etc. Then again, I did ground school 1-1/2 times but only have about three hours stick time full scale, and that goes back a while! So what do I know.
Mike D.
Lycoming actually recommends a lean-of-peak (!) when using turbine inlet temperature, but Usually I go by EGT and "feel". There's always one cylinder that will let you know when you're good.
09-06-2005, 03:24 PM
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RE: I would like to dispel a myth here.
I think what MJD was trying to say is there are less air molecules pulling the fuel molecules in, so why doesn't it just balance out.
ORIGINAL: Rupurt
ORIGINAL: Rupurt
That is what I meant. I understand fuel:air stoichiometry (I design rocket motors for a living..). The only thing causing fuel to be drawn into the engine is suction created by pressure drop. So if air density decreases it seems to me that net pressure drop would also decrease. If it does, then it seems that a smaller mass of fuel would be drawn in per intake stroke as well as less air. Therefore, my first guess is that the two would somewhat balance out. But - even if they did balance out of course horsepower is down because the total mass of fuel:air mixture per stroke is reduced. But I am referring only to stoichiometry, not horsepower; my question is why the ratio shift? That's a physics puzzle beyond my understanding.
09-06-2005, 03:27 PM
#43
RE: I would like to dispel a myth here.
MJD- agreed. Please let me know what you find out, and I will do some digging as well. My guess is that your reasoning is logical on the surface, but there must be some contributing factor to the need for leaning (aside from my overindulgence this weekend).
09-06-2005, 03:41 PM
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RE: I would like to dispel a myth here.
I'll ask my physics guru tomorrow morning, but a quick search indicated that fuel metering is more dependent on the volume of air entering than the mass. That says something but it doesn't fully explain it.
09-06-2005, 03:44 PM
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RE: I would like to dispel a myth here.
Ok guys... you made my brain work for a while....(surprise it still works
) and the answer is (IMNSHO):
The higher the altitude, the lower the amount of oxygen molecules per cubic feet of air. So the air is not only thin as in less "dense" but it is also thin as in "less crowded"; Also, remember than our fliers (love that turn o century term) have positive pressure (for the most part) from the exhaust building in the tank and pushing through the tubing to the carb so even if the pressure drops in the venturi due to the thinner air, the positive pressure of the tank will compensate and over-compensate the loss of pressure.
Essentially at higher altitude the engine gets richer because the fuel has less molecules of oxygen (even wiht the same volume of air passing through) than at lower altitudes... and this is a FACT.. you can ask my buddies that fly near Quito-Ecuador at roughly 3000 masl (or in layman terms 15,000 ft) [8D] and usually lose 20-25% power and 10 to 15% in airlift on their craft so .... basically you lose the engine and you're flying a brick[:'(]
) and the answer is (IMNSHO):The higher the altitude, the lower the amount of oxygen molecules per cubic feet of air. So the air is not only thin as in less "dense" but it is also thin as in "less crowded"; Also, remember than our fliers (love that turn o century term) have positive pressure (for the most part) from the exhaust building in the tank and pushing through the tubing to the carb so even if the pressure drops in the venturi due to the thinner air, the positive pressure of the tank will compensate and over-compensate the loss of pressure.
Essentially at higher altitude the engine gets richer because the fuel has less molecules of oxygen (even wiht the same volume of air passing through) than at lower altitudes... and this is a FACT.. you can ask my buddies that fly near Quito-Ecuador at roughly 3000 masl (or in layman terms 15,000 ft) [8D] and usually lose 20-25% power and 10 to 15% in airlift on their craft so .... basically you lose the engine and you're flying a brick[:'(]
09-06-2005, 04:06 PM
#49
RE: I would like to dispel a myth here.
Rupert- the percentages stay the same almost all the way to space. The reason we need oxygen above a certain altitude is because of pressure (or lack thereof). Did you know that a commercial airliner willusually have a cabin pressure of 5000ft or so when cruising? I'm not sure if they supplement cabin air with oxygen or not.
09-06-2005, 04:10 PM
#50
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RE: I would like to dispel a myth here.
[b]All:
In some of the later posts in this thread the balance between pitch/power and altitude/airspeed has been somewhat clarified.
There is a very old book, "Stick and Rudder" by Wolfganfg Langshweisse, that explains it very well. This book used to be all but required reading for a PP license, wouldn't hurt anyone to read it now. Granted it's from the days of fabric and dope, but the principles will always apply.
In another thread I've listed sources for many modeling supplies, and since it's in "Scratch Building" forum I thought it was a good place for some aerodynamic themes as well. Go [link=http://www.rcuniverse.com/forum/m_1063414/tm.htm]here[/link] and read post number five - it includes a somewhat simplified explanation of the relationship between power and pitch trim.
And yes, it does presuppose starting in a stable condition. But isn't that stable condition what we usually try for?
Bill.
In some of the later posts in this thread the balance between pitch/power and altitude/airspeed has been somewhat clarified.
There is a very old book, "Stick and Rudder" by Wolfganfg Langshweisse, that explains it very well. This book used to be all but required reading for a PP license, wouldn't hurt anyone to read it now. Granted it's from the days of fabric and dope, but the principles will always apply.
In another thread I've listed sources for many modeling supplies, and since it's in "Scratch Building" forum I thought it was a good place for some aerodynamic themes as well. Go [link=http://www.rcuniverse.com/forum/m_1063414/tm.htm]here[/link] and read post number five - it includes a somewhat simplified explanation of the relationship between power and pitch trim.
And yes, it does presuppose starting in a stable condition. But isn't that stable condition what we usually try for?
Bill.