Proper Leaning Procedures in Mountain Flying

[avbug]

I don't know. In which neck of the woods do you reside?

 

[BYUFlyr]

First, I gotta ask what your instructor has been doing all this time that you don't know how or when to lean an engine. This is pre-solo stuff. Really.

Like I said, I've done most of my flying in Florida where there is no need to lean an engine on the ground for better performance. The A/C's checklist states to use a full rich setting during the pre-T/O and pre-landing procedures; to make it even more confusing every pilot I spoke to in Utah had a different procedure. I personally think lean for best performance in T/O and landing....

avbug, thank you for the info... you've been very helpful

 

[westwind]

I don't know. In which neck of the woods do you reside?

I'm that goober that lives near Leadville.

 

[avbug]

Leadville. Lovely scenery this time of year. Come to think of it, any time of year. Do you mine?

Dunno what your image was supposed to be, byu; you'll have to be more creative.

Let's try again. Leaning on the ground for idle is futile; you're not leaning the idle mixture, as that can only be leaned by the mechanic while working on the carburetor...you don't control it.

As for leaning during the runup, it's also wasteful. You need to lean for the power setting to be used for takeoff if you're leaning for takeoff. Not at some other power setting. If you runup at 1,700 rpm at partial throttle, then you're only leaned for that power setting, not for the takeoff power setting.

Many carburetors use an enrichment or economizer valve. This is actually a flat spot that allows a greater fuel flow during full throttle operation, thereby enrichening the mixture. If you lean at a lesser power setting, you still haven't leaned for takeoff. You need to do your leaning at full power, and you need to be aware that if you don't enrich the mixture prior to a power reduction from full power, then you're going to be operating too lean. You might have problems.

Leaning can be done by sound or by RPM loss, but not effectively. Proper leanign requires exhaust gas temp probes, preferably on all cylinders.

Some engines, for the radial crowd, can effectively be leaned by the color of the flame. If you don't have this opportunity, then learn proper leaning techniques, rather than asking every pilot you see. A very rough rule of thumb is to lean until you get the engine faltering then enrichen slightly. This does very little, if anything, to properly set the mixture, but is a very rough leaning technique that may leave you too lean, too rich,or a combination of both (usually the case, as cylinders each receive a different mixture and run at a different temperature).

 

[minitour]

...Let's try again. Leaning on the ground for idle is futile; you're not leaning the idle mixture, as that can only be leaned by the mechanic while working on the carburetor...you don't control it...

Okay, help me out here if I got this wrong.

How fast or how much the air goes through the carburetor is controlled by the throttle. When you open the throttle, a lot of air is going through, drawing a lot of fuel out of the float chamber (obviously considering a float chamber). If you've got the throttle only partially open (or is it partially closed?) then you're not drawing near as much fuel out of the float chamber.

So when you lean the mixture, you're "telling" the carburetor, "hey, I only want 'this much' fuel right now coming out of the float chamber and getting mixed with the air".

If this is right, then avbug, that makes so so much sense.

I'm sure there's a more technical way to explain it with technical terms and scuh, but I'm just wondering if I got the general ideas down. I'd love to explain this to someone at school tomorrow that insists on leaning for taxi when I'm taxiing at 700-800RPM (any more and I get b*tched at for taxiing too fast).

Thanks!

-mini

 

[avbug]

Simply put, when the throttle is open and airflow passes through the carburetor, it passes through a constriction in the throat of the carburetor; we refer to this restriction as the venturi. The venturi sees a speed increase in the airflow and a pressure drop. A fuel port, or jet, is located in this low pressure area. The faster the airflow, the greater the pressure drop, the greater fuel flow that is drawn into the airflow.

So far, so good.

Close the throttle. Very little air flowing through the venturi. The engine is a suction pump, and the throttle is like your hand over the hose of a vacum cleaner. What does the vacum cleaner do when you cap your hand over the hose? The pressure drops in the hose even more (like manifold pressure does when you close the throttle, right??), and the engine strains to get more air.

Same for your engine. The engine still gets fuel, or it wouldn't run, but it's not getting the fuel through the main fuel jet. There's no air flowing past that jet to draw the fuel out. Your mixture controls that jet...your mixture isn't doing anything because that jet isn't doing anything. Your mixture isn't controlling or adjusting fuel flow, because the fuel source that the mixture adjusts isn't in use.

Where the throttle is closed, it doesn't seal perfectly. Otherwise the engine would simply die. Along the edge of the throttle plate a little air gets by. It rushes in due to the low pressure on the engine side of the throttle plate..."suction," if you will. Suction, caused by the suction machine, the engine.

Adjacent to this spot where the air leaks by is the idle jet. This jet also has it's own mixture control, but it's a fixed control set by a mechanic when the meachanic adjusts the idle mixture. The only control you have over this is the idle cutoff function of the mixture control. This is a very rough control; you're actually cutting off the fuel rather than adjusting it's flow, and it's a very, very minute range in which this works.

If your idle mixture is set properly you should be able to set the throttle to idle, and oh-so-slowly retard the mixture toward the cutoff position. At some point the engine rpm should rise about 25 rpm, and the engine should then die. If it doesn't rise at least that, then you're operating with too lean an idle mixure. If it rises much more than that, you're operating with too rich an idle mixture. Either condition needs maintenance.

That little bump, that little 25 rpm rise, is all the mixture adjustment you get in idle. Otherwise, your mixture isn't doing a darn thing no matter how much you twiddle with it. That little 25 rpm jump isn't mixture control, either, but shutting off the fuel, instead. Something entirely different. Far less reliable, far less repeatable, harder to locate, and not consistant.

If you're working an injected engine, the semantics are somewhat different.

 

[minitour]

Thanks

I didn't really buy it before, but I really really thought about it tonight and just sat down and thought about the basics of the principle and wow does it make sense.

Fantastic stuff! Thanks again.

One question though.

Is it better to have the mixture set for "idle" at a SL elevation or where you house the plane?

I'd think SL is where you'd want it and that would mean if you were high enough, you should lean for taxi, etc.

If you didn't set it for a SL altitude and you were at a ~5,000' airport, you'd run too lean coming down to Florida for vacation with your family.

Not sure if that really makes sense, but it does in my head .

Thanks for any insight!

-mini

*edit*
Or would that still not make a difference unless you were up at a really really high altitude during taxi/ground ops?

 

[avbug]

Minitour,

When you wrote the part that said,

I'd think SL is where you'd want it and that would mean if you were high enough, you should lean for taxi, etc.

I think you missed the entire point of the previous discussions about why you can't lean for taxi. Remember?

The idle mixture should be adjusted any time a general climatic change occurs (seasonally) or any time you base the airplane at a different elevation. It's routine maintenance, and should be checked after every flight, and adjusted as required.

 

[minitour]

Eh...I guess I kinda f'd that one up...

I'll try again...

If you have the "idle mixture" set for your airport and you live at a 5,000' airport, when you decide to take the family down to Florida for vacation, you'd be running too lean at full rich. So shouldn't you just have it set for SL Pressure and density and then lean for (what I should have said) Takeoff?

Otherwise, you'd run out of fuel for the engine at sea level, right?

-mini

 

[BYUFlyr]

Dunno what your image was supposed to be, byu; you'll have to be more creative.

I'm sorry I don't understand.... Maybe you misunderstood, I wasn't being sarcastic... thank you for the info. That first time around answered my question.

I was just explaining why my pre-solo written didn't cover leaning procedures for T/O in Florida. It's the same reason why pre-solo writtens in Utah don't cover a whole lot on Florida thunderstorms... different kind of GA flying.