Mixture during maneuvers?

[poadeleted20]

No, it isn't, unless you descend at climb power.

Enriching on descent at high DA can easily flood the engine and cause it to quit, as it is rather difficult to lean precisely at low throttle.

One need only watch the disproportionate drop in EGT's when you pull the throttle back a couple of hundred RPM/a couple of inches MP to commence a descent.

Obviously, if you leave the throttle at the cruise power setting for descent, no change in mixture is required other than very small changes as DA decreases. But if you watch the engine analyzer. you'll see that throttle changes (especially in carbureted engines) have a much greater short-term effect on current EGT in relation to peak EGT than altitude changes.

 

[GeorgeC]

Ron, can you expand on this?

Unless you have a carb air temp gauge so you can modulate carb heat to stay in the green zone, never use partial carb heat -- it might make things worse rather than better.

With carbureted engines, there is no need to enrich during descent unless/until you advance the throttle above where it was in cruise. The reduction in throttle results in a slightly richer mixture all by itself, and enriching it further by advancing the mixture only creates greater potential for plug fouling during the descent.

 

[poadeleted20]

Ron, can you expand on this?

The way a carburetor works, when you reduce throttle, the mass of fuel sucked out of the carburetor jet does not diminish as rapidly as the mass of air flowing past the jet. You'd need to get into a mechanical engineering fluid dynamics book to get a full explanation of why that happens, but as a result, the mass ratio of fuel to air increases as the throttle is closed, and the mixture gets richer.

 

[bobmrg]

Depends on the 172. If you're flying one of the older models with the Continental O-300 engine, absolutely yes, do it that way. With the middle range 172's with the carbureted Lycoming O-320 engine, it's not nearly as critical, and using carb heat only when symptoms of carb icing are detected is OK. And for the later 172's with the injected Lycoming IO-360 engine, you don't even have a carburetor to heat.

As for go-arounds in the carbureted models, just make sure your thumb is catching the carb heat knob and pushing it forward as you advance the throttle and you'll do fine.

That is why humans have opposable thumbs. Not sure where Pipers fall on the evolutionary scale.

Bob Gardner

 

[bobmrg]

No, it isn't, unless you descend at climb power.

Enriching on descent at high DA can easily flood the engine and cause it to quit, as it is rather difficult to lean precisely at low throttle.

This is my position on enriching during descent: Listen to your engine. If it begins to make uncomfortable noises, richen until it is happy. Repeat until in the pattern or on final. Keep that engine toasty warm.

Bob Gardner

 

[Stewartb]

This is my position on enriching during descent: Listen to your engine. If it begins to make uncomfortable noises, richen until it is happy. Repeat until in the pattern or on final. Keep that engine toasty warm.

Bob Gardner

The voice of common sense. And good practical advice. Thanks!

 

[poadeleted20]

That is why humans have opposable thumbs. Not sure where Pipers fall on the evolutionary scale.

Well, you could do it on the old Pipers with push-pull controls rather than the newer ones with the levers and the carb heat out of hand-span on the right, so maybe Pipers are devolving? But it works well on Cessnas and Grummans (and even the AG-5B's with the Piper-style levers have the carb heat as a lever on the quadrant).

 

[poadeleted20]

This is my position on enriching during descent: Listen to your engine. If it begins to make uncomfortable noises, richen until it is happy.

Except that if you're pulling power back, you may need to lean to make it happy again.

 

[MAKG1]

The way a carburetor works, when you reduce throttle, the mass of fuel sucked out of the carburetor jet does not diminish as rapidly as the mass of air flowing past the jet. You'd need to get into a mechanical engineering fluid dynamics book to get a full explanation of why that happens, but as a result, the mass ratio of fuel to air increases as the throttle is closed, and the mixture gets richer.

This only explains a momentary enrichment.

Using the carburetor's main circuit, the steady state fuel flow is proportional to the airflow, with the proportionality determined by the mixture needle.

Bernoulli's Principle (the real one) takes care of this for you. Pressure change in the venturi is proportional to mass air flow (i.e., lb/min, not CFM) through the venturi.

In practice, a carburetor normalizes within a few strokes, and the consequence of snapping the throttle shut is the occasional pop in the exhaust. And then it's done.

 

[bobmrg]

Ron, can you expand on this?

Throttle controls air, mix controls fuel. If you cut back on air and don't touch the mixture, the fuel-air ratio gets richer.

IOW, when you "step on the gas" you are really stepping on the air.

Bob Gardner

 

[Stewartb]

The main jet feeds fuel by suction into the air stream. Air flow controls fuel flow unless it's metered by the mixture control.

Aircraft carb lesson basics. Here's the bowl from my 180's carb. The brass pipe is the main jet. It's fed by the tube on the bottom of the bowl. The slotted piece in the left of the picture is the mixture cam. It fits into the fuel inlet, that tube in the bottom of the bowl. When the slot is oriented toward the jet the mixture is full rich. When it rotates it restricts the fuel feed until it closes it off completely at lean cutoff. The reservoir in the lower right is the accelerator pump chamber. it's normally full of gas until when you move the throttle open when a plunger displaces that fuel through the little nozzle you can see in the carb throat. That nozzle shoots a pressurized stream of raw fuel straight into the intake. That's the basics of an aircraft carb. The idea that closing the throttle enriches the mixture may be true but it would be momentary and unimportant to the average pilot. When I think I may need to advance the throttle and have that pressurized stream of raw fuel shot into my intakes I want the carb heat off.

 

[poadeleted20]

I'm not sure where you get the idea that the change is momentary. The feed pressure on the fuel remains the same, but the airflow is reduced, and both stay that way unless you change either the throttle position or the mixture control. As a result, f/a ratio goes up and stays up unless/until one of those two controls is changed.

 

[Stewartb]

Feed pressure? Pressure from what? You have a float that meters fuel into the bowl and the bowl is right there in the picture. Where does this fuel pressure come from? What keeps my fuel from flowing when I turn off the engine with the key rather than the mixture? These are updraft carburetors. They suck.

 

[poadeleted20]

Feed pressure? Pressure from what?

Gravity. The rest comes from the differential between that head pressure and the suction of the air going by the jet.

 

[Stewartb]

So why isn't fuel flowing out of my carb when I turn the engine off with the mags?

 

[poadeleted20]

So why isn't fuel flowing out of my carb when I turn the engine off with the mags?

What makes you think it won't dribble just a bit unless/until you close the needle valve? However, where do you think it goes under those conditions?

 

[Stewartb]

I know it doesn't. On the practical experience side I use my key to turn the plane off fairly regularly on short stops. And I've worked on the plane and left it for several days with the mixture rich. Not as much as a dribble. Test it on your own plane. I have no reason to make this stuff up.

 

[bobmrg]

Except that if you're pulling power back, you may need to lean to make it happy again.

I pull the power back only to begin descent...roughly 5" or 500 RPM for 500 fpm (YMMV), then I leave it alone.

Bob

 

[bobmrg]

This may come a little late in the thread, but what kind of maneuver are you talking about? Reading the descriptions of chandelles and Lazy 8's in the Airplane Flying Handbook I do not find any mention of extreme power changes.
A touch of power is needed when doing level 360's, but only enough to overcome the increased induced drag.

Enlighten us.

Bob Gardner

 

[MAKG1]

Gravity. The rest comes from the differential between that head pressure and the suction of the air going by the jet.

Usually, there is a needle valve in the float bowl to make gravity irrelevant inside the carb, except perhaps to keep the needle valve under the surface of the gasoline. If there were no float bowl, there would indeed be pressure. But that's not how any carb I've seen works.

The "suction of the air going by the jet" is proportional to airflow. That's the reason the jet is located inside a venturi. The venturi makes the suction.

 

[murphey]

That would preclude you from ever taking off from a low altitude airport. Seems rather limiting for a transportation device.

High altitude procedure is to lean for best power at run-up.

Yes, we lean for altitude at runup in Denver (5500 MSL for this discussion). Lycoming recommends leaning at 5000 MSL. If I'm cruising at 10.5, I'll lean when I get up there. And continue leaning at higher altitudes. Then remember to reverse the process during descent.

Full rich is full rich is sea level. Full rich is defined as the mixture lever advanced to the max, at least at the flight schools around here.

I don't even taxi with mixture at full rich. No point to it.

 

[murphey]

Not MSL. DA. Your airplane doesn't care how high you really are. It only cares what the air density is.

I've seen over 3000 DA at KTCY before. Real sucky day to be doing landings in a Warrior (and it gave me some problems with the heat, so that stunt is on my list of "things to never do again"). And you have to lean your 172 on the ground when it's that hot if you don't want to inspect the cement plant real close.

I'm sitting here, laughing and trying not to spit hot chocolate all over the keyboard. 3000 DA on a hot day. Oh you are spoiled...Let me invite one and all to visit us in Denver. I'll even buy lunch. Summer afternoons at Centennial (KAPA) 5800 MSL, DA can easily be over 8000 on the ground. In fact there's an very large electronic sign at the entrance to the taxiway with the current DA, just to remind people.

There are LSAs (Rotax 90-110 hp) happily flying around here. You learn how to lean. I've flown my cherokee near sea level only once (trip to OSH) and automatically started to lean when I cranked up the engine. My companion (a United pilot) asked me what I was doing. I don't know how to fly without leaning!

 

[murphey]

Consider this - for the PPL manuevers, you're working in an altitude block of perhaps 2000 ft. Why worry about the mixture? There are enough other things to worry about, set the mixture and leave it.

Even for the Commercial, you're still working within that 2000 ft altitude block (usually). Again, set the mixture and concentrate on the flying and the throttle.

 

[MAKG1]

I'm sitting here, laughing and trying not to spit hot chocolate all over the keyboard. 3000 DA on a hot day. Oh you are spoiled...Let me invite one and all to visit us in Denver. I'll even buy lunch. Summer afternoons at Centennial (KAPA) 5800 MSL, DA can easily be over 8000 on the ground. In fact there's an very large electronic sign at the entrance to the taxiway with the current DA, just to remind people.

There are LSAs (Rotax 90-110 hp) happily flying around here. You learn how to lean. I've flown my cherokee near sea level only once (trip to OSH) and automatically started to lean when I cranked up the engine. My companion (a United pilot) asked me what I was doing. I don't know how to fly without leaning!

Dude, you missed the point.

3000 DA is the Cessna boundary between leaning and full rich for takeoff.

And you're spoiled with that long runway. Try Lee Vining on a 10,000 DA day. 4000 feet length and no dummy sign to hold your hand.

 

[poadeleted20]

I know it doesn't. On the practical experience side I use my key to turn the plane off fairly regularly on short stops. And I've worked on the plane and left it for several days with the mixture rich. Not as much as a dribble. Test it on your own plane. I have no reason to make this stuff up.

So you looked inside the carb/air box to see?

 

[poadeleted20]

Usually, there is a needle valve in the float bowl to make gravity irrelevant inside the carb, except perhaps to keep the needle valve under the surface of the gasoline. If there were no float bowl, there would indeed be pressure. But that's not how any carb I've seen works.

The "suction of the air going by the jet" is proportional to airflow. That's the reason the jet is located inside a venturi. The venturi makes the suction.

If the level in the float bowl is above the valve, and the valve is not closed by pulling the mixture, the fuel is going out the jet until the level falls below the valve. Not much there, but enough for a dribble.

 

[murphey]

Dude, you missed the point.

3000 DA is the Cessna boundary between leaning and full rich for takeoff.

And you're spoiled with that long runway. Try Lee Vining on a 10,000 DA day. 4000 feet length and no dummy sign to hold your hand.

Sorry, missed the Cessna boundary definition. I'm used to the Lycoming specs.

Would live to try Lee Vining, where is it? On the other hand we have Leadville. Not a short runway but very interesting in the summer.

I learned to fly here in Denver on a 45 ft wide, 4000 ft long strip with Class B less than a mile from the north end of the strip and a very irate land owner near the other end. Shut down over 10'yrs ago. Made for very interesting TO&L for a student pilot.

 

[N801BH]

Consider this - for the PPL manuevers, you're working in an altitude block of perhaps 2000 ft. Why worry about the mixture? There are enough other things to worry about, set the mixture and leave it.

Even for the Commercial, you're still working within that 2000 ft altitude block (usually). Again, set the mixture and concentrate on the flying and the throttle.

Best answer yet......

If you are doing pattern work, you gain or lose 800-1000 feet... Not any problem for the mixture.... Set it and FORGET it....

 

[MAKG1]

Sorry, missed the Cessna boundary definition. I'm used to the Lycoming specs.

Would live to try Lee Vining, where is it? On the other hand we have Leadville. Not a short runway but very interesting in the summer.

I learned to fly here in Denver on a 45 ft wide, 4000 ft long strip with Class B less than a mile from the north end of the strip and a very irate land owner near the other end. Shut down over 10'yrs ago. Made for very interesting TO&L for a student pilot.

Lee Vining (o24) is on the west shore of Mono Lake, just east of Yosemite National Park. Field elevation is 6800. Upwind terrain has several thirteeners within 10 miles, so winds can be "interesting." It's quite arid and can get into the 90s in summer.

The airport is 4000 feet of pavement, a hangar or two, and several tiedowns. Not much else.

 

[Stewartb]

I like the opposite. An average winter day with reasonable temps can easily have a -4000' density altitude. Our problem is getting enough fuel, not leaning. And talk about good performance. Engines make more power, props make more thrust, and wings make more lift. Put that on slippery skis and this flying thing is even more fun than usual. :wink2:

 

[nosehair]

I do not find any mention of extreme power changes.
A touch of power is needed when doing level 360's, but only enough to overcome the increased induced drag.

Enlighten us.

Bob Gardner

Slow flight, for one.
Sometimes requires extreme sudden power changes on hot thermal days to maintain altitude. From idle to full throttle in some light trainers. And stalls; we are supposed to be learning stalls and stall recoveries from all anticipated configurations of flight.
In training , the pilot is learning control of throttle and elevator on the edge of the stall.
I treat the mixture just as I would in a landing pattern at the approximate density altitude that I am actually at during maneuvers: planning idle power with the expectation of a go-around.

 

[sourdough44]

I flew with a guy recently that was very wrote with his mixture, 'full rich' below 5000, blah, blah.... He was also having some issues with fouling.

I'm hardly ever full rich, my engine runs smooth as warm butter.

 

[Silvaire]

If the level in the float bowl is above the valve, and the valve is not closed by pulling the mixture, the fuel is going out the jet until the level falls below the valve. Not much there, but enough for a dribble.

Uh nope. The needle valve is connected to the float which is adjusted to shut the fuel entering the bowl off at a level that is BELOW the the main jet outlet. If you have a dribble either you have a leaking needle valve or the float level is wrong.