Piston engine ops

[Skaz]

Final appr checks: Reds, Blues, Greens or PUFF: Power, Pitch, Undercarriage, Flaps, Fuel & Fuel Pumps.....

Before commencing the descent I adjust the mixtures too, in the cruise the FF on the 402 is around 90 - 95 lbs/hour, for the descent I set it 100.
I richen then mixture on finals, maybe not full rich, but richer than operation at flightlevel anyway. Climb setting after take-off is 120, so thats a good refrence for me to aim at.

As for the props....this is a bit sticky as it would be heavily biased per personal preference. Top of Green at least, the last little bit that makes the most noise...um, I prefer to push the props full fine, just in case a go-around is needed. You might not alway have time to push all the levers forward, if your hands are on the throttles levers.

With a patient in back (we do medevacs), and stable approach with good conditions, I sometimes leave the props TOG, but usually they go full fine.

right, tell you guys what...I have always done the take-off in the C402 with the mixtures full rich....next one I do ( did 4 today, should have remembered!) I'll lean it out a bit and check the difference.....

 

[lookup]

Prop settings...

On my little ole' O-360 I find that by the time I am down to about 15" MP, the prop has already set itself to fine and moving the prop control full fine makes no difference... other than to be ready for a go-around. During some instruction in a geared engine bird, I was told to NEVER NEVER NEVER let the props drive the engine... ever ever... never never... The SOP in the Commander 520 was to wait until short final (<15" MP, or not at all) for the props to come up..


I do all my mixture settings in small increments going up or down, based on EGT gauge readings. (no FF gauge) MP settings also in slow increments on descent..

It sucks when you're the only one to blame if something breaks AND you're stuck with the entire bill...

LU

 

[340drvr]

Just to clarify:
There's a critical difference in leaning procedures between turbocharged and normally aspirated. Those turbo 520's in the 402 want "full rich" for takeoff at most any elevation (maybe not Tibet or La Paz, but that's another story). I would start with the red knobs full forward, monitoring FF as I approach full power. As long as FF stays in the green, full rich it is. Sometimes things are out of adjustment, and FF goes way over the top, in which case you'd lean back to top of green fuel flow. (Works for the 340, anyway, same TSIO-520)

 

[greyhound]

That's what I thought.....

In a lot of schools, they want you to firewall everything if you lose an engine on a piston twin. Even at cruise.
That did not sound right to me.
If you're going to be on one engine for a while, I'd take good care of it so it can take you to your landing spot.(hopefully not a crash site)
By taking care of it I mean leaning it, and using cowl flaps to help keep it at normal operating temps.

So are we still going to teach everyone to just push the mixtures all the way forward in the event of engine failure on the seminole?

If we are, let's also remind the student to lean for best power after the dead engine is secured.

 

[metrodriver]

Skaz, look out! I think your engines on the 402 areturbo charged.
For these you don't lean on take-off. The sole purpose of the turbocharger is to maintain an airflow into the engine that stays constant all the way from sea level up to its service ceiling (I believe that's what it's called where the maximum altitude is at which the turbo can deliver its rated pressure, there might be another term for it, I forgot). So your engine doesn't know at what altitude it is. This is different for normally aspirated engines.
On a normally aspirated engine the throttle controls the throttle plate in the carburator, which allows the amount of air sucked into the engine. There is also a link to a fuel opening that regulates the amount of fuel being sucked into the airstream in the venturi of the carburator. Now when the plane goes up to altitude the throttle stays in the same position (and so the fuel allowed to go in the airstream), but the airstream itself is getting less because the air is less dense up there. The result is now that you get too much fuel for not enough air (rich fuel mixture). So they designed the mixture control, which controls another 'valve' that controls the amount of fuel going to the fuelcontrol that is attached to the throttle linkage.

Nobody bit into the question I posted on the bottom of page 1 of this thread. If your engine is running rough because of carb ice, FIRST pull back your mixture control till your engine runs smooth, THEN pull your carb heat. Why? All that ice in the carburator was limiting the airflow into the engine, but the amount of fuel was the same as before, so you have a very rich mixture, so rich the engine started to choke on it. If you had pulled the carbheat first you would have allowed warm air (which is less dense then the cold air) and water (engines up to this day don't burn it very well) to make your fuel mixture even richer. Now you probably become a statistic (there are quite a few each year because of this reason): your engine quits because of all the fuel, no air and water, and trying a restart is unsuccesfull because there is no heat source anymore to melt the ice. So after you damage your plane landing in some farm field the FAA decides to test run your engine and it runs perfectly fine and your accident gets as probable cause: failure of the PIC to properly monitor engine conditions.
When the carbheat is melting the ice and you see the rpm go up, slowly push the mixture to where you had it before.

 

[Timebuilder]

That's a great piece of info on the carb ice, metro. It's always a pleasure to learn something new. Thanks!!

 

[avbug]

Why? All that ice in the carburator was limiting the airflow into the engine, but the amount of fuel was the same as before, so you have a very rich mixture, so rich the engine started to choke on it.

Not so, grasshopper.

Fuel flow in a carbureted engine is proportional to the pressure drop at the venturi; that's what draws the fuel into the carburetor throat. The pressure drop is regulated by airflow; restrict the amount of air, (or the velocity through the venturi, and you'll see less pressure, and less fuel.

Plug up a venturi with ice, and you'll see lower fuel flows, and generally eratic fuel flows, too.

If one is to lean prior to applying carburetor heat, the primary reason is that the mixture will be enriched with the application of carburetor heat (due to decreasing air density), not because greater fuel is flowing due to ice.

The engine is nothing more than a big suction machine. It sucks in air, it sucks in fuel. Less air flowing into the engine means less fuel. Don't try to compensate for that with mixture.

That discussion aside, prioritize. If your engine is going to choke on ice, then get rid of the ice. That's the priority. Fine tuning can come later. Take the steps you need to take (carburetor ice...apply as required), and then clean up. You have time.

 

[TurboS7]

With over 9000 hours in props I never thought I would see the day that I would see a thread like this. I love 'em when it is warm and hate 'em when it is cold.

 

[TurboS7]

I guess I will throw in my 2 cents. The procedures for shock cooling are correct but conservative, they will result in long engine life. With a Cont. especially the C402C you can taxi with the mixtures rich. If you take off with them leaned back you will cook your engines. You can check the idle-mixture setting my bringing the mixture back to idle cutoff, if the RPM rises 50RPM the drops off it is set correctly. The big problems we had operating in cold weather with the Cont were engine case cracks and cylinder cracks. Overall I had better luck with Lyc in the cold weather and better luck with the Cont in hot weather. Piston engines are way harder to run than any turbine, it is ironic that the higher the dollar value airplane the easier it gets to operate.

 

[TurboS7]

One more for sub-zero carburetor ops. Don't use carb heat as it will heat up the carb throat and cause carb ice, just the opposite of what it is suppose to be.

 

[metrodriver]

quote: Plug up a venturi with ice, and you'll see lower fuel flows, and generally eratic fuel flows, too.

If one is to lean prior to applying carburetor heat, the primary reason is that the mixture will be enriched with the application of carburetor heat (due to decreasing air density), not because greater fuel is flowing due to ice. end quote

Off course the fuelflow will drop, the air flowing through the venturi can only absorb a limited amount of fuel. When there is no ice, it will be richer than the optimum fuel/ air ratio, but within a range that still allows normal engine ops, and limited by the throttle linkage fuel control, all the fuel allowed to pass through here is being sucked into the airstream. If the venturi starts to be blocked by ice the airflow decreases, but the same amount of fuel is still availabe and the air will try to absorb as much fuel as it can (the mixture is getting richer), but since the air can hold only so much, at some point the amount of fuel being absorbed will also decrease (lower fuel flow). But the fuel mixture is now already so rich that your engine has trouble burning it (running rough).
Your second point is exactly what I said earlier: warm air is less dense and will make your mixture even richer.

This technique of leaning first a bit before applying carbheat was told to me by my first boss. He has over 55000 hrs(25000 as an airline pilot, 30000 in general aviation), has been flying since the 1940's, owns a flightschool since the early sixties, and is a FAA designated examiner. Some other old timers who grew up in aviation told me the same thing.
If all these guys have that kind of experience, that worked well for them, then I will follow that advise. And it has worked for me so far.

I personally have had an occasion where the carburator got blocked by a piece of gasket material that is attached to the carb heat valve in the airbox of a c172. It broke off and ended up in the venturi, blocking all airflow. The engine was shaking so bad I thought it would come of the mounts. Doing all the normal things (mixture rich, carb heat, mag check) didn't do a thing, even made it worse. So finally I pulled the mixture back to almost cut-off and the engine started to run smooth at 1500 rpm maximum power. I managed to bring the plane back to the airport and make a succesful landing. Two weeks later I read a report of folks that experienced the same thing, but did not live to talk about it.
Sometimes it takes steps that nobody will teach you, or that are not in the manual to deal with a certain problem

 

[MarineGrunt]

One more for sub-zero carburetor ops. Don't use carb heat as it will heat up the carb throat and cause carb ice, just the opposite of what it is suppose to be.

So what you're getting at is:

Freezing Air--------->Freezing Carb = no ice

Freezing Air--------->Warm Carb = ice

 

[avbug]

If the venturi starts to be blocked by ice the airflow decreases, but the same amount of fuel is still availabe and the air will try to absorb as much fuel as it can (the mixture is getting richer), but since the air can hold only so much, at some point the amount of fuel being absorbed will also decrease (lower fuel flow). But the fuel mixture is now already so rich that your engine has trouble burning it (running rough).

Again, completely wrong. Reduce the amount of airflow, and the same amount of fuel will NOT be flowing. Fuel is drawn out of the carburetor in proportion to the airflow through it. Reduce the amount of air, and you automatically reduce the amount of fuel. That's the principle of the carburetor. The carburetor doesn't squirt fuel, or inject fuel, or meter any amount of fuel for a given throttle setting. All it does is sit there. Airflow does the rest.

The engine is a vacum machine. It sucks air. To the carburetor, that's all it does. It sucks air.

As air is sucked through the carburetor, a pressure drop occurs in the venturi and slightly beyond, and it is this pressure drop that draws fuel into the airstream. The air doesn't absorb the fuel; it is mixed.

Flow less air through there (as is the case with induction ice), less fuel is drawn out of the carburetor float chamber (or pressure chambers) into the airstream.

You seem to believe that fuel flow is a function of throttle setting; it is not. Throttle setting has nothing to do with fuel flow, but is there to control air flow. The throttle is nothing more than an air valve. The airflow then controls the fuel.

Reduce airflow, either by closing the throttle, or blocking the intake with ice or objects, and you'll see reduced fuel flow automatically, because there is less air there to draw it through.

The engine isn't choking on an overly rich mixture. The fuel flow isn't the same as it might be if there were no intake occlusion. It's reduced, just as the airflow is reduced.

Accordingly, the premise for your comments is incorrect.

Adjusting mixture is necessary and important any time a change is made in power setting, altitude, temperature, or airspeed. Change any operating parameter, and the need for mixture adjustment changes with it. This includes intake blockage, but to state that the mixture is enriched in the manner you describe is incorrect and misleading. It may become leaner, or richer, depending on the type of blockage, or it may stay relatively the same.

Leaning the mixture prior to application of carburetor heat as a blanket premise is wrong. Use of mixture will depend on power setting, and the conditions under which the heat and mixture are to be used. A throttle reduction may be in order first, and sometimes a mixture increase may be in order.

My carb heat experience is limited to float and pressure carburetors on small and large engines, both flat piston, turbo and supercharged piston, and turbocompound radial piston, as a pilot and instructor, and mechanic and inspector. You're right, I'm probably wrong. Then again, a pissing contest over experience is pointless; the operational facts speak for themselves.

Look at the way a carburetor is designed; look at it's function and think about it. 55000 hours of experience won't change that, no matter how good a friend your boss may be.

 

[rbowen]

Currently flying convair 240/580 580 is turbine good airplane however the 240 is piston radial engine. Talking about the mixture issue. You have to start each engine with mixtures in the closed postion. if you don't you will blow out a couple cylinders and likely cause a signifigant amount of damage. We have to pull the mixture to the lean postion if power is going to be below 1500 rpm if going above we go rich.
I would have to say that attempting to heat the cylinders and rings up as fast a possible is foolish. not only is it foolish but extremely hard on the engine.
An aircraft will warm up just fine if you have any amount of taxi time or preheat.

 

[MarineGrunt]

BUMP - (great topic that needs recharging)

 

[enigma]

This string is depressing. I just realized that it's been so long since I flew anything with pistons or carburetors, that I'd be dangerous in one. I guess that I 'll have to find a good instructor before the next time I rent a cherokee.



engima

 

[avbug]

Rbowen,

Starting a radial engine with the mixture in idle cutoff is one way to do it, but by no means the only way. Starting rich and adjusting throttle also works, and no, you're not going to "blow out a couple cylinders and likely cause a signifigant amount of damage." The damage to which you're referring will occur during a backfire if too much throttle is applied to a lean mixture; the danger is in separating the induction tubes, and potential damage to the carburetor. Afterfiring can cause stack damage.

That's purely a matter of pilot technique. Don't let it happen.

You're also operating, I believe, with autorich and autolean positions on your mixture control, something not enjoyed by non-pressure carburetors.