Piston engine ops

[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.