[Bryan Wood]

A friend at work who owns a Cessna 172 was telling me that a topic that is kind of hot right now on the Cessna pilots assoc. groups is flying in cruise with carb heat on and then leaning for a normal flight. From what he was saying, and I'm not sure I'm following the logic is that by doing this they can run full throttle and in effect have the prop running a slightly lower rpm than if the carb heat isn't on. Then they lean as normal and supposedly get better economy from having a fully open Butterfly valve, 100+ rpm drop, and slightly higher man pressure by bypassing the air filter. He said that the folks doing it are claiming no real loss of airspeed, but significant savings of approx. 1gph.

Does anybody have any idea what these guys are doing and is there anything positive about it?

Best,

 

[vlittle]

A friend at work who owns a Cessna 172 was telling me that a topic that is kind of hot right now on the Cessna pilots assoc. groups is flying in cruise with carb heat on and then leaning for a normal flight. From what he was saying, and I'm not sure I'm following the logic is that by doing this they can run full throttle and in effect have the prop running a slightly lower rpm than if the carb heat isn't on. Then they lean as normal and supposedly get better economy from having a fully open Butterfly valve, 100+ rpm drop, and slightly higher man pressure by bypassing the air filter. He said that the folks doing it are claiming no real loss of airspeed, but significant savings of approx. 1gph.

Does anybody have any idea what these guys are doing and is there anything positive about it?

Best,

Bryan, I haven't heard about this, but if they are flying at 100rpm lower, they definitely save fuel.

RV's are a bit different... in a good way. With carb heat on, the air is still filtered and it's being pulled from inside the cowling. I always run with carb heat on the ground so that crud is not sucked into the cowl scoop. I lean it out anyway to prevent plug fouling, so this is a good way to go.

I haven't tried it in flight, though. If I run at full throttle less than 10k feet, my prop will redline. Maybe I can try it the carb heat on (reduces the ram air) and see if that will drop the power enough to run WOT.

Vern

 

[osxuser]

I can give you the Jist of it:

Runnning at full throttle will give slightly better fuel economy because the carb is running with no restrictions. With a fixed pitch prop on most Cessnas, the engine will red-line before full throttle is reached, therefore not allowing full throttle. The addition of carb heat reduces the density of the induction air, and therefore reduces the manifold pressure. This brings the rpm below redline at full throttle in most cases. This supposedly increases efficiency of the engine because of the butterfly is fully opened. It is also said to even out the fuel distribution from the Carb by vaporizing the fuel better initially, thus an even distribution goes to each cylinder, allowing leaning further, and sometimes LOP ops in carbed engines.

Did that do the trick?

 

[Bryan Wood]

Thanks for the explanation. It sounds viable as a way to save gas if the practice works like the explanation. It always amazes me the information that can be obtained here.

Best,

 

[Stephen Lindberg]

Running some carb heat (100F) was SOP for a Cessna 195 I used to own. Evened out the mixture distribution and made the engine run smoother. Not mentioned in the handbook, but part of 195 folklore. Costs a little power but I didn't notice it in cruise. Would this help LOP operation in a carbureted Lyc? Someone knows, I bet.

 

[mahlon_r]

Some thoughts.... hotter induction air should equal less power for the same throttle position, but that hotter air will also decrease detonation margin. So I would be a little cautious while leaning, until I was certain I couldn't hurt the engine especially if I was running any kind or EI. Secondly, most carb heat systems run hot air, from the exhaust system or heated nacelle area, that is unfiltered, highly restricted and with no plenum or ram effect. Is unfiltered air the way to go?
Is the fuel savings from the WOT position that great over the same power setting with partially open throttle?? Might be, I don't know, but I wouldn't think it was that great especially if leaning.
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at you own risk.?

 

[osxuser]

Mahlon,

Your absolutely right, especially about the unfiltered air. This is a practice SOME FG Cardinal drivers use to run LOP, the main thing is to be sure your at a low enough power setting that detonation isn't going to be a big factor, IE I've never tried this, but I might at some point, but only under 60% power AND with a four probe engine monitor.

Stephen

 

[gmcjetpilot]

The C-182 carb heat trick Vs. the C-172 CH trick

A friend at work who owns a Cessna 172 was telling me that a topic that is kind of hot right now on the Cessna pilots assoc. groups is flying in cruise with carb heat on and then leaning for a normal flight. From what he was saying, and I'm not sure I'm following the logic is that by doing this they can run full throttle and in effect have the prop running a slightly lower rpm than if the carb heat isn't on. Then they lean as normal and supposedly get better economy from having a fully open Butterfly valve, 100+ rpm drop, and slightly higher man pressure by bypassing the air filter. He said that the folks doing it are claiming no real loss of airspeed, but significant savings of approx. 1gph.

Does anybody have any idea what these guys are doing and is there anything positive about it?

Best,

I going on the concept that is has nothing to do with icing conditions and is for exonomy or normal operations to get better performance of some kind. I assume this is in cruse WOT at 75% or less. If you use Carb Heat at full power you can induce detonation, especially if you lean.

This is popular with talk among the C-182 crowd for reasons of better fuel distribution. It allegedly allows you to run lean of peak (please no more LOP, other threads for that) with a carborator. My theory is may be the heat helps with distribution and atomization. The Continental in a C-182 is was different than a Lycoming. The continental has COLD runners from the carb to the cylinders. The Lycoming already has a HOT sump, where the intake runners are, so the carb heat may not be needed or (as) effective.

Next in the theory why it helps even the distribution is the airbox it self directs the air towards the leaner cylinders, thus it helps even the distribution out. The AIRBOX on a cessna is WAY DIFFERENT than Vans filtered airbox. The airbox has a large plate to direct cold or hot air into the carb. By slight manipulation of the carb heat you can lean or enrichen the front or back cylinders. HOWVER the C-182 technique is partial carb heat, cracking it NOT full carb heat as the I think you are talking about in a C-172?

I have about 1700 hours in C-172, mostly as a CFI-II. To be honest I never tried this. I have tried it in a C-182 and had mixed results. Regardless the C-182 carb heat "Trick" is to help get even fuel distribution, even or smaller EGT spread between peaks. On the C-172 I have no idea how effective it is, but the Lyc and TCM have different inductions. I suppose that if they use the carb heat and then re-lean, below 75% power in cruise, it can't hurt.

Bottom line is DON'T TRY THIS ON A RV WITH VAN'S FAB.
It will not work because all you will do is choke the air off to the engine. Van's air box and carb heat is a different deal all together. To explain all you have to do is look at the two, inspect the way they work, it will be obvious. The Cessna airbox has the ability to change the air flow at the throat of the carb and provide much more heat. Van's box just closes the inlet off and hopes you can suck some warm cowl air through a small opening. They are two different animals.

 

[osxuser]

I agree that it won't work very effectively with the FAB. But that just follows on my theory that the FAB is ****, why limit yourself with it?