This thread doesn't have to be limited to this discussion but every time I pull carb heat I think about unfiltered air from the engine baffle compartment, bugs, dust and all being force fed through carburetor and into my engine. I push in carb heat after threshold before landing but dust is still getting fed down the skeet tubing to the heat box. Why the hell hasn't someone come up with a small filter at the rear of the baffling that can stop some of that. Our cars have cabin air filters now. Good place for a K&N and a good project for EZ Flap.

We've been inducting unfiltered air into our engines via the carb heat for decades, with no apparent massive failure rate. Actually, assuming that carb heat is properly used, there's really no need for a filter. Once you're airborne, there is very little particulate in the air, and most carb heat systems have a turn or two in them (as in through the heat muff) that should eliminate any bugs, etc. that might find their way into the system. Ever hear of a mechanic finding all sorts of sediments inside the heat muff during an annual? Nope.

I have a friend who removes his induction air filter and replaces it with a screen when he goes onto skis. No real need for filtered air on straight skis, except to keep the big chunks out, and the ram air effect offers a couple more hp.

MTV

I've been taught to push the carb heat in (off) when I Know I have the field made. This is usually well before the thresh hold and high enough that more than likely, no dust will enter the carb. Plus, one less thing to fiddle with if you got to go around.

I can appreciate there is clean air up high or in the winter but off airport and at the last few hundred feet during landing here in the summer months with the farm dust and bugs is what causes the need to clean my windshield. This lower altitude with heat pulled has to be the dirtiest air of the flight and I know the discussion that just K&N vs regular filters can generate. A dusty bee's ass has to be abrasive right?

I agree that unless you're in a dust storm once your airborne the dirt/dust contamination is about nil.

Speaking of carb heat talking to Steve at Pponk about fuel burn he told me he uses carb heat in cruse flight and then adjusts hie mixture accordingly I think he said about 50 ROP and it reduces his burn rate without much penalty to speed. Anybody else been using this technique?

Ya know... After all these years, and all these hours, the carb heat is still a big friggin' mystery to me.

My old gurus told me to, "Check for ice often, use carb heat early, and use it all or nothing." So I do. It seems to have worked so far, but I don't have a solid base to back up why I do it the way I do.

Gump

Glidergeek wrote:I agree that unless you're in a dust storm once your airborne the dirt/dust contamination is about nil.

Speaking of carb heat talking to Steve at Pponk about fuel burn he told me he uses carb heat in cruse flight and then adjusts hie mixture accordingly I think he said about 50 ROP and it reduces his burn rate without much penalty to speed. Anybody else been using this technique?

Are your crazy?? Oh boy!!

GumpAir wrote:Ya know... After all these years, and all these hours, the carb heat is still a big friggin' mystery to me.

My old gurus told me to, "Check for ice often, use carb heat early, and use it all or nothing." So I do. It seems to have worked so far, but I don't have a solid base to back up why I do it the way I do.

Gump

Gump, I wondered about this myself until I ran up a new engine on a test stand. This was a Cont A75. I let this engine run at about 1/4 power while I was checking it out. Happened to look at the intake maifold and it had frost on the entire surface. When I went to full throttle the frost slowly disappeared because of the volume of air passing through. I again went to 1/4 power and it just took about 30 seconds for it to frost up again. This was on a day when it was about 60F outside. No wonder with the right relative humidity it could ice up internally and plug off the intake air to the carburator. Note that the Cont intake is external to the engine whereas the intake is internal on like a Lyc O-320 or O-360 so it gets heat from the oil.
As I say I wondered why you are suppose to use Carb Heat when you throttle back during landing, but now I wonder no more.
HC
PS the only time I had carb ice on my 170 was when I sat at an idle for several minutes while I waited for someone to do a runup.

Glidergeek,
Wow, that is an interesting theory, I can't seem to make any sense out of it. Knopp is a pretty smart guy, he must have it figured out but it doesn't work out in my mind. Heated air will be less dense so you will have to lean more, so you add more power which should bring you right back where you started, unless you are at altitude and you need more power, then you are hurting............

dirtstrip wrote:... I push in carb heat after threshold before landing but dust is still getting fed down the skeet tubing to the heat box. Why the hell hasn't someone come up with a small filter at the rear of the baffling that can stop some of that... .

Ok, now after doing my own homework I have the answer that best satisfies my "Why haven't they"... question.

There are three types of carb ice.
1. Throttle or butter fly ice buildup.
2. Carburetor venturi ice that slowly chokes off the airflow.
3. Impact ice from external sources, freezing rain or snow physically clogging the surface of the main filter. This is the type that could also clog a secondary carb heat filter and thereby eliminate the only alternate source of intake air for the carburetor. Unfiltered air then becomes the lesser of two evils, the first being the slow death of the engine by unfiltered air vs sudden death of pilot and passengers by way of the iced off airfilters.

I must say I now have my curiosity all jacked up over this flying with the carb heat on theory. Interesting.

Here is an explanation that helps understand why running with carb heat on can give better performance. Its about getting the unrestricted airflow through the carb with a fully open butterfly.


"Running 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 fully open. 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."

I would give credit with a footnote to the author of this if I knew who he was.

Running with carb heat on does indeed help with mixture ditribution on some engines. It works best when a carb temp gauge is installed so with some trial and error the pilot can find the sweet spot by pulling carb heat on in cruise to say 45 F +/- or so (but YMMV). Then he can lean as he/she noramllay would. O-470's with carb temp gauges love this technique.

The other thread on the C185 cold weather baffles got me remembering the days of running carbs at really cold temps. As the density altitude goes down in extreme cold, you can only push that mixture knob in so far before it hits the stop. Then, the problem is that you find EGT's climbing and an engine running way too lean. If it's cold enough, you can get so lean the damn thing doesn't want to run. The fix is to pull carb heat to warm the air, and raise the DA to enrichen the mixture.

Gump