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-   -   To carb heat or to not carb heat..... (https://vansairforce.net/community/showthread.php?t=191323)

Latech15 01-19-2021 06:10 AM

To carb heat or to not carb heat.....
 
So, I was taught in an old Cessna, like most of us, and the only time I ever put on carb heat was when entering the pattern and pulling the throttle back. Now I have a very nice RV6A with a Dynon panel which is equipped with a carb temp gauge. The carb temp turns yellow at 40 degrees, and red at 35 degrees. I have been flying this plane for a couple of years and I usually put the carb heat on when it gets below the 35 degrees, regardless of the stage of flight. FWIW - I didn't not build the plane. The original builder said that he never used the carb heat, even in the pattern. As long as the carb temp gauge showed that is wasn't near 32/0 that he flew the pattern and landed with no carb heat. I have been following those instructions for 2.5 years and almost 300 hours.

On a flight yesterday, it was cold the entire trip. I left the carb heat on the whole time. Clear air, no moisture. I found that when I turned it off, adjustments had to be made to the throttle and the mixture to get it back to a EGT, CHT, Fuel Flow setting that I am used to. In speaking to a flying buddy, he recommended that I ask on here as to if I even needed carb heat at all in that situation. I am positive that if I didn't have the carb temp gauge, that I wouldn't have turned it on unless I was experiencing the signs of carb icing, low RPM and engine roughness. So What say you VAF, when do you pull the carb heat knob?

BravoAlphaRomeo 01-19-2021 06:19 AM

I oftentimes use constant carb heat in my 172 with an O-360 when itís cold out. I find that it makes my EGTís much more uniform. Cold 100LL does not like to vaporize very well and the warm air helps. If I were in a dusty environment would certainly exercise caution...

BAR

Steve Melton 01-19-2021 06:20 AM

Quote:

Originally Posted by Latech15 (Post 1495531)
So, I was taught in an old Cessna, like most of us, and the only time I ever put on carb heat was when entering the pattern and pulling the throttle back. Now I have a very nice RV6A with a Dynon panel which is equipped with a carb temp gauge. The carb temp turns yellow at 40 degrees, and red at 35 degrees. I have been flying this plane for a couple of years and I usually put the carb heat on when it gets below the 35 degrees, regardless of the stage of flight. FWIW - I didn't not build the plane. The original builder said that he never used the carb heat, even in the pattern. As long as the carb temp gauge showed that is wasn't near 32/0 that he flew the pattern and landed with no carb heat. I have been following those instructions for 2.5 years and almost 300 hours.

On a flight yesterday, it was cold the entire trip. I left the carb heat on the whole time. Clear air, no moisture. I found that when I turned it off, adjustments had to be made to the throttle and the mixture to get it back to a EGT, CHT, Fuel Flow setting that I am used to. In speaking to a flying buddy, he recommended that I ask on here as to if I even needed carb heat at all in that situation. I am positive that if I didn't have the carb temp gauge, that I wouldn't have turned it on unless I was experiencing the signs of carb icing, low RPM and engine roughness. So What say you VAF, when do you pull the carb heat knob?

I think each carb/engine set up is slightly different and carb ice can form at different conditions for each. for me I notice carb ice when the OAT is between 15-25C and relative humidity is over 50%. I can feel the engine miss very slightly as the ice sheds. I normally do not use carb heat even in these conditions and just let it shed. at higher altitudes I never have carb ice.

one thing I did that may have helped is that I polished the carb venturi. I notice some flash on the carb venturi casting and I spent some time to polish it. if you like to polish metal, that is probably the best location to spend your time.

Scott Hersha 01-19-2021 06:26 AM

I operate mine the way my Lycoming operators manual says to. Check it on the ground before takeoff to make sure it is working. Mine shows a very slight RPM drop. Then in flight, only used if encountering icing conditions. I donít use mine in flight very often, basically never because I donít fly in icing conditions. I have pulled it on in flight to check it, and saw the same indications I see on the ground. I have a carbureted O-360-A1A with a fixed pitch prop.

On my first RV (RV6), I had a carb temp gauge, and I donít think it was accurate. It indicated 35-40* all the time, even in the hot summer. Carb heat on only increased the indication 1-2*.

Latech15 01-19-2021 06:33 AM

According to the book, carb heat can form at up to 70 degrees, when the relative humidity is at 50%. That encompasses a lot of the year in the south.

Freemasm 01-19-2021 07:03 AM

It's local pressure dependent
 
1 Attachment(s)
The throttle position has a great deal to do with it. The carb venturi lowers the static pressure at the throat by design so the fuel will vaporize easier. The molecules only feel the static conditions. Everything else is "relative". Water is special. The lower pressure at the venturi throat raises its freezing point. A closed (even partially) throttle has the same effect compounded with that of the carb venturi so moisture can freeze at a very high temperature. Never use partial carb heat. It can make conditions worse based on the aforementioned principles.

BTW, if you're kicking it old school with a carb and manual mixture control, always cruise WOT and adjust speed with Mixture and RPM. This minimizes your throttling losses. This was religion in Mooney world.

SB attached for reference. It may have been updated but the basis would be the same.

Fly safe.

Taltruda 01-19-2021 07:09 AM

Just because its cold out doesn't mean carb ice is going to form. Google the chart that shows temperature and the probably of carb ice. Flying around with the knob pulled just because the guage shows under 35 degrees is most likely not only a waste of power, but also sucking in unfiltered air that bypasses the air filter. I only use carb heat upon symptoms of carb ice.

DeeCee 57 01-19-2021 07:09 AM

old wives tales die hard... ;)

My experience: Carbed O-360 with standard Vans FAB. Several hundred hours of flying in sometimes pretty hard weather, hot & cold, rain, mist, clouds, icing, etc.
Never had to use carb heat... which sure don't mean I will never have to.

Usual practice is to test it's function during the run-up, then leave it alone for the rest of the flight. It will be used if I ever see an unusual drop of the MP.

As for the dedicated carb T gage, it is an analog relic that will disappear during my next panel update... absolute useless indication.

pa38112 01-19-2021 07:22 AM

What engine do you have? 0-320's and 0-360's with the up-draft carbuarter are not prone to carb ice. I fly a RV6-A with a 0-360-A1A and a Sam James cowl. I have a carb temp probe. I never used carb heat even in carb ice conditions. Colder air is better for your effincency unless you need the carb heat to get a good lean.

Many RV's have very poor carb heat set-ups. The fact that you had to re-adjust your lean when you turned yours off would suggest that yours works well, but have you measures the inlet temp with and without it? I had a 5 degree F tempature change until I re-worked everything and go it up to 30 degrees.

Freemasm 01-19-2021 07:35 AM

Point of clarity
 
Cold air being better for engine efficiency is misleading. There's a lot of variables at play.

It sounds contrary but warmer air and warmer fuel make for more efficient subsonic combustion, which our IC engines are designed for (talking deflagration. Detonation and diesels are a different topic). Colder air allows the engine to make more power as it can burn more fuel. Becoming more or less efficient depends on the specific design point of the subject powerplant. There's real engine guys on the forum who I'm sure will chime in.


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