carburetor heat

[NatCigg]

There are many situations which can cause carb ice, but mainly its due to condensation! If you will remember the Ventura history of air being forced though a "throat" area, which does two things, it increase the velocity and decreases the temperature. Throw in the right atmosphere conditions and wa la, you have ice!
While most of the post are concerning "opposed" engines, the big "iron" also had carb ice problems. No one seemed to know why, but the B-26 engines were famous for carb icing! I don't think it was "common" to the general line of R-2800's, but I used to hear a lot of complaints about that problem with the 26! Might have had something to do with the "baffling" around the engine!
ON most light aircraft I ever flew, the problem of carb ice seem to be more prevalent in the Southeast and Southwest, as opposed to operating in the Northern area's. When the temperature and dew point get within about 10 degrees of each other, that seem to be the most threatening set up for carb ice!
 

Of course I know nothing on the the issue yet think it is interesting to note the evaporation of fuel in the carburetor will also decrease temperature.  Considering the already present decrease in temperature brought on by the venturi effect and a feedback loop of more velocity/cooling caused by ice formation, the total phenomenon comes together in the carburetor.  interesting story. 

[pembquist]

Can someone explain in simple terms what actually causes ice to form in the carburetor?

There are broadly two things that cause carb ice. The first is that as air is cooled it reaches a point where the amount of water it contains as a gas reaches a saturation point and the water changes from a gas to liquid (condensation) or from a gas to a solid/ice (deposition), you have seen this on such things as the inside of your freezer or a propane bottle in use. The second thing is the reduction in temperature that occurs inside a carburetor. The carb has a venturi which is in simple terms a tube with a narrow middle section. Air traveling through the narrow part both accelerates and decreases in pressure compared to air in the rest of the tube, this decrease in pressure causes the temperature of the air to fall, (you may have experienced this using a CO2 fire extinguisher or a product like Dust Off.) In addition to the cooling that this venturi effect causes is the additional cooling that comes from the evaporation of gasoline as it is drawn into the throat of the carburetor by the lower pressure of the venturi and atomized, this effect you will be familiar with from sweating whereby your body cools itself by having sweat turn into vapor thereby absorbing heat from your skin, in the case of the gasoline it absorbs heat from the air in the carburetor.

[Vudu15]

In the R22s we have a Lycoming 360 anytime we go below 18'' of manifold we pull full carb heat, also when flying over water or though....well basically all the time lol. when i did my solos I just kept the carb heat on full.