There is nothing special about Corvairs that makes them more or less susceptible to carb ice than any other aero or auto engine as far as I'm aware.On another thread we have been discussing "carb ice" on certified aero engines. I was wondering if any of the Corvair conversions have had carb ice problems?
My basic thinking is about how to best design an intake system that "won't" ice rather than one that can be deiced when needed. I read the attachment that mcrae0104 provided above on WWs experience with icing. There was a comment that the heat system he uses is to be applied before icing begins, not after there is some buildup. I would like to build a Corvair with fuel injection. There are several reasons I'm thinking about that. One reason is I don't like the idea of a single intake tube that dumps both fuel and air into a "log". The intake tube isn't conducive to equal amounts of fuel or air to all the cylinders. There are long runs, sharp turns and short intake runners. The shape and size of the components vary, changing the flow patterns. I would like to build an injection with individual runner tubes to each cylinder. The longer tubes with gentle curves would provide better velocity (hypothetically) at the lower rpms....and better cylinder filling. Injectors at each cylinder should provide better atomization of fuel with less tendency for fuel pooling. Computer monitoring should provide a more accurate fuel/air ratio and better mileage. Icing would be virtually eliminated, but I want to know how it can be positively eliminated. There would be a throttle body involved and it would have throttle plates. It may/may not have a venturi where the throttle plates reside. I guess that a slide type of throttle body could be employed that would remove any venturi shape from the intake. Usually, any air intake has a rounded shape at its entrance to smooth air flow. I'm wondering if that might cause some venturi action even if there is no actual venturi at the entrance to the intake.The lowest temp on a carb would be just past the venturi (lowest drop in pressure) however the farther out down the tube you go the warmer the air becomes as it pick up heat from the engine compartment (unless you fly in northern MN in wither ) The main issue is if you have a venturi (most slide carbs dont) to produce the sudden temp drop, and if you have an object past the carb for ice to accumulate on - such as a throttle plate. I use to fly my plane with a TORNADO venturi past the carb and never had carb ice with the Aerocarb. The Tornado spun the air/fuel for better disippation. Seemed to work fine. Almost all carb manufacterers recommend carb heat (mostly for insurance reasons) - but I guess it would be better to have it and not need it than.....
The pressure drop causes a temperature drop. But the fuel vaporizing in the carb causes a much larger temperature drop, which is why FI doesn't have much of an icing problem. Lycoming's FI has a venturi at the throttle body inlet to generate reference pressures relative to air density and velocity for the fuel controller's function.FI cant ice up. Throttle plates have no venturi effect. If physical ice gets in there like in an IFR condition you have alternate air source. Mechanical Icing like IFR is going to be impossible for you as you have already proven how conservative you are; you are not building a plane for known icing. Venturi effect carb icing only happens at the venturi when the atmosphere has lots of moisture. Carb icing is just like how the AC in your car and house work. Pressure drop. it is also how they puff up your Fruit Loops. Apples and oranges when talking icing of carbs and FI.
The airflow around the edge of the throttle plate is not at the same pressure as measured by the manifold pressure gauge. It's lower. At idle we see about 10"Hg MP. The air squeezing past the throttle plate will be much lower than that because of its velocity. As it gets past the plate it slows and the pressure rises to what we see on the MP gauge.Much smaller venturi effect on the RSA and it is dry; very low pressure drop. It is there to change the air pressure to measure it. It is also on the walls, being open, no restriction. Throttle plate very small amounts of vacuum at small angles. The carburetor only works if it has high vacuum at the venturi. High pressure drop to pull the fuel out of the bowl. If the impact tube iced up on a RSA the engine would go rich; if both impact and vacuum iced up it would probably not change mixture at all.