Mike, after some thought, and with respect, I have to beg to differ on some points, but please forgive me if I have misinterpreted your post:
I do agree that the "throttle butterfly is downstream from the venturi" but you must recall that you can only get pressure differential with air flow, and it is this pressure differential due to flow that can cause carb icing.
The vacuum within the intake system of a normally-aspirated engine is caused by the intake mechanism of the Otto cycle, in fact the "suck" of the cylinders. The pressure difference between that in the cylinders and atmospheric is, in our circumstances, largely dependent upon the obstruction to airflow between the two, which is the carburettor butterfly valve. This is why, in my case and that of WR, that we get maximum "suck" on our A-65's at closed throttle.
The more closed is the butterfly valve (throttle), the greater the pressure differential (and therefore the pressure drop within the carburettor). If this drop occurs so to cause a sufficient temperature drop for the prevailing relative humidity, then water vapour within the intake mixture can crystalise to form ice, and it will form on any suitable surface within the intake system that is cold-soaked where it can form. The pressure within the carb caused by forward motion is minimal; the intake could be pointed backwards.
All of the above gets worse when fuel vaporisation occurs, because of the latent heat of evaporation of the fuel causes the cold-soaking of all the intake system.
You are indeed correct in pointing out that "when the throttle is wide open more air is passing through the venturi", but of course it is at very close to atmospheric pressure when so doing, so there is little pressure drop. This is why carb ice is rarer at cruise or full-power settings.
F3G: you are correct: "small" Continentals, from the A-40 (yes I have flown one) to the 0-300, are more conducive to carb ice because of the design of the carburettor/intake system in relation to the warm oil sump. In these engines, the carburettor hangs some distance away from the oil sump heat source, and so are succeptible to cold airs. In the typical Lycoming situation, however, the carburettor is located very close to the sump, where it benefits more from the heat of the oil.
shytorque ...Quote: The latter point is why carb heat on an aircraft should be used BEFORE ice builds up. You are also right.
DT
p.s. Mike: I should be very happy to offer you a trip in my A-65 powered machine (as my guest) in order to demonstrate the above; PM me at any time, I am Midlands-based....unfortunately the A-40 is solo only...and in the USA
[edited for typo]
Last edited by DubTrub; 19th Oct 2004 at 07:44.