48 A

Clooney - I nearly turned a Mixmaster O-2A (on of the few in danger of getting a bird strike from behind) into a plow one day by icing carbs at 80% throttle, so seems to me the kinds of stuff inside and the ambients may well have as much bearing as the pressure differential, and that the chill travels from inside out, not vice versa?

Probably more fun to debate whether a dead stick Mixmaster is better as a single bottom (gear up) or two bottom (gear down) plow.

Steve

I guess all you need to keep the spinning side up is a few Collins Radio gyros, eh?

Stan

This was VFR, guess that's when it bites you. IFR is when you expect that sort of stuff.

You're right about those other guys in Iowa making good stuff! Collins and JD, good combination. What do you do when it comes to picking something to drive down the street. They don't make anyting in Iowa that I'm aware of. And if you wanna say that a Sperry would have made me auger in, I'll buy off on that! Ha!

take care,

Steve

Gotta laugh. If the airplane throttle stays constant, then you've never ridden with one of us fighter pilot wannabes! My JD is the set it and forget it throttle!

Understand we're talking about external versus internal frozen water. So what's causing the carb housing to be cold and therefore frost on the outside? I'd think pressure differential as a single causal factor would cause frost to extend a ways into the intake runners, and maybe a ways back up the air cleaner tube. It does extend a fair ways on the coils of air conditioner; admittedly probably a greater pressure differential. As it is, the only place I've seen it is on the body of a carb. Usually right around the body of the throttle, down to the bowl.

Your 65/40 observation is fairly close to the 20/20 rule they preach in flight school. That is, if temp and dew point (I think dew point is about the same number as % humidity at that range) are within 20 of one another, watch out for icing.

I'm assuming an internally icing carb frosts up on the outside as well, just as a glass of ice water causes condensation on the outside of the glass, or frosts in your hand at a football game, but have never had much taste for sky diving or wing walking -- so don't know. Sure is a common sensical sort of idea, though.

Occurs to me maybe we're talking about the same effect at widely spaced parts of the scale. The air density at 5,000 feet at constant temp, dew point, and a constant 30 inches of Hg altimeter settings is a 1:.85 ratio. Said another way, we're 820 ft above MSL here. JD's don't fall out of the sky with frozen carbs, don't even stop running on the ground (we hope). At 65 temp, 40 dew point and 30 in. Hg we're 96% of MSL air density here. If we move the JD to Leadville, CO. - about 9600 ft. above MSL - the same temp, dew and altimeter settings say the air is about 7 tenths as dense as sea level. Relative ambient air density may account for the differences in degree while the effect is the same in kind.

Been a long time since I took physics or knew Mr. Bernoulli very well, but I'm assuming pressure differential will stay constant if not shrink as altitude increases.

So you borrow some Doc's laser spectroscopy gizmo and see if that hole / orifice (redundant? -- borrow the proctologist's gizmo, make it even sound worse) freeze up in there. Meanwhile I'll take up wing walking and see what happens to a carb at 10,000 ft with 20/20, no carb heat, and no touching the mixture control. Or, we find somebody in Leadville, CO with a JD and he can figure it out....

Steve

Lurch,

Don't know about always - but here's what happens sometimes when it's cold and/or wet outside.

When fuel is vaporized with air in a carb, the air gives up roughly 20 degrees of temperature. When the air is laden with moisture and that 20 degree loss takes the air below freezing, ice crystals form and bingo -- you have frost.

I borrowed this chart from an aviation site to show you what sorts of temperature mixed with what sorts of humidity expressed as dew point will cause various degrees of icing. The chart is based on AVgas, so with Mogas, or auto and tractor juice, the results will vary a bit depending on the blending.

The clever JD guys designed in a manifold heat "switch" to redirect some hot exhaust air to surround your intake runners. You should probably try switching yours to the H or Hot position if you haven't already. Once it warms up or dries out, switch it back to cold, as carb heat robs power from the engine.

So much of the stuff I learned to be a pilot - like navigating with a circular slide rule - has become hopelessly obsoleted by things like GPS. It's good to find an instance where the old flight school stuff comes in handy. Thanks.

Steve