wasn't at altitude. If your going up make sure and have a heat box you just never know. I sat on the side of the road because a 64 Scout International frosted up one winter.
The temperature in the venturi and at the throttle plate when it's closed can be more than 70°F lower than ambient, which explains why carb ice is possible when the OAT is 100°F. Air accelerates through the venturi and past the throttle plate's edge when it's closed or nearly so, and higher speed means lower pressure, and lower pressure means lower temperature. Add the evaporation of the fuel being sprayed into the airstream, and more heat is absorbed from the air, lowering its temperature further.
The KC Dawn Patrol Aviators all use this method for carb heat, they have sent be the concept sketches they use. I have attached them, they claim is it is very effective simple and lite. I have a traditional carb heat source on my revflow carb which claims not to need it. I like the insurance and keeps me in practice of the disapline of applying carb heat before pulling power back.
From this I can guess, then, that carb ice is possible in Arizona? (20% humidity at 95f)The temperature in the venturi and at the throttle plate when it's closed can be more than 70°F lower than ambient, which explains why carb ice is possible when the OAT is 100°F. Air accelerates through the venturi and past the throttle plate's edge when it's closed or nearly so, and higher speed means lower pressure, and lower pressure means lower temperature. Add the evaporation of the fuel being sprayed into the airstream, and more heat is absorbed from the air, lowering its temperature further.
Certfied airplanes have to meet this requirement:
Sec. 23.1093
Induction system icing protection.
(a) Reciprocating engines. Each reciprocating engine air induction system must have means to prevent and eliminate icing. Unless this is done by other means, it must be shown that, in air free of visible moisture at a temperature of 30° F.--
(1) Each airplane with sea level engines using conventional venturi carburetors has a preheater that can provide a heat rise of 90° F. with the engines at 75 percent of maximum continuous power;
(2) Each airplane with altitude engines using conventional venturi carburetors has a preheater that can provide a heat rise of 120° F. with the engines at 75 percent of maximum continuous power;
A system that can provide a temperature rise of 90°F is a serious source of heat. A lot of homebuilts can't come anywhere close to that, with the result that engine power loss due to carb ice is far too common. Another factor there is the number of pilots who think carb ice is only a wintertime thing.
Lycomings are not immune to ice. We had a bunch of them, various models in various airplanes, and some, like the O-320-A2D in one of the Citabrias, would ice up in a heartbeat even though its sump and carb and everything was identical to the -E2D models in the 172s. And I had carb ice lots of times in the 172s as an instructor. In the summertime. Sometimes right after startup.
That chart refers to temperature and dewpoint. The % RH is shown there for interest's sake, and at 95°F you're just within the limit at 20%. It would be very unusual and that would apply to a very ice-prone setup. But it's possible. That chart is a general thing and one should get to know his system and watch for ice. I've seen carbs icing up and the pilot has no idea that it's doing so. Get him to pull the carb heat and watch the tach and see the RPM fall, then rise, close the carb heat and see the RPM rise to a higher figure than before the carb heat was pulled and the throttle still right where it was. Carb ice.
Not immune, but enough less likely, I gather, with the intake going through the warm sump, that they don't recommend carb heat as a usual thing, but as a response to any unexplained drop in rpm or manifold pressure.
