Carb heat question

[skonie]

I have a question on carb heat. I have two ducts going to carb, one filtered air and one coming from heat muff. So when I select one, it blocks the air from the other. My question is....is it okay to instead of shutting off air from filtered air is it okay to simply route air from heat muff into same duct? In other words when I select carb heat on, just open a heat valve and not stop air from filtered source?

Thanks

[Av8r3400]

The idea of bypassing the filter when carb heat is applied is to bypass any possible blockage of the filter. I believe most certified aircraft are designed this was as well.

[jtpitkin06]

When you need carb heat you want it HOT. Mixing it with cold air from the filtered intake defeats the purpose.


JP

[Av8r3400]

I took the question to mean he wanted FILTERED, hot air going into the carbs.

[skonie]

I wa planning on making an air box that had two hoses going into it, one from heat muff and one from filtered air but with only having the shutoff to the heated side. So I would simply be adding heat to the filtered side. Some say then that this would not be hot enough air?

[jtpitkin06]

Let's look at doing it either way.

A: If you mix the filtered cold air with heated air it probably will not be hot enough to be effective as carb heat.

B: If you plumb the filtered air to have either a heated path or an unheated path then you have no alternate air intake.

Without an alternate path, the question becomes, "what do you do if you inadvertently fly into freezing rain and the air filter ices up?" It would be nice to have a source of air that bypasses the air cleaner.

Don't fly in cold weather? OK... ever fly into a swarm of bugs during takeoff? Insects can completely clog the air filter in seconds. Happened to a pilot friend in his Cessna 140 . The engine lost most of its power when it was starved for air. He pulled on the carb heat and he got back nearly full power. Landing was a bit tough because the smashed bugs made the windshield IFR. He had to look out the side window like Lindbergh.

I'd stick with the usual design: Filtered cold and unfiltered hot. There's little need to have filtered air when you need carb heat.

John