Carb air box tube

[Laz]

What is the purpose of the long tube that sticks out of the bottom of the Carb heat box on filtered installations ? Some of the exact same heat boxes have the tube with the slant cut in them and some do not. I have seen both on a T-craft and the one listed in the last parts manual does not show the tube.

I have heard that it was to prevent heated air from blistering the paint. The Unheated carb box closes completely and seems to pose no problem.

Any thoughts are appreciated.

[cvavon]

Re: Carb air box tube

that tube is a bypass for the hot air that is coming to the air box from the exhaust that is being pushed with ram air from the front baffles when you switch to carb heat the hot air goes in the engine

[astjp2]

Re: Carb air box tube

Yep!

[Laz]

Re: Carb air box tube

That part I understand, but why is it that some planes J-3 and others have the tube and some do not. The Carb heat is not bypassed on the unfiltered T-tcraft box and many planes like the Cessna 150 use the box with just a hole in it, no tube. I guess what I rely want to know is why the Tube and not just a hole?

[3Dreaming]

Re: Carb air box tube

Most likely because the pressure in the cowling doesn't allow for the cooling air to move through. That's why they use the tube with the bevel. There is also an opening in the lower heat muff around the tail pipe for cooling air to exit, and also for the cabin heat box. For the unfiltered airbox there was an SB to cover over heating of the exhaust on the Lycoming because of not having an exit for the carb heat air. For my BL we made a dump valve located just behind the airbox.