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Standard Vans Heat Selector/Bypass...

Michael Burbidge

Well Known Member
I have a question that's been bugging me for a long time. (TG-10)

It seems like when the heat selector is partially open, that air can come from the heat duct and from inside the engine compartment. Is there something about the pressure coming from the heat duct that prevents that?

Michael-
 
Think about it - Air pressure inside the plenum (which supplies the heat muff/heat box) is greater than the exit air on the firewall side where the box is located.
 
I have a question that's been bugging me for a long time. (TG-10)

It seems like when the heat selector is partially open, that air can come from the heat duct and from inside the engine compartment. Is there something about the pressure coming from the heat duct that prevents that?

Michael-

It absolutely happens on my bird. The air volume through my heat valve reduces significantly once it is fully actuated and completely sealed from the engine compartment. I have considered changing the system design because of it but it is not high enough priority.
 
I thought about the same thing too.
But the air coming from the heat duct is under more pressure than the air in the lower engine compartment. So my theory is that a partially open heat selector would divert some of the air to the cabin, and the rest to the lower engine compartment.

The bigger question I have been wondering about is, if the hot "higher" pressure air is going into the lower engine compartment, doesn't this reduce the pressure differential between the upper and lower areas? And does this make a significant difference in cooling?
 
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I thought about the same thin too.
But the air coming from the heat duct is under more pressure than the air in the lower engine compartment.

Curious how you come to the conclusion that the pressure drop through many feet of scat plus a heater jacket is less than 6" of fin. Memory says the drop across the fins is just a few inches of water. It could easily be that much through the heater. It is true I have no pressure data, only the practical info above.

I think it is possible for different builds to get different results in this area. For instance, I have 3 blast tubes plus I have not trimmed my cowl exit at all. Cyl cooling is fine even though I suspect I have higher pressures than others in the lower cowl.
 
I have no data to confirm my theory, but I would think the scat+heater would have less resistance (friction area) than all the closely places fins. But just a theory.
 
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