Ron Natalie writes:

They control cooling air through the engine cowls. They are flap
shaped "valves" if you want on the outflow of the cooling air.
When open, you get more cooling air but that also means more
drag. In my plane, I can get another five to ten knots when
I close them. I take-off and climb with them open and close
them at cruise.

Do you do this more or less routinely, or do you actually look at the
engine temperatures to see when to open or close them? I get the
impression that it's not a super-critical item except in a few
circumstances.

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