[TomcatViP]

Hi Xnomad,

Imagine that you try to fill an empty bottle of coke with water from the tap.

At the begining, the water will flow in the bottle. Then when the bottle is filled, the water does not flown into the bottle anymore but is spilled around. To get a cte flow of water trought the bottle, you'll need to cut an opening.

Now imagine that the outer section of the bottle of coke is heated and you have to try to cool it's inner volume. You'll manage that by flowing water trought it.


Tht's exactly the same thing with a radial engine cowling. With no exit, the air will flow around and not trought the engine*. With stagnent air the temp will raise around the cylinder up to a destructive point.

Tht's why it is essential to force a flow of air around the cylinders. For that engineers must build an air path where the air could circulate. Cowling flaps are here to regulate the flows and by this way the speed of the air flowing.

The fastest is the flow on the exit (at the coling flaps), the less will be the overall drag.

NACA cowling are also using airfoils sections to generate a lift vector pointing slightly forward. This "lift" force generated that way is a frwrd thrust component that even more reduce the overall drag of the engine

So, closing flaps will reduce the exit section for the air hence increase the airflow overall speed hence reduce the drag. However, the mass flow of air circulating around the cylinders will be less. This reduce massflow will hence absorb less calories produce by the engine meaning that the cooling effect will be reduced. To compensate for the reduce section, the airplane will hve to fly faster to get a same amount of air around the cylinders per second.


* tht's not exactly true since a part of the air flow will penetrate the front cowling section forcing another part to escape trought that same section with a frwd speed. Thta frwd speed is a net drag account that will induce the plane to slow down

~S!