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Consider a piston engine with all its coolers and components within an aircraft engine bay. To cool this engine, cooling intakes are positioned facing the direction of flight. Now, the condition at the inlet surface need not reflect the flight conditions exactly, one reason for this would be since the resistance pressure from within the engine bay would influence the conditions at the inlet itself.

The question is specific to this resistance pressure as a first approximate at least. I know there are a variaty of other reasons as to why the conditions at the inlet surface would change (e.g pressure distribution at location of intake position, inlet type, etc...) but those aren't a concern here yet.

A good answer to this based on actual data, or statistical data or even experience would be greatly appreciated. Sources as to where one may find the answers to this would also be much welcomed. Thanks in advance!

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  • $\begingroup$ I find my engine cooling works best in straight & level flight. Loss of cooling is seen in a climb. Cooling is also affected by how much the cowl flaps are open, or closed. And by how rich the mixture is set. All in all, I try to keep the cylinder head temperatures at 380F or below for best cylinder life, using less severe climb, opening the cowl flaps more, enriching the fuel flow more (could be simple change from 10 gph to 11 gph at 2400 RPM for example), and reduced RPM. I don't have a gage to measure air pressure directly. $\endgroup$
    – CrossRoads
    Apr 14, 2020 at 12:55

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3 main points come to mind:

  1. The space above the cylinders is a pressure plenum so that the air pressure and velocity flowing past the cylinders is the same everywhere.
  2. The space below the cylinders is a low pressure zone and to achieve that the exhaust opening should be larger than the inlet opening(s) to get the highest differential at low speed, but can be reduced as ram pressure increases (hence a cowl flap). The pressure balance has to take into account air bled off the high pressure plenum to pass through coolers or other services that need ram air.
  3. The baffling that controls the flow between the high pressure plenum above to the low pressure exhaust zone below past the cylinders has be tight to the cylinders, actually touching the fins, so that the cooling air is forced between the fins. Cooling air that passes beyond the edges of fins does no good.

As for details and numbers, you should probably obtain this book on piston cowling design.

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  • $\begingroup$ Well my question considers the engine and all its coolers and components within the engine bay as a single package, and the resultant pressure resistance it generates at the cooling intakes is what I'm looking for. I would think it would have to do with the density and temperature within the engine bay as a result of the engine running. Considering this as an incompressible case, I still don't have the temperature in hand at the moment to calculate an estimate number. If there is a rough figure or guide on this, I would appreciate it. $\endgroup$
    – Guha.Gubin
    Apr 14, 2020 at 15:44

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