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For ducts and vents, check Lopresti Speed Merchants and NACA data. NACA looked at cowl flaps and heat vents ~70 years ago, found gently radiused curves that let cooling air rejoin the slipstream flowing as parallel as possible work best and with least drag. Venturi effect and exhaust heat can, if theory and legend are correct, actually make a bit of thrust. That's the story with the P51 radiator, at least.
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