Originally Posted by David-aviator
If good cooling is efficient mass air flow, how is that efficiency defined?
A serious question, so I dug around on my shelf for a reputable one-page reference. This from Aerodynamics, Aeronautics, and Flight Mechanics,
Barnes McCormick...the fundamentals:
Eq 4.45 spells out the most basic concept...mass x momentum loss = cooling drag.
Note the concept in the first paragraph about energy removed and energy added.
For any given power setting the quantity of heat you must carry away is fixed. The best way to decrease required mass flow is to increase the quantity of heat transfered to the mass, ie heat the air as much as possible during its pass through the engine compartment. Doing so requires less mass to carry away the same quantity of heat. Measuring the air temperature increase following a pass through the system (or an individual cylinder baffle, or an oil cooler) is one very real yardstick for cooling efficiency. Heat exchanger efficiency = cooling air temp rise / (CHT-OAT)
Seems like if inlet air is restricted because of internal pressure caused by a small exit area, drag is created in front of the engine compartment as oncoming air creates a wall of pressure at the inlets and spills over the top, bottom and sides of the exterior of the cowl.
Cowl and inlet shape can be done badly of course, but do realize that external diffusion is in itself nearly frictionless.