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Angier:

Scott got it right in his formula for the maximum available ram pressure
(allso called stagnation pressure - that which is achieved when you bring the
flow to a stop without friction).  This is what the pitot tube collects and
what the airspeed indicator is reading.  

However, when you want to induct a flow rate of air and slow it befoe taking
it into the engine for cooling, combustion or into the cabin for ventilation,
you must pay a friction penalty.  The flow is partially slowed in front of
the aircraft (depending on how blunt it is) and then further slowed as it
passes into the diffuser, the passage that expands and further slows the flow
before it is used.  Typically the maximum recovery you can get in in the
range of 75-80% of the stagnation pressure, and you might only get 50% if the
diffuser lossses are high (poor design).  So at the end of the day, at 200
knots IAS you can expect maybe 1.5 inches of mercury, or a bit less, in
manifold pressure boost.  It's worth getting, so design your inlet carefully.
 And don't give it up with an air cleaner that is too small.

Fred Moreno

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