Erm..as I understand it intake momentum drag is also sometimes referred to as Ram Drag.
Consider a typical airliner jet intake, in a wind tunnel.
Immediately behind the intake 'lip' the intake forms a divergent passage ahead of the fan (i.e. it gets wider). This serves to decrease the speed of the incoming airflow and (by Bernoulli) increase the static pressure ahead of the fan, improving the compression performance of the compressor.
Or, if the aircraft were moving in still air, the airflow into the engine is actually accelerated by the divergent intake from stationary to produce the same effect (it is still 'slowed down' relative to the aeroplane's movement).
Broadly speaking, the change in momentum of the air into the engine due to the shape of the intake has to be due to a force applied to it. This force is what is apparent as a 'drag' on the engine.
This increase in static pressure ahead of the fan as the aircraft gains forward speed is also the reason why engine take-off thrust, if set using EPR (engine pressure ratio), must be set by a certain speed. Otherwise, the increasing pressure ahead of the engine (p2) means that actual numerical value of EPR will fall as the aircraft accelerates and this could lead to inaccurate EPR setting if left too late.
Not the clearest of explanations, I know, but I can't find my old Propulsion notes...
Feel free to correct/criticise/abuse y'all!
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...proceeding below Decision Height with CAUTION...