This question (IMD) crops up every so often.
To save you digging back this is what I said the last time and nobody flamed me for it then:
Warning! A physical description of intake momentum drag using only words and no diagrams is not for the fainthearted
All jet engines have intake momentum drag (IMD) because the air as it enters the intake is decelerated violently and redirected by the compressor into a radial flow. If you are an old centrifugal compressor guy (ah! ..Vampires/Meteors… then this is quite easy to visualise because the rotor on those compressors blocked the whole intake and the air had no option but to move out radially on its journey towards the combustion chambers.
With todays axial compressors it seems as if the air could just wiggle largely straight on through. In fact the first stage rotors are designed to grab the incoming air and deliver it at high radial velocity towards the waiting first row stators that act like a brick wall to this largely radial flow. This stator induced reduction in velocity results in a sudden pressure rise (Bernoulli). This now higher pressure but slower air slides off the stators into the path of the next row of rotors which add speed to it again only for the second row stators to “stop” it again. The process is repeated for as many stages as you use, upping the pressure each time.
The overall experience of air as it passes through a jet engine creates some drag forces (IMD being but one) and some thrust forces. The net thrust is just that. The sum of the thrust bits minus the sum of the drag bits.
Any help?
JF