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I understand that the turbine of the engine runs the compressor and the fan of the engine. If the air flows through the compressor undisturbed, can we avoid having an inlet in the gas turbine engine to reduce drag of an aircraft?

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    $\begingroup$ Welcome! What do you mean by "can we avoid having an inlet in the gas turbine engine"? air must enter the engine thru an inlet (edit your question directly to clarify, using the 'edit' link). By the way, air doesn't flow thru the compressor undisturbed, it is compressed, compression is by itself a big disturbance. The cold flow in a turbofan is accelerated, which is also a disturbance. $\endgroup$
    – mins
    Feb 4, 2019 at 11:58
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    $\begingroup$ If there's no inlet, how does the air get... let in? $\endgroup$ Feb 4, 2019 at 14:33

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If you are talking about the inlet (diffuser), this is not a good idea for several reasons

UNIFORMITY OF AIRFLOW THROUGH THE ENGINE

Because of the design of the diffuser, it can eliminate turbulent airflow and, in the case of supersonic aircraft prevent the engine from ingesting shockwaves, causing compressor stalls and flameouts. Diffusers also direct the air into the compressor in a uniform fashion, reducing engine problems if the aircraft is sideslipped or at high angles of attack.

ADDITIONAL RAM AIR COMPRESSION

The diffuser can provide additional compression at higher speeds vis a vis ram air forced through the diffuser slowing it down and compressing it. While this process does create additional aerodynamic drag, it provides the engine with denser air to use and subsequently more thrust.

RCS REDUCTION FOR MILITARY APPLICATIONS

Jet engine compressors and fans are strong radar reflectors, which increase the radar cross section (RCS) of an aircraft. Diffusers can be designed to either hide the engine compressor from view of a radar using a S-shape diffuser or cover the diffuser inlet with a grill having openings narrower than the wavelength of a potential radar beam, much like the grill on a microwave window protects the people around it.

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  • $\begingroup$ Just a detail: "narrower than the width of a potential radar beam", actually than the wavelength. When this condition is obtained, the mesh acts like a mirror (a reflector). $\endgroup$
    – mins
    Feb 5, 2019 at 16:27

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