Air intake doors.....

[Pelican]

Hi,

 

I think most of us Mig 21Bis pilots have noticed that when the aircraft is at rest or below ~38 knots a small intake door opens up on each side of the fuselage at the leading edge of the wing root directly beneath the cockpit.

 

It is visually clear that it provides additional airflow to the engine at low speeds but I am unsure as to why the nose intake isn't sufficient for this purpose.

 

(Going on a hunch here) Is it because the largely dominant nosecone causes turbulent airflow through boundary layer interactions & prevents sufficient air from entering the engine at such low speeds?

 

 

Best,

[Art-J]

The main intake geometry and cross section area is carefully chosen for optimum airflow velocity and volume, max static pressure recovery and uniform pressure field at anything between cruise and top speed with high compressor RPM, as this is where an engine spends most of its lifetime.

 

Especially when the plane was born as a Mach 2 interceptor, like our MiG.

 

For any situations when intake air supply doesn't match the exact engine demand, bypass and bleed doors or valves might/must be added. You will find them on almost every fast plane with center cone (i.e. Su-17, SR-71)... but also on many subsonic planes with no cone at all (Harrier, C-141, B-52).

[Pelican]

Thanks Art-J, that does a lot to clear up my query.... :)