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This is an interesting phenomenon noted for F-35B aircraft during vertical flight and vertical landing. They will do so with their weapons bay doors partially opened.

Of interest, this only occurs during hover (ref 2:48 in the video above) but not in slow flight while the lift fan system is activated.

I have never found the reason why this is so. Does anybody know if Lockheed Martin installed additional auxiliary air intakes inside the weapons bays to provide additional air to the engine during hover?

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    $\begingroup$ Can you point at the time in the video where you observed this and/or provide a still image? $\endgroup$
    – DeltaLima ♦
    Aug 1, 2022 at 9:08
  • $\begingroup$ Only when landing on a ship, not when landing at an airport? $\endgroup$
    – Camille Goudeseune
    Aug 1, 2022 at 13:59

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Somewhat speculative, as I have not (yet) found a fully authoritative source:

Partially open weapons bay doors create a channel that moderates the airflow from the nozzles as the aircraft comes closer to surface, creating a cushion of air between them.

The configuration is similar to AV-8B Harrier's Lift Improvement Device system (LIDs), which was developed to mitigate Hot Gas Ingestion (HGI) in close ground proximity.

I also suspect they limit the spanwise flow of air in the same situation. This flow would adversely affect the efficiency of the auxiliary nozzles in the wings, possibly compromising controllability.

This Youtube video shows the weapons bay doors opening as the aircraft is approaching for landing, note that the plane is already in VTOL configuration.

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  • $\begingroup$ I wonder if for the same reason the landing gear compartment doors stay open even some time after the landing gear is stowed. $\endgroup$
    – U_flow
    Aug 2, 2022 at 13:00

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