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Watching some video of one Mig-31 taking off, one can see those doors circled in red in the picture sourced from it, are those meant to produce net lift and reduce take off and landing distance/speed? Are those surfaces related to landing gear bay doors in the first place, and how are modern airliner's bay door designed to behave regarding lift generation during landing and takeoffs?

I've read or heard somewhere that airliner's landing gear bay door are mostly designed to reduce noise when extended, what about lift?

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    $\begingroup$ An aerofoil of such low aspect ratio and such high incidence will probably produce more drag than lift. As far as airlines are concerned, every airliner that I know of has its undercarriage doors open parallel to the airflow, so they physically cannot produce lift. $\endgroup$
    – Aditya Sharma
    Commented Feb 28, 2023 at 23:11
  • $\begingroup$ in practice there's a wide range of AoA to be flown with those airliners doors opened, they may be designed to be parallel to airflow, but most of the time it's the airflow that's not parallel to the doors. Agreed with the low aspect ratio high incidence of the circled devices, even if ram lift is a thing too (there's still lift after stall) – $\endgroup$
    – user21228
    Commented Feb 28, 2023 at 23:39
  • $\begingroup$ Well, they'd probably do a better job at at acting like airbrakes than wings if they're stalled (which they probably are)... Also, the way airliner gear doors are positioned when open, the AoA probably has no effect on lift. $\endgroup$
    – Aditya Sharma
    Commented Feb 28, 2023 at 23:49

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No they just stick out in the breeze and make drag, and the forward doors probably create some minor trim change from the little bit of lift they might generate, while creating a lot more drag than the sideways doors.

They could have (and probably would have if it was a western fighter) added extra hydraulic sequence valves and controls to make those forward doors close when the gear is locked down. It's a common practice on civilian airplanes to close the doors that cover the wheels when the gear is down, usually done to maximize performance in critical cases, like a go-around single engine.

The designers probably decided to live with the effects of those doors hanging down, the thing having so much power. They may have wanted the drag of the doors as fixed speed brakes, while the gear is down, to increase the power requirement on approach, which can make the descent angle easier to control because the engines are more responsive when at a higher power setting.

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