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P-51 Landing Gear Doors during Startup

Wed Mar 25, 2020 4:47 pm

Was bored during the mandatory stay at home period, so I looked up some videos on YouTube.



Noticed that the landing gear doors of the P51 start to close soon after engine start. Is it because the systems lose hydraulic pressure after a while and the doors fall open and once the engine is started, pressurization starts and the doors revert back to the closed position? If so, why even bother closing them? Aerodynamically more streamlined during takeoff/approach?
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Re: P-51 Landing Gear Doors during Startup

Wed Mar 25, 2020 6:46 pm

I think you answered your own questions correctly. I flew an aircraft where the inboard doors would retract once the hydraulic pumps started turning, but for the life of me can't remember which one-- an airliner or military jet. Reducing drag during takeoff and landing phase is a good thing. That's why most inboard gear doors close after the gear is down.
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Re: P-51 Landing Gear Doors during Startup

Wed Mar 25, 2020 8:37 pm

Closing the door reduces drag, and also cleans up the airflow ahead of the radiator inlet. A cursory internet search discovers a number of forums discussing this very topic, but the consensus seems to be:

Lowering the gear selector handle unlocks the mechanical door lock and it doesn't re-lock the door until the handle is in the UP position. So when the gear is selected down, hydraulic pressure does its thing in sequence to lower the door and gear, and then swing the door back up into the closed position once the gear has come down, but until you want to raise the gear and bring the gear lever up, there is no mechanical uplatch holding the gear door closed. Hence the door will eventually droop as hydraulic pressure drops after engine shutdown. Starting the engine spins the engine-driven hydraulic pump (actually, just turning the engine over starts pressurizing it, and you can see the doors move in the videos before the engine starts), and the gear door goes back up.

Also: There were several models and factory differences. There are probably slight variations between. And who knows what mechanical changes have been made to P-51s in private hands over 70 years.
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Re: P-51 Landing Gear Doors during Startup

Wed Mar 25, 2020 10:53 pm

Thanks for the answers RetiredWeasel and Florianopolis!
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Re: P-51 Landing Gear Doors during Startup

Thu Mar 26, 2020 3:10 am

On the 727 the inboard Krueger leading edge flaps all dropped with loss of hydraulic pressure on engine shutdown and retracted on start up
The best contribution to safety is a competent Pilot.


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