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I wondered whether the final scene in The Memphis Belle where they hurry to extend the gear by hand crank (because their electric system failed) is realistic, but according to this answer the B-17 indeed used a hand crank for manual gear extension.

But why did it (and other aircraft of the era that used similar system) need to manually crank the gear all the way down rather than just pull a latch out and let gravity do the rest, like modern systems generally work?

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  • $\begingroup$ It's realistic that the gear needed to be cranked down due to battle damage, but the urgency behind it was not. In the movie they portrayed landing with one wheel up as being instant death with the airplane exploding. It was dramatic effect, a last second click of the gear locking into place saving the crew, in reality it rarely meant more than a bent airplane having one wheel up. $\endgroup$
    – GdD
    May 15, 2016 at 15:59
  • $\begingroup$ @GdD, … especially since the wheel protruded quite far below the engine even in retracted position. $\endgroup$
    – Jan Hudec
    May 15, 2016 at 16:37

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They had to manually crank down the gear because the electric system was damaged.

"As the B-17 limps closer to base, Dearborn orders the landing gear dropped. Only one wheel descends due to electrical failure caused by battle damage, but the crew are able to manually lower the malfunctioning wheel just before landing."

Free fall only works for hydraulic powered landing gear systems. The B-17 uses an electrically driven motor to raise and lower the gear.

Not all hydraulic landing gear can free fall. PBY-5A's have hydraulic gear and emergency gear extension used a backup hydraulic pump. Additionally you could manually break the up locks and then manually push the gear to the locked position.

I bet many other WWII era aircraft with hydraulic gear were similar. Free falling hydraulic gear probably came much later.

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    $\begingroup$ But what is the mechanical difference between electric and hydraulic actuator that allows disconnecting a hydraulic one, but not an electric one? $\endgroup$
    – Jan Hudec
    May 14, 2016 at 14:20
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    $\begingroup$ Mechanical difference is that electrical systems typically use a snail gear. The snail gear only works easy from one end, but not from another. $\endgroup$
    – kubanczyk
    May 14, 2016 at 15:58
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    $\begingroup$ @JanHudec Free fall on hydraulic actuators work because in free-fall model both sides of the actuator are open to return, so the hydraulic fluid can freely flow from one side of the actuator to the other. Really there is no reason you can't do free fall with electric, there would just have to a way a disengage the electric actuator from the gear. Further, some hydraulic gear actually require cranking to unlock the uplocks and to lock the downlocks, $\endgroup$
    – OSUZorba
    May 15, 2016 at 4:48
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The B-17 used an oil filled strut that extended down and rearward, moved by a screw drive in front of the strut which also was part of the locking mechanism. Gravity was not your friend, as disconnection of the screw drive, by battle damage for example, would let the main wheel pivot rearward into the trailing edge of the wing on landing. The cranks were coaxial with the electric motors that swung the gear and it takes somewhat over 200 turns to get one down. No one cranks them up.

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Because of wind pressure

Disengaging locks will result in landing gear to partially open but gravity is not strong enough to make the gears lock in their extended position. Wind blast or wind pressure will oppose the movement of landing gear and to actually make the gears extend fully when main system has failed, hand crank is the better option as opposed to relying on gravity.

Also modern systems do not solely rely on gravity!

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    $\begingroup$ actually modern systems have to demonstrate the capability to extend the gear relying only on gravity in emergency situations. $\endgroup$
    – Federico
    May 15, 2016 at 12:05
  • $\begingroup$ Is there any commercial aircraft which can lower gears while relying only on gravity? $\endgroup$
    – Fiaz Husyn
    May 15, 2016 at 15:24
  • $\begingroup$ So what do the modern systems require except gravity? $\endgroup$
    – Jan Hudec
    May 15, 2016 at 15:46
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    $\begingroup$ @FiazHusyn, Yes. Look at pretty much every main gear on any commercial airliner they open inboard to outboard, the only thing air pressure is doing is adding friction (possible exceptions are the body gear on DC10s and 747s). Nose landing gear get some benefit from the air pressure, although it is not typically needed. For example, on the 757 when testing alternate extend on the ground, you may put up to 100# of force on the NLG to make it lock, but in the dozens of tests I've witnessed the most that was needed was a slight nudge and usually nothing was needed. $\endgroup$
    – OSUZorba
    May 16, 2016 at 3:24
  • $\begingroup$ @JanHudec manual crank OR Pneumatically Operated OR High Pressure Nitrogen. Concept is to use alternate, separate source of any fluid, stored at high pressure and activated without relying on main electric power source $\endgroup$
    – Fiaz Husyn
    May 17, 2016 at 19:26

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