I once heard, or misheard, this:
There is an emergency propeller that comes down in an emergency of jet-engine aircraft. It is behind the main landing gears.
Is this true or a story somebody made up?
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In case of total loss of all main generators, the RAT is automatically extended and drives the emergency generator via a hydraulic motor.
The location of the ram air turbine varies with different aircraft and some of them (like the popular A320) have them near the landing gear. The following image is from @mins' excellent answer to another question.
Location of Ram Air Turbines in different aircraft, image from aviation.stackexchange.com