Passive pressure stabilization systems for aircraft

A transmission system comprising:

• a casing providing a chamber including a first fluid having a first density and a second fluid having a second density more than the first density, the chamber to provide the second fluid to gears of the transmission system;
• a reservoir tank fluidly coupled to the chamber by a first passageway and a second passageway, the reservoir tank including the first fluid and the second fluid;
• and
• a breather tube coupled to the casing, the breather tube to extend within the reservoir tank to fluidly couple the second passageway between the reservoir tank and the chamber.

An aircraft movable in an upright orientation and an inverted orientation, the aircraft comprising:

• a passive system including:
• a fluid reservoir to store oil and air;
• a gearbox to include a plurality of gears, the gearbox including a cavity to store the oil and the air;
• a first passageway integrally formed with a casing of the gearbox, the first passageway to fluidly couple the fluid reservoir and the cavity of the gearbox;
• a second passageway integrally formed with the casing of the gearbox, the second passageway to fluidly couple the fluid reservoir and the cavity of the gearbox;
• and
• a breather tube coupled to a casing of the fluid reservoir, the breather tube to fluidly couple the second passageway, the fluid reservoir and the cavity of the gearbox when the aircraft is in the inverted orientation, the first passageway to fluidly couple the fluid reservoir and the cavity of the gearbox when the aircraft is in the upright orientation.