What are the two types of oxygen systems commonly found in unpressurized aircraft?

The two types of oxygen systems commonly found in unpressurized aircraft are continuous-flow and pressure-demand types.

Continuous-flow oxygen systems provide a constant flow of oxygen to the user, which is beneficial during low-altitude operations or in situations where there is sufficient cabin pressure. This type of system is straightforward and often used in training aircraft or in situations where supplemental oxygen is required for short durations.

On the other hand, pressure-demand oxygen systems are designed to deliver oxygen only when the user inhales. This system is ideal for higher altitudes where ambient pressure is lower, which allows for efficient oxygen use and minimizes wastage. Users can breathe in the oxygen when they need it without constant flow, making it suitable for environments where maintaining adequate oxygen saturation is critical.

The other choices do not represent the types of oxygen systems utilized in unpressurized aircraft. Clycol and pneumatic systems, for instance, pertain to different types of aircraft operational systems unrelated to oxygen supply. Liquid and gas systems are too broad and do not specifically refer to oxygen systems found in aircraft. Finally, centrifugal and vacuum systems refer to mechanisms for airflow management rather than oxygen delivery systems.