Ram air turbine

A ram air turbine (commonly known by the acronym RAT) is a small turbine that is connected to a hydraulic pump, or electrical generator, installed in an aircraft and used as a power source. The RAT generates power from the airstream by ram pressure due to the speed of the aircraft.

With the exception of crop dusters (see below), modern aircraft only use RATs in an emergency—in case of the loss of both primary and auxiliary power sources the RAT will power vital systems (flight controls, linked hydraulics and also flight-critical instrumentation). Some RATs produce only hydraulic power, which is in turn used to power electrical generators. In some early aircraft (including airships), small RATs were permanently mounted and operated a small electrical generator or fuel pump.

Modern aircraft generate power in the main engines or an additional fuel-burning turbine engine called an auxiliary power unit, which is often mounted in the rear of the fuselage or in the main-wheel well. The RAT generates power from the airstream due to the speed of the aircraft. If aircraft speeds are low, the RAT will produce less power. In normal conditions the RAT is retracted into the fuselage (or wing), and is deployed manually or automatically following complete loss of power. In the time between power loss and RAT deployment, batteries are used.

Ram air

Ram air refers to the principle of using the airflow created by a moving object for power. The term may also refer to any of the following:

See also

Ram-air intake

A ram-air intake is any intake design which uses the dynamic air pressure created by vehicle motion to increase the static air pressure inside of the intake manifold on an internal combustion engine, thus allowing a greater massflow through the engine and hence increasing engine power.

Design features

The ram-air intake works by reducing the intake air velocity by increasing the cross-sectional area of the intake ducting. When gas velocity goes down the dynamic pressure is reduced, while the static pressure is increased. The increased static pressure in the plenum chamber has a positive effect on engine power, both because of the pressure itself and the increased air density that this higher pressure gives.

Ram-air systems are used on high-performance vehicles, most often on motorcycles and performance cars. Ram-air was a feature on some cars in the sixties, falling out of favor in the seventies, but recently making a comeback. While ram-air may increase the volumetric efficiency of an engine, they can be difficult to combine with carburetors, which rely on a venturi-engineered pressure drop to draw fuel through the main jet. As the pressurised ram-air may kill this venturi effect, the carburetor will need to be designed to take this into account; or the engine may need fuel-injection.