Air Driven Generator(Ram air turbine)

A ram air turbine (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
due to the speed of the aircraft. With the exception of crop dusters
(see below), modern aircraft only use
RATs in 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, 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, and if
aircraft speeds are low the RAT will
produce less power. In normal conditions the RAT is retracted into the
fuselage (or wing), deploying
automatically following complete loss
of power. In the time between power
loss and RAT deployment, batteries
are used. RATs are common in military aircraft
which must be capable of surviving
sudden and complete loss of power.
Many modern types of commercial
airliners are equipped with RATs. In
the 1960s the Vickers VC-10 was one of the first types of airliner equipped
with a RAT. The Airbus A380 has the largest RAT propeller in the world at
1.63 m in diameter, but around 80 cm
is more common. A typical large RAT
on a commercial aircraft can be
capable of producing, depending on
the generator, from 5 to 70 kW. Propellers started as two-bladed or
four-bladed models but military (and
increasingly commercial) models now
use ducted multi-blade fans. Smaller,
low airspeed models may generate as
little as 400 watts. In other military uses, pod-fitted
systems such as the M61A1 Vulcan or electronic systems (e.g. the AN/ ALQ-99 TJS) can be powered by a RAT in standard operation. Also, some
free-fall nuclear weapons, such as the
British Yellow Sun and Blue Danube used RATs to power radar altimeters and firing circuits. In non-military use, RATs have been
used to power centrifugal pumps to pressurize the spray systems on
aircraft that are used as crop dusters to deliver liquid agents to cropland.
The major reason for choosing a RAT
is safety; using a RAT allows the FAA-
certified engine and power systems on
the aircraft to remain unmodified.
There is no need to use an engine power takeoff to drive the pump, and
the pump can be placed low or below
the exterior of the airframe greatly
simplifying plumbing, and being the
lowest point in the plumbing, it will
have gravity feed from the spray tanks and never need to be primed. In the
event of a pump failure that could
result in seizure, there is no effect on
the flying ability of the aircraft or its
systems apart from the obvious fact
that the spray systems are non functional.