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servo actuators were introduced when human pilots were unable to
achieve the response rates needed to maintain a steady flight condition,
or were at least unable to sustain such control for extended periods.
In these electrically signalled and hydraulically powered actuators,.
electrical control demands from the flight control computer are
fed to ehsv (electro-hydraulic servo-valves) mounted on the actuator.
These ehsv control the output position of the actuator either directly
or via an amplification system such as the main servo valve(s).
This configuration is the basis of many full-authority FBW systems
from Airbus to Apache. Examples from Claverham include all the Tornado
Primary Flight Control Units, Spearfish torpedo, Apache WAH-64 and
the future FBW version of the SH92 Helicopter.
As an intermediate step, many earlier aircraft
were fitted with limited authority stabilisation systems where
the ehsv was used to control a small series actuator, which added
a high frequency control signal to the pilot’s mechanical
input. The most compact installation of this approach is to fully
integrate the series actuator within the main servo control actuator,
as in equipment supplied by Claverham for Jaguar, and Harrier
combat aircraft and the Lynx Helicopter programmes.
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