EVOLUTION
OF PRIMARY FLIGHT CONTROL SYSTEMS
The earliest aircraft used direct mechanical
control of the flight control surface either using rods or cables
connected to the pilot’s control column. Various devices
such as springs, control tabs and weights were employed to balance
the loads on the surface so that the pilot was physically capable
of flying the aircraft. By the 1940’s as aircraft speed
and weight increased, these mechanical solutions became less and
less effective and hydraulic power-assist devices were introduced
(eg Hawker Hunter and the Canberra Bomber). These early hydro-mechanical
devices in turn evolved into full authority mechanically signalled
power control units used extensively in the 1950’s (eg De
Havilland Comet and Hawk).
Evolution has continued with the introduction
of full authority electrically signalled systems using servo-valves
in the 1970s merging on into Direct Drive Valve systems in the
1980s and 1990s. There is much current interest in the More Electric
Aeroplane. Claverham's contribution to this is in the development
of pure electro-mechnaical PCU (Primary Control Units) for Primary
Flight Controls in manned and unmanned aircraft.
The main classes described are Mechanically
signalled, Electro-hydraulic, Direct
drive, and Electro-mechanical. Many
applications use more complex hybrids, for example many helicopters
and 1960 / 70 vintage combat aircraft use a mechanically signalled
actuator with a limited authority electrically signalled “series”
actuator incorporated to provide stabilisation or some other computer
controlled facility.
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