Report of the Second Piloted Aircraft Flight Control System - Acgsc.org

~dditional means were tried in laboratory tests to improve performance.
Damping in the feedback loop was tried, although better results
were obtained, the installation complications did not warrant
its use. The use of a double om rlng piston also helped to reduce
phase lag by reducing backlash of the system. Although the improvement
did not reach the magnitude desired, the double ROW ring piston
was installed since bacldash uill deterlorate aw system.
The main problem, that of low spring rates of the system, still
remained, and prevented necessary increase in system gain to obtain
better response. This problem can be illustrated by referring to
a basic type power operated control system using a direct feedback
linkage as shown schematically in figure 4. It can be seen that
external disturbance to the control surface or mass results in
deflection of the reaction structure. If the input system is held
fixed while the reaction structure is deflected, the followup linkage
opens the control valve in a direction to oppose this disturbance.
~f the system has high gain, the control 'valve may admit
enough energy to cruse the system to over correct the surface position.
Due to the inertia of the surface the over correction causes
a deflection of the structure In the opposite direction which in
turn again opens the valve and again excess energy is admlttecl ad
the cycle is repeated. If the energy admltted per cycle is equal
to or greater than that extracted by aerodynamic damping and friction
of the system, the surface will continue to oscillate. The
amount of energy that is added to the gystem is prinudly detelc
mined by the flow characteristics of the mstering vahe and mry be
kept small if the flow per unit deflection is emall. Stability
mey, therefore, be achieved by ube of a law gain valve. This
improvement in stabil%ty of the system is obtained at the expense
of speed of response.
As very little can be done about the hydraulic spring rate and as
the structure stiffness can be increased only to a limited degree,
it was evident that some other means should be coxwidered to compensate
for all of the system spring rates.
ADDITION OF STRUCTURAL COMPE2SATION TO THE BASIC TYPE SISTBl
High performance of a servomechanism cannot be obtained if lack of
stability lindts the allowable gain. Therefore, the stability of
the system is the gwerning factor. Improvement can be obtained by
increasing either spring rate or damping or by reducing the mass.
For this system the mass could not be reduced and adding damping to
the output was not considered practical or desirable, leaving the
reaction spring rate as the factor to be considered in order to
Improve stability.