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

IMPROVEMENT OF POWER SURFACE CONTROL SYSTEMS
BY STRUCTURAL DEFLECTION COMPENSATION
Power controls for operation of aircraft control surfaces have
become a necessity as a result of increased airplane sise, speed,
range and control requirements. The direct feedback linkage type
of powered surface controls presents the problem of stability
versus respome. Thie is especially true when a large surface
inertia ie coupled vith a relatively law structural spring rate.
Although these systems nrny be eatisfactory for manual operation,
they msy prove marginal or u~atisfactory when used with automatic
flight control equipment.
A moans for improving the performance of a mechanical-Wdraulic
control syrtm hoving high surface inertia aad low structural
spring rate8 has been dweloped and has proven quite ~uccessful
on an airplane installation. In this discuseion we will first
cover a basic type powered surface control system; Them it will
be shm how a rtructural deflection feedback linkage can be added
to the cystom to imprwe performance.
A BASIC TTPE PCMER OPERATED CONTROL mTEn
A powered surface control system operating a high inertia surface
which had a low reaction spring rate me~y be limited in performance.
To retain stability of the system the gain must be kept low, resulting
in slow response which my make the system Inadequate for
8atisfactoi.g airplane control. This problem was encountered in
the dwelopment of a rudder control system which is described belaw
and shm In figure 1. operation of the pilot~s pedals rotatee
the torque tube Xi by means of cables. A differential linkage ie
l0~8ted at the upper en3 of the torque tube. Since the surface ie
initially stationary, the movement of the torque tube causes movement
of the error link X, which is connected to the Wraulic control
valve. Displacement of the control valve admits pressure to one
side of the piston ecld opens the other side to return. The resulting
pressure differential supplies the force to mom the ndder
against inertia, friction, damping and air load. The follou-up or
feedbncK link connected to the rudder and to the differential
then moves the 3 ifferential to reduce the error to a minimum and
closes the tgdraulic control valve. When the CIVW is a dniru, tb.
valve ie in neutral and the fluid trapped on both side8 of the
pieton holds the rudder in one position against variable loads.
This eystem as first installed in the airplane, fignre 2, prwed
to be unstable. Any disturbance to the control surface or to the
pilot input resulted in a continuous oscillation of the eystem.
To overcome this instability the system was slowed d m by reducing
control valve gain. The net result wae a deterioration in
performance and excessive phase lag. The combined lag of the