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

The hydraulic system itself is a high-gain feedback ayetern. The
load parameters of the hydraulic qystem are the effective mass of' the
elevators, the aerodynamic spring rate and the damping, both the aeromeal
damping and that associated with the =in power piston, In
addition, the compressibility of the oil acts essentiallg in shunt with
the main load, since the flow of oil can go either into compression of
the oil or into motion of the main piston and control surface. Up to
reasonably high frequencies, typically in the order of magnitude of 15
cps or above, the open-loop hydraulic system integrates, or, if we look
at it from the frequencp-response standpoint, the gain drops off inversely
proportional to frequancy, Aa we reach the frequency at which resonance
oocure between the oil compressibiUty and the load, the output tando to
inorease. In other words, in this b a of frequencies the aofnpre5aibUty
effeat is such that it tends to increase the motion of the main piaton.
When the main piston is moving to the right, for example, the oil in the
left cylinder is expanding, tending to give the piston motion an extra push.
This resonance phenomena is very lightly damped due to the low frictional
losses in the systam. When the hydraulic loop is closed, through the
input -age, the system will ordinarily oscUte with an amplitude
driving the system into the nonlinear region of oscillation. In certain
cases, these nonlinearities, inherently preaeat in the system, or the
frictional losses which we have neglected tend to stabilize the wetem,
but the relative stability is poor.
Thie resonance phenomena, and the consequent instability or poor
relative stability occurs at such a hi* frequency compared to the siepificant
frequencies of the remainder of the overall aerodynamical systcrm,
it doe6 not appear troublesome on the surface. The main difficulties arise
because these frequencies are, however, of the same order of magnitude as
the flutter frequencies of the aircraft. The coupling between the flutter
system and the Wdraulic and control system may cause undesirable amgUfication
of the flutter amplitudes.
This coupling between the unstable hydraulic system and the flatter
system and the resulting tendency to increase flutter troubles can be
circuw~ted in either of two ways. First, we might simply cut off the
Wdraulic wstem at a frequency mch lower than the band of frequencies of
significance 'in flutter phenomena. Seconily, w mi@t try to increase the
relative stability of the hydraulic system while maintaining the high lowfrequency
gain. The first solution is certainly the simpler. The principal
disadvantage of this method of cutting off the wdraulic system at low
frequencies is the resultant sluggiah response of the overall aircraft
system, or even of the wstem from the stick to the control cnrrface. This
relationship betwema the bandwidth and the tims de4 or time of the transient
response is nothing quantitative, but in general it can be said that, if the
aystem is suitably damped so that there is not undesirable overshoot in the
transient response, the time required for the wstem to respond to a step
function input and essentially reach the steady state is inversely proportid
to the bandwidth. In particular, if the bandwidth is one cps, as an example,
the system will respond to a transient input in something like one-half to one