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

Magpltude of the desired output. - Because the character of the
response is determined by the magnitude of the desired output, a value
of the output magnitude must be selected for design purposes. In general
this information is obtained from the process which the servomotor is
varying and consists of an estimation of the maximum percent change in
the process tht is required to occur in a given traneient or sine wave
oscillation. This variation should then be translated into the output
stroke required of the servomotor to produce such a change. The total
stroke of the servomotor will usually be larger than the design value
of S or St and corresponds to.the steady state range through which
the process is carried. If a step change or sine wave oscillation larger
than S or St is introduced the resulting inertia index will be larger
than that assumed in the design with the consequent possibility of damage
from high peak cylinder pressures. Protection against such damage can
be obtained by restricting the length of the pilot valve lands.
Selection of no-load time constant and inertia index for frequency
response requirements. - If the frequency response requirements of, the
servomotor are known they in general will take the form of the first
break frequency (f l) and the &oesover frequency (f3). The equations
for T and E' in term of these two frequencies are as follows:
As shown in the analysis the response of a servomotor of this type is
characterized in both attenuation and phase shift by a first order relation
up to the crossover frequency. The phase shift in this frequency
band is therefore limited to a maximum value of 90'. The crossover frequency
f3 can therefore be selected on the basis of sufficient amplitude
attenuation at 90' phase shift to insure stability in the control loop.
With fl and fg defined, the dimensions of the servomotor are established.
Substituting equations (23) and (24) in equation (21) the following
relation is obtained:
4MS1p
2
flf3
(251
From equation (25) it can be seen that at a fixed value of
fl an increased
margin for fg can only be obtained by a proportionate increase in piston
area.
Selection of no-load time constant and inertia index for transient
response requirements. - Transient response requirements can be expressed
in tern of the time to reach 90 percent 'of the final value. The