336
14 CFR Ch. I (1–1–14 Edition)
Pt. 60, App. D
lllllllllllllllllllllll
B
EGIN
I
NFORMATION
3. C
ONTROL
D
YNAMICS
a. The characteristics of a helicopter flight
control system have a major effect on the
handling qualities. A significant consider-
ation in pilot acceptability of a helicopter is
the ‘‘feel’’ provided through the flight deck
controls. Considerable effort is expended on
helicopter feel system design in order to de-
liver a system with which pilots will be com-
fortable and consider the helicopter desir-
able to fly. In order for an FTD to be rep-
resentative, it too must present the pilot
with the proper feel; that of the respective
helicopter. Compliance with this require-
ment is determined by comparing a record-
ing of the control feel dynamics of the FFS
to actual helicopter measurements in the
hover and cruise configurations.
(1) Recordings such as free response to an
impulse or step function are classically used
to estimate the dynamic properties of
electromechanical systems. It is only pos-
sible to estimate the dynamic properties as a
result of only being able to estimate true in-
puts and responses. Therefore, it is impera-
tive that the best possible data be collected
since close matching of the FTD control
loading system to the helicopter systems is
essential. Control feel dynamic tests are de-
scribed in the Table of Objective Tests in
this appendix. Where accomplished, the free
response is measured after a step or pulse
input is used to excite the system.
(2) For initial and upgrade evaluations, it
is required that control dynamic characteris-
tics be measured at and recorded directly
from the flight deck controls. This procedure
is usually accomplished by measuring the
free response of the controls using a step or
pulse input to excite the system. The proce-
dure must be accomplished in hover, climb,
cruise, and autorotation. For helicopters
with irreversible control systems, measure-
ments may be obtained on the ground. The
procedure should be accomplished in the
hover and cruise flight conditions and con-
figurations. Proper pitot-static inputs (if ap-
propriate) must be provided to represent air-
speeds typical of those encountered in flight.
(3) It may be shown that for some heli-
copters, climb, cruise, and autorotation have
like effects. Thus, some tests for one may
suffice for some tests for another. If either or
both considerations apply, engineering vali-
dation or helicopter manufacturer rationale
must be submitted as justification for
ground tests or for eliminating a configura-
tion. For FTDs requiring static and dynamic
tests at the controls, special test fixtures
will not be required during initial and up-
grade evaluations if the sponsor’s QTG shows
both test fixture results and the results of an
alternative approach, such as computer plots
which were produced concurrently and show
satisfactory agreement. Repeat of the alter-
native method during the initial evaluation
satisfies this test requirement.
b. Control Dynamics Evaluations. The dy-
namic properties of control systems are
often stated in terms of frequency, damping,
and a number of other classical measure-
ments which can be found in texts on control
systems. In order to establish a consistent
means of validating test results for FTD con-
trol loading, criteria are needed that will
clearly define the interpretation of the
measurements and the tolerances to be ap-
plied. Criteria are needed for both the under-
damped system and the overdamped system,
including the critically damped case. In the
case of an underdamped system with very
light damping, the system may be quantified
in terms of frequency and damping. In criti-
cally damped or overdamped systems, the
frequency and damping is not readily meas-
ured from a response time history. There-
fore, some other measurement must be used.
(1) Tests to verify that control feel dynam-
ics represent the helicopter must show that
the dynamic damping cycles (free response of
the control) match that of the helicopter
within specified tolerances. The method of
evaluating the response and the tolerance to
be applied are described below for the under-
damped and critically damped cases.
(a) Underdamped Response. Two measure-
ments are required for the period, the time
to first zero crossing (in case a rate limit is
present) and the subsequent frequency of os-
cillation. It is necessary to measure cycles
on an individual basis in case there are non-
uniform periods in the response. Each period
will be independently compared to the re-
spective period of the helicopter control sys-
tem and, consequently, will enjoy the full
tolerance specified for that period.
(b) The damping tolerance will be applied
to overshoots on an individual basis. Care
must be taken when applying the tolerance
to small overshoots since the significance of
such overshoots becomes questionable. Only
those overshoots larger than 5 percent of the
total initial displacement will be considered
significant. The residual band, labeled T(A
d
)
on Figure 1 of this attachment is
±
5 percent
of the initial displacement amplitude, A
d
,
from the steady state value of the oscilla-
tion. Oscillations within the residual band
are considered insignificant. When com-
paring simulator data to helicopter data, the
process would begin by overlaying or align-
ing the simulator and helicopter steady state
values and then comparing amplitudes of os-
cillation peaks, the time of the first zero
crossing, and individual periods of oscilla-
tion. To be satisfactory, the simulator must
show the same number of significant over-
shoots to within one when compared against
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