Pt. 60, App. D

14 CFR Ch. I (1-1-19 Edition)

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BEGIN INFORMATION

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3. CONTROL DYNAMICS
a. The characteristics of a helicopter flight
control system have a major effect on the
handling qualities. A significant consideration 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 deliver a system with which pilots will be comfortable and consider the helicopter desirable to fly. In order for an FTD to be representative, it too must present the pilot
with the proper feel; that of the respective
helicopter. Compliance with this requirement is determined by comparing a recording 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 possible to estimate the dynamic properties as a
result of only being able to estimate true inputs and responses. Therefore, it is imperative 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 described 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 characteristics 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 procedure must be accomplished in hover, climb,
cruise, and autorotation. For helicopters
with irreversible control systems, measurements may be obtained on the ground. The
procedure should be accomplished in the
hover and cruise flight conditions and configurations. Proper pitot-static inputs (if appropriate) must be provided to represent airspeeds typical of those encountered in flight.
(3) It may be shown that for some helicopters, 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 validation or helicopter manufacturer rationale
must be submitted as justification for
ground tests or for eliminating a configuration. For FTDs requiring static and dynamic
tests at the controls, special test fixtures
will not be required during initial and upgrade 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 alternative method during the initial evaluation
satisfies this test requirement.
b. Control Dynamics Evaluations. The dynamic properties of control systems are
often stated in terms of frequency, damping,
and a number of other classical measurements which can be found in texts on control
systems. In order to establish a consistent
means of validating test results for FTD control loading, criteria are needed that will
clearly define the interpretation of the
measurements and the tolerances to be applied. Criteria are needed for both the underdamped 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 critically damped or overdamped systems, the
frequency and damping is not readily measured from a response time history. Therefore, some other measurement must be used.
(1) Tests to verify that control feel dynamics 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 underdamped and critically damped cases.
(a) Underdamped Response. Two measurements are required for the period, the time
to first zero crossing (in case a rate limit is
present) and the subsequent frequency of oscillation. It is necessary to measure cycles
on an individual basis in case there are nonuniform periods in the response. Each period
will be independently compared to the respective period of the helicopter control system 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(Ad)
on Figure 1 of this attachment is Section5 percent
of the initial displacement amplitude, Ad,
from the steady state value of the oscillation. Oscillations within the residual band
are considered insignificant. When comparing simulator data to helicopter data, the
process would begin by overlaying or aligning the simulator and helicopter steady state
values and then comparing amplitudes of oscillation peaks, the time of the first zero
crossing, and individual periods of oscillation. To be satisfactory, the simulator must
show the same number of significant overshoots to within one when compared against

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