456 

14 CFR Ch. I (1–1–19 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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