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Four motion-system configurations were examined for
each of three tasks: (1) translational and rotational
motion, (2) translational without rotational motion,
(3) rotational without translational motion, and (4) no
motion. Figure 11 illustrates, in a plan view, the
simulator cab motion for these configurations for Task 1,
which was the ±15° heading turns. In the Translation+
Rotation case, the cab translates and rotates as if it were
placed on the end of a 4.5-ft vector rotating in the
horizontal plane. This case represented physical reality, or
the truth case. In the Translational case, the pilot always
points in the same direction, as the cab translates in x
and y. In the Rotation case, the cab rotates but does not
translate. Finally, in the Motionless case, the cab does not
move.
18
Translational + Rotation Translational
Rotation Motionless
Figure 11. Simulator cockpit motion configurations in plan
view.
When either translational motion or rotational motion was
present for Tasks 1 and 3 (yaw rotational regulation), it
was the full translational or rotational motion calculated
by the vehicle math model. That is, the cockpit provided
the full accelerations that the math model calculated and
that the visual scene provided, along with the effective
motion delays in equations (1)–(4). This statement was
true, except for the longitudinal motion provided by the
translational motion configuration; for yaw turns about a
point, the longitudinal acceleration at the pilot’s station is
always negative (centripetal acceleration in eq. (6)). These
accelerations, if integrated twice to motion-system
position commands, would cause continual longitudinal
cab movement aft for this motion configuration.
Eventually, the simulator cab would exceed its available
longitudinal displacement. Thus, a second order, high-pass
filter was used in the longitudinal axis so that the cab
would return to its initial position in the steady state.
This type filter is typically used in flight simulation, and
it had the form of
˙˙
( )
x
a
s
Ks
s s
com
xp m m
=
+ +
2
2 2zw w2
(9)
where ˙x˙com is the commanded longitudinal acceleration of
the simulator cab, axp is the math model’s longitudinal
acceleration at the pilot’s position, K is the motion gain,
z is the damping ratio, and wm is the filter’s natural
frequency.
As described earlier, Task 2 (180° hover turn) did not
allow full motion. Thus, a high-pass filter of the same
form in equation (9) was used in all axes. The values of K
and wm were empirically selected to use as much cockpit
motion as available (fig. 6).
For Task 3, the vertical motion was always the full math
model vertical motion, even in the Motionless condition.
That is, Motionless for Task 3 refers to the simulator cab
being motionless in the horizontal plane. Table 1 lists
K and wm for each tested configuration in each axis.
A configuration with K = 1 and wm = 1.0E-5 rad/sec
effectively makes the filter in equation (9) unity for the
tasks, considering the task time-scale. The filter damping
ratio (z) was 0.7 for all configurations.
Each of these tasks could be performed on a typical
hexapod motion system, except for the vertical translations
required in Task 3. In particular, the amount of cab
translation corresponding to the ±15° rotations in Task 1
is less than ±1.2 ft. For Task 2, typical pilot aggressiveness
levels resulted in maximum cab yaw orientations of
less than ±5° and lateral travels of less than ±0.5 ft.
Finally, for Task 3, maximum cab yaw orientations were
also less than ±5° and lateral travels were less than
±0.5 ft. However, the vertical simulator cab translation
for Task 3 was near 10 ft, which could not be
accomplished by today’s hexapods.
Procedure
Pilots were asked to rate the overall level of compensation
required for a task using the following descriptors: not-afactor,
minimal, moderate, considerable, extensive, and
maximum-tolerable. These descriptors were taken from the
Cooper-Harper Handling Qualities Scale (ref. 53), and
were thus familiar to all the test pilots. For analysis,
these adjectives were given interval numerical values from
–1 to 4, respectively.
Next, the pilots rated the motion fidelity according to the
following three categories: (1) Low Fidelity—motion
cueing differences from actual flight were noticeable and
objectionable, (2) Medium Fidelity—motion cueing
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Helicopter Flight Simulation Motion Platform Requirements(15)
