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reported in the no-motion condition, it was reported nearly
50% of the time when only rotational motion was
present.
No rotation Rotation
0%
100%
% of time trans. mot. reported
No translation
Translation
Figure 17. Lateral translational motion perception for
Task 1.
The influences of rotational cues on lateral translational
motion reports may have been a result of the pilots
sensing some actual lateral translational acceleration in the
rotation-only configuration. The pilot’s design eye-point
was less than 0.5 ft forward of the motion-system’s
rotation point. It is possible that depending on the
variation in pilots’ posture, this small offset may have
resulted in their vestibular system registering a translational
acceleration. The maximum rotational accelerations
for the likely worst case (0.5 ft offset and a 20°/sec2 yaw
accelerations, see fig. 12) results in a 0.005-g translational
acceleration. This acceleration is small but perhaps just
within a pilot’s threshold (see appendix A).
Pilot reporting of rotational motion, shown in figure 18,
was also affected by an interaction between actual
rotational and translational motion (F(1,5) = 10.4,
p = 0.023). Rotational motion was reported 30% of the
time when no motion was present. The reporting of
rotational motion increased dramatically to 87% when any
motion was given. Apparently, when combined with
visual cues, the translational motion enhances the onset of
a phenomenon called vection. Vection is visually induced
motion; that is, it is the belief that one is moving
through a scene when no motion is actually present (a
phenomenon first investigated and reproduced in a laboratory
by E. Mach in 1875) (ref. 55). A description of how
the vestibular and visual cues combine to produce vection
has been described by Zacharias and Young (ref. 56).
23
No rotation Rotation
0%
100%
% of time rot. mot. reported
No translation
Translation
Figure 18. Rotational motion perception for Task 1.
To summarize the results for this task, translational
motion was clearly the most important motion variable.
Translational motion improved pilot-vehicle performance,
lowered control activity, lowered pilot compensation,
improved pilot impression of motion fidelity, and caused
pilots to believe that rotational motion was present when
it was not. The addition of rotational motion showed no
statistically significant improvement, with the exception
of a marginal statistically significant decrease in the
number of overshoots.
Task 2: 180° Hover Turn
Objective Performance Data. Figure 19 is a
representative time-history of several key variables for the
Translation+Rotation motion and Motionless conditions
in Task 2. Peak math model, and thus visual, yaw rates
for this turn were 50°/sec (not shown). These rates were,
of course, attenuated by the motion system (table 1) so
that it remained within its displacement constraints. In the
Motionless configuration, an increase in yaw overshoots
is noted, which is evident in the displacements, rates,
accelerations, and control inputs. These trends are
consistent with those of Task 1.
Figure 20 depicts, for the four motion conditions, the
means and standard deviations of the number of times
pilots overshot the ±3° heading criterion about the runway
centerline during the 180° turns. When translational
motion was added, the decrease in the number of overshoots
was marginally significant statistically (F(1,4) =
5.40, p = 0.081). Interestingly, in this case, the addition
of rotational motion made the performance worse, and this
result was statistically significant (F(1,4) = 13.26,
p = 0.022). Rotational and translational motion did not
interact in this measure.
These results are not easily explained; however, it must be
remembered that in this task the motion platform never
presented the pilots with full math-model motion. It is
therefore possible that some false cueing in rotation,
owing to the motion filter and its selected parameters, had
a negative effect on performance in this case. A rough
approximation confirms this possibility. For instance, if
one modeled the yaw rotation between 0° and 180° by
y = p w
2
sin t (10)
then the peak yaw rate would be (p/2)w. Since the peak
yaw rates were 50°/sec, this gives a natural frequency of
approximately 0.6 rad/sec. This frequency is a reasonable
approximation of the heading time-histories, if one
discounts the holding times at both 0° and 180°. That is,
　
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