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opposite direction, it is no longer providing confirmation,
but it is instead providing conflict. Consistent with this
explanation is the dictum “bad motion is worse than no
motion” in that spurious motion cues destroy any vection
that has been generated by the other motion and visual
cues (ref. 9).
Earlier it was stated that Meiry’s yaw experiment (ref. 22)
showed that the addition of rotational motion improved
performance. It is possible that the difference in experimental
setup between that experiment and the one reported
here could account for the difference, although the results
of Task 1 in the present study did agree with those of
Meiry by showing a marginal improvement in performance.
In Meiry’s experiment, pilots countered a considerable
yaw rotational disturbance (white noise with a
15° rms), did not control a model representative of a
helicopter (the model resulted in yaw rate proportional to
pilot input at all frequencies), and the visual system was a
line on an oscilloscope. So, with this deprived-cue visual
system and with only a yaw rotational cue, yaw rotational
motion might have helped. However, Meiry’s study did
not determine if the lateral translational motion cue could
have provided an equivalent substitute for the yaw
rotational cue, which was the principal result shown here.
Effect of Results
The effect of these results is twofold. First, for simulators
with independent-axis motion drives, that is, dedicated
servos for each axis, excluding the yaw platform rotational
degree-of-freedom capability would result in a cost savings
to manufacturers. To users of motion simulators that
already have a yaw rotational degree of freedom, less time,
if any, needs to be spent configuring and tuning that axis
for a given application.
Second, for simulators without independent-axis motion
drives, such as the common synergistic hexapod motion
systems (fig. 33), not using the yaw platform rotational
degree of freedom allows for more available displacement
in the other motion axes. Since the same set of actuators
is used to move the platform to a desired position or
orientation, the available displacement or orientation of
any axis is a function of the displacement or orientation in
another axis.
An example of the dependency is shown in figure 34,
which was generated by Cooper and Howlett (ref. 40).
Loss of available motion in the longitudinal, lateral, and
vertical axes also occurs for rotations about the yaw axis.
Thus, users should disable yaw rotation and thereby gain
additional benefits in axes that provide added value in
flight simulation.
Figure 33. Typical hexapod motion drive system.
60 y = 0°
y = 20°
y = 40°
y = 60°
–60
–60 0
Pitch attitude, deg
Roll attitude, deg
60
0
Figure 34. Effect of yaw angle orientation on pitch and roll
angles.
31
4. Vertical Experiment I:
Altitude Control
Background
For helicopters, only one study has focused extensively on
the vertical axis (ref. 24). In that study, the effects of the
motion-filter natural frequency were examined, but almost
always with a high-frequency gain of unity. The research
described here also examined motion-filter natural
frequency, but in addition evaluated the combined effects
of motion-filter gain.
The setup of the experiment is described, including a task
description, the math model, and the simulator cueing
systems. This description is followed by the results,
which subsequently validate a revised motion-fidelity
criterion for the vertical axis.
Experimental Setup
Task
The vertical task required the pilot to increase aircraft
altitude 10 ft. To do so, the pilot used visual cues to place
the horizon between two red squares on an object 50 ft
away, as shown in figure 35. These sighting objects were
used previously, in both flight and simulation, for vehicle
model validation (ref. 57). The red region had a height of
0.75 ft, which was the final altitude tolerance.
0.75 ft
2.25 ft
10 ft
Bob-ups
Red
Horizon
Figure 35. Sighting object for vertical tracking.
An altitude displacement, or bob-up, of 10 ft was chosen
for two reasons. First, 10-ft bob-ups could be performed
in the VMS without any attenuation of the math model
accelerations. This situation is referred to as 1:1 motion,
since the simulator cockpit motion is the same motion as
that calculated by the vehicle model, and the motion is
also the same as that shown by the visual scene. Second,
　
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